Introduction
Thünen explains:Herring Trapped in Climate ChangeWhy herring is lacking recruitment
A Multimedia feature by Nadine Kraft, Patrick Polte, Annemarie Schütz and Christopher Zimmermann
Translation: Cornelius Hammer and Dina Führmann
Herring – Silver of the Baltic Sea
Not only
for the shining colour of its scales, herring is the “Silver of the Seas”.
It was the
foundation of the growth and power of the medieval Hanseatic Trade League.
Clupea
harengus, the
herring, induced technological development and still today sustains the
livelihood of many people around the Baltic Sea.
In the Western Baltic Sea, however, it is increasingly lacking recruitment. This is what we first noticed 15 years ago during our annual Rügen Herring Larvae Survey.
Since then, we have been systematically investigating the reasons of declining offspring production. By now it has become evident that the warming of the sea along the migration routes (Ref. 1), as well as the shift of the yearly successions, the so-called phenology (2), are the main reasons for the decline, caused most probably by man-made climate change.
read more about the stock development
In the Western Baltic Sea, however, it is increasingly lacking recruitment. This is what we first noticed 15 years ago during our annual Rügen Herring Larvae Survey.
Since then, we have been systematically investigating the reasons of declining offspring production. By now it has become evident that the warming of the sea along the migration routes (Ref. 1), as well as the shift of the yearly successions, the so-called phenology (2), are the main reasons for the decline, caused most probably by man-made climate change.
read more about the stock development
Importance for the Coastal Fishery
Importance for the Coastal Fishery
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Fishers,
fishing vessels and freshly caught fish offered at the quayside are still decisive
for the flair of the harbours along the Baltic Sea coast, although the number
of vessels continuously declines. In the federal state of Schleswig-Holstein, the cod catches dominate
the landings of the fishery, whereas in Mecklenburg-Western Pomerania it is the
herring that is the mainly targeted fish and constitutes the basis of the business.
In recent years, however, the quotas had to be reduced repeatedly, between 2017 and 2021 by 94 percent. As a consequence, the fishery in Mecklenburg-Western Pomerania experiences currently its second large structural change since the German reunification.
In recent years, however, the quotas had to be reduced repeatedly, between 2017 and 2021 by 94 percent. As a consequence, the fishery in Mecklenburg-Western Pomerania experiences currently its second large structural change since the German reunification.
Impact of Climate Change on Fish Stocks
Of all
marginal seas of the world, it is the Baltic Sea, the second largest brackish
sea in the world, that is warming fastest since the 1980s. Its mean water level
increased, while its salinity has markedly decreased. For
herring this poses a serious challenge. Winters have become warmer than they
used to be only a few years ago. The elevated temperature brings the spawning
and the production of its offspring out of sync with the other ecological
processes. In contrast, the decreased salinity poses no problem for the herring.
It can spawn successfully even in the Gulf of Bothnia at a salinity of 0.2
permille.
The impact of climate change on the fisheries differs considerably by region. An overview:
The impact of climate change on the fisheries differs considerably by region. An overview:
Herring research
The Herring Fishery
Being
asked, many fishers don’t share our concerns regarding the Western Baltic herring.
This, because the herring is a perfect example for a pelagic fish species: It
lives and migrates in great schools to the spawning grounds. Therefore, fishers
will continue to catch the herring in great numbers with full nets, and will do
so, in principle, even up to the very last school, giving the false impression
of vastly abundant fish. This is seemingly a paradoxon, since the catches do
not reflect the true stock status of the fish in the distribution area.
The Rügen Herring Larvae Survey
The Rügen Herring Larvae Survey
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A more
realistic picture of the biomass is provided by long time-series of the fishery
research. Such a time series is the Rügen Herring Larvae Survey (RHLS) in the
Greifswalder Bodden and the adjacent Strelasund, being one of the longest and
most extensive research surveys on herring early life history in the world. The
survey started in 1977, and since 1992 we annually determine the number of
herring larvae by weekly surveys throughout the entire spawning season
(March-June), covering at least 30 stations each time. Such a long time series
with extensive sampling of this magnitude makes it a survey of highest temporal
and spatial resolution.
The Rügen Herring Larvae Survey
The Western Baltic herring
stock is highly migratory: In summer the schools
are feeding in the Skagerrak and Kattegat and even as far as in the northeastern
North Sea. During winter they form schools in The Sound, the straight between
Denmark and Sweden. From here they migrate to the spawning grounds at the
southern coast of the Western Baltic Sea in spring. Their most important nursery
is a 500 square kilometre lagoon, the Greifswalder Bodden at the German Baltic
Sea coast. Here, herring spawn since hundreds of years with great continuity,
attaching their eggs in the shallow zones at submerged plants. However, since
about 15 years the number of herring larvae decreases continually. Amongst all
it might imply, one thing is sure: Less herring larvae will lead to less adult
fish three years later that can eventually be caught by fishers. (5)
Watch the interview with Dr. Patrick Polte, Herring recruitment group at the Thünen Institute of Baltic Sea Fisheries.
Watch the interview with Dr. Patrick Polte, Herring recruitment group at the Thünen Institute of Baltic Sea Fisheries.
Basic Data for Setting Catch Quotas
The data
derived from the Rügen Herring Larvae Survey provide a recruitment index that
is made available to the International Council for the Exploration of the Sea (ICES).
Together with other indicators and internationally collected data, the
respective workings groups of ICES use this index to compute and assess the
state of the stock of the Western Baltic herring and of many other stocks of
the North Atlantic. Based on the results, ICES develops an advice for the
politicians in charge, recommending total maximum catches for the stocks for
the next years. The scientific expertise is therefore the basis for a sustainable
management of the stocks.
After 2006 the politicians have reduced the total allowable catches for herring in the Western Baltic Sea initially only hesitantly. Between 2011 and 2017 they followed the scientific recommendation strictly. Since 2018, the EU-Council of Ministers limits the catches in the southern management area to a quantity that should allow the stock to recover within a couple of years. For 2022, the EU and Norway agreed to follow the same approach in the northern management area.
