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Rapid recovery of exhausted adult coho salmonafter commercial capture by troll fishing

Anthony P. Farrell, Patricia E. Gallaugher, and Richard Routledge

Abstract: To reduce mortality in the by-catch of commercial salmon fisheries, techniques are being explored to revivefish before live release and improve survival. By measuring blood and muscle variables, we demonstrated that captureof coho salmon (Oncorhynchus kisutch) by commercial trolling methods resulted in severe exhaustion and stress, e.g.,muscle lactate reached 46.1 mmol·kg–1 while muscle phosphocreatine (PCr) decreased to 6.1 mmol·kg–1. Nevertheless,coho salmon recovered rapidly by swimming in a cage alongside the vessel while fishing activity continued. In particu-lar, there were significant increases in muscle glycogen and PCr levels, and a decrease in muscle lactate after twohours. Notably, and in contrast to when exhausted fish are held stationary during recovery, plasma lactate remained low(<4 mmol·L–1) during recovery, a phenomenon observed in earlier laboratory studies with rainbow trout (Oncorhynchusmykiss). There was no postcapture delayed mortality after 24 h. Therefore, we have established that postexhaustion ac-tivity promotes a rapid recovery in wild salmon and this result might find application in nonretention commercial andrecreational fishing.

Résumé: On est actuellement à la recherche de techniques pour diminuer la mortalité des prises accessoires dans lapêche commerciale au saumon, en réanimant les poissons avant de les libérer pour augmenter leur survie. La mesuredes variables du sang et des muscles nous a permis de démontrer que la capture des Saumons coho (Onchorhynchuskisutch) par les méthodes commerciales de pêche à la traîne produit un épuisement et un stress graves; e.g., laconcentration de lactate dans le muscle atteint 46,1 mmol·kg–1, alors que celle de la phosphocréatine du muscle estréduite à 6,1 mmol·kg–1. Néanmoins, les Saumons coho se sont rétablis rapidement en nageant dans une cage fixée surle côté du navire, alors que la pêche se poursuivait. Il y avait, en particulier, des augmentations significatives desconcentrations de glycogène et de phosphocréatine dans les muscles et une réduction du lactate musculaire après deuxheures. Contrairement aux poissons épuisés gardés au repos durant la récupération, les poissons actifs ont maintenu desconcentrations de lactate plasmatique basses (<4 mmol·L–1) durant la récupération, un phénomène observéantérieurement en laboratoire chez la Truite arc-en-ciel (Oncorhynchus mykiss). Il n’y avait pas de mortalité différée24 h après la capture. Nous avons donc conclu qu’une activité après l’épuisement favorise une récupération rapide chezles saumons sauvages; nos résultats peuvent sans doute s’appliquer à la gestion des poissons qui ne sont pas retenusdans les pêches commerciales et sportives.

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Introduction

Commercial fisheries worldwide are estimated to discard38% of fish caught, i.e., they are thrown back with noattempt at revival (Alverson et al. 1994). Of those fish notalready dead, many may either suffer postcapture delayed

mortality, be captured by predators, or because of theirweakened state, be recaptured by commercial fishers. TheCanadian selective fishing policy, instituted in 1998 to con-serve certain salmon stocks that are at low levels, includesrevival and nonretention of nontarget coho salmon (Onco-rhynchus kisutch).

In collaboration with Fisheries and Oceans Canada, wehave been assessing coho salmon revival on board commer-cial salmon fishing vessels in British Columbia (Farrell et al.2000, 2001; S. Buchanan, A.P. Farrell, J. Fraser, P.E.Gallaugher, R. Joy, and R. Routledge, unpublished data).“Blue box” revival tanks are presently used on board gillnetvessels because they can promote low 24-h delayed mortal-ity rates (Farrell et al. 2000), even though they were ratherineffective at promoting physiological recovery on boardgillnet, seine, and troll vessels, in that there was no signifi-cant recovery of muscle concentrations of phosphocreatine(PCr), glycogen, and lactate after 1 h. A modified revival

Can. J. Fish. Aquat. Sci.58: 2319–2324 (2001) © 2001 NRC Canada

2319

DOI: 10.1139/cjfas-58-12-2319

Received September 28, 2001. Accepted November 9, 2001.Published on the NRC Research Press Web site athttp://cjfas.nrc.ca on November 23, 2001.J16554

A.P. Farrell.1 Department of Biological Sciences, SimonFraser University, Burnaby, BC V5A 1S6, Canada.P.E. Gallaugher. Continuing Studies, Simon FraserUniversity, Burnaby, BC V5A 1S6, Canada.R. Routledge.Department of Statistics and Actuary Science,Simon Fraser University, Burnaby, BC V5A 1S6, Canada.

