VERTICAL AND ONSHORE-OFFSHORE DISTRIBUTIONAL PATTERNS OF TUNA LARVAE IN RELATION TO PHYSICAL HABITAT FEATURES

The seasonal and spatial distributions of larval scombrids in Waters off Oahu, Hawaii, were studied in September and December 1985 and April and June 1986 to determine vertical and onshore-offshore patterns. Horizontal distribution was resolved by sampling transects of 3 stations on the windward (east; 3.7, 9.3, and 27.8 km offshore) and leeward (west; 1.8, 9.3, and 27.8 km offshore) sides of the island. Vertical distribution was resolved by sampling with MOCNESS and Manta nets in 8 depth strata (0 to 80 m) at the 2 nearshore stations and 9 depth strata (0 to 200 m) at the 4 offshore stations. Scombrid larvae were abundant in all months except December. Collections in September and June were dominated by Thunnus spp.; about half were large enough to be identified to species and were predominantly T. albacares. Thunnus spp. generally were most abundant closest to shore on the leeward side of the island. Larval Thunnus were not taken at > 60 m, were most abundant in the upper 20 m, and were captured in the neuston at night but rarely in daytime. The other 2 dominant larval scombrids, Katsuwonus pelamis and Auxis spp., were most abundant in slightly deeper water. K. pelamis were most abundant at offshore stations, but Auxis spp. showed no clear onshore-offshore pattern. In April, Auxis spp. were most abundant and comprised over 99% of all larval scombrids. Diel vertical distribution patterns were not consistent between cruises or sides of the island for any species studied. No evidence was found for diel Migratory behavior. The physical structure of the water column, particularly temperature and salinity, altered the spatial and temporal distribution patterns of larvae on short time and space scales and also on the seasonal Scale. Seasonal patterns of spawning were likely dependent upon availability of appropriate thermal habitat. Larval Thunnus spp. and K. pelamis, in particular, were abundant only when temperatures were relatively warm. Auxis spp. larvae were found over a wider range of temperature than were the other genera. We suggest that the high concentrations of scombrid larvae, particularly Thunnus spp., near islands may be related to habitat characteristics that promote higher larval survival than in oceanic waters.