After 2006 the politicians have reduced the total allowable catches for herring in the Western Baltic Sea initially only hesitantly. Between 2011 and 2017 they followed the scientific recommendation strictly. Since 2018, the EU-Council of Ministers limits the catches in the southern management area to a quantity that should allow the stock to recover within a couple of years. For 2022, the EU and Norway agreed to follow the same approach in the northern management area.
Why is there less Herring Recruitment?
The search for
causes started when we noted drastically declining numbers of 20 millimetre
larvae in the catches of the Rügen Herring Larvae Survey from 2006. This is the
size fraction of larvae used to calculate the recruitment index. Since then,
six hypotheses were tested by means of extensive fundamental ecological research.
The Baltic Sea is too warm
The Baltic Sea is too warm
Surface temperature of the Baltic Sea in February: Average of 1990-2020 (left); 2020 (right), Source: BSH.
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In addition
to the extensive data series obtained from our own survey, we have utilized all
available data on the Greifswalder Bodden and the Strelasund, such as water
temperature, nutrients and primary production, and applied them to our
statistical models. We aimed at evaluating why herring arrive at its spawning
grounds earlier than 30 years ago and deposit their eggs earlier.
The results of the modelling identified weaker and later-in-the-year winter periods as main cause: The change in temperature explains more than 50 percent of the declining recruitment of the stock and is therefore the most important single factor. The remaining 50 percent are explained by a number of other factors (1).
Over the past 30 years the February surface temperature of the Baltic Sea was never as high as in 2020. At the same time the lowest larvae index N20 was determined.
The results of the modelling identified weaker and later-in-the-year winter periods as main cause: The change in temperature explains more than 50 percent of the declining recruitment of the stock and is therefore the most important single factor. The remaining 50 percent are explained by a number of other factors (1).
Over the past 30 years the February surface temperature of the Baltic Sea was never as high as in 2020. At the same time the lowest larvae index N20 was determined.
Surface temperature of the Baltic Sea in February: Average of 1990-2020 (left); 2020 (right), Source: BSH.
From Spawn to Herring
The warmer
water not only induces earlier spawning. Rather, the deposited eggs develop
faster and as a result, herring larvae hatch earlier. After hatch, larvae are
able to feed from their yolk sack. But due to the higher temperatures their
metabolism is elevated as well, causing the yolk sack supply being depleted
earlier than previously. As soon as this stage is reached, external food must
be available.
As a consequence the larvae now require external food about three weeks earlier, as compared to 30 years ago. This food consists of the early stages of larvae of zooplankton crustaceans. First results of our additional studies show however, that at such early time in the year, these prey items are not yet available in sufficient abundance. While the spawning activity of herring is mainly driven by temperature, zooplankton larvae depend on the occurrence of phytoplankton and thus on the light regime. As a result, many herring larvae starve and for this reason the number of adult herring decreases further from year to year.
However, even for larvae hatching later, the warmer environment into which they are born poses a threat and causes increased mortality, for example due to cardiac arrhythmia (13).
As a consequence the larvae now require external food about three weeks earlier, as compared to 30 years ago. This food consists of the early stages of larvae of zooplankton crustaceans. First results of our additional studies show however, that at such early time in the year, these prey items are not yet available in sufficient abundance. While the spawning activity of herring is mainly driven by temperature, zooplankton larvae depend on the occurrence of phytoplankton and thus on the light regime. As a result, many herring larvae starve and for this reason the number of adult herring decreases further from year to year.
However, even for larvae hatching later, the warmer environment into which they are born poses a threat and causes increased mortality, for example due to cardiac arrhythmia (13).
From Spawn to Herring
Impact on the Fishery
-
The Western Baltic herring stock can
be managed sustainably, even if its recruitment remains low. About 20,000 tons could
be harvested sustainably annually from the Western Baltic Sea under the prevailing
ecological conditions. Although such a quantity is only about half of what was
caught 30 years ago, it is still ten times as much as harvested in 2021.
-
With the knowledge of the causes of
reduced productivity, it remains in our hands to improve the ecological conditions
for better herring recruitment in spite of climate change.
-
Without herring the coastal fishery
of Mecklenburg-Western Pomerania faces its end. It is worth to undertake all
conceivable effort to save the fishery, for economic, cultural and even ecological
reasons.
This is how the Herring can recover
This is how the Herring can recover
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The drastic
decline of the herring stock in the Western Baltic Sea cannot entirely be
attributed to high fishing pressure. During the past 15 years we at the Thünen
Institute could show that the reduced productivity of the herring is a result
of the warming of the sea and a temporal shift of the seasons. These changes
have an immediate influence specifically on the coastal fishery. With a decreasing
stock, the number of fish available to the fishery decreases accordingly. Although
man-made climate change cannot be mitigated in the short term, there are at
least two potential ways how to support Western Baltic herring in its recovery:
- With significantly reduced fishing
pressure in all management areas the stock is capable to recover, even if its productivity
remains low. It will be possible to harvest the stock again sustainably in a
couple of years and it will then provide about half the yield of what was
harvested 30 years ago.
- The productivity of the stock can be
enhanced by reducing other stressors. Even the reduction of the nutrient load
in the water capture area of the rivers discharging into the Greifswalder
Bodden would help to increase the resilience of the stock.
