132 



Fishery Bulletin 100(1) 



Discussion 



Many factors affect the survival of lobsters discarded in 

 commercial trap fisheries, including capture, handling, 

 and discarding processes. In our experiment we focused on 

 handling factors to assess their impacts on mortality in 

 the NWHI commercial lobster fishery. 



If the 2- and 3-h dry experimental treatments typify 

 commercial handling practices, then the mortality of dis- 

 carded spiny lobster from handling practices on commer- 

 cial vessels is extreme, ranging from an average of 25% to 

 45% and 70% to 77%, respectively, depending on how da- 

 ta are pooled. Handling mortality for slipper lobster also 

 appears high (estimated at 31%) but is considerably less 

 than that for spiny lobster; thus spiny lobsters may have a 

 lower handling tolerance than slipper lobsters. 



Although there are no published estimates of handling 

 mortality for P. i7iarginatiis and S. squa?nmosus, studies 

 on other lobster species suggest that handling mortality 

 in the NWHI lobster fishery is high. Lyons and Kennedy 

 (1981), reporting on P. argiis. estimated that 12.3% of lob- 

 sters died after 30 minutes of exposure to direct sunlight 

 and an average 24.1% after 1-4 hours of exposures. They 

 also found that lobsters exposed for 2-4 hours tended to 

 die within a week following exposure, whereas those ex- 

 posed for only 1 hour survived longer. Laboratory experi- 

 ments by Brown and Caputi ( 1983) on small western rock 

 lobster, Paniiliri/s cygniis, exposed to direct sunlight re- 

 sulted in an expected time to 50%f. mortality that decreased 

 with increasing temperature, ranging from 233 minutes at 

 27°C to 99 minutes at 31-35°C. Time to 50% mortality was 

 387 minutes for lobsters held in the shade at 26.5-32°C. 



Handling mortality, however, represents only a portion 

 of the total mortality of discarded lobsters resulting from 



their capture, shipboard processing, and subsequent re- 

 lease in the NWHI. Additional mortality resulting from 

 habitat displacement, predation, and other factors associ- 

 ated with discarding might result in total discard mortal- 

 ity estimates that approach 100%. Qualitative evidence 

 suggests that discarded lobsters are subject to high preda- 

 tion from the giant trevally, Caranx ignohilis, which ag- 

 gregate around vessels during fishing operations (Good- 

 ing, 1985). 



Sorting and discarding lobsters immediately after they 

 are placed on deck appears to reduce total discard mortal- 

 ity. The discarded lobsters would need to be returned to 

 the general vicinity of their capture and as close to the 

 sea floor as possible to avoid the gauntlet of predators in 

 the water column. Brown and Caputi (1986) reported a 

 reduction in recapture rates of displaced undersized rock 

 lobsters compared with nondisplaced rock lobsters and re- 

 lated the reduction directly to predation mortality. 



Adoption of the retain-all fishery by the WPRFMC 

 in 1996 significantly reduced fishery-induced handling 

 mortality. Although sporadic discarding occurs, current 

 discard rates are less than 1% and will have no detect- 

 able consequence at the population level. The research 

 does, however, provide insight into past fishery-induced 

 handling impacts, which likely contributed to the de- 

 cline in NWHI lobster catches. If future management 

 again reverted to mandatory discarding practices, this 

 research provides information to assess its impact on 

 fishery-induced mortality. However, to fully understand 

 the synergistic effects of catching, handling, and dis- 

 carding practices on mortality in the NWHI lobster fish- 

 ery, additional research to assess the impacts associat- 

 ed with shipboard sorting and releasing (e.g. postrelease 

 predation) is required. 



