Wade: Estimates of incidental kill of dolphins by the purse-seme tuna fishery 



347 



Lo and Smith ( 1986) solved for Equation 2 from Equa- 

 tion 1 for the following reasons: 



• MPS was assumed to be constant within a stra- 

 tum during this period because they found no sig- 

 nificant differences in MPS by year, and some years 

 had no data on MPS available. 



• The vessel logbooks did not show use or not of the 

 backdown procedure, so the number of sets could 

 not be stratified on this variable. Instead, the pro- 

 portion of use of backdown procedure, P t , was esti- 

 mated from the MPS data where it was assumed 



' that backdown 1) increased linearly between the 

 known use of 0.0 in 1959 and the observed propor- 

 tion of 0.79 in 1964—65; 2) equaled the proportion 

 of 0.89 observed in 1966-71; and 3) equaled the 

 proportion of 0.93 observed in 1972. 



• Unsuccessful sets were not frequent and usually cap- 

 tured few or no dolphins. Backdown was thus not often 

 used during these sets, which killed relatively few 

 dolphins. Therefore, MPS was pooled across stratum 

 k, the use or not of backdown, for unsuccessful sets. 



• The use or not of the backdown procedure was as- 

 sumed to have the same magnitude of effect on small 

 and large vessels. Therefore, the ratio of MPS with 

 no backdown to MPS with backdown, C, was pooled 

 across vessel stratum to increase the sample size. 



To confirm that I was correctly duplicating Lo and 

 Smith's ( 1986) method, I recalculated their kill esti- 

 mates using my calculations of MPS and the num- 

 ber of sets reported in their Table 3. 



I modified their method in the following ways. The 

 first modification was to account for the revised stocks 

 of offshore spotted dolphin. IATTC provided the num- 

 ber of sets by year previously reported by Punsley ( 1983 ) 

 but stratified geographically into separate totals for the 

 northeastern and western/southern areas (Fig. I). 6 



6 Data provided by M. G. Hinton, Senior Scientist, Inter-Am. Trop. 

 Tuna Comm. 7« Scripps Inst. Oceanogr., La Jolla, CA 92093, 24 

 June 1993. A typographical error in Punsley (1983) led to the 

 inadvertent use of an incorrect value for the total number of 

 dolphin sets in 1959 in Lo and Smith ( 1986). The correct value 

 of 391 has been used here in place of 591. 



30* 



150' 



130" 120* 110° 



WEST LONGITUDE 



100' 



90* 



Figure 1 



Newly defined areas for offshore stocks of pantropical spotted dolphins, Stenella attenuata. 

 The outer line is the eastern tropical Pacific study area as defined by the National Marine 

 Fisheries Service. The inside solid line represents the new boundary between the north- 

 eastern and western/southern stocks of spotted dolphins. Sightings of offshore spotted 

 dolphins to the north of 5°N and to the east of 120°W are assigned to the northeastern 

 stock, and sightings outside of that area are assigned to the western/southern stock. The 

 dashed line represents the old boundary between the previously defined northern and 

 southern stocks. The circles represent the location of observed dolphin sets used to esti- 

 mate mortality per set in this paper, consisting of one fishing trip in 1968, five trips in 

 1971, and 12 trips in 1972. The exact location of sets from the 1964 and 1965 trips were not 

 available, but the 1964 trip was stated to be 200 miles off the coast of Acapulco, Mexico. 



