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.

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Nephrops norvegicus exposed to starvation and hypoxia in the field as well as in the laboratory exhibited marked differences in glycogen reserves, haemolymph haemocyanin concentrations, and tissue copper and manganese contents. Muscle glycogen concentration was reduced by ca 50% in lobsters collected from hypoxic field conhtions. Glycogen concentrations were reduced to ca 3 "/o of initial values in muscle and 10% in midgut gland following 7 mo starvation in the laboratory. Haemolymph haemocyanin concentration was markedly reduced in N. nowegicusexposed to severely hypoxic conditions although haemolymph copper concentration remained virtually unchanged. Starvation resulted in a slower reduction of haemocyanin concentration than was associated with hypoxia. Starvation resulted in a net loss of copper after 7 mo. Experimental exposures to moderate or severe hypoxia were not associated with net losses of copper, although in long-term moderate hypoxia the whole body copper load was redistributed, especially to the midgut gland increasing the concentration there 4 times. Redistribution of copper to the midgut gland did not occur in N. norvegicus exposed to severe hypoxia in the field. Threeto four-fold lower copper concentrations were found in the midgut glands of hypoxic lobsters (compared to controls from normoxic areas). As well as low copper concentrations, these lobsters had 2 to 4 times higher manganese concentrations in internal tissues. In the absence of sediment, manganese concentrations and contents were not influenced by laboratory exposure to hypoxia.