882 



Fishery Bulletin 97(4), 1999 



(Kaimmer, 1994) were used, halibut less than 82 cm 

 in length were hooked more often in locations other 

 than the jaw than were larger halibut. Although this 

 difference was small, it suggests a functional rela- 

 tion between the mechanical operation of the hook 

 and the size of the fish's mouth in relation to overall 

 hook and bait dimensions. A higher rate of hooking 

 success for smaller halibut might have been seen if 

 smaller bait had been used. This was not tested in 

 the present experiment because bait size was held 

 constant to the standard used in IPHC setline sur- 

 veys and representative of the commercial halibut 

 fishery. 



It is clear that an increased hooking success for 

 larger fish has a dramatic effect on the size selection 

 of longline gear. Much of the length-based selectiv- 

 ity assumed for halibut setlines can be explained by 

 the differences in hooking success by fish length dem- 

 onstrated by this study. The selection curve gener- 

 ated from the present observations is the first to be 

 determined for Pacific halibut through direct ob- 

 servation of hook attacks. 



The large number of bait attacks by species other 

 than halibut compensated for the very low hooking 

 success for these species. In fact, we caught more of 

 these species than halibut during the experiment. 

 Although their presence or interaction with baits 

 short of hooking did not seem to affect the attack 

 rate of halibut that were present, the removal of 

 available baits by interspecies competition should be 

 included in any model of hook-and-line CPUE. 



Acknowledgments 



The author expresses his thanks to S. Lokkeborg for 

 comments on an early draft of this paper, as well as 

 three anonymous reviewers who gave many useful 

 suggestions. I would also like to thank Craig Rose 

 and Scott Mclntire, National Marine Fisheries Ser- 

 vice, Seattle, for loan of the camera gear and for in- 

 valuable assistance in training me in its use, and 

 most especially for their long-distance troubleshoot- 

 ing during the project. 



Conclusions 



We consider the results of this study to be qualita- 

 tive, not quantitative. Their application to commer- 

 cial or experimental longline sets or CPUE indices 

 should take into account the limited nature of the 

 experiment, both in terms of number of hooks fished 

 and in terms of lack of seasonal or wide areal varia- 

 tion in the observations. Although it is likely that 

 the results are generally applicable to halibut, they 

 could vary significantly for fish with different feed- 

 ing histories in a different life stage. Attack rate par- 

 ticularly could be susceptible to the condition offish 

 in relation to recent feeding or spawning activity. The 

 primary objective of this project was to determine 

 estimates for the attack rate and hooking success of 

 Pacific halibut on gear typical of those used in the 

 commercial fishery. Results show that although these 

 parameters may be estimated, their values are in- 

 fluenced by a number of factors, most notably by the 

 presence of bottom current and direction of approach 

 in relation to that current (for attack rate) and fish 

 length (for hooking success ). The behavioral sequences 

 observed for Pacific halibut were fairly limited and in- 

 cluded searching into a current for food, looping and 

 lying around or near bait items, and a vigorous biting- 

 rushing sequence that often resulted in hooking. The 

 relative absence of physical contact with the bait prior 

 to biting (such as tasting) might indicate that texture 

 is less important for food selection by halibut. The role 

 of vision in halibut feeding was not tested. 



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