Growth, movements and diet of the terapontid Amniataba caudavittata in an Australian estuary

Abstract

Samples of Amniataba caudavittata were collected from the Swan Estuary in south-western Australia between February 1977 and December 1980, using seine nets in inshore shallow waters and gillnets and otter trawls in offshore waters. Although fish could only be aged using otoliths until they were 18–22 months old, they had by that time reached c. 75% of their final length. The mean lengths of the cohorts determined by using MIX, which analysed each of the monthly length-frequency distributions independently, were similar to those yielded by MULTIFAN, which constrains the means of each of the sequential and corresponding cohorts to a von Bertalanffy growth curve. Consequently, the von Bertalanffy growth curve parameters determined by MULTIFAN and those derived from the use of the means produced by MIX were similar. The clear cut trends exhibited by the progression of modal lengths in successive samples, and their close correspondence to the trends shown by the growth curves, demonstrate that many A. caudavittata live for at least 3 years. Growth was seasonal and confined to the warmer months of the year. The apparent negative growth recorded in winter can be attributed to a tendency for the smaller representatives of the different cohorts to remain inshore at that time and thus to be more susceptible to capture by seine netting, the main sampling method. Offshore movements by larger fish in the winter allow those fish to remain in the high salinities found beneath the pronounced haioclines that form in the deeper waters of this estuary during the heavy freshwater discharge that occurs typically at that time of the year. Adult A. caudavittata move into the upper estuary where spawning occurs, with considerable numbers of the resultant juveniles then moving downstream into the middle estuary. A, caudavittata is a benthic omnivore, with the 0+ age class ingesting algae and a range of small crustaceans, while older fish prey to a greater extent on polychaetes.