Sources and figures
Sources
Sources
Fullscreen
(1) Dodson JJ, Daigle G, Hammer C, Polte P, Kotterba P, Winkler G, Zimmermann C (2019) Environmental determinants of larval herring (Clupea harengus) abundance and distribution in the western Baltic Sea. Limnol Oceanogr 64(1):317-329,
DOI:10.1002/lno.11042
(Thünen Institute internal funding and EU Data Collection Framework)
(2) Polte P, Gröhsler T, Kotterba P, Nordheim L von, Moll D, Santos J, Rodriguez-Tress P, Zablotski Y, Zimmermann C (2021) Reduced reproductive success of Western Baltic herring (Clupea harengus) as a response to warming winters. Front Mar Sci 8:589242,
DOI:10.3389/fmars.2021.589242
(Thünen Institute internal funding and EU Data Collection Framework)
(3) Cheung WWL, Reygondeau G, Frölicher TL (2016) Large benefits to marine fisheries of meeting the 1.5°C global warming target. Science 354 (6319): 1591-1594,
DOI: 10.1126/science.aag2331
(4) Kniebusch M, Meier HEM, Neumann T, Börgel F (2019) Temperature variability of the Baltic Sea since 1850 and attribution to atmospheric forcing variables. J. Geophys. Res. Oceans 124: 4168-4187,
DOI: 10.1029/2018JC013948
(5) Oeberst R, Klenz B, Gröhsler T, Dickey-Collas M, Nash RDM, Zimmermann C (2009) When is year-class strength determined in western Baltic herring? – ICES Journal of Marine Science, 66: 1667–1672.
DOI: 10.1093/icesjms/fsp143
(Thünen Institute internal funding and EU Data Collection Framework)
(6) Kanstinger P, Beher J, Grenzdörffer G, Hammer C, Huebert KB, Stepputtis D, Peck M (2018) What is left? Macrophyte meadows and Atlantic herring (Clupea harengus) spawning sites in the Greifswalder Bodden, Baltic Sea. Estuar Coast Shelf Sci 201:72-81,
DOI:10.1016/j.ecss.2016.03.004
(Project DONG Power Plant Environmental Effects Assessment)
(7) Bauer RK, Stepputtis D, Gräwe U, Zimmermann C, Hammer C (2013) Wind-induced variability in coastal larval retention areas: a case study on Western Baltic spring-spawning herring. Fisheries Oceanogr 22(5):388-399, DOI:10.1111/fog.12029
(Thünen Institute internal funding)
(8) Kotterba P, Moll D, Hammer C, Peck M, Oesterwind D, Polte P (2017) Predation on Atlantic herring (Clupea harengus) eggs by the resident predator community in coastal transitional waters. Limnol Oceanogr 62(6):2616-2628, DOI:10.1002/lno.10594
Kotterba P, Kühn C, Hammer C, Polte P (2014) Predation of threespine stickleback (Gastrosteus aculeatus) on the eggs of Atlantic herring (Clupea harengus) in a Baltic Sea lagoon. Limnol Oceanogr 59(2):578-587, DOI:10.4319/lo.2014.59.2.0578
(EU-Projects BONUS Bio-C3 and Inspire)
(9) Moll D, Kotterba P, Nordheim L von, Polte P (2018) Storm-induced Atlantic herring (Clupea harengus) egg mortality in Baltic Sea inshore spawning areas. Estuaries Coasts 41(1):1-12, DOI:10.1007/s12237-017-0259-5
(EU Data Collection Framework)
(10) Nordheim L von, Kotterba P, Moll D, Polte P (2020) Lethal effect of filamentous algal blooms on Atlantic herring (Clupea harengus) eggs in the Baltic Sea. Aquatic Conserv 30(7):1362-1372, DOI:10.1002/aqc.3329 (Stipend Deutsche Bundesstiftung Umwelt)
(11) Finke A et al in prep
(Stipend Studienstiftung des deutschen Volkes)
(12) Līvdane L et al in prep
(Thünen Institute internal funding and EU Data Collection Framework)
(13) Moyano M, Illing B, Polte P, Kotterba P, Zablotski Y, Gröhsler T, Hüdepohl P, Cooke SJ, Peck M (2020) Linking individual physiological indicators to the productivity of fish populations: A case study of Atlantic herring. Ecol Indic 113:106146,
DOI:10.1016/j.ecolind.2020.106146
DOI:10.1002/lno.11042
(Thünen Institute internal funding and EU Data Collection Framework)
(2) Polte P, Gröhsler T, Kotterba P, Nordheim L von, Moll D, Santos J, Rodriguez-Tress P, Zablotski Y, Zimmermann C (2021) Reduced reproductive success of Western Baltic herring (Clupea harengus) as a response to warming winters. Front Mar Sci 8:589242,
DOI:10.3389/fmars.2021.589242
(Thünen Institute internal funding and EU Data Collection Framework)
(3) Cheung WWL, Reygondeau G, Frölicher TL (2016) Large benefits to marine fisheries of meeting the 1.5°C global warming target. Science 354 (6319): 1591-1594,
DOI: 10.1126/science.aag2331
(4) Kniebusch M, Meier HEM, Neumann T, Börgel F (2019) Temperature variability of the Baltic Sea since 1850 and attribution to atmospheric forcing variables. J. Geophys. Res. Oceans 124: 4168-4187,
DOI: 10.1029/2018JC013948
(5) Oeberst R, Klenz B, Gröhsler T, Dickey-Collas M, Nash RDM, Zimmermann C (2009) When is year-class strength determined in western Baltic herring? – ICES Journal of Marine Science, 66: 1667–1672.
