Overexploitation causes profound demographic changes to the protandrous hermaphrodite king threadfin (Polydactylus macrochir) in Queensland’s Gulf of Carpentaria, Australia

Abstract

Fishing pressure is a significant driver of the demography of exploited fish populations. Here, we examine temporal patterns in the demography of king threadfin (Polydactylus macrochir), a large, protandrous teleost, in Queensland’s south-eastern Gulf of Carpentaria, Australia, between two periods: 1986–1990 (Period 1) and 2007–2009 (Period 2). Significant age truncation was evident from Period 1 (max age = 14 years, modal age = 5 years) to Period 2 (max age = 8 years, modal age = 3 years). Fish >5 years constituted 58% of the catch in Period 1, and less than 4% of the catch in Period 2. Length and age at sex change differed considerably between periods. In Period 1, the relationships between length and sex ratio and age and sex ratio were best described by a logistic model with 50% of the population changing sex to female at 889 mm fork length and at 6.3 years of age. In Period 2, there were substantially more females in small length and younger age classes with no obvious length or age at which sex change occurred. The predicted mean asymptotic length (L∞) from the best-fit models of growth was 361 mm greater in Period 1 than in Period 2, while the growth rate (K) was more than twice as high in Period 2 than in Period 1. Fishing mortality during Period 2 was estimated to be 2–3.5 times higher than that of natural mortality (M) and 2.6–5.4 times higher than that of Period 1, depending on the maximum age used to calculate M. No significant trends were evident in total annual rainfall or air temperature in the region over the study period, suggesting the observed changes in demography were not related to contrasting patterns in rainfall or temperature between sampling periods. These results suggest that heavy fishing pressure has likely had a profound effect on the demography of P. macrochir in the study region over a period of approximately 20 years, with the use of gillnets resulting in the selective removal of larger, older, female P. macrochir and subsequent reductions in age structures and sex change schedules. Accordingly, management intervention is urgently required to reduce fishing pressure and restore the natural demography of the population.