Influence of water temperature and fish length on the maximum swimming speed of sand flathead, Platycephalus bassensis: Implications for trawl selectivity

Abstract

The maximum swimming speed of Australia's commercially important sand flathead ( Platycephalus bassensis) was assessed by video observation of fish swimming in a tank and measurement of muscle contraction time. Knowledge of the maximum swimming speed is important because it influences the ability or otherwise of fish to escape from various parts of the trawl. In our study fish were collected locally by angling and divided into three groups based on total length. Each group was tested at three water temperatures (10, 15 and 20 °C) to reflect typical changes in seasonal water temperature. Tank-held fish were first induced to swim in a raceway to measure tail beat frequency (in Hz) and distance swum per tail beat (stride length). The highest measured swimming speed was 336.6 cm −1 for a 24.1 cm fish at 20 °C. At this speed the recorded tail beat frequency was 17.9 Hz. The observed swimming speeds of each group of P. bassensis were linearly related with tail beat frequency, and a statistically significant ( P < 0.05) increase in stride length was detected with increased swimming speed. For all size groups, tail beat frequency increased with an increase in water temperature. The time taken for the anaerobic swimming muscle blocks dissected from newly euthanased P. bassensis to complete their contraction following an electric stimulus (5 V, 2 ms) was measured at each water temperature. Water temperature significantly ( P < 0.01) affected mean contraction times for all fish tested, but fish length did not affect contraction time ( P = 0.49). The mean pooled contraction times at each water temperature were 30.2 ms at 10 °C, 22.5 ms at 15 °C and 20.0 ms at 20 °C. For all size groups, the temperature effect on the muscle activity made tail beat frequency greater from 16.6 Hz at 10 °C to 25.0 Hz at 20 °C on average. We then used this data to calculate the maximum tail beat frequency and swimming speed of each group of fish at each water temperature. All estimated maximum swimming speeds were in excess of the typical trawl towing speed of 1.5 m s −1, and the implications of these results for seasonally varied towing strategies are discussed.