Seasonal and spatial changes in the larval fish fauna within a large temperate Australian estuary

A total of 66814 fish larvae, representing 37 families and 74 species, were collected in samples taken monthly between January 1986 and April 1987 from 13 sites located at frequent intervals throughout the large Swan Estuary in south-western Australia. The Gobiidae was the most abundant family, comprising 88.2% of the total number of larvae, followed by the Clupeidae (3.4%), Engraulididae (2.9%) and Blenniidae (1.0%). The most abundant species were Pseudogobius olorum (53.3%), Arenigobius bifrenatus (31.2%) and Engraulis australis (2.9%). Abundance of fish larvae in the lower, middle and upper regions of the estuary each reached a maximum between mid-spring and early summer, 2 to 4 mo before the attainment of maximum temperatures. Larvae of species such as Nematalosa vlaminghi and Apogon rueppellii were collected only between November and February, whereas those of others such as P. olorum, E. australis and Leptatherina wallacei were present over many months. The times and locations of capture of larvae have been related to the distribution and breeding periods of the adults of these species. The mean monthly number of species was far greater in the lower than upper estuary (14.7 vs 2.7), whereas the reverse was true for mean monthly concentration (42 vs 197 larvae per 100 m3). Classification, using the abundance of each of the 74 species recorded at the different sites, showed that the composition of the larval fish fauna in the lower, middle and upper estuary differed markedly from each other. Most larvae caught in the lower estuary belonged to marine species, whereas those in the upper estuary almost exclusively represented species that spawn within the estuary. The fact that the larvae of the 59 species of marine teleosts recorded during this study were restricted mainly to the lower estuary, and yet contributed only 6.2% to the total numbers for the whole estuary, helps to account for the relatively high species diversity in this region. The lack of penetration of many of these larvae beyond the first 12.5 km of the estuary presumably reflects the weak tidal effect in the wide basins of the middle estuary and saline regions of the tributary rivers. The larvae of the 13 teleosts that typically spawn within the estuary contributed 93.8% to the total numbers of larvae. Most of these estuarine-spawned larvae belong to teleosts that deposit demersal eggs and/or exhibit parental care (egg-guarding and oral and pouch-brooding), characteristics which would maximize their chances of retention within the estuary