Length Structure of Deep-Pelagic Fishes Sheds New Light to their Life Histories

Here we use a new technique to study life history variation in deep-pelagic fishes from a mid-ocean ridge system. Shape of length distribution in a population is to a significant extent determined by the degree to which an average individual approaches its asymptotic maximum size. Analysing the material from the pelagic trawl hauls taken during the 2004 Mar-Eco expedition along the northern Mid-Atlantic Ridge, we show that length distributions in many deep-pelagic fish species are characterised by negative skew (the left tail of the distribution is longer). In other words, a large proportion of individuals had a size close to species-specific maximum size. Provided that our sampling can be considered representative, this finding suggests that deep-pelagic fishes have a low mortality rate relative to the rate at which they grow towards their asymptotic size.