Behavior of Age-0 Northern Rock Sole (Lepidopsetta polyxystra) in a Stratified Water Column

Northern rock sole (Lepidopsetta polyxystra) is a commercially important flatfish species whose settlement dynamics are poorly understood. Recently collected field data from known nursery sites show that newly settled juveniles are initially settling in the shallowest region of juvenile nursery areas (< 3 m in depth) where substrates are too coarse to permit burial and generally contain low densities of infaunal prey items. Temperature has been recognized as a vital component to larval growth and rock sole larvae could be delaying settlement by behaviorally avoiding the swift changes in temperature associated with stratified water in deeper ocean zones, and instead choose to settle in mixed coastal areas where they do not need to cross such a steep gradient.

In this study, we will examine the effect of thermocline strength on settlement behavior of Northern rock sole. The behavior and position of individual larvae will be observed in vertical settlement columns with varying degrees of thermal stratification: weak, strong, and absent.  I hypothesize that the larvae will behaviorally avoid the thermocline and largely remain above the thermocline in the tank; moreover, I expect that larger fish are more likely to cross the thermocline, because they may be less sensitive to temperature changes and more ready to settle. I expect to determine if larvae will behaviorally respond to a thermocline, and if this response is dependent on size. The results of this study will be used to evaluate the importance of behavior in flatfish settlement.