1Corresponding author (e-mail: [email protected]).

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tank in which there is a laminar flow of water over the fish(a Fraser box) is accepted as an improvement over the bluebox because when used in conjunction with modified gillnetgear and methods, the Fraser box promotes significant re-covery of muscle PCr and lactate concentrations in 1 h andrestores swimming performance, as well as keeping 24-h de-layed mortality rates low (Farrell et al. 2001; S. Buchanan,A.P. Farrell, J. Fraser, P.E. Gallaugher, R. Joy, and R.Routledge, unpublished data). The present study reports onanother successful revival technique for coho salmon. Troll-caught fish were transferred to and revived in a submergedcage attached to the side of the vessel while fishing activitycontinued. Recovery in a submerged cage had an additionalbenefit over the blue box and the Fraser box of maintainingplasma lactate at very low levels during recovery.

Materials and methods

Coho salmon were caught with commercial troll gear using bar-bless hooks by the vessel Sherry Shan in Barkley Sound, B.C., Can-ada during 1998 and 1999. Fish were either sacrificed for tissuesampling immediately as they reached the surface or quickly trans-ferred without air exposure to an aluminum-framed cage attached tothe side of the troll vessel. The cage (~1.22 m deep) was almostcompletely submerged and had light canvas sides and a see-throughmesh top. A hinged door on top of the cage swung open to allowfish to be flicked off of the hook and into the cage. Fishing activitiescontinued with the vessel moving at a variable speed of 1–2 knots.Therefore, to keep pace with the vessel, the fish (60–80 cm inlength) swam at speeds of ~0.75–1.5 fish body lengths·s–1 (bl·s–1).After a recovery period of about 2 h, fish were either removed fromthe cage with a dipnet for tissue sampling or transferred to a com-mercial netpen (12 m × 12 m × 12 m) to complete a 24-h recovery.For transfer, the cage was detached from the fishing vessel and theback end was slid into the netpen. A vertical panel in the back endof the cage was lifted and the front end was raised, allowing the fishto swim into the netpen. Water temperature ranged between 12°Cand 16°C.

Blood and muscle samples were taken to assess physiologicalstatus. Tissue collection and analytical techniques followed thosefully described in Farrell et al. (2000, 2001). Briefly, fish werestunned at the water level by a blow to the skull. Then a musclesample from a location below and slightly anterior to the dorsal finwas quickly (in <35 s) removed and frozen between precooledmetal tongs. The frozen tissue was placed in aluminum foil andstored on dry ice. Directly following the muscle biopsy, blood wasdrawn by caudal puncture into a 3-mL heparinized vacutainer andtwo heparinized hematocrit tubes were filled immediately from theblood sample. Plasma aliquots were frozen on dry ice. Skeletalmuscle and plasma samples were subsequently stored at –80°Cprior to analysis in the laboratory. Plasma lactate and glucose con-centrations were measured using a YSI 2300 STAT Plus glucoseand lactate analyzer (YSI Inc., Yellow Springs, Ohio). Plasmacortisol was measured in duplicate after suitable dilution using acommercial ELISA kit (enzyme-linked immunosorbent assay,Neogen Co., Lexington, Ky.). Plasma chloride concentrations weremeasured in duplicate using a model 4425000 digital chlorido-meter (Haake Buchler Instruments, Saddle Brook, N.J.). Concen-trations of plasma sodium and potassium ions were measured usinga model 510 Turner flame photometer (Palo Alto, Calif.). Plasmaosmolality was measured using a model 5500 vapour pressure me-ter (Vapro, Wescor, Logan, Utah). Muscle glucose and lactate con-centrations were determined with perchloric acid extracts using theglucose and lactate analyzer, and glycogen was determined asglucosyl units after digestion with amyloglucosidase. Muscle PCr

and adenosine triphosphate (ATP) concentrations were determinedenzymatically. All procedures were approved by the Simon FraserUniversity Animal Care Committee in accordance with the Cana-dian Council on Animal Care.

