Effect of density and habitat structure on nocturnal swimming activity in juvenile northern rock sole Lepidopsetta polyxystra
Lampi, Clara Y.
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Previous work on habitat preferences of flatfish has focused on daytime activities; however, the functionality of habitat may change with prevailing light conditions. This paper explores the relationship between benthic habitat and nocturnal swimming behavior in the water column by juvenile northern rock sale, Lepidopsetta po/yxystra, one flatfish species. Fish were tested at densities of 1.2, 2.4, 6.9, and 12 fish/m2 on bare sand and in gravel, sponge, and eel grass habitats at a density of 6.9 fish/m2. All fish were initially placed on one half of the tank and the number of fish that ended the trial on the opposite side of the tank gave an indication of fish activity in the water column. A partial barrier required fish to swim in the water column to cross from one side of the tank to the other. Density did not have an effect on nocturnal swimming except at the highest density, 12 fish/m2, where crossings increased by 13%. While other experiments have demonstrated that rock sale show a preference for benthic structure during the day, our results indicate that this association weakens at night. Fish swam out of the eel grass treatment and showed no preference for or against the sponge habitat. Fish swam away from gravel substrates, consistent with their behavior during the day. Our results highlight the importance of considering nocturnal behavior when determining essential habitat. Integrating these results with knowledge of daytime behavior of rock sale allows for a general comprehensive model of movement and short scale habitat selection.