Boggs: Estimating capture depths of longline-caught pelagic fish 



653 



The area (lat. 17°-18°N) of highest catch rates for 

 bigeye tuna was on the south edge of a northward tran- 

 sition to a deeper thermochne and oxycHne (Fig. 10). 

 The north-south pattern is typical of the central Pacific 

 Ocean at these latitudes, whereas the highly variable 

 pattern in the thermocline between lat. 19.4° and 20°N 

 was probably caused by the proximity to the lee side 

 of the island of Hawaii. 



With regard to the other species, the thermal struc- 

 ture of the habitat (Fig. 10) and the confirmed depth 

 distribution of fish (Figs. 4, 7, and 8) suggested that 

 yellowfin tuna were most abundant in the mixed layer 

 (24°-25°C) and the steepest part of the thermocline 

 down to about 15°C. Striped marlin appeared to be 

 most abundant in the mixed layer and the top of the 

 thermocline to ~20°C. Spearfish appeared to occupy 

 a habitat between that of yellowfin tuna and striped 

 marlin, and mahimahi occupied the mixed layer. 



Standardized gear efficiency 



For bigeye tuna in 1989-90, the CPUE ranges for stan- 

 dardized deep gear and proposed new gear were about 

 3.1-4.0 and 4.1-5.6 times, respectively, as great as 

 those for regular gear (Table 5). Shallow gear on aver- 

 age was about half as efficient as regular gear in catch- 

 ing bigeye tuna, whereas it was about 40% more effi- 

 cient than regular gear in catching spearfish and 

 striped marlin. Deep gear was only about half as effi- 

 cient as regular gear in catching striped marlin and 

 spearfish, and the proposed new gear was only about 



20% as efficient for striped marlin and about 30% as 

 efficient for spearfish. 



The numbers of yellowfin tuna and mahimahi caught 

 in 1989 were much lower than in 1990, so the latter 

 year provided better data for calculating gear efficiency 

 for these species (Table 5). Shallow gear was about 3.0 

 times as efficient at catching mahimahi, and deep and 

 new gear reduced efficiency to about 90% and 75% in 

 comparison with regular gear. For yellowfin tuna, 

 shallow gear was about 25% more efficient than reg- 

 ular gear, whereas the deep and new gear types were 

 each about 65% as efficient. 



Discussion 



Habitat deptli 



Hook timers are useful in confirming whether fish are 

 caught while longline hooks are sinking, settled, or ris- 

 ing. Combined with capture depths from TDRs, hook 

 timers offer a new method for establishing the habitat 

 depth of large pelagic fishes. Stock assessments (Suzuki 

 1989) depend on the estimation of effective effort, 

 defined as fishing effort corrected for differences in 

 efficiency due to gear and habitat depth (Suzuki et al. 

 1977). Improving the definition of tuna and billfish 

 habitats and the estimation of effective effort in those 

 habitats should lead to significant improvements in 

 assessing true abundance. 



Comparisons of CPUE by two gear types provide 

 only qualitative information on habitat depth. For 



