Diel and seasonal patterns of Nephrops norvegicus (Decapoda: Nephropidae) catchability in the western Mediterranean

Catchability patterns of western Mediterranean Nephrops norvegicus were studied in relation to depth and season by sampling animals during 4 continuous 24 h cycles of trawl hauls at 100 to 110 m and 400 to 430 m in October (near the autumn equinox) and June (near the summer solstice). Bottom light intensity was recorded between consecutive catches. Catch data were transformed into density estimates of animals emerging from their burrows. Results revealed a marked emergence rhythmicity: nocturnal with crepuscular peaks on the continental shelf, and diurnal on the continental slope. On the slope, peaks of catch occurred in phase with light intensity, whereas this did not happen on the shelf. In particular, both benthic environments presented diel fluctuations in light intensity that differed within several orders of magnitude. These data indicate a differential physiological adaptation of N. norvegicus eyes to local photic conditions, implying a revision of the optimum fight intensity concept, already proposed in the literature. Emergence patterns were almost identical for males, females and berried females, and these were not size-dependent.