Trnsctions of the Americn Fisheries Society 141:907 918, 2012 C Americn Fisheries Society 2012 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2012.675899 ARTICLE The Effect of Nonntive Slmonids on Socil Dominnce nd Growth of Juvenile Atlntic Slmon Jessic A. Vn Zwol nd Bryn D. Neff* Deprtment of Biology, University of Western Ontrio, 1151 Richmond Street, London, Ontrio N6A 5B7, Cnd Chris C. Wilson Aqutic Reserch nd Development Section, Ontrio Ministry of Nturl Resources, Trent University, 2140 Est Bnk Drive, Peterborough, Ontrio K9J 7B8, Cnd Abstrct Nonntive species hve been shown to negtively impct the ntive community in which they re introduced. In the Gret Lkes, competition with nonntive slmonids my be hindering the restortion efforts of Atlntic slmon Slmo slr, once-ntive top predtor in Lke Ontrio. We exmined the effects of brown trout S. trutt nd rinbow trout Oncorhynchus mykiss, two nonntive fishes in Lke Ontrio, on the socil dominnce nd growth rte of juvenile Atlntic slmon from three strins being used for reintroduction efforts in Lke Ontrio. Using seminturl strem chnnels, we found tht the presence of either rinbow trout or brown trout reduced ggression, dominnce, nd food consumption of the Atlntic slmon. Brown trout hd the strongest effect, incresing ggression levels in the chnnels by fctor of two nd shrply reducing the dominnce of Atlntic slmon. When both nonntives were present, initited ggression by Atlntic slmon decresed by fctor of three nd food consumption hlved s compred with when the slmon were lone. Consequently, over 7-d time period, stndrd growth rte of the Atlntic slmon dropped from no chnge in mss when lone to vlue of 0.3% per dy when with the nonntive species. Of the three strins tested, one strin ws lest ffected by the nonntive trouts, implicting genetic differences mong the strins nd suggesting tht one strin my hve greter poststocking success in Lke Ontrio tributries with nturlized popultions of brown trout nd rinbow trout. The introduction of nonntive species cn negtively ffect individuls through competition nd displcement (Hmilton et l. 1999), s well s entire communities or ecosystems by ltering productivity nd nutrient cycling (D Antonio nd Vitousek 1992). Reductions in biodiversity nd bundnce of biot within the ecosystem typiclly follow (D Antonio nd Vitousek 1992; Olden et l. 2004). For exmple, estblishment of nonntive species like the zebr mussel Dreissen polymorph nd Eursin wter milfoil Myriophyllum spictum hve shrply decresed species diversity in impcted ntive communities with drmtic species loss (Mdsen et l. 1991; Riccirdi et l. 1998). Nonntive species often overwhelm ecosystems leving ntive species to cope or risk extirption (Riccirdi et l. 1998). Among fishes, slmonids re mong the most widely introduced species round the world (Crwford nd Muir 2008). Indeed, Edge et l. (1993) nd Dewld nd Wilzbch (1992) showed tht ntive fishes fed less in the presence of brown trout Slmo trutt, while Nkno et l. (1998) found tht introduced brook trout Slvelinus fontinlis shifted forging frequency, microhbitt selection, nd rection distnces of ntive bull trout Slvelinus confluentus. Similrly, Kitno (2004) found tht rinbow trout Oncorhynchus mykiss, brown trout, nd brook trout hve reduced ntive popultions of whitespotted *Corresponding uthor: bneff@uwo.c Received April 26, 2011; ccepted Jnury 18, 2012 Published online June 20, 2012 907