DOI: 10.1093/icesjms/fsp143
(Thünen Institute internal funding and EU Data Collection Framework)
(6) Kanstinger P, Beher J, Grenzdörffer G, Hammer C, Huebert KB, Stepputtis D, Peck M (2018) What is left? Macrophyte meadows and Atlantic herring (Clupea harengus) spawning sites in the Greifswalder Bodden, Baltic Sea. Estuar Coast Shelf Sci 201:72-81,
DOI:10.1016/j.ecss.2016.03.004
(Project DONG Power Plant Environmental Effects Assessment)
(7) Bauer RK, Stepputtis D, Gräwe U, Zimmermann C, Hammer C (2013) Wind-induced variability in coastal larval retention areas: a case study on Western Baltic spring-spawning herring. Fisheries Oceanogr 22(5):388-399, DOI:10.1111/fog.12029
(Thünen Institute internal funding)
(8) Kotterba P, Moll D, Hammer C, Peck M, Oesterwind D, Polte P (2017) Predation on Atlantic herring (Clupea harengus) eggs by the resident predator community in coastal transitional waters. Limnol Oceanogr 62(6):2616-2628, DOI:10.1002/lno.10594
Kotterba P, Kühn C, Hammer C, Polte P (2014) Predation of threespine stickleback (Gastrosteus aculeatus) on the eggs of Atlantic herring (Clupea harengus) in a Baltic Sea lagoon. Limnol Oceanogr 59(2):578-587, DOI:10.4319/lo.2014.59.2.0578
(EU-Projects BONUS Bio-C3 and Inspire)
(9) Moll D, Kotterba P, Nordheim L von, Polte P (2018) Storm-induced Atlantic herring (Clupea harengus) egg mortality in Baltic Sea inshore spawning areas. Estuaries Coasts 41(1):1-12, DOI:10.1007/s12237-017-0259-5
(EU Data Collection Framework)
(10) Nordheim L von, Kotterba P, Moll D, Polte P (2020) Lethal effect of filamentous algal blooms on Atlantic herring (Clupea harengus) eggs in the Baltic Sea. Aquatic Conserv 30(7):1362-1372, DOI:10.1002/aqc.3329 (Stipend Deutsche Bundesstiftung Umwelt)
(11) Finke A et al in prep
(Stipend Studienstiftung des deutschen Volkes)
(12) Līvdane L et al in prep
(Thünen Institute internal funding and EU Data Collection Framework)
(13) Moyano M, Illing B, Polte P, Kotterba P, Zablotski Y, Gröhsler T, Hüdepohl P, Cooke SJ, Peck M (2020) Linking individual physiological indicators to the productivity of fish populations: A case study of Atlantic herring. Ecol Indic 113:106146,
DOI:10.1016/j.ecolind.2020.106146
Figures and photographs
1. Herring in the net: ©Annemarie Schütz/Thünen Institute
2. Silver of the Baltic Sea: ©Daniel Stepputtis/Thünen Institute
3. Herring Recruitment Index N20: ©Christopher Zimmermann/Thünen Institute
4. Stock development: ©www.fischbestaende-online.de
5. Distribution and management areas: ©Christopher Zimmermann/Thünen Institute
6. Herring fishers Gr. Zicker: ©Andrea Müller/Thünen Institute
7. Significance for Mecklenburg-Western Pomerania: ©Christopher Zimmermann/Thünen Institute
8. Western Baltic Sea from the ISS: Image courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center,
http://eol.jsc.nasa.gov, file STS099-751-33_3.JPG, accessed 13 Feb 2021
9. Winners and losers of climate change: after Cheung et al. 2016, redrawn and modified
10. Lofoten cutter: ©lowe99/stock.adobe.com
11. Lofoten cod fisher: ©Ulf Berglund/MSC
12. Tropical fishery: ©MSC
13. Sea bass: ©ILYA AKINSHIN/stock.adobe.com
14. Beach fishery Baltic Sea: ©Christopher Zimmermann/Thünen Institute
15. Baltic herring pairtrawl fishery: ©Lena Ganssmann/MSC
16. Herring haul: ©Christopher Zimmermann/Thünen Institute
17. Larvae net Bongo: ©Harry Strehlow/Thünen Institute
18. RHLS station grid: ©Christopher Zimmermann/Thünen Institute
19. Interview P. Polte, FFS Clupea: ©Annemarie Schütz/Thünen Institute, using video material by Annemarie Schütz/Thünen Institute and a figure by Christopher Zimmermann/Thünen Institute
20. ICES logo: ©ICES.dk
21. Herring larvae: ©Dagmar Stephan/Thünen Institute
22. Distribution map of water plants GWB 1938: Map based on species distribution reported by Seifert (1938), Subklew (1955) and Munkes (2005), cited in Kanstinger et al. 2018
23. Distribution map of water plants GWB 2009: Görres Grenzdörffer/Univ. Rostock, cited in Kanstinger et al. 2018
24. Baltic Sea storm: ©Daniel Stepputtis/Thünen Institute
25. Larval drift: ©Robert Bauer/Thünen Institute and Ulf Graewe/Leibniz IOW
26. Stickleback feeding experiments: ©Paul Kotterba/Thünen Institute
27. Egg aggregations washed ashore: ©Dorothee Moll/Thünen Institute
28. Overgrown herring spawn: ©Lena von Nordheim/Thünen Institute
29. Copepods and nauplia: ©Gesche Winkler/UQAR
30. Diatom bloom in the Baltic Sea: ©NASA Ocean Color Image Gallery (2020, August), https://eoimages.gsfc.nasa.gov/images/imagerecords/147000/147135/baltic_oli_2020228_lrg.jpg, accessed 12 Feb 2021
31. Prerow Baltic Sea coast: ©Cornelius Hammer/Thünen Institute
32. Sea surface temperature in February: ©Bundesamt für Seeschifffahrt und Hydrographie/Remote Sensing Group
33. Phenology shift: ©Christopher Zimmermann/Thünen Institute
34. Animation phenology shift: ©Annemarie Schütz, Nadine Kraft, Harry Strehlow/Thünen Institute
35. Interview Christopher Zimmermann: ©Annemarie Schütz/Thünen Institute using a figure by Christopher Zimmermann/Thünen Institute, video material by Annemarie Schütz/Thünen Institute and a photo by ©Lena Ganssmann/MSCVideo
36. Herring in a trawl: ©Annemarie Schütz/Thünen Institute
37. Herring recruitment stressors: ©Christopher Zimmermann/Thünen Institute