Results and discussion

Analysis of plasma and skeletal muscle samples clearlyshowed that coho salmon were severely stressed and meta-bolically exhausted immediately after capture (Fig. 1). Im-portantly, muscle lactate (46.1 mmol·kg–1) was ~40% higherthan that observed in exhausted rainbow trout (Onco-rhynchus mykiss) in laboratory studies (Milligan 1996;Kieffer 2000; Milligan et al. 2000) and similar to the levelsobserved in angled wild Atlantic salmon (Salmo salar)(Booth et al. 1995).

Despite being severely exhausted, coho salmon showedsignificant metabolic recovery after a 2-h period whileswimming alongside the fishing vessel (Fig. 1). Specifically,muscle glycogen increased threefold (from 4.0 to12.7 mmol·kg–1), muscle PCr increased more than twofold(from 6.1 to 15.0 mmol·kg–1), and muscle lactate decreasedtwofold (from 46.1 to 21.2 mmol·kg–1) (Fig. 1). Plasma lac-tate decreased significantly to 3.6 ± 0.5 from 6.6 ±1.4 mmol·L–1. This result contrasted with our earlier work(Farrell et al. 2000, 2001), where plasma lactate increased toabout 20 mmol·L–1 during revival. Plasma cortisol and chlo-ride concentrations increased significantly (Fig. 1).

These findings clearly show that wild adult salmon cap-tured by commercial troll gear can be revived while the fish-ing vessel continues its fishing activity at sea. Consequently,a cage beside a troll fishing vessel, similar to a Fraser boxon board a gillnet vessel (Farrell et al. 2001), provides aneffective means to promote physiological recovery of com-mercially caught coho salmon. The submerged cage offeredan advantage over the Fraser box of keeping the plasma lac-tate concentration low during recovery. By allowing the fishto recover muscle metabolites before release, we anticipatethat fish are better able to avoid predators than if they werereleased directly to the sea in an exhausted state. Theremight be an added benefit of reducing delayed mortality, butthis would be hard to verify experimentally.

Revived fish (N = 14 for 2-h recovery;N = 14 for 24-h re-covery) showed no postcapture delayed mortality after 24 h.This compares very favorably with the delayed mortalityrates of ~2% with both the blue box and the Fraser box.With the exception of plasma sodium, the physiological pro-files of the plasma and muscle tissue were essentially thesame in fish sampled after 24-h and 2-h recovery periods(Fig. 1). By using a dipnet for sampling we may have under-estimated the recovery of variables such as muscle PCr, lac-tate, and glycogen because of fish activity. Also, holdingadult salmon in large pens created additional difficulties inobtaining fish without altering their physiological statusthrough activity.

It was not possible to obtain samples from fully restedadult coho salmon on board a commercial vessel, so an evalu-ation of the extent of the physiological recovery depends oncomparisons with other studies. Also, a comprehensive litera-ture review of variables in connection with the Fraser box waspublished earlier (Farrell et al. 2001). Therefore, rather than

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repeat this comparison, we have compared the results of fiverecovery methods graphically; three methods involved com-mercially caught adult coho salmon and two methods in-

volved laboratory studies with rainbow trout (Fig. 2). For thiscomparison, and where possible for each revival method, wehave presented the absolute difference of the recovered vari-

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Fig. 1. (a) Physiological status of skeletal muscle and (b, c) plasma in mature wild coho salmon (Oncorhynchus kisutch) sampled aftercapture by commercial troll fishing methods. Sampling was performed immediately after capture (0 h), after a 2-h recovery periodwhile swimming in a cage beside the fishing vessel (+2 h), and after completing a 24-h recovery in a large netpen (+24 h). Hematocrit(39.8 ± 1.5%;N = 41) and muscle glucose (1.30 ± 0.16 mmol·kg–1; N = 38) were also measured. Mean values are presented forN =11 to 14 fish, except the ATP value for 0 h whereN = 4. The standard error of the mean is indicated by the vertical line. Statisticallysignificant differences (P < 0.05) from the initial samples are indicated by an asterisk. Statistical tests for differences were donethrough an analysis of variance (JMP, SAS Institute Inc., Cary, N.C.) with a 5% significance level. All analyses were followed by ap-propriate multiple comparison tests to determine which means were significantly different (with Tukey or Bonferroni adjustments to re-duce the experimentwise error rate to at most 5%).



able from an average resting value obtained for rainbow trout(Milligan 1996). Ideally, the smaller the absolute difference,the better the recovery. (Note: comparisons could not be made

among ionic and osmotic variables because this would in-volve a comparison between freshwater and saltwater fish.)The temperature ranges of the five studies overlapped, but we

