North Atlantic demersal deep‐water fish distribution and biology: present knowledge and challenges for the future

This paper summarizes knowledge and knowledge gaps on benthic and benthopelagic deep‐water fishes of the North Atlantic Ocean, i.e . species inhabiting deep continental shelf areas, continental and island slopes, seamounts and the Mid‐Atlantic Ridge. While several studies demonstrate that distribution patterns are species specific, several also show that assemblages of species can be defined and such assemblages are associated with circulatory features and water mass distributions. In many subareas, sampling has, however, been scattered, restricted to shallow areas or soft substrata, and results from different studies tend to be difficult to compare quantitatively because of sampler differences. Particularly, few studies have been conducted on isolated deep oceanic seamounts and in Arctic deep‐water areas. Time series of data are very few and most series are short. Recent studies of population structure of widely distributed demersal species show less than expected present connectivity and considerable spatial genetic heterogeneity and complexity for some species. In other species, genetic homogeneity across wide ranges was discovered. Mechanisms underlying the observed patterns have been proposed, but to test emerging hypotheses more species should be investigated across their entire distribution ranges. Studies of population biology reveal greater diversity in life‐history strategies than often assumed, even between co‐occurring species of the same family. Some slope and ridge‐associated species are rather short‐lived, others very long‐lived, and growth patterns also show considerable variation. Recent comparative studies suggest variation in life‐history strategies along a continuum correlated with depth, ranging from shelf waters to the deep sea where comparatively more species have extended lifetimes, and slow rates of growth and reproduction. Reproductive biology remains too poorly known for most deep‐water species, and temporal variation in recruitment has only been studied for few deep‐water species. A time series of roundnose grenadier Coryphaenoides rupestris recruitment spanning three decades of fisheries‐independent data suggests that abundant year classes occur rarely and may influence size structure and abundance even for this long‐lived species.