Sea Grant 3rd Annual Science Symposium Lobsters as Model Organisms for Interfacing Behavior, Ecology, and Fisheries: Discussion Session Summary on Metapopulation Dynamics and Implications for Management

Understanding patterns of connectivity among marine populations has emerged as a dominant research focus in the last decade as interest in metapopulation dynamics in the oceans has increased. For our purposes here we will adopt a broad definition of a metapopulation as a collection of spatially distributed populations linked through dispersal processes. One of Stan Cobb's earliest contributions to lobster behavior and population biology in fact concerned connections between offshore lobster groups and near shore populations (Rogers et al., 1968). Dispersal was inferred by carefully tracking larval stage distributions along an inshore-offshore transect and noting the predominance of early stage larvae offshore and settlement stage lobsters inshore in these synoptic surveys. Subsequent work by Stan and colleagues elucidated the behavioral mechanisms favoring directional larval transport (Cobb et al., 1989) and the linkage between hydrodynamics and dispersal potential (Katz et al., 1994). Building on these themes, Rom Lipcius of the College of William and Mary, led an insightful discussion of metapopulations and the links to fishery management, drawing examples from different crustacean taxa from tropical to temperate seas. Metapopulations can exhibit substantial resilience to external stressors (both natural and anthropogenic), if they exhibit some degree of asynchrony in their fluctuations. A population driven to low levels can be ‘rescued’ by a subsidy from a linked population and understanding the dispersal pathways is critical to understanding metapopulation dynamics. A central theme in the discussion concerned the need to identify source and sink populations as a prelude to effective spatial management of these resources. We can frame this issue in terms of vital rates (births and deaths) and dispersal patterns. We will consider a source population as one in which the birth and emigration rates exceed the death and immigration rates. Such a population is self-sustaining. Conversely, a sink population is one characterized by a dominance of loss terms in such a way that persistence of the population depends on an influx from other sources. In some systems, a dominance of immigration rates can control local dynamics of even a self-sustaining population, creating a ‘pseudo-sink’ population under the above definition. Over-exploiting a source population can adversely affect any sink populations dependent on it in addition to the loss in yield and productivity of the source population itself. While Lipcius indicated that some models indicate that an optimal strategy may be to completely protect source populations, Rhode Island lobster fisherman, Dick Allen, noted that carefully regulated exploitation of source populations may be feasible without harming the metapopulation structure. A spirited discussion followed a suggestion that sink populations might be heavily exploited without major consequence. Kevin Hovel, San Diego State University, noted that changes in oceanographic conditions or other factors such as predation fields could alter the inter-relationships among linked populations and that the status of a presumed sink population might change. Mark Butler, Old Dominion University, further noted that we need information on dispersal patterns at all life history stages to understand the patterns of connectivity among populations and that often, this information is lacking not only for eggs and larvae but for older age groups. Decisions concerning the appropriate harvest levels of a sink population may in the end center on factors such as optimizing yield per recruit and considerations related to the capacity for self-replenishment while constantly monitoring for changes in environmental conditions that could alter source-sink relationships. Broad concern was voiced for any deliberate over-exploitation of presumed sink populations. Consideration of spatial dynamics and dispersal patterns is of course directly relevant to an important practical problem: understanding the potential efficacy of Marine Protected Areas (MPAs) as a management tool. Bob Steneck, University of Maine, raised the question of whether there are any solid examples of closed areas providing a larval subsidy to areas open to fishing. The central difficulty is in tracking larval dispersal patterns because of the problems of marking and following the early life stages. Approaches used to date to infer decapod dispersal in general have been based on larval distributions in relation to spawning locations and ontogenetic changes in the spatial distribution of successive larval stages (as in Rogers et al., 1968), the use of coupled biological-physical models used to track potential dispersal trajectories (as in Katz et al., 1994), and information on genetic structure. Emerging technologies such as elemental fingerprinting may also prove to be useful in some circumstances. Of the existing approaches, the use of Lagrangian particle tracking approaches could most readily be applied to examine the potential role of MPAs as larval source areas. Mike Fogarty, NMFS, noted that this approach has been successfully applied to sea scallops in closed areas on Georges Bank. The increased incidence of disease in a number of crustacean populations (notably in southerly populations of the American lobster) engendered considerable discussion in the context of source-sink dynamics and the possibility of a rescue effect from less impacted populations in the metapopulation. Conversely, Butler indicated that diseases can spread through the interchange of infected larvae. It's clear that the interplay between different stressors (e.g., disease and exploitation) can become particularly important in evaluating the stability and resilience of these populations. The complexities inherent in understanding metapopulation dynamics and, in particular, the dispersal pathways at different life stages naturally led to a discussion of the role of precautionary management of metapopulations. Bob Miller (DFO Canada), Kathy Castro (URI) and Dick Allen noted that management choices hold real economic consequences for fishers that cannot be ignored, and that there are distinct but intersecting scientific and policy issues involved. Fogarty noted that the scientific issues concern framing alternative hypotheses about states of nature and the connectivity of marine populations, while the policy issues involve making choices based on estimates of the risks of harvesting strategies associated with alternative hypotheses. The challenge is to define win-win solutions to coping with uncertainty. The threads of Stan's career and many accomplishments can be woven into the fabric of this discussion on metapopulations, from his earliest contributions to his recent interests in the interplay of animal behavior, ecology and resource management. We are grateful to him for pointing the way.

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