This study is the first to focus solely on the biology and ecology of the Leiognathidae, a diverse teleost family that is abundant in the Gulf of Carpentaria, northern Australia. It is also the first broad-based biological study of any family of demersal fish assemblages that comprises a large portion of the bycatch of the Gulfs prawn trawlers. The results will contribute to a better understanding of some of the factors important in structuring demersal fish assemblages of soft-bottom, shelf ecosystems and are used to assess some of the potential impacts of commercial trawling for prawns in the Gulf of Carpentaria.
Life-cycles of leiognathid species were determined by a) interpreting patterns in their Gulf-wide distribution, b) examining the relationships between fish size and depth in a relatively small area of the Gulf where leiognathids are diverse and abundant and c) examining published information on the occurrence of some species in estuaries and shallow coastal areas. Data from demersal trawls at 261 sites carried out between 1991 and 1993 in depths ranging from seven to 63 m were used to describe the Gulf-wide distribution patterns of leiognathids. The results indicate that eight species, Gazza minuta, Leiognathus decorus, L. equulus, L. fasciatus, L. leuciscus, L. smithursti, L. splendens and Secutor mconius, are usually restricted to coastal areas, while four species, L. bindus, L. moretoniensis, Leiognathus sp. and S. insidiator, are common throughout the Gulf.
The depth-related patterns of fish-size (mean weight and minimum and maximum lengths) were investigated using data from 356 trawls carried out in and near Albatross Bay between 1986 and 1994, in depths ranging from six to 58 m. Most of the coastal species were rarely caught in depths >40m and exhibited linear increases in size with depth. This pattern is consistent with the existence of estuarine and/or nearshore nursery areas, and supports previous observations that juveniles of these species occur in the nearshore (≤5 m) and inshore (6-20 m) areas of Albatross Bay and/or the Embley River estuary. The requirement for nearshore and/or estuarine nursery areas may be an important factor in restricting these species to coastal areas. In contrast, L. bindus, L. moretoniensis and S. insidiator exhibited approximately quadratic relationships between fish-size and depth, resulting from small fish living in both the shallow inshore areas and deeper offshore areas. The existence of offshore nursery areas for these and other widespread species may be an important factor aiding their distribution throughout the Gulf It is possible that the different life-cycles displayed by leiognathids in the Gulf of Carpentaria may help to reduce potential competition among the abundant closely related, sympatric species, especially during the juvenile life stages.
Despite the contrasting distribution patterns displayed by leiognathids, there is a large potential for competition for food resources in shallow (≤40m) coastal areas, such as Albatross Bay, where the family is most abundant and diverse. Diets of sympatric species collected during November 1993 and May 1994 were compared using gravimetric and frequency of occurrence methods, and species were grouped into four dietary guilds using multi-dimensional scaling analysis. The largest guild comprised five species; L. decorus, L. equulus, adult L. leuciscus, L. moretoniensis and L. splendens. These fish consumed mainly polychaetes and a variety of other benthic invertebrates. Leiognathus bindus, L. elongatus and S. insidiator together formed a guild of zooplanktivores, consuming mainly calanoid copepods. Secutor ruconius and juvenile L. leuciscus formed a second guild of zooplanktivores, but larvaceans were the main prey. Finally, G. minuta was the only piscivorous species.
Potential intra-family competition for food is reduced by the formation of these dietary guilds. However, the similarities in the diet of sympatric species belonging to the same guild suggest that the potential for competition for food is high, or that food is not limiting. It is likely that populations of leiognathids and other potential competitors are kept below levels at which intra-guild competition would occur because of high rates of mortality, especially from the region's demersal trawl fishery. Thus competition for food is unlikely to be the most important factor responsible for structuring leiognathid assemblages. Furthermore, the distribution of species belonging to the zooplanktivore and benthic invertebrate-feeder guilds throughout the Gulf suggest that food availability does not provide a simple explanation for the restriction of other species in the same dietary guilds to coastal areas.