908 VANZWOLETAL. chr Slvelinus leucomenis, Dolly Vrden Slvelinus mlm, msu slmon (lso known s cherry slmon) Oncorhynchus msou, nd Skhlin timen (lso known s Jpnese huchen) Hucho perryi, either directly by predtion, or indirectly by competition for resources. In fct, rinbow trout nd brown trout hve hd such widespred negtive effects on ntive ecosystems tht they hve been listed mong the top 100 of the world s worst invsive lien species (Lowe et l. 2000). In the Gret Lkes, nonntive slmonids my lso be impcting the restortion efforts of ntive fishes (Crwford 2001). Specificlly, Atlntic slmon Slmo slr ws once ntive top predtor in Lke Ontrio but ws extirpted t the end of the 19th century through combintion of hbitt loss nd exploittion (McCrimmon 1977). During the pst century, there hve been numerous ttempts to restore this species, but nturlly reproducing popultion hs yet to be estblished (Stnfield nd Jones 2003). Conversely, nonntive rinbow trout, brown trout, chinook slmon O. tshwytsch, coho slmon O. kisutch, nd sockeye slmon O. nerk, hve been routinely stocked in Lke Ontrio tributries to enhnce recretionl fisheries (Crwford 2001; Stewrt nd Schner 2002). Rinbow trout nd brown trout were both introduced strting in the erly 1900s from different popultions cross North Americ nd developed nturlized popultions (McCrimmon 1977; Crwford 2001; Kerr 2006). The presence of these nonntive slmonid species could be dversely ffecting Atlntic slmon restortion efforts (Grieg et l. 2003). Indeed, Scott et l. (2005) exmined brief interctions (<1 d) between some of these nonntive slmonids nd Atlntic slmon nd noted dverse impcts on socil behvior of juvenile Atlntic slmon (lso see Scott et l. 2003). Other reserch reveled tht interctions with rinbow trout my heighten ggression, territorility, nd competition for resources in strem becuse of niche overlp with Atlntic slmon nd, in some cses, these two species hve been considered ecologicl equivlents (Gibson 1981; Hern nd Kynrd 1986). In Europe, brown trout coexist with Atlntic slmon (Armstrong et l. 2003) nd re the more ggressive nd socilly dominnt of the two species (Strdmeyer et l. 2008). Dominnt individuls typiclly hve preferentil ccess to resources, which cn led to incresed growth nd survivorship (e.g., Ens nd Goss-Custrd 1984). In ddition, in Lke Ontrio the presence of rinbow trout, brown trout, nd other nonntive slmonids increses species richness in the strems nd lke which cn increse competition for the sme resources nd likely lters the crrying cpcity for Atlntic slmon simply through density effects (Crwford 2001). Understnding vrition in behvior within species is crucil for determining the role phenotypic differences ply in restortion efforts nd for understnding the impct of nonntive species (Curio 1996; Cro 1999). Knowledge derived from such behviorl studies cn id in creting effective mngement strtegies for the estblishment of species. As such, we exmined the potentil effect tht nonntive nd ecologiclly similr slmonids hve on Atlntic slmon during the juvenile life stge. Using seminturl strem environments, we observed gonistic nd feeding behviors of juvenile Atlntic slmon in the presence of juvenile brown trout nd rinbow trout to determine if their presence hindered Atlntic slmon ggression, food consumption, or growth. Although rinbow trout nd brown trout represent tx tht re subdivided into mjor lineges (e.g., Berntchez 2001; Blnkenship et l. 2011), our study focuses on the impct of the nturlized popultions of these species in the Lurentin Gret Lkes wtershed. We exmined the comprtive performnce of three different strins of Atlntic slmon, (LHve River, Rivière ux Sumons [Lc Sint-Jen], nd Sebgo Lke) tht re being used s prt of lrge-scle effort to reestblish Atlntic slmon in Lke Ontrio (Grieg et l. 2003). Thus, we were ble to look t popultionspecific genetic differences in Atlntic slmon behvior nd performnce when in competition with the nonntive slmonids nd ssess the potentil importnce of performnce differences within nd mong strins for reestblishing this formerly ntive species in Lke Ontrio. METHODS Study species. In this study, brood stocks were used from three Atlntic slmon popultions. Pst restortion efforts hve focused on stocking only one Atlntic slmon strin originting from the LHve River in Nov Scoti (Stnfield nd Jones 2003; Dietrich et l. 2008). This strin ws chosen primrily becuse of its vilbility s broodstock, rther thn specific ecologicl considertions (Grieg et l. 2003). The LHve strin of Atlntic slmon is ndromous, life cycle thought to be different from the originl strin tht