2. Silver of the Baltic Sea: ©Daniel Stepputtis/Thünen Institute
3. Herring Recruitment Index N20: ©Christopher Zimmermann/Thünen Institute
4. Stock development: ©www.fischbestaende-online.de
5. Distribution and management areas: ©Christopher Zimmermann/Thünen Institute
6. Herring fishers Gr. Zicker: ©Andrea Müller/Thünen Institute
7. Significance for Mecklenburg-Western Pomerania: ©Christopher Zimmermann/Thünen Institute
8. Western Baltic Sea from the ISS: Image courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center,
http://eol.jsc.nasa.gov, file STS099-751-33_3.JPG, accessed 13 Feb 2021
9. Winners and losers of climate change: after Cheung et al. 2016, redrawn and modified
10. Lofoten cutter: ©lowe99/stock.adobe.com
11. Lofoten cod fisher: ©Ulf Berglund/MSC
12. Tropical fishery: ©MSC
13. Sea bass: ©ILYA AKINSHIN/stock.adobe.com
14. Beach fishery Baltic Sea: ©Christopher Zimmermann/Thünen Institute
15. Baltic herring pairtrawl fishery: ©Lena Ganssmann/MSC
16. Herring haul: ©Christopher Zimmermann/Thünen Institute
17. Larvae net Bongo: ©Harry Strehlow/Thünen Institute
18. RHLS station grid: ©Christopher Zimmermann/Thünen Institute
19. Interview P. Polte, FFS Clupea: ©Annemarie Schütz/Thünen Institute, using video material by Annemarie Schütz/Thünen Institute and a figure by Christopher Zimmermann/Thünen Institute
20. ICES logo: ©ICES.dk
21. Herring larvae: ©Dagmar Stephan/Thünen Institute
22. Distribution map of water plants GWB 1938: Map based on species distribution reported by Seifert (1938), Subklew (1955) and Munkes (2005), cited in Kanstinger et al. 2018
23. Distribution map of water plants GWB 2009: Görres Grenzdörffer/Univ. Rostock, cited in Kanstinger et al. 2018
24. Baltic Sea storm: ©Daniel Stepputtis/Thünen Institute
25. Larval drift: ©Robert Bauer/Thünen Institute and Ulf Graewe/Leibniz IOW
26. Stickleback feeding experiments: ©Paul Kotterba/Thünen Institute
27. Egg aggregations washed ashore: ©Dorothee Moll/Thünen Institute
28. Overgrown herring spawn: ©Lena von Nordheim/Thünen Institute
29. Copepods and nauplia: ©Gesche Winkler/UQAR
30. Diatom bloom in the Baltic Sea: ©NASA Ocean Color Image Gallery (2020, August), https://eoimages.gsfc.nasa.gov/images/imagerecords/147000/147135/baltic_oli_2020228_lrg.jpg, accessed 12 Feb 2021
31. Prerow Baltic Sea coast: ©Cornelius Hammer/Thünen Institute
32. Sea surface temperature in February: ©Bundesamt für Seeschifffahrt und Hydrographie/Remote Sensing Group
33. Phenology shift: ©Christopher Zimmermann/Thünen Institute
34. Animation phenology shift: ©Annemarie Schütz, Nadine Kraft, Harry Strehlow/Thünen Institute
35. Interview Christopher Zimmermann: ©Annemarie Schütz/Thünen Institute using a figure by Christopher Zimmermann/Thünen Institute, video material by Annemarie Schütz/Thünen Institute and a photo by ©Lena Ganssmann/MSCVideo
36. Herring in a trawl: ©Annemarie Schütz/Thünen Institute
37. Herring recruitment stressors: ©Christopher Zimmermann/Thünen Institute
Climate Change and Marine Fish
Impact of Climate Change on Fish Stocks WorldwideWinners and losers
By now we
have a reasonably good understanding of the impacts of climate change on fish
stocks for many parts of the high seas. The distribution areas of all
commercially exploited stocks will shift gradually towards the poles. This
shift will mostly leave losers, specifically in the tropics. In contrast, the
few fish stocks profiting from climate change are found in the high latitudes in
areas so far covered by ice. (3)
Impact of Climate Change: The North Polar Sea
Impact of Climate Change: The North Polar Sea
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The
northern extensive shelf areas so far covered by ice will experience an
increase in productivity. For this reason, the polar and subpolar seas will be
profiting from climate change, as it will be possible to catch more fish. For the western
Europeans, who source fish mainly from this region, such a shift is likely to
be advantageous.
Impact of Climate Change: The Tropics
The impact of Climate Change will have its strongest effects in the tropics. Fish stocks move to higher latitudes without being replaced by species that can sustain even higher temperatures. In addition, people in the tropics depend on fish as a source of animal protein more than anywhere else.
Impact of Climate Change on Coastal Fisheries
The coastal
fisheries of our latitudes are losers as well, because generally they are not
able to follow the emigrating fish stocks. Their fishing rights are limited to
coastal areas in their proximity. Newly immigrating species will hardly be able
to fully compensate the loss during the upcoming decades. An example is the
European sea bass that only played an insignificant role in the fisheries of
the north European seas, now however being caught in greater numbers in The
Channel. Although this fishery is highly profitable, it cannot even marginally compensate
for the losses in the cod fishery.
Impact of Climate Change: Baltic Sea
Impact of Climate Change: Baltic Sea
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Producing a
forecast of climate change effects for a marginal sea like the Baltic Sea is a difficult
task (4), since the influence of the surrounding land plays a much bigger role
than for the open oceans.
