179 



Abstract— Our analyses of observer 

 records reveal that abundance esti- 

 mates are strongly influenced by the 

 timing of longline operations in rela- 

 tion to dawn and dusk and soak time — 

 the amount of time that baited hooks 

 are available in the water. Catch data 

 will underestimate the total mortal- 

 ity of several species because hooked 

 animals are "lost at sea." They fall off, 

 are removed, or escape from the hook 

 before the longline is retrieved. For 

 example, longline segments with soak 

 times of 20 hours were retrieved with 

 fewer skipjack tuna and seabirds than 

 segments with soak times of 5 hours. 

 The mortality of some seabird species 

 is up to 45% higher than previously 

 estimated. 



The effects of soak time and timing 

 vary considerably between species. 

 Soak time and exposure to dusk periods 

 have strong positive effects on the catch 

 rates of many species. In particular, the 

 catch rates of most shark and billfish 

 species increase with soak time. At the 

 end of longline retrieval, for example, 

 expected catch rates for broadbill 

 swordfish are four times those at the 

 beginning of retrieval. 



Survival of the animal while it is 

 hooked on the longline appears to be an 

 important factor determining whether 

 it is eventually brought on board the 

 vessel. Catch rates of species that 

 survive being hooked (e.g. blue shark) 

 increase with soak time. In contrast, 

 skipjack tuna and seabirds are usu- 

 ally dead at the time of retrieval. Their 

 catch rates decline with time, perhaps 

 because scavengers can easily remove 

 hooked animals that are dead. 



The results of our study have impor- 

 tant implications for fishery manage- 

 ment and assessments that rely on 

 longline catch data. A reduction in soak 

 time since longlining commenced in the 

 1950s has introduced a systematic bias 

 in estimates of mortality levels and 

 abundance. The abundance of species 

 like seabirds has been over-estimated 

 in recent years. Simple modifications 

 to procedures for data collection, such 

 as recording the number of hooks 

 retrieved without baits, would greatly 

 improve mortality estimates. 



Manuscript approved for publication 

 22 September 2003 by Scientific Editor. 



Manuscript received 20 October 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:179-195 (2004). 



Fish lost at sea: the effect of soak time 

 on pelagic longline catches 



Peter Ward 



Ransom A. Myers 



Department of Biology 



Dalhousie University 



Halifax, B3H 4JI Canada 



E-mail address (for P Ward) wardiSmathstat.dal ca 



Wade Blanchard 



Department of Mathematics and Statistics 

 Dalhousie University 

 Halifax, B3H 44 Canada 



Our knowledge of large pelagic fish in 

 the open ocean comes primarily from 

 tagging and tracking experiments and 

 from data collected from longline fish- 

 ing vessels since the 1950s. Abundance 

 indices for pelagic stocks are often 

 derived from analyses that model catch 

 as a function of factors such as year, 

 area, and season. However, the amount 

 of time that baited hooks are available 

 to fish is likely to be another important 

 factor influencing catch rates (Deriso 

 and Parma, 1987). 



The activity of many pelagic animals 

 and their prey vary with the time of 

 day. Broadbill swordfish, for example, 

 feed near the sea surface at night. They 

 move to depths of 500 m or more during 

 the day (Carey, 1990). Other species may 

 be more active in surface waters during 

 the day (e.g. striped marlin) or at dawn 

 and dusk (e.g. oilfish). Longline fishing 

 crews take a keen interest in the tim- 

 ing of their fishing operations and soak 

 time (the total time that a baited hook 

 is available in the water). However, as- 

 sessments have not accounted for those 

 factors in estimating the abundance 

 or mortality levels of target species or 

 nontarget species. 



In many assessments that use pelagic 

 longline catch rates, fishing effort is as- 

 sumed to be proportional to the number 

 of hooks deployed. The effects of soak 

 time and timing may have been omit- 

 ted because a clear demonstration of 

 their effects on pelagic longline catch 

 rates is not available. The few pub- 

 lished accounts on soak time in pelagic 

 longline fisheries have been based on 



limited data and a few target species. 

 For example, in analyzing 95 longline 

 operations or "sets" by research vessels 

 Sivasubramaniam ( 1961) reported that 

 the catch rates of bigeye tuna increased 

 with soak time, whereas yellowfin tuna 

 catch rates were highest in longline seg- 

 ments with intermediate soak times. 



In contrast to the limited progress in 

 empirical studies, theoretical approach- 

 es are well developed for modeling fac- 

 tors that may influence longline catch 

 rates. Soon after large-scale longlining 

 commenced. Murphy (1960) published 

 "catch equations" for adjusting catch 

 rates for soak time, bait loss, escape, 

 hooking rates, and gear saturation. He 

 suggested that escape rates could be es- 

 timated from counts of missing hooks 

 and hooks retrieved without baits. 

 Unfortunately, such data are rarely col- 

 lected from pelagic longline operations. 



More recently, hook-timers attached 

 to longline branchlines have begun to 

 provide information on the time when 

 each animal is hooked and also whether 

 animals are subsequently lost, e.g. 

 Boggs (1992), Campbell et al. 1 - 2 Such 

 data are particularly useful to under- 



1 Campbell, R., W. Whitelaw, and G. Mc- 

 Pherson. 1997. Domestic longline fish- 

 ing methods and the catch of tunas and 

 non-target species off north-eastern 

 Queensland (1st survey: October-Decem- 

 ber 1995). Report to the Eastern Tuna 

 and Billfish Fishery MAC. 71 p. Aus- 

 tralian Fisheries Management Authority, 

 PO Box 7051, Canberra Business Centre, 

 ACT 26 10, Australia. 



2 See next page. 



