California Sea Grant College Program

Although many shark species are currently fished past sustainable levels, we lack the data to fully assess their status under traditional frameworks. There is an increasing need to develop and validate new methods to assess shark populations. In chapter one, I tested how well the ad hoc methods that use linear models would reflect sustainability as seen from a more conventional fisheries point of view. I compared two examples of linear demographic models to a conventional approach, that directly measures a species' sustainability, by assessing 26 shark species' vulnerability to fishing pressure using each method. I found that the linear demographic models resulted in different rankings and numerical values than the conventional approach. Because these ranking are used to inform management decisions, it is important to use approaches similar to the conventional metric, which directly measure a species' vulnerability. In chapter two, I developed a method to use catch-per-unit-effort and size-structure data from the common thresher shark, Alopias vulpinus, in an age-structured model to determine the fishing mortality (0.76 y"1) and the resulting recruitment decline (0.20 recruits egg"1). I determined the status of the population based on the fraction of current biomass (22%) and lifetime reproduction (0.47) relative to unfished levels. These values indicated that thresher sharks were overfished in 1992. This method is useful for other new fisheries with similar kinds of available data. In chapter three, I developed a new method to estimate the population abundance of white sharks, Carcharodon carcharias, off central California, where no fishery data exist. Because a white shark's fin allows each individual to be uniquely identified and they showed predictable annual fidelity at coastal sites, I was able to compute a mark-recapture abundance estimate using a Bayesian algorithm. I collected 321 photographs identifying 131 unique individuals. The abundance off central California was estimated to be 251 individuals ([197,360] 2.5% and 97.5% quantiles). My methods can be readily expanded to include juvenile and young-of-the-year sharks and sharks from other locations, over extended time- series, and to monitor the status, population trends and protection needs for white sharks.