What determines the growth of tropical reef fish larvae in the plankton: food or temperature?

The late-stage larvae of the reef fishPomacentrus coelestiswere collected using light traps at stations on a cross-shelf transect near Northwest Cape, Western Australia, during the spring and summer months (October to February) of 1997–98 and 1998–99. Physical (water temperature, wind) and biological (chlorophylla, zooplankton abundance) variables were measured concurrently at each station. In 1997–98, environmental conditions were characterised by intrusive upwelling onto the shelf, relatively cooler water temperatures and higher chlorophyllaand zooplankton biomass. During the 1998–99 summer, water temperatures were warmer, and chlorophyllaand zooplankton biomass were relatively low. Catches ofP. coelestiswere much lower in the first summer (197 fish) than the second (1483 fish). Records of planktonic growth were obtained from otoliths of subsets of larvae from both summers. Growth varied among months within each summer; however, on average, larvae grew more slowly in the 1997–98 summer than the 1998–99 summer (0.48 mm d–1vs 0.53 mm d–1respectively), despite the presumed food (copepods) being more abundant in the plankton. Partial correlation analysis showed that water temperature explained approximately 30% of the variation in growth of larvalP. coelestis. In contrast, chlorophyllaand zooplankton abundance explained much less (4.1 and 3.5%, respectively).