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Temperature effects on the vertical distribution of lobster postlarvae
(Homarus americanus)

In situ observations of postlarvae of the American lobster (Homarus americanus) were used to quantify behavioral depth regulation with respect to intrinsic and extrinsic factors. Postlarvae spent 65% of the time near the surface (0–0.5–m depth), which is less than expected based on previous plankton net surveys. The proportion of time spent at the surface decreased over the season (∼0.80 to ∼0.57) and was correlated with increasing depth of the 12°C isotherm. Postlarvae remained in waters above 12°C, suggesting that it may serve as a minimum temperature threshold. The seasonal trend was removed, and the residuals were used to examine the daily variation about the seasonal trend. These daily variations in the residuals were correlated with the depth of the thermocline. Residual data were also used to examine vertical distribution as a function of time of day. The proportion of time spent near the surface was lowest at midday and greatest in the morning and late afternoon. This shift in vertical distribution was not correlated with light intensity, indicating that it may be part of an endogenous rhythm. When the data were parsed with respect to light intensity, and developmental stage, an ontogenetic shift was observed from positive to negative phototaxis.