inhbited Lke Ontrio, which my hve spent its entire life cycle in freshwter (Blir 1938; Prsons 1973). The freshwter Sebgo Lke strin from Mine is both physiologiclly nd (now) physiclly lndlocked (Wrd 1932; Wtts 1999). Finlly, the Lc Sint-Jen strin from Quebec lives entirely in freshwter much like the originl extirpted strin of Lke Ontrio ws believed to hve been (Blir 1938; Gge 1963). As juvenile mortlity mong slmonids is high (Elliott 1990; Good et l. 2001), restortion efforts in Lke Ontrio stock vrious ge-groups of Atlntic slmon including 1.5-yer-old individuls. Our behviorl trils involved 1.5-yer-old Atlntic slmon (N = 504), brown trout (N = 180), nd rinbow trout (N = 180). All fish were rered from brood stocks estblished by the Ontrio Ministry of Nturl Resources (OMNR). LHve Atlntic slmon (N = 168) nd brown trout were obtined from the OMNR Hrwood Fish Culture Sttion (Hrwood, Ontrio), while Lc Sint-Jen (N = 168) nd Sebgo Lke (N = 168) Atlntic slmon nd rinbow trout cme from the OMNR Normndle Fish Culture Sttion (Normndle, Ontrio). Fish were of the sme ge nd culture history s those routinely stocked in strems feeding Lke Ontrio. As such, the yerlings of the three species differed in size (see below) s they do under locl nturl conditions. Prior to the strt of the experiment, fish were held for
EFFECTS OF NON-NATIVE SALMONIDS ON ATLANTIC SALMON 909 one month t the OMNR Codrington Fisheries Reserch Fcility (Codrington, Ontrio) in flow-through tnks with n verge density of 0.6 fish/l, exposed to nturl light cycle, nd fed trout chow (Corey Aqufeeds, Fredericton, New Brunswick). Experimentl setup. Seminturl strems were used to perform six behviorl trils in blocks between My nd July 2009 t the Codrington htchery. The strems were designed to provide substrte nd flow conditions similr to those used by Atlntic slmon nd trout found in southern Ontrio nd re strems (Gibson 1973; Hern nd Kynrd 1986). Ech strem chnnel hd n overll length of 2.4 m with riffle nd pool section. The upstrem riffle section ws 1.6 m long, 0.4 m high, nd 0.5 m wide with wter depth of pproximtely 0.2 m nd flow velocity of 0.18 ± 0.05 m/s. Substrte in the riffle consisted of 7 10 cm river rock nd two 15 18 cm rocks to provide potentil cover. The riffle section ws followed by pool section mesuring 0.8 m long, 0.8 m high, nd 0.5 m wide. The pool wter depth ws 0.6 m with surfce current of 0.027 ± 0.025 cm/s. Pool substrte consisted of river rock rnging in size from 2 to 10 cm. Wter from the htchery s surfce wter hed pond (grvityfed system) ws piped to the strem chnnels through hedbox inside the htchery, which ensured equl flow to ll strem chnnels. Wter temperture ws 9.8 ± 1.4 C (men ± SD). Strem chnnels were set up in two prllel series of six chnnels ech. Wter flowed from the hedbox through the first two chnnels nd then into subsequent chnnels in both series. Chnnels were connected using two 10-cm PVC pipes, which were covered with wire mesh on one end to prevent the movement of fish between chnnels. Ech tril block ws composed of 12 tretments, with 12 fish per tretment. Ech Atlntic slmon strin underwent four tretments: Atlntic slmon lone (12 fish), Atlntic slmon with brown trout (+BT; 6 slmon, 6 trout), Atlntic slmon with rinbow trout (+RT; 6 slmon, 6 trout), nd finlly, Atlntic slmon with both brown trout nd rinbow trout (+BTRT; 4 slmon, 4 of ech trout species). Density in the strem chnnels ws 10 fish/m 2, which is the upper end of densities found in the field (Frnsen et l. 1993), but by holding density constnt, we were still ble to determine the reltive strengths of intrspecific nd interspecific competition mong ecologiclly similr species (Fusch 1998). The three Atlntic slmon strins were considered seprtely in ll trils (not mixed) in order to independently evlute their comprtive performnce. There were seven tril strt dtes (one tril block hd pir of dtes, due to logisticl constrints t the onset of the experiment, with the commencement of four tretments followed by eight