Swimming endurance of American plaice (Hippoglossoides platessoides) and its role in fish capture

Abstract

The swimming capability of fish plays an important role in determining the selectivity and efficiency of capture by mobile bottom-trawl fishing gears. The authors test the hypotheses that the swimming endurance of American plaice (Hippoglossoides platessoides) is independent of fish size and water temperature. Using a swimming flume, swimming endurance was examined across a range of fish sizes (14-44 cm) and water temperatures (-0.2 to 9.7°C). All trials were conducted at a swimming speed of 0.30 ms-1. This is comparable to the swimming speeds required by flatfish during herding by bottom-trawl sweeps. The data were analysed using failure time (survival) analysis, a more robust statistical technique for endurance-type data. The advantage of this technique over traditional regression analyses is that censored observations can be included in the analysis. An important distinction, however, is that the dependent variable being modelled was not endurance per se, but the instantaneous rate of exhaustion, or hazard rate at a given time t. The results showed that both fish length and water temperature had a negative effect on the endurance hazard rate. This is equivalent to an increase in the probability of American plaice achieving a given endurance with increasing fish length and water temperature. This was sufficient evidence to reject the null hypotheses that the swimming endurance of American plaice is independent of these variables. Probability curves for the distances the fish would be capable of swimming were calculated for different fish sizes and water temperatures. The findings suggest that the herding efficiency of American plaice by bottom-trawl sweeps may be largely size- and temperature-dependent.