Onshore-offshore distribution and abundance of tuna larvae (Pisces: Scombridae: Thunnini) in near-reef waters of the Coral Sea

Large-scale (100s km) distributions of tuna larvae (family Scombridae), particularly of the commercially important genera Thunnus and Katsuwonus, have been extensively investigated because of the need to identify spawning locations and the possibility of estimating spawning stock biomass from surveys of larvae (Strasburg, 1960; Richards, 1976; Scott et al., 1993). Despite this effort, and the apparent abundance and fecundity of T. albacares (yellowfin tuna) and K. pelamis (skipjack tuna) in the western and central Pacific Ocean, relatively low mean concentrations (2–4 larvae/ 100 m3, Leis et al., 1991) and abundances of tuna larvae (2–8 larvae/10 m2, Leis et al., 1991) have been found during most sampling programs. Low numbers of tuna larvae in any one site or region may simply reflect a broadly distributed, but sparse, spawning pattern of adults, as indicated by the wide geographical ranges of T. albacares and K. pelamis larvae. Other possible explanations, however, are that previous sampling scales of 100s km between samples were too coarse to account for spatial variability in abundances (Davis et al., 1990a), and that tuna larvae are more abundant in undersampled near-reef areas (Leis et al., 1991). Nearly all sampling of tuna larvae has been conducted in the open ocean because of the oceanic distributions of adults; however relatively high concentrations of tuna larvae have been found in previously undersampled tropical near-reef (<5 km offshore) locations in three studies (Miller, 1979; Leis et al., 1991; Boehlert and Mundy, 1994). Concentrations of larval T. albacares were up to two orders of magnitude greater within 2 km of leeward (west) Oahu Island, Hawaii, than published concentrations in surrounding oceanic waters (Miller, 1979), and Thunnus spp. and K. pelamis larvae were up to 100 times more concentrated within 200 m of coral reefs in French Polynesia than in oceanic waters of the central Pacific Ocean (Leis et al., 1991). Because adjacent oceanic locations were not simultaneously sampled in these near-reef studies, a direct comparison of these two habitats was not possible. Furthermore, comparisons between the near-reef values of larval tuna concentration determined by Miller (1979) and Leis et al. (1991), and those determined for oceanic habitats in the central Pacific Ocean in other studies, may also be confounded because of differences in sampling methods employed between the two Onshore-offshore distribution and abundance of tuna larvae (Pisces: Scombridae: Thunnini) in near-reef waters of the Coral Sea