Feeding patterns of transforming and juvenile myctophids that migrate into neustonic layers

Abstract

Adult and juvenile myctophids feed at night in the epipelagic zone (<200 m) and are more dispersed in the mesopelagic zone (200-1000 m) during the daylight hours. In contrast, larvae inhabit the upper 200 m throughout a 24 h period and have daylight feeding patterns. Transforming stages occur both at the surface and in the mesopelagic zone and show less-defined feeding patterns. In this study, we analysed the trophic ecology of transforming and juvenile stages of 4 myctophids that occupy the neustonic layers (first 0-20 cm of the water column) during their nightly vertical migrations: Dasyscopelus asper, Gonichthys cocco, Myctophum affine, and M. nitidulum. Day and night neuston samples were collected across the equatorial and tropical Atlantic in April 2015. Transforming and juvenile stages occurred at night in the neuston, where they fed, but were absent from this layer during the day. The highest prey ingestion was observed between 01:00 and 04:00 h (UTC). Feeding incidence and the number of prey ingested increased from transformation stages to juvenile stages. Although the maximum prey size increased with fish body length, there was no significant increasing trend in mean prey sizes, but a great variability in the sizes of consumed prey. Diets of the 4 species mainly comprised a variety of copepod genera, usually dominated by Oncaea species. There was no evidence of resource partitioning among the 4 myctophid species. Estimates of daily feeding rations, based on the relationship between carbon content per gut and carbon content of fish body, throughout the night feeding period, showed that these species in these early stages ingested between 0.43 and 5.78% of their body carbon weight daily. We suggest that the occurrence and feeding of these early stages in the neuston may contribute to reducing trophic competition between migrating myctophids by space segregation.