No abstract

Beginning in the late 1980s, lobster (Homarus americanus) landings for the state of Maine and the Bay of Fundy increased to levels more than three times their previous 20-year means. Reduced predation may have permitted the expansion of lobsters into previously inhospitable territory, but we argue that in this region the spatial patterns of recruitment and the abundance of lobsters are substantially driven by events governing the earliest life history stages, including the abundance and distribution of planktonic stages and their initial settlement as Young-of-Year (YOY) lobsters. Settlement densities appear to be strongly driven by abundance of the pelagic postlarvae. Postlarvae and YOY show large-scale spatial patterns commensurate with coastal circulation, but also multi-year trends in abundance and abrupt shifts in abundance and spatial patterns that signal strong environmental forcing. The extent of the coastal shelf that defines the initial settlement grounds for lobsters is important to future population modeling. We address one part of this definition by examining patterns of settlement with depth, and discuss a modeling framework for the full life history of lobsters in the Gulf of Maine.

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 (ϳ0.80 to ϳ0.57) and was correlated with increasing depth of the 12ЊC isotherm. Postlarvae remained in waters above 12ЊC, 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.Larval settlement often drives the abundance of benthic populations (Gaines and Roughgarden 1985), and we are increasingly aware of the importance of larval behavior in the successful transport, delivery, and settlement of planktonic larvae to the benthos (Sponaugle et al. 2002). Larvae frequently encounter a nonhomogeneous environment, and their behavioral response to changing environmental conditions may have a profound effect on settlement. Unfortunately, our understanding of the interplay between larval behavior and hydrography in the period leading to settlement is often hindered by the inherent difficulty of sampling a planktonic phase that is hyper-dispersed and patchily distributed with unpredictable vertical distribution. The vertical distribution of larvae is particularly important as it may influence net transport, depth of settlement, and larval abundance as estimated from plankton samples.The postlarva of the American lobster (Homarus ameri-