Biology of Western Butterfish (Pentapodus vitta) in Cockburn Sound, Western Australia

Billy E Bowe

The Western Butterfish, Pentapodus vitta Quoy & Gaimard (1824) is a subtropical nemipterid whose abundance has greatly increased on the lower-west coast of Western Australia. The aims of this study were to determine the pattern of abundance and biology of P. vitta in a temperate embayment (Cockburn Sound) and compare the results to those of a previous study undertaken in a subtropical embayment (Shark Bay) to test the predictions from the temperature-size rule (TSR). Pentapodus vitta displayed sexually dimorphic growth with females attaining, on average, significantly smaller lengths at age than males, with the latter sex predominating the larger size classes. Although males attained a larger maximum size of 265 mm total length (TL) compared to 237 mm for females, females reached a greater maximum age (12 vs 9 years). The TL at 50% and 95% maturity were >20 mm smaller for females (127 and 148 mm TL) than males (147 and 181 mm TL). Mean monthly gonadosomatic indices were elevated for females and males from September, reaching peaks in November/December before declining rapidly in January. The trend in GSI and the presence of maturing and mature gonads during October to December, strongly suggests spawning largely occurs over these three months. Comparisons of the abundance of P. vitta caught in otter-trawls in 2007/8 and 2021-23 demonstrated that there was a 30-fold increase in mean abundance and an increase in the frequency of occurrence from 44% to almost 100%. This indicated this species was now among the most abundant (33.4% of the total catch) and ubiquitous teleosts in Cockburn Sound having previously been mainly restricted to Owen Anchorage. Comparisons of the biology of P. vitta between Cockburn Sound and Shark Bay show that in the former temperate embayment, this species grow slower to a larger maximum size and attains an older age than in the subtropical Shark Bay, where P. vitta reach their asymptotic length faster. This finding matches the hypothesis of the temperature-size rule where fish in cooler waters grow slower but reach larger sizes compared to those in warmer waters which grow faster but attain smaller maximum sizes. These findings on the growth and reproduction of P. vitta in Cockburn Sound provide strong support for the hypotheses on changes in growth between sub- tropical and temperate environments. This information can now be used by management to better understand the impact of a sub- tropical generalist increasing in abundance at an astronomical rate in a temperate embayment, Cockburn Sound.

Biology of Western Butterfish (Pentapodus vitta) in Cockburn Sound, Western Australia

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