Impact of discard mortality in Crangon fisheries on year-class strength of North Sea flatfish species

Abstract

In the long-term by-catch sampling programme of the Federal Research Centre for Fisheries (Germany), four flatfish species occurred regularly and in large numbers in the catches depending on fishing ground and season: Pleuronectes platessa, Limanda limanda, Platichthys flesus, and Solea solea. Another five flatfish species were netted occasionally and in much lower numbers: Scophthalmus maximus, Scophthalmus rhombus, Buglossidium luteum, Microstomus kitt, and Glyptocephalus cynoglossus. Mortality estimates per year class for the Schleswig-Holstein part of the German North Sea coast regarding the impact of selective trawls and by-catch mortality were in the same order of magnitude as the ones based on a differently designed by-catch sampling programme (1982 and 1988–1992). The worst-case mortality estimate for 0-group plaice (e.g. lowest rate given for natural mortality, highest rate determined for by-catch mortality, no re-catch) based on Virtual Population Analysis (VPA) data for the I-group was in the order of 3 to 11% of the year class. For 0-group sole, the corresponding figures ranged from 1 to 8%. The shrimp fleet in Schleswig-Holstein represents 50% of the entire German shrimp fleet. However, catches in Schleswig-Holstein are generally higher than in Lower Saxony for biological reasons and due to differences in fleet structure. If the results for 0-group plaice obtained in Schleswig-Holstein were extrapolated to the total German shrimp fleet, the range would be 6 to 18%. In other words, even a complete stoppage of the German shrimp fishery is unlikely to have a detectable beneficial effect on the stock. However, the discard mortality caused by the combined German, Dutch and Danish shrimp fisheries, as well as by the sole and plaice fisheries, can be considered to have a significant effect.