Feeding ecology of myctophid fishes in the northern Scotia Sea

The diets of 9 species of myctophid fishes, Electrona carlsbergi, E. antarctica, Gymnoscopelus fraseri, G. nicholsi, G. braueri, Protomyctophum bolini, P. choriodon, Krefftichthys anderssoni and Nannobrachium achirus, were investigated during austral autumn in the northern Scotia Sea. Based on the percent index of relative importance (%IRI), the data suggest dietary specialisation in some species, which may permit resource partitioning. Hierarchical agglomerative cluster analysis of Bray-Curtis similarity (60% threshold) separated the myctophid community into distinct feeding guilds. One group (G. braueri and E. antarctica) fed principally on Themisto gaudichaudii, another (P. choriodon and G. fraseri) primarily on copepods (Metridia spp. and Rhincalanus gigas), and a third group (G. nicholsi and P. bolini) consumed copepods and euphausids (mostly Metridia spp. and Euphausia frigida). The diets of E. carlsbergi and K. anderssoni differed from the other species, with E. carlsbergi being the only species that consumed salps. There was a general switch in diet from copepods to euphausiids and amphipods as the myctophid predator size increased. Dietary specialisation is likely the result of a combination of predator size, gape size, filtering capacity of the gill rakers and the vertical distribution of predators and prey. Antarctic krill were only consumed by the larger myctophids, which represented a numerically minor part of the myctophid community, supporting the concept that myctophids can provide a krill-independent link between secondary production and higher trophic levels. However, the northern Scotia Sea is dominated by adult krill, which are only suitable as prey for larger fish. In the northern Scotia Sea, myctophid predation had a very small impact on copepod production but a higher impact on macrozooplankton, with a best-estimate of 4% of the daily production of Themisto gaudichaudii and 6% of that of Euphausia superba being consumed.