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Fig. 2. (a) Absolute differences of plasma variables and (b) muscle variables from a control value for various methods of revivingsalmonids after exhaustive activity. The smaller the absolute difference from the control values (based on average resting values forrainbow trout; Milligan 1996), the smaller the metabolic disturbance remaining for that particular method. From left to right, the fishrevival methods were as follows: the present study in which commercially caught wild coho salmon were sampled after swimming for2 h beside a troll fishing vessel (solid bar, + 2 h troll swim; 12–16°C); rainbow trout sampled 2 h after being chased to exhaustion ina laboratory and allowed to swim moderately during the recovery period (hatched bar, + 2 h trout swim; 12–16°C; Milligan et al.2000) (values for plasma potassium and glucose and muscle PCr and ATP were unavailable); commercially caught wild coho salmonthat were sampled after being revived for 1 h in a blue box onboard a fishing vessel (shaded bar, + 1 h blue box; 12–19°C; Farrell etal. 2000) (muscle ATP value unavailable); commercially caught wild coho salmon that were sampled after being revived for 2 h in aFraser box on board a gillnet fishing vessel (hatched bar, + 2 h Fraser box; 12–16°C; Farrell et al. 2001) (muscle ATP valueunavailable); rainbow trout sampled 2 h after being chased to exhaustion in a laboratory and held stationary in a recovery box (openbar, + 2 htrout static; 12–16°C; Milligan 1996; Milligan et al. 2000; C.L. Milligan, Department of Zoology, University of WesternOntario, London, Ont., personal communication) (muscle ATP value unavailable).



cannot exclude temperature differences between experimentsas a possible confounding factor in the comparison of recov-ery methods. We note that the rates of recovery of musclelactate, glycogen, and ATP are all temperature dependent,while peak muscle lactate and rate of recovery of PCr are in-dependent of temperature (Kieffer 2000).

The comparison presented in Fig. 2 shows that, comparedwith coho salmon revival in a blue box, physiological recov-ery was much better when coho salmon recovered eitherwhile swimming in a cage or when placed in a Fraser box.Only a 1-h recovery period was examined earlier with theblue box and this complicates the comparison with the pres-ent study where only a 2-h recovery was examined. Even so,the physiological recovery observed after 1 h with a Fraserbox was not observed with the blue box after 1 h (e.g., mus-cle lactate was 53.9 mmol·kg–1 for a blue box comparedwith 39.9 mmol·kg–1 for the Fraser box, and muscle PCr was2.5 mmol·kg–1 for the blue box compared with12.2 mmol·kg–1 for the Fraser box), thus the differences wenote are unlikely to be a consequence of a longer recoveryperiod. The blue box also performed more poorly in revivingasphyxiated fish (a >50% delayed mortality rate versus a<7% mortality rate for the Fraser box) (Farrell et al. 2001).Thus, when compared with the blue box recovery method,coho salmon were revived faster, to a greater extent, andwith lower delayed mortality rates using both a Fraser boxon board a gillnet vessel and caged fish alongside a troll ves-sel.

Although commercially caught coho salmon showed greatermuscle lactate levels than rainbow trout in laboratory experi-ments, coho salmon held in a cage alongside a troll vesselcould recover certain metabolic variables to the same extentas rainbow trout (Fig. 2). In fact, plasma lactate remainedlower in swimming coho salmon than in static rainbow trout.Nonetheless, commercially caught coho remain severelystressed during recovery because cortisol levels were ele-vated to a much greater degree.

Figure 2 shows that coho salmon swimming in a cagealongside a troll vessel recovered as well as or better thancoho salmon held for 2 h in aFraser box. Besides enhancedrecovery of muscle metabolites there were lower levels forplasma lactate and cortisol for swimming coho salmon(Fig. 2) as well as 0% versus 2.3% delayed mortality after24 h (Farrell et al. 2001). We believe that an enhanced re-covery compared with the Fraser box relates directly tomoderate swimming activity during recovery, notwithstand-ing the important differences in capture methods betweenthe two studies. This conclusion is consistent with a recentlaboratory study on rainbow trout (Milligan et al. 2000) inwhich moderate continuous swimming activity (0.9 bl·s–1 ina swimming flume) promoted a threefold faster recovery inrainbow trout (Fig. 2). Although we could not control swim-ming speed, we estimate that the coho salmon swam at aspeed (0.75–1.5 bl·s–1) comparable to that used earlier withrainbow trout. Furthermore, we can now identify two physi-ological characteristics shared by fish that swim during theirrecovery. First, plasma lactate remains very low(<4 mmol·L–1 in this study; <5 mmol·L–1 for rainbow trout;<4 mmol·L–1 for sockeye salmon (Oncorhynchus nerka):Farrell et al. 1998) compared with stationary fish (10–15 mmol·L–1 for rainbow trout and 24.6 mmol·L–1 for coho

