Southern Ocean mesopelagic fish communities in the Scotia Sea are sustained by mass immigration

The biomass of mesopelagic fish in the Southern Ocean is one of the largest of any ocean region and is dominated (both in terms of diversity and biomass) by myctophids (lantern-fish). Despite their high ecological importance both in this region and globally, our understanding of the life cycles and distribution of myctophids remains limited. We examined length-frequency data from trawl nets collected across a major sector of the Southern Ocean (the Scotia-Weddell sector) in different seasons to determine patterns of recruitment and growth. There was an absence of larval myctophids, of any species, in net catches, while larger, older individuals became increasingly dominant with increasing latitude. Very few specimens were found to contain mature gonads, indicating that individuals do not reach reproductive condition in this region. Most myctophid species that occurred within the survey regions neither recruited locally nor were self-supporting. Myctophids are prey to a large number of higher predators (penguins, seals and cetaceans) in the Scotia Sea, and are a major predator of zooplankton and krill. We show that this vital part of the Southern Ocean food-web is dependent on mass immigration from lower latitudes in the region. By implication, the sensitivities of this system depend not only on local conditions but also on levels of connectivity to other oceanic regions.