Two examples: With increasing global temperatures more water evaporates, leading generally to more precipitation. In the catchment area of the Baltic more freshwater running into the Baltic Sea will therefore lower its salinity. Marine fish, such as cod or plaice, are physiologically adapted to higher salinities and will become less productive in fresher water. But climate change might also induce stronger and more frequent winds from the west during the autumn storm season. This might, as a consequence, push more salt water of North Sea origin into the Baltic Sea, inducing an increase of the salinity, which, as a result would increase the recruitment of many marine species. back
Two examples: With increasing global temperatures more water evaporates, leading generally to more precipitation. In the catchment area of the Baltic more freshwater running into the Baltic Sea will therefore lower its salinity. Marine fish, such as cod or plaice, are physiologically adapted to higher salinities and will become less productive in fresher water. But climate change might also induce stronger and more frequent winds from the west during the autumn storm season. This might, as a consequence, push more salt water of North Sea origin into the Baltic Sea, inducing an increase of the salinity, which, as a result would increase the recruitment of many marine species. back
Hypotheses on the Reduction of Recruitment
Before/after view
Comparison of occurrence of water plants in the Greifswalder Bodden 1938 and 2009.
Are there too few Aquatic Plants?
Herring attach their extremely sticky spawn to the leaves and stems of
so-called macrophytes. In the Greifswalder Bodden these are various sea grass
species and larger aquatic plants. Still 80 years ago these were abundantly
available even in the deeper parts of the area, as is shown in a publication from
1938. In an arial survey of 2009 plants were visible in near-shore areas only,
and we raised the question whether herring still finds enough aquatic plants to
attach their eggs to (6). A comparison shows however, that, although the
herring stock was in good shape, there were considerably fewer aquatic plants even in
the 1980s and 1990s as compared to the 1930s. A lack of plants can therefore not
explain the decline of the decreasing herring recruitment alone that is
observed since the early 2000s.
Are the Larvae flushed out of the Bodden?
The Greifswald Bodden is a semi-closed lagoon and as such an ideal
nursery ground for juvenile herring. The lagoon is shallow, relatively large
but still almost closed from the open sea. This bears two great advantages:
Firstly, the larvae swim in a “soup of prey particles”. Secondly, they cannot be flushed into
the open sea where they would hardly find as much prey as within this juvenile
retention area.
However, during the past years the storms have increased in strength and frequency, leading to the question, whether more larvae than previously are flushed out of the Bodden into the Pomeranian Bight and starve there? In an analysis we could show that the larvae were indeed moved faster and stronger within the lagoon (7) but that they are not flushed out of the Greifswalder Bodden in greater numbers. This hypothesis for the declining recruitment is therefore rejected.
However, during the past years the storms have increased in strength and frequency, leading to the question, whether more larvae than previously are flushed out of the Bodden into the Pomeranian Bight and starve there? In an analysis we could show that the larvae were indeed moved faster and stronger within the lagoon (7) but that they are not flushed out of the Greifswalder Bodden in greater numbers. This hypothesis for the declining recruitment is therefore rejected.
Has the Number of Predators on the Spawn increased?
A third possibility for the declining recruitment might be an increased
number of predators, such as birds or other fish. The number of spawn-eating
long-tailed ducks has declined however to such an extent that they can hardly
be made responsible for the weak recruitment of herring.
Remains the predatory fish, as for example the stickleback: They are small, are not fished since they are economically of little importance and consume herring spawn in great quantities. Have the sticklebacks multiplied to such an extent since the early 2000s that they have become a threat for the herring spawn? Due to the fact that there are no data on stickleback abundance in the area prior to 2006, this hypothesis can hardly be verified.
Based on our investigations, we can, however, demonstrate that the stickleback plays a much bigger role as predator on the spawn than previously assumed (8). In addition, the sticklebacks seem to consume even more herring eggs, the higher their density on water plants is.
Remains the predatory fish, as for example the stickleback: They are small, are not fished since they are economically of little importance and consume herring spawn in great quantities. Have the sticklebacks multiplied to such an extent since the early 2000s that they have become a threat for the herring spawn? Due to the fact that there are no data on stickleback abundance in the area prior to 2006, this hypothesis can hardly be verified.
Based on our investigations, we can, however, demonstrate that the stickleback plays a much bigger role as predator on the spawn than previously assumed (8). In addition, the sticklebacks seem to consume even more herring eggs, the higher their density on water plants is.
Is Spawn washed up the Shore by Storms?
Another hypothesis takes up with the increasing number of storms. Could
it be that the storms and the swell tear off the aquatic plants now only
located in shallow water and wash them ashore, where the eggs will quickly die?
We indeed found aquatic plants in great quantities covered thickly with eggs on the shore (9). However, we can not affirm this as the single cause for declining herring recruitment since data prior to 2006 are not available.
We indeed found aquatic plants in great quantities covered thickly with eggs on the shore (9). However, we can not affirm this as the single cause for declining herring recruitment since data prior to 2006 are not available.
Are increased Nutrient Loads the Cause?
According to scientific assessments, the
greatest problem for the Baltic Sea and its coastal lagoons is eutrophication, the excessive
load of nutrients being discharged from land by more than 60 small and larger
rivers. They bring not only freshwater but also phosphorus and nitrogen
compounds, which stem mainly from agriculture, though from traffic as well.
The most conspicuous effect of the eutrophication is the strong growth of very small planktonic algae in the Greifswalder Bodden. This phytoplankton reduces the light penetration in greater depths, which is the reason why aquatic plants suitable as spawning surface for herring are only found in the shallow littoral zones close to the shore, where light can still reach the bottom.
Eutrophication also promotes the growth of filamentous algae and fungi. They overgrow the aquatic plants and even the large egg deposits of the herring, reducing the gas exchange of the eggs and the growth of the plants. Moreover, we could show that the overgrowing algae emit poisonous substances that inhibit the development of the herring embryos (10). One conclusion from this study is that if it was possible to stop the nutrient inflow from the Peene river into the Greifswald Bodden during the spawning season, this would have immediate positive effects by reducing the excessive growth of the algae.
The most conspicuous effect of the eutrophication is the strong growth of very small planktonic algae in the Greifswalder Bodden. This phytoplankton reduces the light penetration in greater depths, which is the reason why aquatic plants suitable as spawning surface for herring are only found in the shallow littoral zones close to the shore, where light can still reach the bottom.
