Investigating the ecological consequences of stock recovery

Some fish stocks are rebuilding -due to management plans among other factors- and are likely to have higher abundance and biomass than we have seen in recent times. What effects will this have through trophic interactions both up and down the foodweb?
Published: 18 July 2019

​In recent years, investigating the ecological consequences of stock recovery has been a focus point for the Working Group on the Ecosystem Effects of Fishing Activities (WGECO), with particular emphasis on benthivorous fish and invertebrates (fish that feed on benthic prey). 

What happens when fish stocks recover?

Jeremy Collie, University of Rhode Island, and Stefán Áki Ragnarsson, Marine and Freshwater Research Institute , co-chairs of WGECO explain the group's recent work. “If there is a large increase in benthivorous fish, will this have an impact on benthic productivity and biodiversity? To consider this, we required data on the spatial distribution of benthivorous predators, their prey consumption rates, and diet composition. We also needed data on the abundance and production of benthic fauna."

Food limited stocks

One hypothesis suggests that a stock could become so large that there is no longer enough food for it. To examine this, the group estimated how much benthivourous fish eat and related that to the available food in the environment. WGECO used North Sea plaice as an example. According to Tobias van Kooten, Wageningen University and member of WGECO, “The recovery of fish stocks like plaice provides an incredible opportunity to learn about the driving mechanisms in marine ecosystems, which is essential to successfully apply ecosystem based fisheries management".   

Collie is happy with the group's progress, “The result we have found is that these benthivourous fish are eating 5–20% of the available prey. While we wouldn't say that there is plenty of food available, we would say that we don't think that the amount of food available is limiting the recovery of stocks."

Larger stock size, smaller fish?

As the abundance of the stock increases, does this affect the growth rate of the fish? WGECO reviewed the methods to test for this. “We found the methodology to be confusing because it isn't standardized; different people got different answers" explains Collie, “We standardized the methods and from that we do see density-dependent growth in some stocks, such as Georges Bank haddock, but it's not generalized across all stocks or species."

This is consistent with the group's findings on food limitation. “As people have approached this with different methods, we haven't fully resolved it yet but there is evidence of density dependent growth in some stocks but not all."

Warmer waters, smaller fish?

In 2018, work from the group suggested that the decreasing growth rate (weight at length) of North Sea place was density-dependent. Further analysis by the group this year offered another argument: that the decreasing fish size is temperature dependent, not density dependent. “While food scarcity doesn't appear to be a reason not to rebuild stocks -stocks will not run out of food- there are confounding factors", Ragnarsson explains, “At the same time as we are rebuilding stocks, the oceans are warming, which affects the metabolism of fish. So it is possible that the fish are spending their energy on metabolism as a response to increased temperature."

Simon Greenstreet, Marine Scotland Science and member of WGECO, agrees that the decline in plaice growth is temperature related and argues, "The support for density dependence just is not there, but the evidence of a temperature effect is overwhelming".

Full details of WGECOs work on the ecological consequences of stock rebuilding, along with their work on spatial distribution indicators for fish and benthos can be found in their 2019 report.

An ICES Viewpoint​ on the consequences of stock recovery is currently in development and due to be published later in 2019.​ ​

The work of WGECO focuses on the ecosystem effects of fisheries, especially with regard to the application of the ecosystem approach and contributes to the following ICES science priorities:

 -Ecosyste​m scienceattempts to advance and shape understanding of the structure, function, and dynamics of marine ecosystems — to develop and vitalize marine science and underpin its applications.

-Seafood production generates evidence and advice for management of wild-capture fisheries and aquaculture to help sustain safe and sufficient seafood supplies.

Read more about ICES science priorities in our Science plan.​


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Photo: Plaice on board of RV Belgica from the southern North Sea. Hans Hillewaert.

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Investigating the ecological consequences of stock recovery

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