Table 5. — Fishing intensity and numbers of marked shrimp recovered, Tortugas grounds, 



December 1962 to February 1963 



' In analysis, this number adjusted to 2,314 for the mean release date of 12/14/62. 



- Per (mean) 9,450 hours of fishing intensity. 



' Recoveries during this final period were not used in analysis. 



shrimp in the water mass \vill affect their avail- 

 ability to a trawl fishing on or near the bottom. 

 Over short periods of time then, we may 

 normally expect considerable variabihty in catches 

 of pink shrimp (unmarked or marked) which is 

 independent of the decrease in a population with 

 time. 



DEFINITIONS OF NOTATIONS 



Notations and symbols suggested by Beverton 

 and Holt (1957) and Holt (1960) are used in our 

 analysis and summary. Definitions follow: 



Z= Instantaneous total mortality coefficient 

 F= Instantaneous coefficient of mortality 



caused by fishing 

 M= Instantaneous coefficient of mortality by 



(natural) causes other than fishing 

 X= Instantaneous coefficient of other loss in 

 marking theory, i.e., losses in the ex- 

 perimental population due to all causes 

 except recapture (true natural mor- 

 tality plus losses of individuals which 

 for any reason become unavailable for 

 recapture)' 

 /= Fishing intensity (fishing effort per unit 

 area) 

 A^o= Initial size of the experimental popula- 

 tion (number in batch of marked 

 shrimp liberated at time zero) 

 n=Number of marked shrimp recaptured 



in a given period, e.g., 

 ri,, = Number in fu-st period, n2=Number in 

 second period, etc. 



ANALYSIS 



Variations in catch per unit fishing intensity 



' As defined by Beverton and Holt (1957). Holt (1960) gave a varied mean- 

 ing. 



described previously do not nuUify the value of 

 this information in deriving mortahty estimates. 

 Population decreases with time during the experi- 

 mental period are reflected well by these data. An 

 analytical technique designed to yield mean instan- 

 taneous mortality coefficients, accordingly, was 

 chosen. 



The numbers of marked shrimp recaptured each 

 2-week period varied considerably in both experi- 

 ments in response to fluctuating fishing intensity. 

 As suggested by Kutkuhn (1966), the numbers of 

 recoveries that accumulate each period may be 

 corrected to unit fishing intensity and analytical 

 methods given by Beverton and Holt (1957, pp. 

 185-191) applied. A factor of 45.6, the average 

 number of boat-nights ' of fishing intensity per 

 2-week period, was applied to Sanibel recoveries 

 to convert them to a unit basis (table 4) ; a factor 

 of 9,450, the average number of hours of fishing 

 intensity per 2-week period, was used to convert 

 the Tortugas recoveries (table 5). We recognize, 

 however, that this method of analysis is better 

 suited to the Tortugas experiment than to the 

 Sanibel experiment where fishing intensity fluc- 

 tuated considerably from period to period. 



Lines fitted to the natural logarithms of ad- 

 justed recoveries (figs. 7 and 8) indicate that the 

 dechne in numbers of both experimental groups 

 followed a hnear trend. Regression fines have a 

 slope equal to minus (F-t-X) or Z. For Z, the 

 instantaneous total mortahty coefficient, the re- 

 gression equations yielded values of 0.233 for 

 Sanibel and 0.357 for Tortugas. Values of 153.4 

 and 121.5 (designated Ui and n2) were obtained by 

 substituting appropriate units of time in the 

 regression equation for Sanibel. These figures are 



' Sanibel fishing effort was reported to us in boat-nights rather than in 

 hours. No valid conversion factor was available to convert boat-nights to 

 hours; therefore, Sanibel effort was used in the form of boat-nights as original- 

 ly reported. 



PINK SHRIMP MORTALITY ON SANIBEL AND TORTUGAS 



499 



