The American lobster settlement index at 20 years: looking back - looking ahead

We review the accomplishments and future challenges of larval settlement monitoring for the American lobster (Homarus americanus), following a workshop convened in June 2009 observing the programme’s twentieth anniversary. In the late 1980s the new emphasis on “supply-side ecology” energised researchers to look to larval transport and settlement processes to explain population dynamics of marine species with complex life cycles. At that time, larval settlement indices for spiny lobsters (Panulirus cygnus) in Australia were already demonstrating a capacity to forecast subsequent harvest trends and motivated the search for similar predictive power in the other lobster fisheries. The American Lobster Settlement Index (ALSI) was initiated in 1989 soon after diver-based suction sampling proved an effective way of sampling newly settled lobsters in shallow, cobble-boulder nursery habitats. The survey has expanded from a few sites in coastal Maine, USA, to encompass other lobster-producing regions of the Northeast United States and Atlantic Canada. Supported by state and provincial marine resource agencies, monitoring is conducted annually at the end of the late summer-early autumn postlarval settlement season. The settlement index has been the springboard for numerous research projects, contributing to some 24 peer reviewed publications and technical reports to date. Because of the interest in evaluating the predictive power of the time series for subsequent fishery trends, much research has focused on the pre- and post-settlement processes influencing lobster population dynamics. Owing in part to steep gradients in environmental conditions throughout the species’ range, it is becoming evident ocean-atmospheric processes that influence annual fluctuations in larval supply vary on much larger spatial scales (100-1000 km) than do post-settlement processes, such as predation, disease and intra- and interspecific competition. Thus, unlike the Australian case, forecasting models for the American lobster will likely need to be regionally customised to account for differing regional dynamics. Since 2005, vessel-deployed passive postlarval collectors (plastic coated, wire mesh trays filled with cobbles) have been tested and deployed widely as an alternative to suction sampling to assess lobster settlement in waters where diving is unsafe or impractical. Indeed, collectors may become the tool of choice for settlement monitoring in some regions. As we enter the third decade of the region-wide collaboration, it will be important to continue to assess the value of the index as a tool for stock assessment, forecasting, and a mechanistic understanding of lobster recruitment processes.

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