Defining the Extent of Larval Exchange among Kelp Rockfish (Sebastes atrovirens) Populations Using Otolith Microchemistry

Field: Agricultural and Biological Sciences (Life Sciences)

Julie D. Standish, Page 1 Defining the Extent of Larval Exchange among Kelp Rockfish (Sebastes atrovirens) Populations Using Otolith Microchemistry Julie D. Standish University of California, Santa Barbara Department of Ecology, Evolution and Marine Biology (Advisor: Robert Warner) Introduction Over 60 species of rockfish (Sebastes) inhabit the waters along the California coast. Rockfish are an important economical component of the state’s commercial and recreational fisheries. Over the past two decades, rockfish populations along the coast of California have declined sharply leading to heightened concern over the management policies for rockfish and sustainability of existing stocks nearshore (Ralston 1998, Dorn 2002). Several measures, including a statewide network of marine protected areas (MPAs), are being considered to prevent overexploitation of nearshore stocks. In the last decade, the live-fish fishery that targets nearshore species, especially rockfishes, has grown dramatically yet little is known regarding the effect of the fishery on stock levels of the nearshore rockfish species. Critical to the effectiveness of fisheries management, including ecosystem-based fisheries methods and MPAs, is knowledge of the extent of larval movement and larval connectivity among populations. Information regarding the spatial scale of larval dispersal is required to determine the appropriate spacing between MPAs and the size of individual MPAs within a network in order to effectively sustain and replenish exploited stocks (Gaines et al. 2003). However, explicit information regarding the dispersal patterns of larvae remains unknown for any commercially important marine species. Given the small size of fish and invertebrate larvae and their potential for long distance dispersal, tracking larvae as they disperse has been a very challenging task. Consequently, most of the information on larval dispersal has come from a variety of indirect methods, including correlations between settlement and oceanographic patterns (Meekan et al. 1993, Wing et al. 1995a, Wing et al. 1995b, Dixon et al. 1999, Morgan et al. 2000), the rate of range expansion following invasion of exotic species (Geller 1994, Grosholz & Ruiz 1995), and population genetics (Moberg & Burton 2000, Kinlan & Gaines 2003, Buonaccorsi et al. 2004, Sotka et al. 2004). Although these methods have provided valuable insights into the larval dispersal process and estimates of the degree of exchange among populations, they do not provide information on the specific origin of individuals. Alternative techniques that can explicitly determine the natal source and movement of larvae are needed (Jones et al. 1999). Recently, environmental markers such as the isotopic and elemental composition in the otoliths of fishes have shown great promise as a means to track the history and quantify the movement among populations. Formed during the embryonic stage, otoliths grow by the daily accretion of calcium carbonate into a proteinaceous matrix, generating a pattern of concentric rings around the central nucleus. The central nucleus forms the core of the otolith as the fish grows. Trace elements from the environment are incorporated into the calcium carbonate matrix of the otolith. The elemental signatures within otoliths may be used as a natural tag of past environments experienced by an individual. Otolith microchemistry has been used to determine stock structure within marine species (Campana et al. 1994, Campana et al. 2000, Gillanders 2001, Rooker et al. 2003, Fowler et al. 2005, Ashford et al. 2006), to detect anadromy (Kalish 1990, Zimmerman & Reeves 2000,