Eutrophication also promotes the growth of filamentous algae and fungi. They overgrow the aquatic plants and even the large egg deposits of the herring, reducing the gas exchange of the eggs and the growth of the plants. Moreover, we could show that the overgrowing algae emit poisonous substances that inhibit the development of the herring embryos (10). One conclusion from this study is that if it was possible to stop the nutrient inflow from the Peene river into the Greifswald Bodden during the spawning season, this would have immediate positive effects by reducing the excessive growth of the algae.
Or is there too little Prey available for the Larvae?
Maybe herring larvae do not find enough food? We observe that presently herring
larvae still hatch successfully in great numbers. However, a little later, when
reaching the next phase of their development, when they start feeding, their
number is largely reduced. In a number of studies, we therefore explore the
feeding behaviour of the larvae, as well as the prey. We have investigated for
the first time how large their prey items may maximally be for a herring larva.
The preliminary results show that they cannot be larger than the first larval
stage of the small planktonic crustaceans (11). If prey items are larger, the
herring larvae will starve quasi with a fully laid table, since they are not
able to capture and ingest them.
It is therefore evident that if the herring larvae appear three weeks earlier in the Bodden, the suitable crustacean larvae are not yet available. The development of the zooplankton larvae depends on the abundance of drifting microscopic unicellular algae, the phytoplankton. The phytoplankton is not affected by the warming of the water and develops in synchrony with the increasing sunlight during springtime. The microalgae appear therefore at the usual time, whereas the herring larvae are too early and miss the zooplankton.
In another comprehensive study the abundance of zooplankton at all different developmental stages is currently analysed for the Greifswalder Bodden. First results show that considerably fewer small crustaceans occur throughout the past years (12). The underlying reasons need still be clarified though, the warmer winter most likely play a role here as well.
It is therefore evident that if the herring larvae appear three weeks earlier in the Bodden, the suitable crustacean larvae are not yet available. The development of the zooplankton larvae depends on the abundance of drifting microscopic unicellular algae, the phytoplankton. The phytoplankton is not affected by the warming of the water and develops in synchrony with the increasing sunlight during springtime. The microalgae appear therefore at the usual time, whereas the herring larvae are too early and miss the zooplankton.
In another comprehensive study the abundance of zooplankton at all different developmental stages is currently analysed for the Greifswalder Bodden. First results show that considerably fewer small crustaceans occur throughout the past years (12). The underlying reasons need still be clarified though, the warmer winter most likely play a role here as well.
Conclusion: Many Factors have an Impact on the Ecosystem
Each single factor does, apparently, not have a decisive negative impact
on the herring recruitment. However, all factors together may synergistically
change the ecosystem: The eutrophication of the coastal waters causes aquatic
plants to grow only in the very shallow nearshore zones. Herring deposit their
eggs on these water plants. This area is, however, much smaller than some
decades ago.
For the stickleback the high spawn concentrations apparently offer new opportunities: it consumes much more herring spawn than previously. At the same time the aquatic plants are more easily torn off from the sediment and deposited at the shore by means of the increasing storms. Moreover, due to the eutrophication the algae and fungi grow practically without limit, increasing the egg mortality due to the overgrowth.
At the end of this chain of individual factors there is a loser: the herring, that is not able to adapt quickly enough to a changing environment.
back
For the stickleback the high spawn concentrations apparently offer new opportunities: it consumes much more herring spawn than previously. At the same time the aquatic plants are more easily torn off from the sediment and deposited at the shore by means of the increasing storms. Moreover, due to the eutrophication the algae and fungi grow practically without limit, increasing the egg mortality due to the overgrowth.
At the end of this chain of individual factors there is a loser: the herring, that is not able to adapt quickly enough to a changing environment.
back
Stock Development
Development of Herring Recruitment
Development of Herring Recruitment
Fullscreen
The
recruitment index (N20-larvae index) provides the most important information on
recruitment of Western Baltic herring for the ICES stock assessment. The index reflects
the cumulated number of herring larvae reaching a length of 20 millimetre
during the spawning season (March-June). There is a good relationship between
the number of larvae at this length and the number of three-year old herring entering
the fishery. This allows us to use the recruitment index for a catch prognosis.
Recruiting year classes have been fluctuating in the past as well. However, since 2006 the stock of adult herring decreases continuously and the 2020-year class was the weakest in the 30-years-time series. back
Recruiting year classes have been fluctuating in the past as well. However, since 2006 the stock of adult herring decreases continuously and the 2020-year class was the weakest in the 30-years-time series. back
Stock Development of the Western Baltic Spring Spawning Herring
Stock Development of the Western Baltic Spring Spawning Herring
Fullscreen
The ICES stock assessment for Western Baltic herring only starts in 1991. Previously, this stock could not be
separated from the North Sea stock. During summer both stocks partly mix in the
northern distribution area.
Since the beginning of the 1990s this herring stock has decreased in size (upper left figure) and, according to the present perception, is now far too small. Since 2006 the spawning stock biomass is permanently below the reference point according to the concept of the Maximum Sustainable Yield (MSY) and at present only about half of the value of the limit reference point (Blim). Blim should be avoided by all means.
The fishing pressure was far too high since the beginning of the times series (upper right figure). It has only been reduced to less than the MSY reference point (Fmsy) from 2020.
The catches from the entire stock were reduced within the past 30 years, from 200,000 tons to less than 5,000 tons (lower left figure).
The values for recruitment (numbers of juveniles of the age group 0, lower right figure) deviate from those of the recruitment larvae index (N20), because they reflect the result of the stock assessment which includes additional information. The decreasing trend is however identical.
back
Since the beginning of the 1990s this herring stock has decreased in size (upper left figure) and, according to the present perception, is now far too small. Since 2006 the spawning stock biomass is permanently below the reference point according to the concept of the Maximum Sustainable Yield (MSY) and at present only about half of the value of the limit reference point (Blim). Blim should be avoided by all means.
