A. D. McKinnon

The potential of tropical paracalanid copepods as live feeds in aquaculture

The small calanoid copepods Bestiolina similis and Parvocalanus crassirostris (Paracalanidae) are compared to Acartia sinjiensis, a species of the copepod genus most commonly cultured to provide larval diets for tropical snappers and groupers. All species are easily maintained in culture, though cultures of Acartia spp. are easier to establish because of the positive phototactic behaviour of this genus. B. similis and P. crassirostris are smaller than A. sinjiensis, and consequently, their developmental stages are more suited to larval fish requiring small prey items. In addition, siphon-avoidance experiments indicated that adults of these species were more vulnerable to predation, though this was not the case for juvenile copepods. Egg production was maximised with larger algal cells, especially Rhodomonas sp. and Heterocapsa niei: B. similis fed H. niei achieved the highest egg production rates (48 eggs female-1 day-1). Lipid composition of all species was low and variable (11–26 mg g-1) under our culture conditions, in contrast to results from temperate copepod species or from wild-caught copepods. All three species studied had DHA/EPA/ARA ratios that met or exceeded those recommended for marine larval fish feeds (DHA/EPA/ARA-14:3:1, 20:9:1 and 25:6:1 for A. sinjiensis, P. crassirostris and B. similis, respectively). On the basis of size of developmental stages, susceptibility to predation, growth rate and nutritional composition, B. similis was the best candidate for larval fish diets.

Rapid changes in shelf waters and pelagic communities on the southern Northwest Shelf, Australia, following a tropical cyclone

Abstract A pronounced shift in water column characteristics and in the composition of plankton communities was observed following the passage of Tropical Cyclone Tiffany along the margin of the southern Northwest Shelf, Australia in January 1998. Satellite-derived images of sea surface temperature, meteorological and hydrographic data indicate a southward movement of shelf waters into the study area near North West Cape (21°46′S). Changes in water mass temperature and salinity characteristics also occurred as a result of local heating and evaporation. Local in situ growth was likely to have caused increases in micro-phytoplankton abundance, biomass and primary production on the shelf. A diverse, copepod-dominated shelf mesozooplankton community changed to a less diverse assemblage dominated by copepods usually found in shallow nearshore habitats. Post-cyclone larval fish catches included families absent or rare in pre-cyclone samples. In the case of copepods and larval fish, southward transport of water masses along the shelf was most likely to have caused the observed changes. Long-shore water transport forced by cyclonic winds may be a recurrent, but episodic mechanism of planktonic dispersal on the North West Shelf.

High mortality in a copepod population caused by a parasitic dinoflagellate

Infection of copepods by parasitic dinoflagellates has been known for many years, but the ecological consequences of this parasitism have been largely neglected. We estimated mortality rates in the copepodParacalanus indicus Wolfenden due to parasitism by the dinoflagellateAtelodinium sp. by applying laboratory mortality rates to a field population of infected copepods in Port Phillip Bay, Australia, sampled in 1982–1985. Adult female copepods were most often infected, with an incidence of 0 to 28.5% (median 6.2%). Stage V female copepodites were less often infected, and males were never infected. The median mortality rate in females was about 7% d−1, or about one-third of total mortality, and the maximum was 41% d−1. The frequent occurrence of dinoflagellate parasitoids in some species of copepod implies an important, species-specific mechanism for the regulation of populations.

Temporal patterns in distributions of tropical fish larvae on the North-west Shelf of Australia

Information on the temporal distributions of tropical fish larvae is scarce. Early stage larval fishes were sampled using towed bongo plankton nets at sites on the southern North West Shelf of Australia (21°49′S, 114°14′E), between October and February of 1997/98 and 1998/99. The first summer was characterised by El Nino–Southern Oscillation-driven upwelling and high primary productivity, whereas in the second summer water temperatures were warmer and primary production was lower. Benthic percoid shorefishes dominated surface assemblages in both summers and this pattern may be typical of tropical shelf environments.The 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), whereas 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. Thus, seasonal changes in abundance may reflect differences in the spawning activities of adult fishes and/or larval survival.

Determinants of pelagic metabolism in the Timor Sea during the inter-monsoon period

The Timor Sea is a major conduit of the Indonesian Throughflow characterised by large internal waves and tides. To ascertain whether these result in high pelagic productivity, we conducted experiments to determine the metabolic balance between net community production (NCP) and community respiration (CR) on the Sahul Shelf, the Sahul Shoals and the Yampi Shelf, an area of active hydrocarbon seeps. The barrier to vertical mixing of subthermocline nutrients represented by the halocline allowed new production to dominate in March 2004, whereas production in June 2005 depended on recycled nutrients. CR was correlated with dissolved organic carbon (DOC) in 2004, but with chlorophyll in 2005, suggesting that respiration was dominated by microheterotrophs in 2004 but by autotrophs in 2005. Overall, area-specific CR averaged 120 ± 92 (s.d.), 101 ± 52 and 61 ± 6 mmol O2 m–2 day–1, NCP averaged 109 ± 85 (s.d.), 32 ± 41 and 57 ± 10 mmol O2 m–2 day–1, and average gross primary production (= CR+NCP) : R ratios were 1.9, 1.4 and 1.9 on the shelf, at the Sahul Shoals and the Yampi Shelf, respectively. We suggest that differences in water column structure and internal wave activity drive intermittent high production events in a predominantly oligotrophic sea.