Boggs: Estimating capture depths of longline-caught pelagic fish 



647 



Figure 3 (left) 



Sample records from time-depth recorders (TDRs) 

 measuring the deep positions on three sets (each with 

 two TDRs) in 1989, illustrating the typical sinking 

 time (0.5 h), variation in settled depth, and typical ris- 

 ing time (0.5 h). 



Figure 4 (below) 



Hook depths for catches of 14 frequently-caught taxa 

 in a study off Hawaii, winter 1989 and 1990 (com- 

 bined). Settled hook depths are shown for all hooks 

 that caught fish (unconfirmed) and for those hooks 

 that caught fish while settled (i.e., not sinking or ris- 

 ing) as indicated by hook-timer data (confirmed). 



Fig. 3) and <40m for the regular sets. 

 Also, the gear sometimes took more 

 than 0.5h to rise (e.g., the first TDR on 

 set 16; Fig. 3) or sink. Such deviations 

 contributed to the variation in estimated 

 capture depths. Capture depths of fish 

 caught with hook timers on baskets with 

 TDRs were based on the TDR depth 

 at the time of capture; however, most 

 catches were made by baskets without 

 TDRs. 



A comparison between the uncon- 

 firmed depths of all hooks that caught 

 fish and those confirmed to have caught 

 fish while settled (Fig. 4) showed that 

 without hook-timer confirmation, many 

 fish appeared to be caught at greater 

 depths than they actually were. For ex- 

 ample, mahimahi Coryphaena hippurus 

 had unconfirmed capture depths of 

 <420m and confirmed capture depths 

 of <190m. Most confirmed capture 

 depths were <100m for mahimahi and 

 skipjack tuna Katsuwonus pelamis. 

 Striped marlin Tetrapturus audax, 

 whitetip shark Carcharhinus longima- 

 nus, blue shark Prionace glauca, and 

 wahoo Acanthocybium solandri had un- 

 confirmed depths of < 350-420 m and 

 confirmed depths of < 200-230 m. Spe- 

 cies having a preponderance of con- 

 firmed capture depths of < 150 m were 

 yellowfin tuna Thunnus albacares, 

 striped marlin, and spearfish Tetraptu- 

 rus angustirostris. Most confirmed cap- 

 ture depths were >200m for thresher 



