Distributions and diets of the larvae of tropical shorefishes near the northwest cape of Australia

The importance of understanding the factors influencing growth and survival of larval fishes and their effect on subsequent recruitment has been recognised in temperate areas since the late 1800’s. Despite this, our knowledge of these topics is severely limited for tropical larval fishes. In this study, early stage larval fishes were sampled using towed bongo plankton nets at sites on the southern North West Shelf of Australia (NWS) (21º49'S, 114º14'E), between October and February of 1997/98 and 1998/99. The first summer was characterised by El Niño - Southern Oscillation (ENSO) driven upwelling and high primary productivity, compared to the second summer when water temperatures were warmer and primary production was lower. I examined 9944 fish larvae from 76 families captured with the bongo nets. Benthic percoid shorefishes dominated surface assemblages in both summers and this pattern may be typical of tropical shelf environments. Abundance and diversity of larval fishes were lowest in October and increased from November through to February. Assemblages displayed weak cross-shelf patterns with a few taxa being more abundant at inshore sites (e.g. monacanthids), but others were more abundant offshore (e.g. scombrids). Although the composition of assemblages remained relatively consistent, many taxa (e.g. pomacentrids and carangids) showed differences in abundance between summers. Multivariate analyses found no relationships between abundance patterns of larval fishes and biophysical variables such as temperature, salinity and zooplankton biomass. Seasonal changes in abundance may thus reflect differences in the spawning activities of adult fishes and/or larval survival.\ud \ud Knowledge of the diets of tropical fish larvae is limited to only a few taxa. Here, we describe the diets of 591 individuals from 50 families of tropical larval shorefishes collected off the Northwest Shelf of Australia (21º49'S, 114º14'E), effectively doubling the number of families for which have we dietary data. The diversity of prey items eaten differed significantly among families. The majority of fish larvae ate copepods but there were some interesting exceptions. Chaetodontids ate only chaetognaths, acanthurids and nemipterids ate appendicularians, and tetraodontids ate predominately non-copepod prey (44% decapod larvae, 20% bivalves and 15% protists). Within the fish families that specialised on copepod prey there were marked differences in the types of copepod prey, with a clear preference shown for calanoid copepods, particularly small calanoids such as Bestiolina similis and Temora spp. Copepod communities in the area were food-limited and we suggest that the ability of some larval fishes to feed on components of the microbial food web may be an important determinant of their success.\ud \ud Further research into the feeding ecology of tropical larvae should consider the relationship between fish condition and prey type within the overall biophysical environment of the larvae. Identification of tropical larvae to species is still problematic and the use of genetic techniques may improve taxonomic resolution. Increasing our knowledge of the behaviour of tropical fish larvae will assist in interpretation of predator-prey relationships