Distribution, growth, and population genetics of the glacier lanternfish (Benthosema glaciale) in Norwegian waters: Contrasting patterns in fjords and the ocean

The mesopelagic fish Benthosema glaciale was found to be widely distributed in the Norwegian Sea and several west Norwegian fjords, but mostly absent above the continental shelf. Areas with bottom depths of less than 300 m were almost totally devoid of B. glaciale. There were significant differences in allozyme frequencies and growth between fjords and the Norwegian Sea, with a slower growth towards a larger asymptotic length in the oceanic population. Allozyme and growth differences among fjords were smaller yet significant after pooling samples within each fjord, which suggests there is only limited connectivity among these fjords. The physical barrier to gene flow represented by the continental shelf probably serves as an isolating mechanism between the fjordic and oceanic populations, while shallow sills at the mouths of fjords may restrict gene flow, and in particular limit exchange of adults among fjords. The catch per unit effort of B. glaciale was generally higher in fjords, and in some cases as much as one to two orders of magnitude higher than in the Norwegian Sea. We discuss physical characteristics of fjords that may make them a beneficial habitat for B. glaciale and other mesopelagic fish compared to the Norwegian Sea.