Verification Of Multispecies Interactions In The North Sea By Trawl Survey Data On Norway Pout (Trisopterus)

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Extensive stomach sampling programmes of North Sea fish during the recent 2 decades have shown that cod (Gadus morhua), whiting (Merlangius merlangus) and saithe (Pollachius virens) are by far the main predators on Norway pout (Trisopterus esmarkii) of age 1 and older. As the stock sizes of cod, whiting and saithe have decreased significantly over the period this offers a unique opportunity to test whether a decrease in natural mortality of Norway pout can be detected in mortality estimates obtained directly from abundance data of Norway pout. Two surveys, which cover the Norway pout distribution well, have been analyzed in this regard. Both showed clear decrease in total mortality consistent with the decrease in amounts of predators. The level of predation mortality is similar to that from the ICES North Sea Multispecies model (MSVPA), but the MSVPA does not reflect the variation in mortality over time. It is speculated that this might be a consequence of the model ignoring a very high mortality of Norway pout of age 2 and older possibly due to spawning or growth stress.