Seasonal patterns in the nocturnal distribution and behavior of the mesopelagic fish Maurolicus muelleri at high latitudes

Abstract

Acoustic scattering layers (SL) ascribed to pearlside Maurolicus muelleri were stud- ied in Masfjorden, Norway, using upward-looking echo sounders cabled to shore for continuous long-term measurements. The acoustic studies were accompanied by continuous measurements of surface light and supplemented with intermittent field campaigns. From autumn to spring, young M. muelleri formed an SL in the upper ~75 to 150 m in the daytime, characterized by migra- tion to near-surface water near dusk, subsequent 'midnight sinking', followed by a dawn ascent before a return to the daytime habitat. Light levels were ~1 order of magnitude lower during the dawn ascent than for ascent in the afternoon, with the latter terminating before fish reached upper layers on ~1/3 of the nights from late November to mid-April. Adults showed less tendency of migration during autumn and winter, until the SLs of young and adults merged in late spring, and thereafter displayed coherent migration behavior. The midnight sinking became progressively deeper from autumn to winter but was strongly reduced from mid-May when the darkest noctur- nal light intensity (PAR) at the surface was above 10 �3 µmol m �2 s �1 . The pearlside took on school- ing in upper waters during the even lighter nights in early June, with minimum light of ~5 × 10 �3 to 10 �1 µmol m �2 s �1 at the surface. Nocturnal schooling ceased in early July, and midnight sinking reappeared in mid-August. We suggest that the strong variation in nocturnal light intensity at high latitudes provides changing trade-offs between visual foraging and avoiding predators and hence varying time budgets for feeding in the upper, productive layers.