In an earlier report, we presented our work designing models to better estimate “natural mortality” of fish stocks in the North Sea from grey and harbour seals and harbour porpoise – in other words, how many fish these marine predators are eating. Our new report expands on this with natural mortality models calibrated for the differing contexts of the North Sea, the Celtic Sea, Bay of Biscay and Western waters, to identify spatial and temporal trends in natural mortality around Europe – ultimately furthering our knowledge of complex food web interactions in these waters.
As part of our Ecological Effects on Fisheries theme, we’ve been modelling multi-species predator-prey interactions to provide data to feed into stock assessments. In this report we expand the net to explore other predator and prey species dynamics by looking at fish-on-fish predation as well as that by marine mammals, and incorporating environmental factors such as temperature in our natural mortality models. This will help us provide fisheries managers with a more accurate understanding of the impacts of management strategies at an ecosystem level and work towards effective Ecosystem Based Fisheries Management (EBFM).
SEAwise Research
To decipher the diets of our focus species, we analysed scat and stomach contents to unearth squid and cuttlefish beaks, and fish ear bones (otoliths) – both of which can be used to identify different species and reveal exactly what predators are eating and in what quantities. Alongside this diet data, predator-prey abundance and distribution data were used to see where species overlap.
In the North Sea we used an updated Stochastic Multispecies model (SMS) to explore predator-prey relationships on and between stocks and reveal natural mortality at multiple levels. Included in these models were the impacts of fish-on-fish consumption, and consumption from other predators such as seabirds, rays, seals and porpoises. Updates to the North Sea SMS incorporated estimates of seabird consumption and provided, for the first time, estimates for the impact of grey seals on plaice. North Sea models were also expanded to include marine mammal diet and prey abundance data for grey and harbour seal, and harbour porpoise. Headline results include that sandeels are a favourite food item for porpoises and seals, and that harbour seals switch between different prey more readily than grey seals. Importantly, EBFM seeks to reduce bycatch of marine mammals, which can be facilitated by knowing which areas they feed in, where fishing is also taking place.
By using an updated FLBEIA model we also modelled recruitment – the number of fish surviving to adulthood – and natural mortality of cod, saithe, whiting and haddock in the North Sea. This revealed a complex and perhaps surprising story: if fleets were to choose to catch fewer of these fish in response to their numbers falling, predatory fish such as cod and saithe would also be caught less – enabling them to put more pressure on intermediate predators and prey species and ultimately destabilising the system.
In the Celtic Sea we used Ecopath with Ecosim to model the impact of predators on commercial species, and included an extensive range of animal groups including: seabirds, marine mammals, fish, cephalopods and more. While the results need further analysis, they have already begun to reveal future trends in natural mortality for multiple fish species in the region including cod.
In the Bay of Biscay – also in the Western Waters – we are developing a methodology to model natural mortality using OSMOSE. Within this model we have included 18 species or species groups that are commonly fished, and environmental factors such as sea temperature.
SEAwise has also developed a new model specifically for European seabass, called DEB-IBM. Based on the assumption that catch rates will remain constant up to 2099, the model predicts that the two distinct seabass stocks will experience faster growth and likely greater survival due to increasing temperatures – meaning that stock biomass will continue to increase over time.
The results in this report have broadened our understanding of the complex predator and prey dynamics across the North Sea, as well as the Celtic Sea and Bay of Biscay in Western Waters. The report also provides more accurate and precise data on how changes in the abundance, distribution and behaviour of predators affects their prey, including key commercial fish stocks. Ultimately, improving data on natural mortality alongside our work to model fisheries and management impacts on stocks will help to strengthen the predictive abilities of our ecosystem models as we work towards developing effective EBFM across Europe.
Read the full report here.
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