The ages of 331 fish belonging to eleven species were estimated by counting daily growth increments on otoliths (sagittae). The species can be divided into four groups based on approximate life-spans. Leiognathus elongatus and S. insidiator are the shortest lived species, with life-spans of up to approximately one year, while L. equulus is the longest-lived species, with a life-span of two to three years. Leiognathus decorus, Leiognathus sp. and S. ruconius live to approximately one to 1.5 years. Gazza minuta, L. bindus, L. leuciscus, L. moretoniensis and L. splendens live between 1.5 and two years.
Growth of all species was described well by the von Bertalanffy growth equation. Estimates of L∞ ranged from 74 mm for L. elongatus to 155 mm for L. equulus. Comparisons of growth patterns between species were made using a re-parameterised version of the von Bertalanffy growth equation. The three slowest growing species up to 85 days of age (S. ruconius, L. equulus and L. decorus) are coastal with estuarine and nearshore nursery areas, while three of the four fastest growing species (S. insidiator, L. elongatus and Leiognathus sp.) are widespread with offshore and inshore nursery areas. These observations support the view that fast growth rate is an advantageous strategy in young fish in offshore nursery areas, but not in estuaries. This life-history strategy may be related to differences in the relative risk of predation in the different nursery areas.
Examination of the reproductive characteristics of leiognathids concentrated on the patterns exhibited by female fish. The minimum size at maturity of females was determined by histological examination of gonads and by comparing the gono-somatic index (GSI) of all fish with a threshold GSI defining maturity. Only one of the five smallest and fastest (<6 months) maturing species, S. ruconius, has estuarine and nearshore nursery areas and coastal distribution patterns. Three of the five, L. bindus, L. moretoniensis and S. insidiator, have offshore and inshore nursery areas and adults are spread throughout the Gulf of Carpentaria. Little is known about the distribution of the fifth species, L. elongatus, although it does not appear to have estuarine nursery areas. In contrast, five of the six largest and slowest (>7 months) maturing species, G. minuta, L. decorus, L. equulus, L. leuciscus and L. splendens, have estuarine and nearshore nursery areas. These observations suggest it may be more advantageous for species that use estuaries as nursery areas to have higher fecundity at first spawning than to reach spawning size quickly, while it may be more advantageous for species with offshore and inshore nursery areas to spawn at least once, than to delay spawning until fecundity is higher.
The seasonal reproductive patterns of nine species were investigated by monitoring GSI throughout the year and by estimating the proportion of the population classed as mature on each sampling occasion. The nine species examined can be divided into four groups. The reproductive season of L. moretoniensis continues throughout the year, including the post-wet season, when all fish examined were classed as mature. Reproductive activity of L. equulus, L. leuciscus and L. splendens is lowest during the post-wet season, but continued through the remainder of the year. Reproductive activity of G. minuta, L. bindus, L. decorus, and S. ruconius is also lowest in the post-wet season, but remains low in the early (G. minuta and L. bindus) to middle (L. decorus and S. ruconius) dry season as well. Secutor insidiator has the most restricted reproductive season, with high reproductive activity in the pre-wet season only. The differences in the seasonal timing of spawning do not explain the differences in lifecycles, particularly the distribution of nursery areas, displayed by the coastal and widespread species of leiognathids in the Gulf of Carpentaria.
The demersal habits of leiognathids make them susceptible to capture by prawn trawlers, and thus they comprise a large portion of the Northern Prawn Fishery's bycatch. However, most trawling in the Gulf of Carpentaria occurs in depths of 20-40 m. This feature decreases the potential impact of trawling on populations of leiognathids by causing incidental spatial refuges, especially for juvenile fish. For widespread species, these refuges are mainly in the deep offshore waters of the Gulf, while for coastal species, they are the shallow nearshore and estuarine waters. Consequently, commercial trawlers catch mainly adult fish, which suggests that individual fish have a high probability of having reproduced before capture and is another factor that suggests leiognathid populations are able to cope with current temporal and spatial patterns of trawling in the Gulf of Carpentaria. This is not the case in parts of south-east Asia, however, where intensive fishing in all depths and habitats, including estuaries and the shallow coastal areas, has almost certainly contributed to the dramatic decline of the region's leiognathid populations.