Do larvae of mesopelagic fishes in the Arabian Sea adjust their vertical distribution to physical and biological gradients?

Abstract

Size-specific vertical distributions of larvae of the myctophid species Benthosema pterotum, Bolinichthys longipes, Diaphus arabicus, Diogenichthys pan urgus, 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 Pahstan 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.

Assemblages of fish larvae and mesozooplankton across the continental shelf and shelf slope of the Andaman Sea (NE Indian Ocean)

“…However, smaller fish are distributed higher in the water column (Roe & Babcock 1984), and a study of vertical distribution patterns of tropical fish larvae by Smith & Suthers (1999) showed that the majority of the larvae occurred above the 100 m sampling depth. A study on the vertical distribution of larval myctophid species in the Arabian Sea showed that the average vertical distributions were well above 100 m water depth (Röpke 1993). Röpke (1993) did not find significant differences between vertical distributions during day or night; however, daytime catches were much below those obtained during the night (~25%), in agreement with our observations.…”

“…A study on the vertical distribution of larval myctophid species in the Arabian Sea showed that the average vertical distributions were well above 100 m water depth (Röpke 1993). Röpke (1993) did not find significant differences between vertical distributions during day or night; however, daytime catches were much below those obtained during the night (~25%), in agreement with our observations. While larval fish avoidance of towed nets during daylight periods is a well-known problem, the transparency of upper water masses in the tropics apparently magnifies the problem.…”

Distribution and abundance of fish larvae in the northern Ionian Sea (Eastern Mediterranean)

“…This latter species spawns at higher temperatures (11-15°C) and generally prefers highly saline waters. In general, the larvae of mesopelagic fish showed deeper distributions in the water column than the shelf-dwelling species, which had already been indicated from other geographical areas (Röpke 1993;Sabates 2004). The larvae of Argyropelecus hemigymnus showed deep distributions, as reported by Olivar et al (1998) during the stratified summer period of the Mediterranean.…”

“…For example, the larvae of Notoscopelus elongatus showed a more surface-oriented distribution, whereas the larvae of Myctophum punctatum, belonging to the Myctophinae subfamily, showed a deeper distribution until 100 m. Also, the most abundant species identified, Benthosema glaciale, belonging to the Myctophinae subfamily, showed a deeper distribution. Röpke (1993) indicated a similar distribution trend, even if in another area as the Arabian sea. An exclusively deep occurrence was shown by larvae of Argyropelecus hemigymnus, as indicated by Olivar et al (1998) during the summer stratified period in the Mediterranean.…”

“…Given the homogeneity of the physical characteristics throughout the water column, and given that all the sampling collections were carried out during the daytime, except station 20 that was sampled in the afternoon, the fish larvae were situated near the surface where they found higher concentrations of food. Röpke (1993) concluded that prey densities seem to be more critical in the vertical distribution of fish larvae than physical gradients.…”