378 



Fishery Bulletin 88(2), 1990 



Tag recoveries by yearly intervals after the release 

 period, estimated annual instantaneous fishing and 

 natural mortality coefficients, and rates of exploitation 

 are shown in Table 5. Note that these are all of the 

 tag recoveries which could be assigned to the time 

 intervals; other tabulations of recoveries in this paper 

 may differ because of various missing information 

 components. 



Yield and stock replacement per recruit 



Shown in Figure 4 is the yield-per-recruit isopleth dia- 

 gram forM= 0.28 during the first year of life and 0.14 

 thereafter. Age at first capture ranges from 4 to 13 

 years, and F runs from 0.05 to 0.28. The graph in- 

 dicates a fairly flat yield contour between F = 0.15 and 

 F = 0.28, with age at first capture between 5 and 10 

 years. 



Also in Figure 4 are isopleths showing percent pop- 

 ulation replacement per recruit over the same param- 

 eters used for the yield computations. For use in cal- 

 culating population replacement, the fitted curve (Fig. 

 5) relating Ackerman's data on number of embryos to 

 parental weight was 



P, = 22.64 - (7592) (0.4208)"'<' ', 



where P, is number of embryos, iv{t ) is maternal 

 weight (kg) at age t, and the standard error of estimate 

 equals 0.38313. The value for w(t ) was obtained from 



w{t) = 0.00000305 



X (172.4[l-exp(-0.0717[/-H2.302])]}3"5. 



Figure 4 



Isopleths showing percent population replacement of leopard sharks 

 tagged in San Francisco Bay, California, in numbers (dashed lines) 

 and yield per recruit in kilograms (solid lines), for M = 0.28 during 

 the first year of life and 0.14 thereafter. 



Replacement increases steadily as F tends toward 

 and age at first capture increases (equation 2). The 

 shape of this surface is not unexpected, but clearly 

 some type of density-related compensation would be 

 needed to increase mortality or reduce fecundity as the 

 population increases; otherwise in the absence of fish- 

 ing the sea would be filled with leopard sharks. This 

 mechanism might be in the form of higher juvenile mor- 

 tality, less frequent litter bearing, or reduced average 

 litter size. Holden (1973, 1977) discusses some possi- 

 ble mechanisms of this type and cites examples from 

 the literature of marine mammals and elasmobranchs. 

 We do not have information to examine any of these 

 possibilities in the present case. 



Discussion 



If it is assumed that the tagged population approxi- 

 mates the real population, these results suggest that 

 San Francisco Bay leopard sharks are mostly resident, 

 with some moving out of the bay during fall and winter. 

 Judging from the single Moss Landing recovery, limited 

 population exchange evidently occurs between Elkhom 

 Slough and San Francisco Bay leopard sharks. Also, 

 previously undocumented evidence was received dur- 

 ing the course of this study from L. Talent (Oklahoma 

 State Univ., Stillwater, OK 74078, pers. commun., July 