The fishing pressure was far too high since the beginning of the times series (upper right figure). It has only been reduced to less than the MSY reference point (Fmsy) from 2020.
The catches from the entire stock were reduced within the past 30 years, from 200,000 tons to less than 5,000 tons (lower left figure).
The values for recruitment (numbers of juveniles of the age group 0, lower right figure) deviate from those of the recruitment larvae index (N20), because they reflect the result of the stock assessment which includes additional information. The decreasing trend is however identical.
back
Distribution and Management
Stock development of the Western Baltic Spring Spawning Herring
Stock development of the Western Baltic Spring Spawning Herring
Fullscreen
The ICES stock assessment for Western Baltic herring only starts in 1991. Previously, this stock could not be
separated from the North Sea stock. During summer both stocks partly mix in the
northern distribution area.
Since the beginning of the 1990s this herring stock has decreased in size (upper left figure) and, according to the present perception, is far too small. Since 2006 the spawning stock biomass is permanently below the reference point according to the concept of the Maximum Sustainable Yield (MSY) and at present only about half of the value of the limit reference point (Blim). Blim should be avoided by all means.
The fishing pressure was far too high since the beginning of the times series (upper right figure). It has only been reduced to less than the MSY reference point (Fmsy) from 2020.
The catches from the entire stock were reduced within the past 30 years, from 200,000 tons to less than 5,000 tons (lower left figure).
The values for recruitment (numbers of juveniles of the age group 0, lower right figure) deviate from those of the recruitment larvae index (N20), because they reflect the result of the stock assessment which includes additional information. The decreasing trend is however identical.
back
Since the beginning of the 1990s this herring stock has decreased in size (upper left figure) and, according to the present perception, is far too small. Since 2006 the spawning stock biomass is permanently below the reference point according to the concept of the Maximum Sustainable Yield (MSY) and at present only about half of the value of the limit reference point (Blim). Blim should be avoided by all means.
The fishing pressure was far too high since the beginning of the times series (upper right figure). It has only been reduced to less than the MSY reference point (Fmsy) from 2020.
The catches from the entire stock were reduced within the past 30 years, from 200,000 tons to less than 5,000 tons (lower left figure).
The values for recruitment (numbers of juveniles of the age group 0, lower right figure) deviate from those of the recruitment larvae index (N20), because they reflect the result of the stock assessment which includes additional information. The decreasing trend is however identical.
back
Distribution and Management Areas
Distribution and Management Areas
Fullscreen
Western Baltic herring is widely distributed
and migratory: It spawns in spring in the shallow water areas of the Southern
Baltic Sea, spends its juvenile phase in summer in the Arkona Sea and
migrates for feeding as adult fish to the north into the Kattegat, Skagerrak
and even the north-eastern North Sea. In winter, the stock retracts to The
Sound and starts the spawning migration from there (left figure).
During its annual migration, the herring crosses several management areas (right figure): The western Baltic Sea, Kattegat and Skagerrak, and the North Sea. Mixing with neighbouring North Sea and Central Baltic stocks occurs at the fringes of the distribution area.
Separate total allowable catches are set for each of these management areas. The fishing mortality on the stock is however determined by the total catch from all management areas. back
During its annual migration, the herring crosses several management areas (right figure): The western Baltic Sea, Kattegat and Skagerrak, and the North Sea. Mixing with neighbouring North Sea and Central Baltic stocks occurs at the fringes of the distribution area.
Separate total allowable catches are set for each of these management areas. The fishing mortality on the stock is however determined by the total catch from all management areas. back
Scroll down to continue
Swipe to continue
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Herring Trapped in Climate Change
Herring – Silver of the Baltic Sea
Importance for the Coastal Fishery
Impact of Climate Change on Fish Stocks
The Herring Fishery
The Rügen Herring Larvae Survey
The Rügen Herring Larvae Survey
Basic Data for Setting Catch Quotas
Why is there less Herring Recruitment?
The Baltic Sea is too warm
From Spawn to Herring
From Spawn to Herring
Impact on the Fishery
This is how the Herring can recover
Sources
Impact of Climate Change on Fish Stocks Worldwide
Impact of Climate Change: The North Polar Sea
Impact of Climate Change: The Tropics
Impact of Climate Change on Coastal Fisheries
Impact of Climate Change: Baltic Sea
Are there too few Aquatic Plants?
Are the Larvae flushed out of the Bodden?
Has the Number of Predators on the Spawn increased?
Is Spawn washed up the Shore by Storms?
Are increased Nutrient Loads the Cause?
Or is there too little Prey available for the Larvae?
Conclusion: Many Factors have an Impact on the Ecosystem
Development of Herring Recruitment
Stock Development of the Western Baltic Spring Spawning Herring
Distribution and Management Areas
Herring Trapped in Climate Change
Herring – Silver of the Baltic Sea
Importance for the Coastal Fishery
Impact of Climate Change on Fish Stocks
The Herring Fishery
The Rügen Herring Larvae Survey
The Rügen Herring Larvae Survey
Basic Data for Setting Catch Quotas
Why is there less Herring Recruitment?
The Baltic Sea is too warm
From Spawn to Herring
From Spawn to Herring
Impact on the Fishery
This is how the Herring can recover
Sources
Impact of Climate Change on Fish Stocks Worldwide
Impact of Climate Change: The North Polar Sea
Impact of Climate Change: The Tropics
Impact of Climate Change on Coastal Fisheries
Impact of Climate Change: Baltic Sea
Are there too few Aquatic Plants?
Are the Larvae flushed out of the Bodden?
Has the Number of Predators on the Spawn increased?
Is Spawn washed up the Shore by Storms?
Are increased Nutrient Loads the Cause?
Or is there too little Prey available for the Larvae?
Conclusion: Many Factors have an Impact on the Ecosystem
Development of Herring Recruitment
Stock Development of the Western Baltic Spring Spawning Herring
Distribution and Management Areas
