Changes in the stock composition and abundance of spawning Gemfish Rexea solandri (Cuvier), Genpylidae, in South-eastern Australian waters

Abstract

Data collected during a study of the New South Wales trawl fishery showed a marked increase in landings of gemfish Rexea solandri (Gempylidae) during the late 1970s, which was due to the exploitation of winter spawning aggregations. Catches peaked at 5100 t in 1980, but declined to about 3000 t over the following few years, rising again to around 4000 t in 1987. Daily landings data for individual vessels were analysed for the period 1973-86. Fishing effort (target days) was standardized by comparison of each vessel's catch per target day with that of an arbitrary standard vessel. Catch per standard day declined by about 70% between 1980 and 1983. The mean caudal fork length of gemfish from spawning run catches declined from 79 cm to 71 cm over the period 1975-85, reflecting a reduction in the number of large fish in the catch and the recruitment of smaller fish into the spawning aggregations. Ages of gemfish were estimated from counts of hyaline zones on the sagittal otoliths. For age classes 1 to 3, otolith-determined ages were validated by means of modal analysis of monthly length-frequency data, but no validation was possible for the older age classes. Males sampled from spawning-run catches were estimated to be predominantly 4-8 years of age, with a maximum age of 11, whereas females were estimated to be predominantly 5-10 years old, reaching a maximum age of 16. For each sex, the proportion of older fish in the catch declined between 1980 and 1986, and there was a corresponding increase in the prominence of newly recruited age classes. It was concluded that there had been a substantial decline in the relative abundance of spawning gemfish, and that significant changes had also occurred in the size and age composition of the stock, consistent with the effects expected to result from exploitation.