Biology and commercial exploitation of the estuarine catfish, Cnidoglanis macrocephalus (Valenciennes) in South Western Australia with emphasis on the seasonally closed Wilson Inlet

Abstract

Wilson Inlet is a seasonally closed estuary on the southern coast of Western Australia. It comprises a large basin (48 km^2), which is fed by two main tributary rivers and opens to the ocean by a narrow 1 km inlet channel. The sand bar which forms across the mouth of the estuary during dry periods is artificially breached when water levels reach 1.015 m Australian Height Datum. The length of time the estuary is open to the sea, which ranges from about two to 11 months, is related to the time of opening, i.e. the earlier the opening (which corresponds to high winter fresh water build up), the longer the bar remains breached. Samples of the estuarine catfish Cnidoglanis macrocephalus were collected by gill net and beach seines from the basin of Wilson Inlet in each month between September 1987 and April 1989. Cnidoglanis macrocephalus is a benthic carnivore, feeding mainly on molluscs, crustaceans and polychaetes. Diet changed from soft bodied prey {e.g. crustaceans, mytilids, nereids and gastropods) to robust bivalves {e.g. tellinids and venerids) with increasing body size and it also changed with season. Spawning occurred within the estuary, predominantly between mid-spring (October) and mid-summer (January), with peak spawning activity late November and early December. Sexual maturity was typically first reached at an average total length of 425 mm, at which size the fish were nearing the end of the third year of life. The eggs of C. macrocephalus were deposited in burrows and the males subsequently guarded both the eggs and larvae. While the growth of C. macrocephalus in Wilson Inlet was more rapid than in a nearby inshore marine area, it was similar to that in the Swan Estuary on the lower west coast of Western Australia further to the north. Commercial catches of C. macrocephalus in Wilson Inlet rose markedly from an annual average of 34,89 kg between 1976 and 1979 to 58,431 kg between 1985 and 1989. During these two periods, the percentage contribution of C. macrocephalus catches in Wilson Inlet to the total catch of this species in Western Australia increased from 15.6 to 46.0%. Commercial catches in Wilson Inlet varied seasonally, being greatest in spring and autumn and lowest during winter. They also varied markedly among years, with values being high in 1975 and 1986 (60,000 to 80,000 kg) and low between 1977 and 1983 (<35,000 kg). The total lengths and numbers of C. macrocephalus caught in the closed and open fishing waters of Wilson Inlet, using a gill net comprising a wide range of mesh sizes, were recorded. These data were then compared with those obtained for the commercial fishery. The results strongly indicate that (I) the density and percentage contribution of large fish are lower in open than in closed fishing waters, (II) female fish were more readily caught than male fish and (III) the minimum legal mesh size and minimum legal length for C. macrocephalus should be increased to prevent fish being caught before they reached sexual maturity