Epsilon Open Archive

Abstract

Stock assessment is the synthesis of information on life history, fishery monitoring, and resource surveys. It is a critical management tool for monitoring the abundance of commercial fish populations, as well as for predicting the consequences of policy decisions. However, without the key biological reference parameters, e.g., growth rate and age, the estimation of spawning stock biomass and fishing mortality rate is unpredictable. During the past two decades, a number of changes in biology and ecological conditions has affected the Eastern Baltic cod stock, raising concerns among fisheries scientists and managers. Deteriorated quality of key biological parameters for stock assessment, such as true age, growth rates, and reproductive traits, in combination with changes in environmental and ecological conditions, has led the failure of the analytical stock assessment in 2014, leaving the present stock status unclear. Currently, it is unknown whether the drop in mean size and the disappearance of large individuals is due to a decrease in growth rates or by increased natural mortality. Therefore, whether the stock has suffered a decrease in productivity or an increase in mortality (fishing pressure or other natural causes) is an open dilemma, with large implication for fisheries management.  During my PhD, I will make use of results from historical tagging experiments, as well as new international tagging program to provide the necessary information on growth and mortality of Eastern Baltic cod to aid solving the issues with stock assessment. In addition, I will investigate the effects of the changes in growth and condition on cod reproductive potential. Finally, I will try to integrate all the gained biological knowledge (growth, mortality and reproductive potential) in new assessment models for a better management of the Baltic Sea cod.