tretments). At the beginning of ech tril, fish were rndomly selected with similr ctch effort nd nesthetized with MS-222 (tricine methnesulfonte). Once sedted, the initil mss nd totl length of ech fish were recorded. In order to observe nd record individul behvior nd feeding, ech fish ws tgged with colored 2-cm vinyl nchor tg (Floy Tg & Mfg., Settle, Wshington). Tgs were pplied using fine fbric gun (Avery Mrk II Fine Fbric Pistol Grip) with mximum insertion depth of 0.95 cm. Tgs were pplied to either the left or right side of the fish just below the dorsl fin to ensure ll the fish within ech chnnel could be uniquely identified. Between fish, the needle ws disinfected with hydrogen peroxide nd rinsed with wter. The fish were relesed into flow-through holding tnk to recuperte before being plced in the pproprite strem chnnel. A rndom number genertor ws used to determine the plcement of ech tretment in the 12 chnnels for ech tril block. Behviorl observtions begn the dy fter the fish were tgged (dy 1) nd continued for 7 d. Behviors were monitored ech dy in both morning nd fternoon session using rig mde up of three high definition cmcorders (Sony HDR- XR200V) set up bove strem chnnel: one cmer bove the pool nd two eqully spced out bove the riffle section. The cmcorder rig could esily be moved from chnnel to chnnel nd ws situted pproximtely 1 m bove the wter. Two rigs were constructed (six cmers totl), which enbled two strem chnnels to be simultneously recorded before moving the rigs to the next pir of chnnels. The fish were given 15 min to cclimte to the presence of the cmcorder rig before recording begn. Aggressive nd feeding behviors were then recorded for 30 min. In the morning session (0800 1230 hours), ech dy for 7 dys, fish were fed trout chow (Corey Aqufeeds) nd frozen bloodworms (Chironomide; Hikri, Jpn). Specificlly, every minute for the first 10 min of recording session, either 50 100 bloodworms or 1 g of trout chow were lterntely relesed t the top nd middle of the strem chnnel, with the current crrying food items through the chnnel to simulte nturl invertebrte drift ( 2% of biomss in ech strem chnnel). Cre ws tken to void being seen by the fish. The fternoon recordings (1400 1830 hours) did not involve food. The order tht chnnels were filmed ws rndomized using rndom number genertor for ech dy. On dy 8 of ech tril, fish were collected from the strem chnnels for finl mss nd length mesurements. Collection of fish begn t the chnnels frthest from the hedbox to prevent disturbnce. Netted fish were sedted with MS-222 before finl msses nd lengths were recorded. The initil nd finl mss mesurements were used to clculte stndrd growth rte (%/d) using the following eqution (Bernier et l. 2004): Stndrd growth rte = 100 [log e (finl mss) log e (initil mss)]/d fed. Video nlysis. Anlysis of the videos focused on ggressive nd feeding behviors. Aggressive behviors monitored comprised chsing, chrging, nd nipping (see Keenleyside nd Ymmoto 1962 for definitions of behviors). Feeding observtions were of the number of items consumed. Aggressive nd feeding behvior dt from 4 dys of ech tril were nlyzed, comprising dy 1, 3, 5, nd 7. Approximtely 864 h of video
910 VANZWOLETAL. were observed in rel time nd pused every time n ction occurred, with ctor, ct, nd recipient recorded. Sttisticl nlysis. Agonistic nd feeding behviors cross the 4 dys were summed nd converted to rte by dividing by the totl time of observtion. Dominnce ws clculted using Dvid s score, which cretes n index for individuls within socil hierrchy bsed on n individul s initited nd received ggressive cts, while ccounting for repeted interctions mong group members (Dvid 1988; see Gmmell et l. 2003 for detils of the clcultion). Differences in initil mss nd totl length of the Atlntic slmon strins were nlyzed using one-wy nlysis of vrince (ANOVA) models, with strin s the min effect. Student s t-tests were used post hoc to determine differences between pirs of strins. Dt of initited ggression, received