Biologie et écologie du bec de cane, Lethrinus nebulosus (Forsskål), en Nouvelle-Calédonie. Rapport d'opération ZoNéCo. IRD, Nouméa.

IRD, Noumea reportAbstract : Biology and ecology of spangled emperor, Lethrinus nebulosus (Forsskål), in New Caledonia. The present work is a synthesis of the knowledge cumulated over the last twenty years on the biology and ecology of spangled emperor, Lethrinus nebulosus (Forsskål), in New Caledonia, augmented with new information on its population genetics, to establish the bases for rational resource management. Spangled emperors were captured in the northern and southwestern lagoons of Grande Terre and in the Ouvea lagoon, mainly using hand-lines, bottom long-lines, trawls and rotenone poisoning. In total, 2975 fishes were examined for length, weight, sex and gut content. Females were larger than males by 5 cm on average. The longest specimen caught was 69.5 cm FL and the heaviest weighed 5.5 kg. In Ouvea, sizes were smaller than in the southern lagoon or the Northern Province. Average size increased with distance to the coast, depth, coral cover and hard-substrate cover, but diminished with algal cover. The largest fishes were caught at 30-40 m. For a given size, individuals were lighter in the Northern Province than in Ouvea or the southern lagoon. For a given size, individuals were heavier in May- July, that is, before reproduction. Growth was nearly linear until first reproduction, occurrngi at 4-5 years of age. In the southern lagoon, spangled emperors ate more fish, crustaceans and echinoderms, while they ate more mollusks in Ouvea and the Northern Province. Diet varied with individual size but heterogeneously across regions. The proportion of full stomachs decreased in September-October, that is, during the reproductive period. Sex-ratio was 58% females, increased with size (70% females at FL > 60 cm) and varied with regions. First reproduction took place at 35-45 cm FL and seemed to occur earlier in Ouvea. Samples from the northern and southern lagoons and Ouvea were analysed at the mitochondrial locus (nucleotide sequences of the control region and SSCP analysis of a fragment of 16S rDNA) and at three intron loci. No genetic heterogeneity was detected. The biological and ecological differences observed between those three geographic sites therefore could be ascribed to phenotypic plasticity, but they nevertheless allowed the distinction of three stocks. Catches were the most important in Ouvea, in terms of either number, weight, or proportion of total catch; bottom long-line catches were more important in the southern lagoon than in the Northern Province; trawl catches indicated an absence of spangled emperor juveniles on soft bottoms, although rotenone catches and visual observations underwater demonstrated the importance of shallow sea-grass and algal beds, as well as rubble, for juveniles. Spangled emperors amount to less than 1% of the total fish biomass or density, except on mid-lagoon soft bottoms where their biomass was 3.5% of the total. Spangled emperor stocks may well reach around 10 000 tons for the entire New Caledonia - Loyalty region (the Chesterfield area excluded). Spangled emperors might well be over-exploited in those areas which have the highest human densities (i.e. southern lagoon and West coast)