The fish faunas of Wellstead Estuary

Fish faunas in the nearshore, shallow and offshore, deeper waters of Wellstead Estuary, a normally-closed estuary on the south coast of Western Australia, were sampled seasonally between spring 2005 and spring 2008 and additional sampling was undertaken in 2009. The sand bar at the mouth of the estuary was open to the ocean at the beginning of the study, before closing in autumn 2006 and then remaining closed for the duration of the sampling period. Salinities in corresponding seasons were invariably higher in the second year of sampling, when the estuary mouth was closed and rainfall relatively low. Water temperatures were highest in summer and lowest in winter, with the reverse trend pertaining for dissolved oxygen concentrations. A total of 17 fish species were recorded in the nearshore, shallow waters of the basin and eight species recorded in the river of Wellstead Estuary. Estuarine-spawning species overwhelmingly dominated both regions, with the Hardyhead Atherinosoma elongata contributing 82% in the basin. This species, together with four other estuarine-spawning species, i.e. Leptatherina wallacei, Pseudogobius olorum, Favonigobius lateralis and Afurcagobius suppositus, collectively constituted > 99% of the fish fauna in this region. Leptatherina wallacei dominated the fish fauna in the nearshore, shallow wasters of the river and, together with A. elongata, accounted for > 94% of the fish in that region. Seventeen species were recorded in the offshore, deeper waters of the basin, which were dominated by Yelloweye Mullet, with Black Bream and Sea Mullet also making substantial contributions in this region. Black Bream dominated the fish fauna of the offshore, deeper waters of the river, followed by Sea Mullet and then Yelloweye Mullet. Overall comparisons of the fish faunal compositions in 2005-07 with data collected at the same sites in 1996-98, demonstrated that the significant differences between the two periods were driven by consistently greater catches of the three same species i.e. Black Bream, Yelloweye Mullet and Sea Mullet, in 2005- 07. Such differences may be attributable, in part, to a combination of relatively reduced salinities and increased primary productivity in the later period. Comparisons of the fish faunal composition in offshore, deeper waters in 2006, 2007 and 2009 demonstrated significant differences among years. These differences were due, inter alia, to reduced numbers of marinespawning species in 2009. The oldest Black Bream captured in Wellstead Estuary was 15 years old, with each age class between 0+ and 15+ being represented in the catches. Growth between females and males was not significantly different. Growth of this species in 2005-07 was significantly different to 3 that recorded in 1996-98, with length at age typically being > 10% lower in the later period and the difference sometimes exceeding 15%. Such differences may be attributable to increased densities of this sparid and thus increased competition for such resources as food and shelter. Black Bream is an estuarine-spawning species that breeds between late winter and late spring, with spawning peaking in October in Wellstead Estuary. The L50 at maturity of this species in Wellstead Estuary was estimated at 146 mm for females and 138 mm for males and the majority of individuals were mature at the end of their second year of life. These findings indicate that, in Wellstead Estuary, Black Bream have the opportunity to spawn at least once before reaching their minimum legal length for retention (250 mm). Annual spawning success and/or recruitment of Black Bream is highly variable, with the strong year classes of 1998, 2002 and 2005 being associated with periods of relatively high rainfall and thus, presumably relatively reduced salinities, prior to and during the spawning season. The fact that the other two abundant species, Yelloweye Mullet and Sea Mullet, are both marine-spawning species, demonstrates that these mugilids must have a strong affinity for estuaries and that they use such systems predominately as nursery areas. Juveniles of these two species presumably entered the estuary during the months immediately prior to and during the beginning of the study period, when the mouth of this normally-closed estuary was open to the ocean. Growth of the juveniles of these species in Wellstead Estuary was higher than that recorded for the corresponding species in other areas of their distributions and may be attributable to increased resources as a result of high primary productivity in this system. The fact that no individuals with gonads in spawning condition and that no small juveniles were captured during the periods when the sand bar was closed supports the findings of other studies that these species spawn only in marine waters.