ggression, nd food consumption were normlized using logrithmic (x + 1) trnsformtion. Next, we conducted liner mixed models to test the effects of strin nd tretment on initited nd received ggression, Dvid s scores, food consumption, nd stndrd growth rte. The interction between strin nd tretment ws included while initil mss ws entered s covrite in the models. Tril block nd chnnel number were entered s rndom effects. We used vrince components covrince structure nd denomintor degrees of freedom were clculted using Stterthwite pproximtion (Stterthwite 1946). When min effects were significnt or significnt interction existed between strin nd tretment, Student s t-tests were used post hoc to determine differences in vribles. To test the effect of dominnce on growth prmeters, liner regression nlysis ws used to compre Dvid s score nd food consumption or stndrd growth rte. To exmine the effect of nonntive trout species on ech Atlntic slmon strin in multivrite spce, we used direct discriminnt function nlysis (DFA; Duntemn 1984). The DFA exmined the vrition in the five ggression nd growth vribles (initited nd received ggression, Dvid s score, food consumption, nd stndrd growth rte) to ssess how the three Atlntic slmon strins clustered when lone or with nonntive trout species by grouping ech strin by the presence or bsence of nonntive trout species (e.g., LHve individuls lone or LHve individuls with nonntives). All nonntive tretments were grouped together for this nlysis. All five dependent vribles were included in the nlysis s predictors nd the pooled within-group structure mtrix ws nlyzed to determine which vribles most strongly correlted with the discriminnt functions. A two-wy ANOVA ws then used to exmine the effects of tretment (lone versus nonntive) nd strin on the first two DFA xes. All sttistics were performed using JMP 4 (version 4.0.2, SAS Institute Inc., Cry, North Crolin), SPSS 16.0 (SPSS Inc., Chicgo), or Microsoft Office Excel 2003 (Microsoft Corportion, Redmond, Wshington). Presented P-vlues re two-tiled probbilities. RESULTS The strins of Atlntic slmon differed significntly from one nother in initil mss nd totl length (mss: F 2, 297 = 71.5, P < 0.001; totl length: F 2, 297 = 37.8, P < 0.001, Tble 1). Atlntic slmon from the Sebgo Lke strin were the lrgest, followed by Lc Sint-Jen fish, while those from the LHve strin were the smllest. Overll, the verge mss of Atlntic slmon ws 40 ± 16 g (men ± SD), while the verge length ws 164 ± 22 mm. Brown trout hd n verge mss of 39 ± 14 g nd length of 151 ± 18 mm, while rinbow trout were on verge 21 ± 10 g nd 126 ± 19 mm in length. Both mss nd length differed mong the three species (initil mss: F 2,861 = 130.7, P < 0.001; length: F 2,861 = 212.5, P < 0.001), with Atlntic slmon being longer but not hevier thn brown trout (length: t 401 = 7.85, P < 0.001; mss: t 365 = 0.72, P = 0.47), while both species were longer nd hevier thn rinbow trout (Atlntic slmon length: t 370 = 21.9, Atlntic slmon mss: t 520 = 19.5, brown trout length: t 358 = 13.0, brown trout mss: t 322 = 15.1; P < 0.001 for ll comprisons). TABLE 1. Summry of phenotypic nd behviorl chrcteristics of three strins of juvenile Atlntic slmon in four tretments in seminturl strem behviorl trils. Mens ± SDs cross ll four tretments to which the strins were exposed (N = 168 in ech strin) re shown. Different lowercse letters indicte significnt differences mong strins (Student s t-test; P < 0.05). Atlntic slmon strins Chrcteristic LHve Lc Sint-Jen Sebgo Lke Initil mss (g) 30.9 ± 11.7 x 36.6 ± 9.7 y 53.2 ± 16.1 z Totl length (mm) 150 ± 22 x 162 ± 15 y 181 ± 18 z Initited ggression/h 2.8 ± 4.3 z 3.6 ± 4.4 z 1.3 ± 2.8 y Received ggression/h 6.8 ± 6.2 z 6.6 ± 5.6 z 3.1 ± 3.2 y Dvid s score 4.7 ± 13.5 3.7 ± 14.8 1.5 ± 7.8 Food consumption (items/h) 10.7 ± 10.5 z 8.8 ± 7.0 zy 7.4 ± 7.3 y Stndrd growth rte (%/d) 0.12 ± 0.99 0.06 ± 0.85 0.15 ± 0.64
