Conservation and management of exploited shark populations based on reproductive value

Abstract

Several life history traits of sharks result in juveniles being particularly vulnerable to exploitation. However, population level impacts of harvests on juvenile sharks have not been well quantified. This paper examines a range of harvest strategies, including those targeting juveniles. Reproductive value and yield per recruit are used to compare the harvests, which are represented by Leslie matrix models with a harvest matrix. Two species are used as examples: the short-lived Rhizoprionodon taylori and the long-lived Squalus acanthias. Harvests that maintain a stationary population size cause reproductive values to change in opposing ways, but they remove equal fractions of the population's reproductive potential. A new theorem gives population growth as a function of the fraction of reproductive potential removed by a harvest, a relationship useful for comparing harvests on juveniles and adults. Stochastic projections indicate that the risk of depletion is associated with the fraction of reproductive potential removed annually, a measure which encompasses the information in both the selectivity and the rate of fishing mortality. These results indicate the value of focusing conservation efforts on preserving reproductive potential.

Size of catch, reproduction and feeding of the small-eye smooth-hound, <em>Mustelus higmani</em> (Carcharhiniformes: Triakidae), in Margarita Island, Venezuela

“…This behaviour could explain the incidence of gravid females in the catches, as this artisanal fishery operates mainly in-shore. Gallucci et al (2006) indicated that in these fisheries the risk of collapse of shark stocks increases because a large reproductive fraction of the population is captured annually. According to the current data, the risk could be even higher because catches also include a large proportion of juveniles.…”

Evaluating catch and mitigating risk in a multispecies, tropical, inshore shark fishery within the Great Barrier Reef World Heritage Area

“…The concept of the gauntlet fishery has been proposed as an effective method of harvesting long-lived species, providing that fishing mortality on adults remains extremely low (Simpfendorfer 1999;Prince 2005). Although such a harvest strategy is unlikely to provide the maximum sustainable yield (Gallucci et al 2006), it may be preferable, depending on the goals of the fishery. In an artisanal fishery, for example, the harvest of large sharks is unlikely to contribute to food security given that the flesh from these animals often contains high levels of mercury and may not be suitable for human consumption (Lyle 1984;Clarkson 1997).…”

Baseline Health Parameters and Species Comparisons Among Free-Ranging Atlantic Sharpnose (Rhizoprionodon Terraenovae), Bonnethead (Sphyrna Tiburo), and Spiny Dogfish (Squalus Acanthias) Sharks in Georgia, Florida, and Washington, Usa

“…Elasmobranchs are an integral component of the marine ecosystem, though many populations are in decline (Gallucci et al, 2006;Camhi et al, 2009). Commercial and recreational fisheries affect the populations of the Atlantic sharpnose (Rhizoprionodon terraenovae), bonnethead (Sphyrna tiburo), and spiny dogfish sharks (Squalus acanthias; Trent et al, 1997;Rulifson, 2007).…”