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

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.

Indirect Effects of Conservation Policies on the Coupled Human-Natural Ecosystem of the Upper Gulf of California

“…Vaquita population trajectories under alternative fisheries policies, obtained using an ecosystem model that incorporated vaquita age structure and diets, were consistent with results of the single-species model [25]. However, calls for fishery management to address broader ecological and conservation goals in the Gulf [26]–[28] and globally [29]–[31] suggest the need to weigh the impacts of fisheries policies on the broader ecosystem.…”

Status of J stock minke whales (Balaenoptera acutorostrata)

“…The PBR concept is widely used internationally to guide conservation of marine mammals (e.g. Gales 1995;Taylor et al 2000;Marsh et al 2004;Thompson et al 2007;Underwood et al 2008), and has been used by the European Union (ASCO-BANS 2000) and by New Zealand (Slooten and Dawson 2008) to evaluate bycatch of cetaceans. Japan's National Research Institute of Far Seas Fisheries has proposed a PBR-type method to set quotas on Dall's porpoises (Phocoenoides dalli) hunted in Japan, and a simulation study on how to apply this method is described in Okamura et al (2008).…”

Short‐ and long‐term population response to changes in vital rates: implications for population viability analysis

“…There are limited data on movement among these populations and such data are generally difficult to obtain (Baguette et al 2000). Entanglement in fishing gear represents a significant threat to sea lion populations in the Gulf of California (Zavala-Gonza´lez and Mellink 1997, Aurioles-Gamboa et al 2003, Underwood et al 2008. Changes in climate, disease and poisoning are among other threats to sea lions in the Gulf (Heath 2002).…”