Problem and Opportunity

The rate at which individuals are exchanged among populations is a key factor in the extent of gene flow among those populations and their persistence and resilience over time (Jablonski 1986, Slatkin 1987, Strathmann et al. 2002). The concept of a population, defined as “a group of individuals of the same species that live together in an area of sufficient size to permit normal dispersive and/or migration behavior,” is integral to ecology and essential for effective management (Berryman 2002). The information currently available on normal dispersive and migratory behavior in many marine species, however, is insufficient to define realistic spatial scales for populations. Empirical estimates of larval dispersal distances, for example, do not exist for most marine species. Although the population concept is fundamental to both basic and applied ecology, current knowledge of larval dispersal in the ocean and associated population structure is inadequate for most management and conservation efforts.

 Fig. 1. Examples of exchange among coastal populations under two scenarios; a) an open model with extensive larval exchange and b) a stepping-stone model with the greatest exchange among populations in closest proximity.

Accurate, empirical estimates of larval dispersal have been hampered by inadequate analytical techniques. Individual tagging studies are expensive, logistically difficult, and typically not feasible for marine larval stages, during which mortality often exceeds 90% [but see (Jones et al. 1999) ]. Genetic techniques offer only indirect information on larval sources and dispersal distances and can underestimate the extent of population structure (Buonaccorsi et al. 2002, Palumbi 2003, Kinlan & Gaines 2004) . Current assumptions regarding larval dispersal in marine populations include that (1) dispersal is primarily passive, (2) dispersal distances are typically long, (3) recruitment into a population comes from outside sources, and, therefore, (4) populations are primarily open (Cowen et al. 2000) .

Recent empirical evidence suggests that larval retention near local populations may be more common in marine species with planktotrophic, or feeding, larvae than previously believed (Scheltema et al. 1996, Jones et al. 1999, Swearer et al. 1999, Swearer et al. 2002). Studies on tropical reef fish with planktonic larval stages of ≈30 d durations found that 10 to 60% of newly settled juveniles self recruited, i.e., they either returned to or never left their natal populations. More recent genetic studies have found population structure in Caribbean cleaner goby (Elacatinus evelynae) at scales ≤25 km (Taylor & Hellberg 2003). These previous research efforts focused on island populations, where oceanographic features are known to set up retention mechanisms (Bakun 1986, Wolanski & Hamner 1988, Bakun 1996). The extent of self-recruitment in populations with extended planktonic larval periods that reside along continental margins has received little attention. Our recent work on otolith microchemistry in black rockfish (Sebastes melanops), a NE Pacific species with an extended (3 to 6 mos.) planktonic larval and juvenile period, indicates that larval dispersal can be limited, ≤125 km, along the NE Pacific continental margin (Miller and Shanks 2004). Given the broad continental distribution of black rockfish, such relatively small dispersal distances may result in the majority, about 80%, of larvae remaining in areas near parent populations. We are building upon our previous efforts with additional research on black rockfish by studying populations at finer spatial scales and new research on the otolith microchemistry of canary rockfish (Sebastes pinniger).

Overall, our work addresses a number of key issues in Oregon and on the west coast, including 1) declines in Sebastes spp. and the recent widespread harvest closures along the west coast continental shelf; 2) the ongoing discussion of and planning for marine protected areas as both fisheries management and conservation tools; and 3) the general lack of knowledge regarding larval dispersal in the majority of marine fish. Furthermore, the work specifically addresses two of Oregon Sea Grant’s goals, including 1) fisheries biology and ecology, particularly population dynamics with implications for behavior and distribution of larvae and adults, and 2) understanding and managing estuaries and nearshore marine systems.