DO LARVAE OF MESOPELAGIC FISHES IN THE ARABIAN SEA ADJUST THEIR VERTICAL-DISTRIBUTION TO PHYSICAL AND BIOLOGICAL GRADIENTS

Size-specific vertical distributions of larvae of the myctophid species Benthosema pterotum, Bolinichthys longipes, Diaphus arabicus, Diogenichthys panurgus, Hygophum proximum, and Myctophum aurolaternatum, and of the photichthyid species Vinciguerria nimbaria were analyzed from 3 hydrographically and ecologically different regions of the northern Arabian Sea (Indian Ocean) during the intermonsoon period (March-June) 1987, using a MOCNESS-1 net system to 150 m depth under comparable circumstances. Regional data on the vertical centres of mass were compared by species and length class. Concurrent measurements of physical stratification and of prey abundance and distribution in the water column were related to larval distribution. Results indicate that larvae of mesopelagic species are found at relatively deep depth. They move downward during early development, adapting to their later life in the mesopelagic zone. Species- and size-specific depth selection was responsible for much of the interregional differences in vertical distribution of fish larvae in general, since the species composition was different between regions. Most of the myctophid and photichthyid species avoided the upper mixed layer, which contained the highest concentrations of potential prey organisms, and their distribution was also not directly related to pycnocline depth (except for B. pteroturn). Below the mixed surface layer the abundance and vertical distribution of potential prey was more important in determining the vertical distribution of the larvae than the gradient of physical stratification. The low abundance of prey on the shelf off Pakistan was probably responsible for the strong concentration of larvae just below the mixed layer and for their poor nutritional condition in that area. Larvae of identical species and length class occurred on average about 20 m deeper in the central oceanic region compared to the coastal areas off Oman and Pakistan. This was probably due to a deeper and broader distribution of prey organisms.