Non-predation natural mortality of Norway pout ( Trisopterus esmarkii ) in the North Sea

Based on age disaggregated data on catch rates in bottom trawl surveys, commercial catches, and the number consumed by the North Sea piscivorous predators, new estimates of non-predation natural mortality, M1, are obtained for Norway pout [Trisopterus esmarkii (Nilsson)]. Simple log catch ratio analysis and rough maximum likelihood procedures are applied.

The analysis focus on the year classes 1977–1981 and 1987–1991, which are represented in the extensive stomach sampling of North Sea piscivorous fish in 1981 and 1991. Although the M1 of Norway pout varied between the two periods, in both periods it increased with age and was very high for age 2 and older fish (0.10 for age 1, 1.74 for age 2, 2.58 for age 3 and 3.05 for age 4 for the 1977–1981 year classes and 0.10 for age 1, 2.03 for age 2, 3.04 for age 3 and 4.39 for age 4 for the 1987–1991 year classes).

This difference between the two periods is not significant. Survey data from each quarter of the year show that the main mortality takes place between the 1st and the 2nd quarter of the year, i.e. from before to after spawning, thus pointing at spawning as the main factor.