salmon; Fig. 2). Second, plasma cortisol levels remain lower(Fig. 2). Although absolute plasma cortisol levels weremuch higher in coho salmon than in rainbow trout (perhapsas a result of the greater degree of initial stress or becausemature Pacific salmon can have much higher cortisol levels),plasma cortisol did not increase nearly as much in swimmingversus stationary coho salmon (Fig. 2). Thus, moderatepostexercise activity, the equivalent of a “cool-down” periodfor ectothermic fish, appears to have substantial benefits tothe recovery process in salmonids and brings into doubt thelongstanding belief that metabolic recovery in severely ex-hausted fish is necessarily prolonged (e.g., Black 1957;Dobson and Hochachka 1987; Milligan 1996).

An association was made many years ago between ele-vated plasma lactate levels in exhausted fish and delayedmortality. Subsequently, Wardle (1978) showed that an in-jection of adrenaline in exhausted plaice (Pleuronectesplatessa) reduced the appearance of lactate in the plasmaand decreased the occurrence of delayed mortality. In thepresent study, we provided a simpler means of getting thesame results. Mild exercise post-exhaustion appears to be theanswer to reducing lactate release into the plasma and pre-venting delayed mortality in salmon.

The exact mechanisms that keep plasma lactate and cortisollow and prevent postcapture delayed mortality are unre-solved at this time but have been discussed by Milligan et al.(2000), who suggested a relationship between the plasmacortisol and lactate dynamics. Interestingly, swimming rain-bow trout recovered muscle lactate to a lower level(~10 mmol·kg–1) than did coho salmon (21.2 mmol·kg–1)and with a lower plasma cortisol level (Fig. 2). This differ-ence again may reflect a linkage between plasma cortisoland lactate dynamics, but we cannot exclude the confound-ing effect of dipnetting fish in open ocean conditions andtaking samples without the aid of anesthesia or cannulation.Although the higher muscle lactate load in coho salmoncould have been a factor, with the intramuscular conversionof lactate back to glycogen being a rate-limiting process, themuscle lactate load in both studies decreased by 21–25 mmol·kg–1 during a 2-h recovery period (from 46 to21 mmol·kg–1 versus from 32 to 10 mmol·kg–1).

In summary, revival of commercially caught nontarget cohosalmon prior to release could yield dividends for fish conser-vation. Consideration, however, must be given to the type ofrecovery conditions, because they can clearly influence therate of recovery. We have established that moderate activitypromotes rapid recovery of exhausted wild salmon and hopethat this result finds application in nonretention commercialand recreation fishing. The absence of delayed mortality af-ter 24 h in the present study is an important new finding be-cause the commercial Pacific troll fishing fleet is currentlyassigned a mortality rate of 25% for coho salmon by-catch,which is based, in part, on earlier scientific studies that re-ported delayed mortality rates of 34–52% for coho and 40–80% for chinook salmon (Oncorhynchus tshawytscha) fromtroll fishing (Parker et al. 1959). Armed with this knowl-edge, both recreational and commercial fishers should be en-couraged to further develop methods that optimize recoveryof their by-catch prior to release, thereby increasing thechances of fish survival. Furthermore, if future work candemonstrate that this type of rapid recovery is associated

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with a restored swimming and migratory ability and withoutimpairment of reproductive potential, major progress willaccrue for future salmon conservation efforts.

Acknowledgements

We wish to thank the skipper and crew of the Sherry Shanfor assisting us with the field sampling. Special thanks go toDanielle Pike, Petra Heppna, and Kim Vanderhoek for excel-lent field and laboratory technical assistance, to AlisonHadwin for statistical analyses, and to Wade Parkhouse andJohn Blackmore for the use of their analytical facilities. Wealso want to express our appreciation of the advice and as-sistance received from Gordon Curry, Brent Hargreaves, andDon Lawseth of Fisheries and Oceans Canada. Funding wasprovided by Fisheries and Oceans Canada and by NaturalSciences and Engineering Research Council of Canada(NSERC) grants to R. Routledge and A.P. Farrell.

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