Age structure and changes in abundance of three important species of teleost in a eutrophic estuary (Pisces: Teleostei)

Abstract

Comparisons have been made between the catch statistics for the commercial gill and haul net fisheries of two large neighbouring estuaries in south‐western Australia. The results indicate that during the 1970s the abundance offish in the Peel‐Harvey system in south‐western Australia rose as a result of the effects of nutrient enrichment, which included a massive increase in macroalgae (Cladophora sp. and Chaetomorpha spp.). In terms of mean monthly catch per boat per annum, which is shown to be a good reflection of the annual catch per unit effort, the total fishery increased by 1.8 times from 738 kg in the 10 years prior to 1969 to 1327 kg between 1970 and 1979. The comparable values for the three most important commercial species, i.e. the Yellow‐eye mullet (Aldrkhetla forsteri), Sea mullet (Mugil cephalus) and Cobbler (Cnidoglanis macrocephalus), increased by 1.9, 21 and 3.3 times, respectively. This contrasts with the situation in the nearby large Swan‐Avon estuary which has not seen prolific macroalgal growth but whose fishery uses the same techniques and is exposed to similar market demands. Thus, in the Swan‐Avon the mean monthly catch per boat for the total fishery increased in the 1970s by only 1.2 times and no significant change occurred in this parameter for the important Sea mullet and Cobbler. Length‐frequency data showed that all three species grew rapidly in the Peel‐Harvey estuary, with the result that most individuals had reached the minimum legal length for capture within two years. In the case of the Sea mullet, this length was sometimes achieved by the end of the first year of life. Since growth rates of the main fish species in the Peel‐Harvey were similar to those in the Swan‐Avon, the increase in weight of fish caught is probably attributable to a rise in fish abundance rather than a faster growth rate. While the rise in abundance may reflect a greater food availability, it could also represent a decline in predation from the large, local, piscivorous bird populations as a result of the development of extensive macrophyte cover.