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Estimating Sustainable Bycatch Rates for California Sea Lion Populations in the Gulf of California

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Estimating Sustainable Bycatch Rates for California Sea Lion Populations in the Gulf of California Contributed Paper Estimating Sustainable Bycatch Rates for California Sea Lion Populations in the Gulf of California JARED G. UNDERWOOD,∗‡ CLAUDIA J. HERNANDEZ CAMACHO,∗† DAVID AURIOLES-GAMBOA,† AND LEAH R. GERBER∗ ∗Ecology, Evolution and Environmental Science, School of Life Sciences, Arizona State University, College & University Drive, Tempe, AZ 85287-1501, U.S.A. †Centro Interdisciplinario de Ciencias Marinas-IPN, Av. IPN s/n Col. Playa Palo de Santa Rita, La Paz BCS 23096, Mexico´ Abstract: Commercial and subsistence fisheries pressure is increasing in the Gulf of California, Mexico. One consequence often associated with high levels of fishing pressure is an increase in bycatch of marine mammals and birds. Fisheries bycatch has contributed to declines in several pinniped species and may be affecting the California sea lion (Zalophus californianus) population in the Gulf of California. We used data on fisheries and sea lion entanglement in gill nets to estimate current fishing pressure and fishing rates under which viable sea lion populations could be sustained at 11 breeding sites in the Gulf of California. We used 3 models to estimate sustainable bycatch rates: a simple population-growth model, a demographic model, and an estimate of the potential biological removal. All models were based on life history and census data collected for sea lions in the Gulf of California. We estimated the current level of fishing pressure and the acceptable level of fishing required to maintain viable sea lion populations as the number of fishing days (1 fisher/boat setting and retrieving 1 day’s worth of nets) per year. Estimates of current fishing pressure ranged from 101 (0–405) fishing days around the Los Machos breeding site to 1887 (842–3140) around the Los Islotes rookery. To maintain viable sea lion populations at each site, the current level of fishing permissible could be augmented at some sites and should be reduced at other sites. For example, the area around San Esteban could support up to 1428 (935–2337) additional fishing days, whereas fishing around Lobos should be reduced by at least 165 days (107–268). Our results provide conservation practitioners with site-specific guidelines for maintaining sustainable sea lion populations and provide a method to estimate fishing pressure and sustainable bycatch rates that could be used for other marine mammals and birds. Keywords: bycatch, California sea lion, fisheries, gill-net entanglement, Gulf of California, population viability, Zalophus californianus Estimacion´ de Tasas de Captura Incidental Sustentables para Poblaciones de Lobos Marinos en el Golfo de California Resumen: La presion´ de las pesquer´ıas comerciales y de subsistencia esta´ aumentando en el Golfo de Califronia, M´exico. El incremento de la captura incidental de mam´ıferos y aves marinas a menudo es una consecuencia asociada con altos niveles de presion´ de pesca. La captura incidental de pesquer´ıas ha contribuido a las declinaciones de varias especies de pin´ıpedos y puedee estar afectando a la poblacion´ de lobo marino de California (Zalophus californianus) en el Golfo de California. Utilizamos datos sobre pesquer´ıas y enmallamiento de lobos marinos en redes agalleras para estimar la presion´ de pesca actual y las tasas de pesca bajo las que se pudieran sostener poblaciones de leon´ marino en 11 sitios de reproduccion´ en el Golfo de California. Utilizamos 3 modelos para estimar tasas de captura incidental sustentables: un modelo de crecimiento poblacional simple, un modelo demografico,´ y una estimacion´ de la remocion´ biologica´ potencial. Todos los modelos se basaron en datos de la historia de vida y de censos de lobos marinos en el Golfo de California. Estimamos el nivel de la presion´ de pesca actual y el nivel de pesca aceptable requerido para mantener poblaciones viables de lobos marinos como el numero´ de d´ıas de pesca (1 pescador/barco ‡email [email protected] Paper submitted December 6, 2006; revised manuscript accepted November 5, 2007. 701 Conservation Biology, Volume 22, No. 3, 701–710 C 2008 Society for Conservation Biology DOI: 10.1111/j.1523-1739.2008.00919.x 702 Fisheries Bycatch and Sea Lions colocando y recuperando el valor de 1 d´ıa de redes) por ano.