Verification of multispecies interactions in the North Sea by trawl survey data on Norway pout ( Trisopterus esmarkii )

Extensive stomach sampling programmes of North Sea fish during the recent two 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 the expected decrease in natural mortality of Norway pout can be detected in mortality estimates obtained directly from abundance data. Two surveys, that cover the Norway pout distribution well, have been analyzed and both showed a clear decrease in total mortality consistent with the decline in predator populations. The estimated average predation mortality is similar to that obtained from Multispecies Virtual Population Analysis (MSVPA), but MSVPA does not reflect the variation in mortality over time. It is speculated that this might be a consequence of the model ignoring a potentially high mortality of age 2 and older fish caused by spawning or growth stress.