Bathymetric trends in demersal fish size: is there a general relationship?

We examined data on size of dominant demersal fish species in the SE Atlantic (44 species) and the NW Mediterranean (31 species) to determine whether there is a general tendency to increasing size towards deeper waters.Our results demonstrate significant positive size-depth relationships for most species examined (63 % SE Atlantic and 74 % NW Mediterranean).The relationships examined involved both a tendency towards greater size with ~ncreasing depth and a tendency towards smaller size towards shallower bottoms.The average ( * standard error) rate of increase in fish length with increasing depth was found to be 0.09 f 0.01 cm length (m depth)-' for the SE Atlantic species and 0.06 k 0.007 cm length (m depth)-' for the NW Mediterranean species.In addition, we found the slope and intercept of these relationships to scale approximately to the 3/4 power of the maximum and minimum fish size respectively, showing that interspecific differences in the nature of this relationship depend on the size range of the different species.Consideration of several hypotheses to account for this general pattern suggests that it reflects a migratory (or diffusive) movement towards deeper waters during ontogeny, where fish benefit from the extended Lives and lower metabolism at lower temperatures.We suggest this pattern is evolutionary In nature, and due to inherited behaviour.