Accounting for correlated observations in an age-based state-space stock assessment model

Abstract Fish stock assessment models often rely on size- or age-specific observations that are assumed to be statistically independent of each other. In reality, these observations are not raw observations, but rather they are estimates from a catch-standardization model or similar summary statistics based on observations from many fishing hauls and subsamples of the size and age composition of the data. Although aggregation mitigates the strong intra-haul correlation between sizes/ages that is usually found in haul-by-haul data, violations of the independence assumption can have a large impact on the results and specifically on reported confidence bounds. A state-space assessment model that allows for correlations between age groups within years in the observation model for catches and surveys is presented and applied to data on several North Sea fish stocks using various correlation structures. In all cases the independence assumption is rejected. Less fluctuating estimates of the fishing mortality is obtained due to a reduced process error. The improved model does not suffer from correlated residuals unlike the independent model, and the variance of forecasts is decreased.</jats:p

Avoiding grey seal depredation in the Baltic Sea while increasing catch rates of cod

The escalating conflict between gillnet fisheries and the growing seal populations in Baltic Sea has increased the need to reduce direct effects in form of reduced or damaged catches. This study describes the design and catch rates of 20 different seal safe cod (Gadus morhua) pot designs suitable for the Baltic Sea. Catches in numbers of individuals and total catch weight were recorded in 2762 pot hauls with pots varying in terms of size, shape, number of entrances, entrance length, pot placement in relation to the sea floor and catch holding chamber. The aim was to investigate the how the different designs and soak times affected the catch rates under similar conditions. Volume and soak time was significantly positively correlated with catch rates and round bottomstanding pots with a fish holding chamber, funnels and 5 entrances had the highest catch rates. The findings are important for the development of cod pots, which can allow the fishers to continue fishing in areas of high seal density with static gear and hence low-carbon emission and minimum bycatch of marine mammals and sea birds