Fish prey of sub-Antarctic fur seals Arctocephalus tropicalis at the Tristan da Cunha Islands, South Atlantic Ocean

Some top predator populations in the South Atlantic and South Indian oceans are in decline, presumably contingent upon reduced food availability, precipitated by climate change. This phenomenon impacts on the positions of major ocean frontal zones which are hypothesised to act as natural dispersal borders for fish in the Southern Ocean. We investigate this hypothesis by establishing the fish diet of sub-Antarctic fur seals, Arctocephalus tropicalis, at Tristan da Cunha Island (37°15′S, 12°25′W) and Gough Island (40°19′S, 9°57′W), South Atlantic Ocean. The diets of these island populations, located on either side of the Subtropical Convergence, are compared with published dietary information from populations further south on islands located within the Polar Frontal Zone. To this end, fur seal scats were collected and analysed for remains of hard parts from prey in 2012–2013. The myctophid fish Gymnoscopelus piabilis, Protomyctophum tenisoni and Symbolophorus barnardi predominated in the diet. Lampichthys gemellarii, Myctophum aurolaternatum, S. barnardi and the Diaphus genus are recorded for the first time in the diet of A. tropicalis. Sub-Antarctic fur seal populations clustered around the Subtropical Convergence (~ 41°40′S), compared with those in the Polar Frontal Zone (~ 47°25′S to ~ 50°47′S), showed a considerable difference in the myctophid fish prey taken. The latitudinal differences in the fish diet of sub-Antarctic fur seals support suggestions that major frontal zones act as natural dispersal borders for fish in the Southern Ocean.