Estimates of Fishing and Natural Mortality of Black Sea Bass, (Centropristis striata), in the Mid-Atlantic based on a Release-Recapture Experiment

Black sea bass in the Mid_Atlantic Bight, are exploited by recreational and commercial fisheries. To evaluate mortality rates, a tag release/recapture study was conducted with 13,794 tagged black sea bass (12,310 legal-size) released between Massachusetts and Cape Hatteras, NC from 2002 to 2004. Of these legal-size releases, 1,683 were recaptured during 2002 to 2007. An instantaneous rates configuration of a Brownie band recovery model was used to estimate both fishing and natural mortality. A seasonal model of fishing mortality, adjusted for non-mixing, and a constant natural mortality best explained the tag recoveries. Fishing mortality estimates were between 0.3 and 0.4 whereas the natural mortality estimate was greater than 1.0. The estimate of natural mortality includes the effects of all unaccounted tag losses, however the results suggest that natural mortality is likely greater than 0.2 which has been assumed based on a maximum age of 15. Higher overall rates of natural mortality could result from increased vulnerability at sexual transition in this hermaphroditic species. Introduction Stock assessments of marine fish populations have long been a key component in managing fishery resources. Information regarding past rates of exploitation, along with potential productivity, allow managers to determine how much future exploitation can be allowed. Traditionally, catch based population models have been the tool of choice in stock assessments but in recent years tag based models have been increasingly used either as independent estimates of exploitation (Latour et al. 2001; Lambert et al. 2006; Jiang et al. 2007) or in conjunction with catch data (Polacheck et al. 2006). If implemented within the framework of a properly designed experiment, tagging programs are capable of providing estimates of exploitation and population size as accurately as catch at age models (Pine et al. 2003). In the Northwest Atlantic, black sea bass (Centropristis striata), support both commercial and recreational fisheries. Although black sea bass are distributed from the Gulf of Maine to the Gulf of Mexico, fish north of Cape Hatteras, NC are considered part of a single management unit. Commercial landings for this stock have remained relatively steady around 1400 mt since 1970, although landings in 1952 peaked at 9,900 mt (Shepherd 2007). Recreational landings, available since 1982, average about 1,600 mt annually. The species affinity for bottom structure during its seasonal period of inshore residency increases the availability to hook and line or trap fisheries while decreasing the susceptibility to bottom trawl gear commonly used for scientific surveys. In autumn when water temperatures decline, black sea bass migrate offshore to areas along the edge of the continental shelf. During this offshore period, sea bass are vulnerable to otter trawl gear as part of a multispecies fishery (Shepherd and Terceiro 1994). Black sea bass are protogynous hermaphrodites and can be categorized as temperate reef fishes (Steimle et al. 1999). Transition from female to male generally occurs between the ages of two and five (Lavenda 1949; Mercer 1978). Males can follow one of two behavioral pathways, either becoming dominant males, characterized by a larger size and a bright blue nuccal hump during spawning season, or secondary males which have few distinguishing features. Spawning in the Middle Atlantic peaks during spring (May and June) when the fish reside in coastal waters. The social structure of the spawning aggregations is poorly known although some observations suggest that large dominant males gather a harem of females and aggressively defend territory