EFFECTS OF NON-NATIVE SALMONIDS ON ATLANTIC SALMON 911 TABLE 2. Summry of liner mixed model results for the frequency of gonistic, forging, nd growth chrcteristics of three strins of juvenile Atlntic slmon in four tretments in seminturl strem behviorl trils. Strin nd tretment were coded s min fctors; initil mss ws treted s covrite. Dependent vrible Independent Degrees of freedom F-sttistic P-vlue Initited ggression/h Tretment 3,380.0 8.60 <0.001 Strin 2,355.1 28.1 <0.001 Initil mss 1,482.7 7.41 0.007 Strin tretment 6,168.4 6.89 <0.001 Received ggression/h Tretment 3,474.3 27.6 <0.001 Strin 2,462.9 26.0 <0.001 Initil mss 1,488.8 9.90 0.002 Strin tretment 6,366.9 6.67 <0.001 Dvid s score Tretment 3,491.0 26.0 <0.001 Strin 2,491.0 2.61 0.07 Initil mss 1,491.0 0.63 0.43 Strin tretment 6,491.0 2.22 0.04 Food consumption (items/h) Tretment 3,466.7 13.29 <0.001 Strin 2,451.7 6.76 0.001 Initil mss 1,489.8 0.61 0.43 Strin tretment 6,334.7 3.64 0.002 Stndrd growth rte (%/d) Tretment 3,472.7 5.16 0.002 Strin 2,472.8 0.21 0.81 Initil mss 1,486.6 18.6 <0.001 Strin tretment 6,368.4 1.94 0.07 Agonistic Interctions nd Dvid s Score Tretment significntly influenced initited nd received ggression nd Dvid s score of Atlntic slmon (Tble 2). Across ll strins, Atlntic slmon juveniles initited more ggression when lone thn in either +BT nd +BTRT tretments (+BT, Student s t-test, t 258 = 2.28, P = 0.02; +BTRT, Student s t-test, t 284 = 5.40, P < 0.001) nd were significntly more ggressive in the + RT tretment thn in the +BTRT tretment (Figure 1). Initited ggression by Atlntic slmon vried between the +BT nd +BTRT tretment with ggression observed to be higher in the +BT tretment (Figure 1). Atlntic slmon lso received much less ggression when lone or in the + RT tretment compred with either +BT (lone, Student s t-test, t 132 = 5.51; + RT, Student s t-test, t 165 = 4.37, P < 0.001 for both comprisons) or +BTRT tretments (lone, Student s t-test, t 106 = 5.48; + RT, Student s t-test, t 178 = 3.90, P < 0.001 for both comprisons; Figure 1b). When Atlntic slmon were lone or in the + RT tretment, they scored higher Dvid s scores thn in the +BT nd +BTRT tretments; Dvid s scores in the +BT tretment were lower thn the +BTRT tretment (Tble 2; Figure 1c). The strin of Atlntic slmon lso influenced initited nd received ggression (Tbles 1, 2). The LHve nd Lc Sint- Jen strins both initited significntly more ggression thn Sebgo Lke, wheres there ws no difference in ggression between Lc Sint-Jen nd LHve strins (Tble 1). Anlogously, Sebgo Lke received significntly less ggression thn both LHve nd Lc Sint-Jen, which did not differ from one nother (Tble 1). An interction between strin nd tretment ws lso found for both initited nd received ggression nd Dvid s score (Tble 2, Tble A.1 in the ppendix). Sebgo Lke initited nd received the lest ggression when lone or with brown trout, wheres in the + RT tretment, Lc Sint-Jen initited significntly more ggression thn either LHve or Sebgo Lke (Figure 1). There ws no difference mong the strins in either initited or received ggression when they were with both brown trout nd rinbow trout (Figure 1). Sebgo Lke individuls scored significntly higher Dvid s scores thn either of the two other strins in the +BT tretment nd in the +BTRT tretment, this strin scored significntly higher thn LHve individuls (Figure 1c). As covrite, initil mss of Atlntic slmon influenced gonistic interctions: hevier individuls both initited more, nd received fewer, ggressive cts (Tble 2). However, initil mss did not influence Dvid s score (Tble 2). Food Consumption nd Stndrd Growth Rte Food consumption nd stndrd growth rte of Atlntic slmon individuls were significntly influenced by tretment (Tble 2). Food consumption ws highest when Atlntic slmon individuls were with conspecifics, followed by consumption in the + RT tretment, nd ws lowest in the two tretments contining brown trout (+BT nd +BTRT; Tble 2; Figure 2). Stndrd growth rte lrgely mirrored the food consumption dt: it ws highest in the lone nd + RT tretments nd ws thelowestinthe+bt nd +BTRT tretments (Figure 2b).
