Marine Research Findings of the VECTORS Project

This website provides access to the research results of the VECTORS project, which can be used to support marine management decisions, policies and governance as well as future research and investment. VECTORS was a large scale project that brought together more than 200 expert researchers from 16 different countries. It examined the significant changes taking place in European seas, their causes, and the impacts they will have on society.

Impact of changes in the upper trophic North Sea food web on commercially important fish

The aim of this study was to demonstrate how climate and fishery induced changes in the upper North Sea food web influenced the stock trajectories of commercially important fish stocks in the last 30 years. To disentangle effects of natural and anthropogenic pressures it was also analysed whether natural mortality or fishing has contributed most to total mortality. Predator-prey interactions were analysed with the multi species assessment model SMS1. We analysed the consequences of a strong increase (outbreak) in abundance of an indigenous species - grey gurnard - for the stock dynamics of commercially important fish stocks. As second example, changes in the diet composition of predators between 1981 and 1991 were analysed. Results from work in VECTORS WP 1, 2.2 and 3.1 give rise to the assumption that the composition of the prey fields has changed between 1981 and 1991 due to environmental changes.

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The importance of predation relative to fishing mortality has been increasing in recent years. The percentage of natural mortality in total mortality is a useful food web indicator disentangling natural and anthropogenic pressures on fish stocks.

With the exception of older cod and sprat, natural mortality (M) represented an increasing proportion of the total mortality (F+M) in recent years (Fig. 1). Due to a successful reduction in fishing mortality (F) for many stocks, natural mortality is becoming the dominant source of mortality in the North Sea. For most species age combinations natural mortality has been more than 50% of total mortality in recent years.

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Outbreaks of indigenous predators like grey gurnard can have a serious impact on the stock dynamics of commercially important fish species as demonstrated for the interaction between grey gurnard and juvenile cod or whiting.

The abundance of grey gurnard increased substantially over the 1990s. The IBTS index shows a threefold increase. The outbreak of the grey gurnard stock had considerably consequences for the predation on commercially important fish species. Grey gurnard became the most important predator for 0-group cod in recent years (Figure 2). Also for juvenile whiting predation caused by grey gurnard became a serious issue. Other prey species (e.g. herring, sprat, sandeel) were less influenced by the increase of grey gurnard abundances. When assuming a decrease in grey gurnard abundance to levels observed before 1991, lower predation mortalities for 0-group cod compared to results of the standard keyrun forecast were observed. The higher survival rate of recruits during the first year led to a stronger increase in cod SSB. The probability to be above Bpa was around 70% in the forecast with reduced Grey gurnard abundances compared to 50% in the standard for ecast. Fishing at F=0.3 led to a collapse of the whiting stock in all runs apart from the run where grey gurnard abundance was assumed to decrease. Whiting suffers from the high predation mortality caused by grey gurnard and the recovery of the cod stock further deteriorates the situation for whiting in the model.

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Changes in the diet composition of predators caused by changes in the spatial distribution of prey form an additional mechanism potentially explaining how changes in climate translate into changes in the productivity of fish stocks.

For cod, whiting and saithe it was found that the relative stomach content for "other food" (crustaceans, molluscs, other fish) in 1991 was significantly lower than in 1981 and predators shifted their diet towards commerically important fish prey. The regime shift around 1987 was characterised by changes in the species composition of zooplankton2 and benthic communities3 representing a large part of “Other Food” found in the stomachs45. For example, euphausiids were an important prey species for saithe in 1981. In the 1991 stomach data significantly fewer euphasiids were found in the stomachs. Euphausiids were most likely less available for North Sea saithe in 1991 since after the regime shift in the late 80s warmer water temperatures prevailed in the North Sea6 and the cold water adapted euphausiids were observed to decrease in abundances7.

As a consequence of the diet shift from "other Food" to commercially important fish prey, in run-81 (parameterised with stomach data from 1981) the probability of a cod recovery above precautionary limits was 70% in 2020. Run-91 (parameterised with stomach data from 1991) only showed a 40% probability to recover above Bpa around 2018, but the stock was forecasted to decline again and the probability is lower than 30% in 2025.

Estimates of Fmsy for cod in a multi species context were robust to observed changes in the upper trophic North Sea Food web between 1981 and 1991. However, in a multi species context, trade-offs in yield between different species lead to different options for Fmsy.

In the keyrun predictions, the cod catch in tonnes summed over the period between 2012 and 2025 was highest at a target fishing mortality of 0.5 when all other stocks were fished at single species targets in line with the MSY concept (Fig. 3). However, the summed catch at F of 0.4 or 0.6 was only marginally lower. Similar results were found for run-81. The level of yields in run-91 was lower compared to the other two runs but highest yields were again found at F values from 0.4 to 0.6.

The choice of a certain target fishing mortality for cod had an impact on other stocks (Fig. 4). Especially inside the mixed demersal fishery lower F values for cod had a positive effect on cod SSB, but a negative one on haddock and whiting due to predator-prey relationships. In contrast, the SSB of herring was not influenced to a large extent as a larger cod stock means a lower whiting stock leading to a compensation of higher predation mortalities from a higher cod stock. This demonstrates that the MSY esimtated in single species models cannot be reached for all stocks simultaneously. In a multi species context additional political decisions are needed to decide what species yield has the highest priority.

 

Relevance for Policy:
  • Common Fisheries Policy
  • Marine Strategy Framework Directive

References

Lead Author:

Alexander Kempf
(alexander.kemnospampf@ti.bund.de)
vTI-Institute of Sea Fisheries (vTI-SF)
Date of research: January 2015

Related articles:

Population dynamics of sprat in the Baltic Sea 

The drivers of a common sole population 

Cod, recruitment, temperature and zooplankton

Connectivity: plaice spawning and nursery areas 

Early life stage survival of Baltic cod

Functional responses of herring and sprat to prey 

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This project has received funding from the European Union’s Seventh Framework Programme for research, technological development and demonstration under grant agreement no 266445
© Vectors 2015. Coordinated by Plymouth Marine Laboratory.