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Fishery Bulletin 102(1) 



hooks retrieved after 1 5 hours 

 have many swordfish 



soak times 

 not observed 



hooks retrieved before 6 hours 

 have few swordfish 



10 



15 



Soak time (h) 



No captures after deployment 

 Soak time coefficient <0 

 e.g. seabirds 



Captures exceed losses 

 Soak time coefficient >0 

 e.g. swordfish 



Losses eventually exceed captures 

 Soak time coefficient <0 

 e.g. skipjack 



Captures balance losses 

 Soak time coefficient -0 

 eg lancetfish 



20 



Figure 2 



Illustration of different patterns in the theoretical relationship between longline catch rates and soak time. The 

 probability of an animal being on a hook when a branchline is retrieved (the "catch rate") is estimated from 

 Equation 4 by using soak times iT) ranging from to 20 hours and three different combinations of values forP n 

 (probability of capture), « (capture rate), and /3 (loss rate). For seabirds, the probabilities were estimated from 

 Equation 6. The probabilities are not on the same scale for all species. 



Another approach might be to fit separate logistic regres- 

 sions to each operation and then to combine the parameter 

 estimates. This would overcome the problems of normality 

 and homogeneity of variance. However, the separate re- 

 gressions would not incorporate information that is com- 

 mon to all operations. 



Instead, we used a logistic regression with random ef- 

 fects. The key advantage in using random-effects models 

 in this situation is that they carry information on the cor- 

 relation between longline segments that is derived from 

 the entire data set of operations. 



Data and methods 



Fisheries 



We analyzed observer data from six different fisheries in 

 the Pacific Ocean to determine the effects of soak time 

 and timing on longline catch rates (Table 1, Fig. 3). These 

 fisheries involve two different types of longline fishing 

 operation: 1 ) distant-water longlining involves trips of 

 three months or longer and the catch is frozen on board 



the vessel; and 2) fresh-chilled longlining, which involves 

 small vessels (15-25 m) undertaking trips of less than four 

 weeks duration, and the catch is kept in ice, ice slurries, or 

 in spray brine systems. The fresh-chilled longliners deploy 

 shorter longlines with fewer hooks (-1000 hooks) than 

 the distant-water longliners (-3000 hooks per operation) 

 (Ward, 1996; Ward and Elscot, 2000). 



The six fisheries share many operational similarities, 

 such as the types of bait used and soak time. However, 

 they are quite different in terms of targeting, which is 

 determined by fishing practices, e.g. the depth profile of 

 the longline, timing of operations and the area and season 

 of activity. South Pacific bluefin tuna longliners operate in 

 cold waters ( 10-16°C) in winter to catch southern bluefin 

 tuna. In the South Pacific yellowfin tuna longliners tar- 

 get tropical species, such as yellowfin and bigeye tuna, in 

 warmer waters (19-22°C) (Ward, 1996). To target bigeye 

 tuna, longlines in the Central Pacific bigeye fishery are 

 deployed in the early morning with hook depths ranging 

 down to about 450 m. The depths of the deepest hook are 

 much shallower (-150 m) in the North Pacific swordfish 

 fishery where the longlines are deployed late in the after- 

 noon and retrieved early in the morning (Boggs, 1992). 



