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Fishery Bulletin 102(1) 



and observer programs may provide estimates of those pa- 

 rameters. Broad limits for the probability of capture may 

 also be obtained if observers were to report the number of 

 branchlines that are retrieved with missing baits or miss- 

 ing hooks. 



For most species, capture rates must balance or outweigh 

 loss rates. In this case, captures result from the increased 

 exposure of animals to the longline as a result of movement 

 and, perhaps, the dispersal of chemical attractants during 

 the operation. However, we must stress that losses are also 

 likely to be occurring for the species that have positive co- 

 efficients. The analyses indicate the relative levels of loss 

 between longline segments of varying soak time. Other 

 than those for seabirds, we cannot estimate the levels of 

 catch that are lost. 



Adding to the uncertainty over loss rates is the unknown 

 fate of lost animals. For seabirds it is known that most 

 drown soon after being hooked. The few seabirds that sur- 

 vive while hooked eventually drown during longline re- 

 trieval (Brothers, 1991). However, it is not known whether 

 other lost animals are dead or alive. 



Results of our analyses may also be useful for monitoring 

 programs. Observers are increasingly being placed on long- 

 liners to collect data on bycatch and to independently verify 

 data reported in logbooks. A sampling approach is neces- 

 sary in some fisheries because observers are often unable 

 to monitor the entire longline retrieval. Indications that 

 catch rates of some species at the end of the retrieval are 

 double those at the beginning necessitate care in designing 

 observer monitoring protocols and in the interpretation of 

 the data. Observers could also collect information on the 

 number of hooks retrieved without baits. Such data would 

 greatly improve the estimates of a and fi required for the 

 theoretical model. For the empirical model, catch rate data 

 from research surveys where longline segments have very 

 short (<4 hour) soak times would improve estimates of 

 soak-time coefficients. 



Historical changes 



The interaction of year and soak time was rarely significant 

 for the random effects models of the seven species exam- 

 ined in detail. This might suggest that soak-time-catch- 

 rate relationships are stable over time. However, the range 

 of years that we analyzed was limited to 1992-97. Over 

 larger time scales there have been large variations in the 

 abundance of individual species and the mix of species 

 comprising the pelagic ecosystem. We cannot predict how 

 soak-time-catch-rate relationships would change with 

 those long-term variations. 



Our original motivation for examining the effects of 

 soak time was the hypothesis that the number of hooks 

 per operation and soak time have increased since longlin- 

 ing commenced and that this may have resulted in an 

 overestimation of billfish catch rates in early years. Ward r ' 

 presented information on temporal trends in soak time 



5 Ward, P. 2002. Historical changes and variations in pela- 

 gic longline fishing operations, http://fish.dal.ca/-myers/pdf 

 papers.html. (Accessed 22 February 2003.1 



and timing for several longline fleets. Although there is 

 uncertainty over the early operations, the available infor- 

 mation indicates significant historical changes in Japan's 

 distant-water longline operations. Average soak time 

 shows a decline from over 11.5 hours before 1980 to 10.0 

 hours in the 1990s. For species with a negative soak-time 

 coefficient, this apparently modest reduction in soak time 

 would inflate catch rate estimates for recent years. It would 

 result in reduced catch-rate estimates for species with posi- 

 tive coefficients. For example, the expected catch rate for 

 swordfish is 0.94 (CI ±0.06) per 1000 hooks for a soak time 

 of 11.5 hours compared to 0.82 (CI ±0.06) per 1000 hooks 

 for 10.0 hours. 



More significant may be changes in the timing of op- 

 erations. During 1960-80 most baits used with Japan's 

 distant-water longliners were available to fish at dawn 

 whereas about 50% were also available at dusk. Longlines 

 were deployed and retrieved at later times in the 1990s so 

 that about 30% of baits were available at dawn and about 

 70% available at dusk. In the case of swordfish, the changes 

 in timing would moderate the effects of reduced soak time. 

 The expected catch rate for swordfish is 0.89 per 1000 

 hooks in the early operations compared to 0.83 per 1000 

 hooks in the later operations. 



Conclusions 



The results have important implications for fishery man- 

 agement and assessments that rely on longline catch 

 data. Modifications to data collection, such as recording 

 the number of hooks with missing baits during longline 

 retrieval, would greatly improve mortality estimates. The 

 mortality of species like seabirds is significantly higher 

 than previously estimated. Such underestimation may be 

 particularly critical for the assessment and protection of 

 threatened species of seabirds. Furthermore, the changes 

 in timing and reduction in soak time have resulted in a 

 systematic bias in estimates of mortality levels and abun- 

 dance indices for many species. For species like swordfish, 

 where soak time has a positive effect on catch rates, the 

 stocks might be in better shape than predicted by current 

 assessments ( if assessments were solely based on catch and 

 effort data). The opposite situation would occur for species 

 with negative soak-time coefficients: assessments that use 

 long time-series of longline catch data will over-estimate 

 the species' abundance so that population declines are 

 more severe than previously believed. 



Acknowledgments 



Grants from the Pew Charitable Trust, Pelagic Fisheries 

 Research Program, and the Killam Foundation provided 

 financial support for this work. Peter Williams (Secretariat 

 of the Pacific Community). U.S. National Marine Fisher- 

 ies Service staff (Kurt Kawamoto, Brent Miyamoto, Tom 

 Swenarton, and Russell Ito ) and Thim Skousen (Australian 

 Fisheries Management Authority) provided observer data 

 and operational information on the fisheries. We are espe- 



