Unaccounted mortality in northern Baltic Sea herring fishery—magnitude and effects on estimates of stock dynamics

Abstract

Correct catch data are necessary for age-structured assessment models. Those data may be biased among other reasons due to fishing practices such as discarding or selection by gear and subsequent escapee mortality. In a number of fisheries juveniles make a large fraction of catches while their survival is low. We applied length-specific selection and escapee mortality functions to estimate “underwater discarding” and the actual total removals from the herring ( Clupea harengus L.) stock in the northern Baltic Sea. Contemporarily an array of fleet selection scenarios was investigated to estimate the impact of underwater discarding on perceived stock dynamics. The analysis showed that in ages 0 and 1 year herring are discarded underwater in larger numbers than landed. Unaccounted mortality involves a marked seasonal pattern because the growth rate of immature herring is high. During the first quarter of the year the first fully recruited age group is 1 year instead of 2 years as indicated by catch samples. The effect of fishing induced mortality decreases as a function of age and size so that the impact on estimated recruitment and fishing mortality at age 1 year is considerable, but irrelevant at age 2 years and older. The actual fishing mortality at age 1 year is estimated to be more than twice higher than estimates of F based on the unadjusted data. The practical effect of underwater discarding on evaluation of stock status, the stock–recruitment function, and reference points is minor. However, changes in codend mesh size, induced by management actions or fishing strategies, should be recognized and their effects considered in stock assessment, in short term forecasts, and in management advice.