Effects of oxygen depletion on the ecology, blood physiology and fishery of the Norway lobster Nephrops norvegicus

Abstract

Biomass, population structure, food selection and blood (haemolymph) physiology of the Norway lobster Nephrops norvegicus (L.) were investigated in SE Kattegat, an area where low oxygen concentrations (t 2 m1 1-', 30 % 02-saturation) have occurred in the bottom water for 1 to 3 mo periods in most years in the 1980's. During the study period (October 1984 to September 1989) lobster biomass decreased in the area from 10.8 kg h-' (catch per u n ~ t effort) to zero (estimated during the last 12 nlo of the investigation). Males contributed on average 78 % of the population density, except in September 1988 during severe hypoxia when a reversed sex ratio was found and females (even berried) dominated (75 % of density). The food of N. norvegicus belonged to 4 major groups; crustaceans, echinoderms, molluscs and polychaetes. The dominant species eaten within these groups were also found to be dominants in the benthic infauna. This suggests that N. norvegicus are not feeding selectively but taking available organisms indiscriminately. In the field and in laboratory experiments N. norvegicus increased blood pigment (haemocyanin, Hcy) concentration in moderate hypoxia (20 to 40 '10 0 2 saturation), and reduced it in severe hypoxia (10 to 20 % 02-saturation). At 02-saturations below 15 % N. norvegicus ceased feeding and had empty stomachs. Thus in low oxygen concentrations the lobsters suffer from hypoxia-induced starvation rather than lack of food. Survival of N. norvegicus exposed to 15 and 10 % 02-saturation was 4 wk and 2 to 4 d, respectively. After return to normoxia recovery of blood Hcy concentration was slow, probably due to lack of copper in the diet, which is essential for Hcy synthesis. We consider blood Hcy concentration to be a promising 'in situ' biomarker with ecological relevance.