Responses to hypoxia of plaice, Pleuronectes platessa, and dab, Limanda limanda, in the south-east Kattegat: distribution and growth

Plaice, Pleuronectes platessa, and dab, Limanda limanda, were sampled with a Glommen lobster trawl at 25 to 40 m depth in the SE Kattegat during spring and autumn of 1984 to 1990. During autumn, hypoxia (O2-concentration <3 mg I−1) occurred in the bottom water below the halocline for four to ten weeks every year, except in 1984 when moderate hypoxia (O2-concentration 3–5 mg 1−1) occurred. Biomass of both species was shown to be negatively correlated with oxygen concentration during autumn. Further, a decrease in population mean total length was observed during the study period in both spring and autumn samples. Laboratory studies of growth of juvenile plaice and dab, at 15° C and 30–34%, showed that growth is reduced at 50 and 30% O2-saturation for both species during a 20 d period. There was some adaptation to hypoxia resulting in less reduction of growth during the second half of the experiment. The frequency of fish eating was reduced in plaice at 30% O2-saturation. Reduced mean total length of the plaice and dab population of the SE Kattegat is discussed in view of sublethal effects of oxygen deficiency.