Selecting a subset of the commercial catch data for estimating catch per unit effort series for ling (Molva molva L.)

Ling is an important species for the Norwegian longline fishery. Motivated by an apparent steep decline in a catch per unit of effort (CPUE) series for ling based on commercial longline data during the periods 1971–1993 and 2000–2003, ling was classified as Near Threatened and placed on the Norwegian Red List, which caused difficulties for the marketing of ling. To examine the validity of the conclusion that this CPUE series indicated that the ling stock was Near Threatened, we estimated CPUE series for the period 2000–2012 based on extensive logbook data available since 2000.

The Norwegian longline fishery is a mixed-species fishery: therefore, we constructed three different CPUE series based on; (1) all catches including zeros, (2) selected catches that appeared to have target ling and (3) selected vessels for each year that seemed to have often targeted ling during that year. It was concluded that these CPUE series indicated that the abundance of ling in Norwegian waters has been fairly stable or increasing since at least 2000, and hence there was no compelling evidence that the abundance of ling was declining.

Based on our analysis, ling was removed from the Norwegian Red List. Even though all three series indicated increasing abundance, their precision varied, and estimates of how much the ling stock has increased depended on the subset of the commercial data on which the CPUE series was based.