NOTES 



INCORPORATING SOAK TIME INTO 



MEASUREMENT OF FISHING EFFORT IN 



TRAP FISHERIES 



While it is recognized that soak time (number of 

 days a trap is allowed to fish before it is retrieved) 

 is an important fishing strategy decision for the 

 individual fisherman, there is surprisingly scarce 

 information on the subject. Little data is available 

 on the relationship between catch and soak time. 

 Similarly, the implications of variable soak times 

 have not been widely discussed. 



This paper develops a model to determine the 

 profit-maximizing soak time for an individual 

 fisherman in the Florida spiny lobster, Panulirus 

 argus, fishery. This establishes the relative im- 

 portance of soak time as one of the components of 

 fishing effort in trap fisheries and leads to 

 suggestions for incorporating soak time into the 

 traditional measurement of trap days to more 

 accurately reflect fishing effort in trap fisheries. 



Profit-Maximizing Soak Time 



Catch per trap day was regressed on soak time 

 with the data collected by Robinson and Dimitriou 

 (1963). The best statistical fit using ordinary least 

 squares is in the form of Equation (1) (Figure 1). 



C = <L (i) 



D SP 



where C = catch per trap haul 



D = days fished for the sample 

 S = soak time in days 

 a = 2.94, i a = 5.40 

 = 0.90, in = 11.25 



year: 1963 

 n = 25 

 R 2 = 0.86. 



Since the number of days fished (D) in this field 

 experiment was synonymous with the soak time 

 (D = S), then: 



C = aS { 



(2) 



Taking the first and second derivatives of Equa- 

 tion (2) with respect to the soak time: 



dC ( 1 - /8) a 

 dS ~ 



SH 



> 



(3) 



d 2 C (P 2 - P) a 



dS'< 



S<l+/3> 



< 0. 



(4) 



Equations (3) and (4) imply the catch per trap haul 

 increases at a decreasing rate with respect to the 

 soak time (Figure 2). This relationship seems 



C 

 D 



FIGURE 1. — Catch per day with respect to the soak time. 



FIGURE 2. — Catch per haul with respect to the soak time. 



213 



