Smith and Abramson: Leopard shark tag-recovery data from San Francisco Bay 



379 



Figure 5 



Fitted monomolecuiar curve relating mean number of leopard shark 

 embryos in litters to parental weight. Areas of symbols are propor- 

 tional to weighting factors of points used in the fitting process. 



1982), who reported that a leopard shark tagged by him 

 in December 1971 in Elkhorn Slough, Moss Landing, 

 California, was recaptured February 1973 in south San 

 Francisco Bay. 



It is not known to what extent fishing effort influenc- 

 ed the recapture pattern, because effort information 

 on the commercial and recreational catch is not avail- 

 able to the degree of geographic precision needed for 

 such an analysis. We do know that leopard sharks are 

 susceptible to bottom fishing gear, whether it be baited 

 hook, gillnet, or, to a lesser extent, bottom trawl. Judg- 

 ing from the available data on recreational and com- 

 mercial landings, and the much greater proportion of 

 recreational versus commercial tag recaptures, recrea- 

 tional anglers are the primary users of the leopard 

 shark resource in northern California. Angling for bot- 

 tom fishes takes place in most areas inside the bay 

 system year-round (Squire and Smith 1977), although 

 in winter, inclement weather may lower fishing effort 

 within the bay as well as along the open coast. Bottom- 

 fishing effort in the ocean is thought to be comparative- 

 ly less than in the Bay at least in waters shallower than 

 91 meters, the reported depth range of this species. 



The 1983 peak in the tagged fish recovery rate and 

 the associated rise in fishing mortality that we calcu- 

 lated (Tables 4 and 5) was unexpected. We have no 

 evidence that fishing effort increased that year to cause 

 this jump in the tag return rate, but there was a sub- 

 stantial rise in the California commercial catch of 

 leopard shark that year, primarily driven by an increase 

 in San Francisco landings (Table 1). There was also a 

 similar rise in the estimated recreational catch (Table 

 2). The year 1983 was also one of unusual oceano- 

 graphic conditions, when warm, nutrient-poor El Nifio 

 water intruded along the central California coast 

 (McClain 1983, Norton et al. 1985). These El Nino con- 

 ditions may have caused an influx of sharks from more 

 southerly populations, but the increased rate of recov- 

 ery for San Francisco Bay tagged fish should be in- 

 dependent of immigration of fish from other areas. 



The California Department of Fish and Game (1987) 

 and Pearson (1989), conducting trawl surveys of San 

 Francisco Bay fishes, have observed higher trawl 

 catches of leopard sharks during wet as opposed to dry 

 years, and 1983 was indeed a wet year. That year the 

 bay system experienced the highest delta outflows from 

 the Sacramento-San Joaquin River systems in over 10 

 years, and June freshwater outflows were twice that 

 of the previous wet year of 1982 and six times the 

 previous decade's average outflow for that month 

 (Calif. Dep. Water Resour., DAYFLOW Prog. Summ., 

 Sacramento, CA 95816, Sept. 1987). Perhaps the 

 anomalous conditions affected the local distribution and 

 availability of central California leopard sharks and 

 possibly their benthic prey, making the sharks more 

 vulnerable to centers of fishing pressure. 



In estimating the mortality, yield, and stock-replace- 

 ment values, it was neccessary to make many assump- 

 tions and adjustments. Clearly we must assume that 

 the tagged fish rapidly mix with the balance of a 

 relatively closed San Francisco Bay population, and 

 that tagged and untagged animals are equally likely to 

 be caught. But because of the paucity of this type of 

 information on these animals, an apparent high suscep- 

 tibility of elasmobranch stocks to fishing pressure, and 

 the noticeable increase in fisheries targeting on sharks, 

 we felt it appropriate to present our best estimates 

 based on the available data and current knowledge of 

 elasmobranch biology. 



In the case of the leopard shark, high stock main- 

 tenance is surely more important than a large yield per 

 recruit. And while there is obviously something wrong 

 with the stock replacement values which exceed 100% 

 on an equilibrium basis, we would like to accept the 

 100% isopleth as reasonably valid. Noting in Table 5 

 that estimated F's ranged from 0.038 to 0.163 with 

 a mean of 0.084, and observing in Figure 4 that the 

 100% replacement isopleth runs well to the right of the 