˜ Las estimaciones de la presion´ de pesca actual variaron entre 101 (9-405) d´ıas de pesca alrededor del sitio de reproduccion´ Los Machos y 1887 (842–3140) alrededor de Los Islotes. Para mantener poblaciones viables de lobos marinos en cada sitio, el nivel de pesca permisible actual podr´ıa ser aumentado en algunos sitios y deber´ıa ser reducido en otros. Por ejemplo, el area´ alrededor de San Esteban podr´ıa soportar hasta 1428 (935–2337) d´ıas de pesca adicionales, mientras la pesca alrededor de Lobos deber´ıa ser reducida en por lo menos 165 (107–268) d´ıas. Nuestros resultados proporcionar a los profesionales de la conservacion´ directrices espec´ıficas para cada sitio para mantener poblaciones sustentables de leones marinos y proporcionan un m´etodo para estimar la presion´ de pesca y las tasas de captura incidental sustentables que podr´ıan se utilizadas en otros mam´ıferos y aves marinas. Palabras Clave: captura incidental, lobo marino de California, enmallamiento en red agallera, Golfo de Califor- nia, viabilidad poblacional, Zalophus californianus Introduction tion size after only 3 generations (15 years) was >65% for 8 of the 11 rookeries used in the analysis. Because of Obtaining accurate information on fisheries-related by- the uncertainty associated with census data and subse- catch for marine mammals and birds is a global chal- quent population-growth rate estimates, it is difficult to lenge in marine conservation biology (Read et al. 2006). ascertain the true status of this population. Here we present a method for estimating current fish- Recent reports estimate that 30,000 fishers in 18,000 ing pressure and sustainable bycatch rates based on en- boats currently fish in the Gulf and that unregulated gill- tanglement data and life history parameters for marine net fishing has doubled in the last decade and continues species (pinnipeds, cetaceans, or birds). This method is to grow (CONANP 2006). The type of fishing that most illustrated with the California sea lion (Zalophus califor- commonly conflicts with California sea lions is the set nianus) population in the Gulf of California (Gulf) as a gill-net fishery (Julian & Beeson 1998; Aurioles-Gamboa case study. Using this method, an estimate of incidental et al. 2003). In fact, the threat of mortality due to inci- catch and the relative intensity of fishing pressure can be dental catch in gill nets is one of the gravest problems approximated indirectly from the number of entangled facing sea lions in this area (Zavala-Gonzalez & Mellink animals recorded. Sea lion entanglement rates are rela- 1997; Programa de Manejo, Islas del Golfo de California tively straightforward to document, and information can 2000; Aurioles-Gamboa et al. 2003). Although the direct be collected quickly. Assuming that the number of entan- impact of gill-net fishing on the sea lion population in the gled individuals indicates relative fishing pressure, the Gulf is unknown, incidental catch in gill and other fishing use of site-specific data on entanglement and life history nets has contributed to population declines in northern parameters permits one to estimate the level of allowable fur seals (Callorhinus ursinus), harbor seals (Phoca vi- fishing pressure. tulina stejnegeri), and harp seals (P. groenlandica)and The California sea lion is a polygynous pinniped that is at least partially responsible for declines in Steller sea breeds on islands of the eastern Pacific Ocean from British lions (Eumetopias jubatus) (Fowler 1987; Woodley & Columbia in Canada to Las Islas Mar´ıas in Mexico and in Lavigne 1991). In the Gulf, recent observations indicate the Gulf (Odell 1981). After an apparent recovery from an increased rate of entanglement in the northern Gulf exploitation in the 19th and 20th centuries, the current (Zavala-Gonzalez & Mellink 1997). Increases in entangle- status of California sea lions in the Gulf is uncertain. ment almost certainly signal an elevated rate of mortality Szteren et al. (2006) compared the most recent survey due to incidental catch, which may be one of the prin- (24,062–31,159) with a 1994 census (31,393 individuals) ciple reasons why sea lion populations in the northern and found an approximate 20% decline between 1994 to Gulf are in decline. 2004. The most recent census data at the rookery level We devised a method to estimate the current level indicate that increases in several large breeding colonies of fishing pressure and a sustainable bycatch rate are masking rapid declines in several smaller rookeries for 11 breeding colonies found throughout the Gulf. (Szteren et al. 2006). Although census numbers indicate a We assumed movement was limited between breeding declining population, estimates of the population-growth colonies. Although movement between rookeries is pos- rate for the Gulf seem to indicate a stable to slightly in- sible and occasionally does occur, the level of movement creasing population (Gonzalez-Suarez et al. 2006; Szteren between rookeries is thought to be minimal on the ba- et al. 2006). Another way to assess the status of the pop- sis of genetic and mark-recapture studies in the Gulf ulation is to estimate the probability of a reduction in (Maldonado et al. 1995; Gonzalez-Suarez et al. 2006; population size over time. Gonzalez-Suarez et al. (2006) Young et al. 2007; Hernandez-Camacho et al., 2008a). found that the probability of a 30% reduction in popula- This method for estimating sustainable bycatch rates can Conservation Biology Volume 22, No. 3, 2008 Underwood et al. 703 help managers in the Gulf establish a fishing strategy that will allow sea lion populations in the region to persist. These methods could also be applied to other marine bird and mammal species.
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