GROWTH, MORTALITY, AND REPRODUCTIVE SEASONALITY OF CALIFORNIA HALIBUT (PARALICHTHYS CALIFORNICUS) A BIOGEOGRAPHIC APPROACH
Differences in key biological processes, such as growth and reproduction, can greatly influence localized population dynamics. Thus, it is important to characterize spatial variation in life history traits of harvested species to develop management plans that maximize fishery sustainability. We estimated sexand region specific growth, total mortality, and reproductive seasonality to assess biogeographic differences in California Halibut life history. We found that central California Halibut grew faster but attained smaller maximum sizes than those from southern California. Catch curve analysis illustrated no difference in total mortality by sex or region, though females lived longer than males. Year class frequency distributions suggested greater recruitment variability in central California, where abiotic factors (e.g. upwelling strength, sea surface temperature) are likely drivers. Reproductive data indicated that summer spawning seasons peak earliest in Mexico, followed by southern and central California. These results demonstrate a need to assess and manage California Halibut at the regional scale. INTRODUCTION Biogeographic differences in growth, mortality, and reproduction can result in disproportionate effects of fishing (Rice et al. 2005) However, fisheries are often managed at relatively broad (e.g. statewide/nationwide) spatial scales. Although averaging the condition of a species across its range simplifies resource management, spatiotemporal changes in key biological processes can result in localized overor under utilization of the resource (Prince 2010) This is because many fish stocks consist of metapopulations (i.e. groups of individuals that are interconnected through larval dispersal, yet exhibit distinct population dynamics) which are differentially affected by fishing pressure (Levins 1969; Adams 1980; Orensanz et al. 2005; Pascoe et al. 2009) Thus, it is important to understand spatial variation in the life history traits of harvested species, especially those that exhibit widespread distributions spanning multiple biogeographic regions. California Halibut, Paralichthys californicus (family Paralichthyidae) is an economically important species that can be found as far north as the Quillayute River in Washington and as far south as Magdalena Bay in Baja California Sur, Mexico (Allen 1990) However, most individuals in U.S. waters are encountered between Bodega Bay, California and the US Mexico border. After a short larval duration of 20 to 29 d, juveniles settle into bays, estuaries, and shallow waters of the open coast (Allen 1988; Kramer 1990) Adults are primarily found nearshore (typically less than 60 m water depth) over sandy habitats that are adjacent to hard substrate or biogenic structures (e.g. sand dollar, Dendraster excentricus, beds) (Allen 1988; Allen 1990) Female California Halibut grow faster and mature later than male conspecifics (Haaker 1975; MacNair et al. 2001) Males reach sexual maturity between 19 and 32 cm (1 to 3 yr) whereas females mature between 36 and 59 cm (2 to 7 yr) (Love and Brooks 1990) California Halibut are multiple, broadcast spawners that have been noted as living to 30 yr, though recent data have demonstrated a lifespan of only 23 yr (CDFW unpublished data [2007 to 2014] In 2011, the California Department of Fish and Wildlife (CDFW, formerly California Department of Fish and Game) conducted its first comprehensive stock assessment for California Halibut to determine population size and the effectiveness of existing management actions (Maunder et al. 2011) As part of the assessment, fishery independent and dependent data were synthesized and incorporated into statistical models developed for two separate stocks, one north and one south of Point Conception (a well known biogeographic boundary between central and southern California) Although substantial amounts of biological information were made available during the assessment, life history data pertained primarily to fish collected off southern California (e.g. Allen 1988; Allen and Herbinson 1990; Allen et al. 1990; Kramer 1990; Domeier and Chun 1995; Valle et al. 1998; MacNair et al. 2001) This forced assessment scientists to parameterize both stock models based primarily on the southern California population. To assess the effect of biogeography on California Halibut life history characteristics, we estimated sex specific growth and instantaneous total mortality for fish collected both north and south of Point Conception. We also compared BARNES ET AL. GROWTH, MORTALITY, AND REPRODUCTIVE SEASONALITY OF CALIFORNIA HALIBUT (PARALICHTHYS CALIFORNICUS) CalCOFI Rep. Vol. 56, 2015 111 ity of fish from southern California were obtained from commercial fisheries (table 1) Date, location, fishery (i.e. commercial or recreational) gear type, sex, fork length (mm) and wet body weight (kg) were recorded in the field. The majority of fork lengths were obtained from whole fish sampled at shore based facilities. However, a number of California Halibut were received from recreational fishers as filleted carcasses. Fork lengths for filleted fish were recorded and converted to pre fillet lengths using the relationship y 1.37 0.99x (R2 0.999, p 0.001) where x represents post fillet fork length (mm) and y represents prefillet fork length (mm) In the laboratory, sagittal otoliths were extracted and thin sectioned according to procedures outlined by the Committee of Age Reading Experts (CARE 2006) Because eyed side otoliths tended to exhibit distorted patterns of growth, blind side otoliths were selected for ageing whenever possible. Without prior knowledge of size or sex, two or three readers independently aged each fish to the nearest year. Rounding decisions were based upon the amount of marginal growth relative to immediately adjacent annuli (i.e. otoliths with a margin representing greater than half the distance between neighboring annuli were rounded up and margins representing less than half the distance between neighboring annuli were rounded down) From 2007 to 2011 and again in 2014, two CDFW staff independently read each otolith until a minimum of two identical (withinreader) age estimates were made. If age determinations by the two readers did not agree, a digital image of the thin section was prepared and both readers discussed the various aspects of the otolith until a final age was agreed upon or they decided to exclude the otolith from analyses due to issues associated with poor readability. In 2012 and 2013, one CDFW and one MLML reader followed the same procedures detailed above. However, if reader specific age determinations conflicted with one another during these years, a third (CDFW) reader was introduced to settle the disagreement. If the third reader did not agree with either of the other two readers, a digital image of the thin section was prepared and all readers either came to a unanimous agreement or region specific estimates of reproductive seasonality by evaluating temporal changes in gonadosomatic index for females collected off central California and corrected larval density data obtained by California Cooperative Oceanic Fisheries Investigations (CalCOFI) surveys along southern California and Mexico. MATERIALS AND METHODS California Halibut were collected between San Francisco Bay, CA and the US Mexico border from 2007 to 2014 (fig. 1) Fish were collected using both fisheryindependent and fishery dependent sampling methods. A diversity of gear types (i.e. hook and line, trawl, gill net, seine, and spear) was used to collect fishes of both sexes and from various size classes. Although fisheryindependent methods were used, we (CDFW [2007 to 2014] and Moss Landing Marine Laboratories [MLML, 2012 and 2013] procured most specimens from commercial and recreational fisheries using a combination of stratified random and opportunistic sampling designs (CDFW 2013; Barnes 2015) Fish from central California were collected in nearly equal proportions from commercial and recreational fisheries, whereas the majorMoss Landing Monterey Santa Cruz San Francisco