652 



Fishery Bulletin 90|4). 1992 



Figure 9 



Comparison between indices of fish abundance from different types 

 of longline sets, calculated as the number of fish caught/1000 hooks 

 without regard to capture depth of individual fish (CPUEg). Three 

 types of longline sets were categorized on the basis of the deepest 

 hooks, but every set contained some hooks as shallow as 40-80 m. 

 Data are combined for 1989 and 1990. 



Although relatively few mahimahi were captured 

 with timer data, these data indicated maximum abun- 

 dance was at 40-80 m in 1990 (only one fish was caught 

 on a settled hook in 1989; Fig. 7, Table 4). Pooled data 

 clearly indicated that CPUEd was highest at 40-120m 

 (Fig. 8). At >200m, all mahimahi with timer data were 

 caught on moving hooks. 



Examining the CPUEg data as if the only available 

 depth information were the set type (Fig. 9) made it 

 difficult to correctly qualify the relative abundance of 

 fish in relation to depth. For example, mahimahi ap- 

 peared almost as abundant in deep sets as in shallow 



Regular 

 (60-200 m) 



10 - 



o 

 o 

 o 







*> flj t- 2 *^ 

 CO .3 



o a. 



a B 







Deep 

 taOO-330 m; 



Very Deep 

 (330-5300 m) 



Aa 



C = o jd ."J 

 G .,3 a « ? 



o o. 



&£ 



03 



^ 



Latitude (°N) 



sets, and spearfish appeared more abun- 

 dant in very deep than in deep sets, il- 

 lustrating that it is impossible to cor- 

 rectly describe fish depth distributions 

 without data on catch by hook position, 

 hook depth, and capture time. 



Oceanographjc habitat 



The temperature profile in 1990 (Fig. 

 10) was representative of the study area 

 in both years, except the bottom of the 

 thermocline (i.e., the 12°C isotherm) 

 was ~40m deeper in 1989. In both 

 years, the highest catch rate of bigeye 

 tuna with confirmed capture depths 

 occurred at lat. 17°-18°N at 360-400m 

 in temperatures of 8°-10°C (Fig. 10). 

 The oxycline in 1990 (Fig. 10) also was 

 similar to that in the previous year (i.e., 

 the S.Omg/L isopleth was only 10-20 m 

 deeper in 1989). Most bigeye tuna were 

 caught at DO concentrations of 2-6 

 mg/L. In both years, the highest catch 

 rate was at 2-3 mg/L. 



Figure 10 



Temperature and dissolved oxygen profiles of the 

 study area (lat. 14°-20''N, long. 148°-159°W) in 

 1990 (similar to 1989). Confirmed capture depths 

 of bigeye tuna in 1989 (D) and 1990 (<>) are 

 indicated. 



