Not just food for people, the species targeted in fisheries are often key prey for other marine species. The loss of fish to predators is one of the most significant sources of what scientists call “natural mortality”. It is therefore essential to take natural mortality of commercial species into account in fisheries management. Traditional management approaches – especially those focusing on single commercial species at a time – have often failed to incorporate important ecological effects such as the amount of fish being taken by predators. Ecosystem-Based Fisheries Management (EBFM) seeks to correct this.
Under our Ecological Effects on Fisheries theme, this report describes work undertaken by SEAwise to develop enhanced models for estimating natural mortality, with a focus on predator-prey interactions. Results from such models could be fed directly into stock assessments, providing fisheries managers with a more accurate understanding of how their management strategies may affect fish stocks and wider ecosystems.
Our research focused mainly on the cases of three marine mammals in the North Sea for which ample data exists – harbour porpoise, grey seal, and harbour seal. We experimented with several different models to predict how many of each fish species the three predators were eating, and how this varied in space and time. For example, the diets of both seal species showed significant variation across space and time, although grey seals overall consumed many more sandeels than harbour seals did. Notably, greater availability of sandeels meant that seals ate fewer of other fish species such as herring – highlighting how increases or decreases in one commercial fish species can affect the biomass of other commercial species via the mediating effect of predators.
With this enhanced understanding of North Sea food webs involving seals and porpoises, we then modelled and explored the dynamics between the predators, their prey, and fishing intensity, taking into consideration additional complicating factors such as potential shifts in the distribution of predators under climate change. We found that all existing models struggled to produce reliable results if certain changes occurred, such as a sharp decline in fish biomass, or a reduction in a fishing fleet’s operations leading to greater hunting success by predators – an uncertainty that managers need to be aware of, and take a precautionary approach with, to avoid management strategies which may result in unintended effects.
This research has laid important groundwork for future exploration of predator-prey dynamics, and how these affect and are affected by fisheries. Going forward, SEAwise will develop tailored models for the Celtic Sea, Bay of Biscay, and Western Waters to expand the availability of information on this important topic. Information derived from the modelling will also be compared with other data such as mortality from seabirds, to fine-tune our understanding of the levels of mortality that various commercial fish species are subject to. Ultimately, what the report emphasises is that while it is essential to have an idea of how many fish are being taken by other species in addition to fleets, this is not enough for the implementation of effective EBFM – we also need to know how changes in the abundance, distribution and behaviour of predators affects fisheries, and how fisheries management measures may in turn affect predators.
Read the full report here.
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