Ward et a!.: The effect of soak time on pelagic longlme catches 



191 



Underlying mechanisms 



The broad taxonomic groups taken by longlme each rep- 

 resent a wide range of life history strategies and feeding 

 behaviors. Nevertheless, the results show a tendency for 

 soak time to have a positive effect on catch rates of most 

 shark species (Fig. 4). It also had a positive effect on catch 

 rates of many billfish species, including striped marlin, 

 black marlin, and swordfish. There is no clear pattern in 

 the effect of soak time on catch rates of tuna or other bony 

 fish. It had a negative effect on the four seabird groups. 



The results imply that the ability of a species to stay 

 alive and to escape or avoid scavengers while hooked is 

 important in determining the catch that is actually brought 

 on board. The effect of soak time is significantly correlated 

 with the ability of a species to survive while hooked on the 

 longline in the four fisheries with data on survival (Fig. 7). 

 Soak time has a strong, positive effect on catch rates of spe- 

 cies like blue shark, which are almost always alive when 

 branchlines are retrieved. Species like skipjack tuna and 

 seabirds are usually dead. Soak time had a negative effect 

 on their catch rates. The opposite trend would be expected 

 if escape is a significant process that affects catch rates; if 

 escape is important, soak time should have a negative af- 

 fect on the catch rates of the most active species. Therefore 

 removal by scavengers is likely to be more important than 

 escape in determining catch rates for many species. 



Longline branchlines are usually 20-30 m in length, al- 

 lowing considerable room for a live, hooked animal to evade 

 predators or scavengers. Or, scavengers may be attracted 

 by immobile and dead animals. The scavenger avoidance 

 hypothesis is attractive, but it is difficult to test with ob- 

 server data. Data from hook-timer experiments may help 

 to estimate the total number of animals that are lost or 

 removed from the longline. Data presented by Boggs ( 1992 ) 

 showed a large number of hook-timers that were triggered 

 but which did not hold an animal when the branchline was 

 retrieved, e.g. his data show that 2-4"7r of hook-timers on 

 10,236 branchlines that had "settled" were activated but 

 did not have an animal. It is unclear whether the trigger- 

 ing of hook-timers was due to equipment malfunction or 

 whether it represents high loss rates. Of particular signifi- 

 cance to the question of loss rates is the fact that current 

 hook-timer technology does not identify the species that 

 were lost and whether they were alive or dead. 



We noticed that soak-time coefficients tended to be poorly 

 correlated between fisheries and that the effects of soak 

 time on catch rates were most pronounced in the South Pa- 

 cific bluefin tuna fishery. Our scavenging hypothesis might 

 explain those observations as evidence that the activities of 

 scavengers vary between fisheries. For example, blue shark 

 might be an important scavenger. They are most abundant 

 in temperate areas (Last and Stevens, 1994). Our analyses 

 showed a predominance of negative soak-time coefficients 



