Estimating total mortality and asympotic length of Crangon crangon between 1955 and 2006

Total mortality (Z, year–1) of southern North Sea brown shrimp (Crangon crangon) was determined as Z = K, based on the von Bertalanffy length–growth constant (K, year–1) and derived from length-based methods. Mortality estimates were based on length frequency distributions obtained from four long-term dataseries (1955–2006): German Demersal Young Fish Survey, Dutch Demersal Fish Survey, and two German Bycatch series (Büsum and East Frisia). Four methods to estimate and L were evaluated. Highest total mortality (Z = 8 year–1) was estimated for the early 1990s, and the lowest (Z = 4 year–1) for the 1960s. Accounting for these differences, a median Z rather than mean values was calculated for the whole series, and the value ranged from 5.74 (Ssentongo and Larkin method), through 5.65 (Beverton and Holt method) and 5.64 (Jones and Zalinge method), to 5.35 (length-converted catch curves). Over the whole period, an increase in and a decrease in the proportion of shrimps >60 mm in the catch was observed, whereas asymptotic length L remained constant (at 79.3 mm total length)