912 VANZWOLETAL. ) Initited ggressive cts/hour 6 5 4 3 2 1 A AB BC C b b b LHve Lc Sint-Jen Sebgo Lke b) 0 AS +RT +BT +BTRT 14 A A B B c) Received ggressive cts/hour Dvid's score 12 10 8 6 4 2 0 4 2 0-2 -4-6 -8-10 -12-14 -16 b AS +RT +BT +BTRT A A B C AS +RT +BT +BTRT Tretment b b b b FIGURE 1. Agonistic interctions nd dominnce of three strins of juvenile Atlntic slmon (LHve, Lc Sint-Jen, nd Sebgo Lke) in four tretments in seminturl strem behviorl trils showing () the number of initited ggressive cts per hour, (b) the number of received ggressive cts per hour, nd (c) Dvid s score. The four experimentl tretments include Atlntic slmon lone (AS), Atlntic slmon with rinbow trout (+RT), Atlntic slmon with brown trout (+BT), nd Atlntic slmon with both brown trout nd rinbow trout (+BTRT). Behviorl observtions were summed for individul fish nd then converted to rte by dividing by the totl observtion time for given chnnel. Brs denote men ± SE for ech of the three strins, while dshed lines indicte the men of the three strins for ech tretment. Different uppercse letters indicte significnt differences between tretments, while different lowercse letters indicte significnt differences mong Atlntic slmon strins within specific tretment (t P < 0.05). FIGURE 2. Feeding behviors nd growth of three strins of juvenile Atlntic slmon (LHve, Lc Sint-Jen, nd Sebgo Lke) in four tretments in seminturl strem behviorl trils showing () the number of food items consumed per hour nd (b) the stndrd growth rte. The four experimentl tretments include Atlntic slmon lone (AS), Atlntic slmon with rinbow trout (+RT), Atlntic slmon with brown trout (+BT), nd Atlntic slmon with both brown trout nd rinbow trout (+BTRT; N = 6). Behviorl observtions were summed for individul fish nd then converted to rte by dividing by the totl observtion time for given chnnel. Brs denote men ± SE for ech of the three strins, while dshed lines indicte the men of the three strins for ech tretment. Different uppercse letters indicte significnt differences between tretments, while different lowercse letters indicte significnt differences mong Atlntic slmon strins within specific tretment (t P < 0.05).
EFFECTS OF NON-NATIVE SALMONIDS ON ATLANTIC SALMON 913 The three strins lso differed significntly in food consumption but not stndrd growth rte (Tbles 1, 2). LHve consumed significntly more food thn Sebgo Lke nd more thn Lc Sint-Jen, lbeit the ltter effect ws mrginlly nonsignificnt (post hoc t-test: t 292 = 1.96, P = 0.052). An interction between strin nd tretment reveled tht Sebgo Lke consumed significntly fewer food items thn either Lc Sint-Jen or LHve in ll but the + RT tretment (Figure 2, Tble A.1 in the ppendix). Conversely, the LHve strin consumed more food or equivlent mounts of food s compred to the other two strins cross the four tretments (Figure 2). Despite these differences in food consumption, however, there ws no observed difference in stndrd growth rte mong the strins in ny of the tretments during the 7-d trils (Figure 2b). Dvid s score ws positively relted to both food consumption nd stndrd growth rte of the Atlntic slmon (food consumption, liner regression: R 2 = 0.008, β = 0.09, N = 504, P = 0.05; stndrd growth rte, liner regression: R 2 = 0.01, β = 0.11, N = 504, P = 0.01). Discriminnt Function Anlysis Differences in ggression nd growth predictors mong Atlntic slmon strin groupings were detected by the DFA (χ 2 (25) = 267.7, P < 0.001, Figure 3). The second function ws lso significnt (χ 2 (16) = 109.1, P < 0.001), s were the third (χ 2 (9) = 30.1, P < 0.001) nd fourth functions (χ 2 (4) = 12.1, P = 0.02). The first nd second discriminnt functions of the nlysis ccounted for 62% nd 28% of the vrition, respectively, nd were the focus of our nlysis. The first discriminnt function (DFA 1) ws positively correlted with initited nd received ggression nd negtively with, to lesser extent, Dvid s score (Tble 3; Figure 3). The two-wy ANOVA reveled tht for DFA 1, ll three strins differed significntly from one nother (F 2,498 = 82.8, P < 0.001) with the Lc Sint-Jen strin scoring the highest, followed by LHve nd then Sebgo Lke individuls (Figure 3). Tretment lso influenced DFA 1 (F 1,498 = 7.30, P = 0.007), with higher scores generlly observed in the nonntive tretments. There ws lso, however, n interction between strin nd presence of nonntives (F 2,498 = 11.6, P < 0.001): Sebgo Lke nd Lc Sint-Jen, but not LHve individuls hd higher DFA 1 vlues in the nonntive versus lone tretments (Figure 3). The second discriminnt function (DFA 2) ws positively correlted with initited ggression, food consumption, growth rte, nd Dvid s score nd negtively with received ggression (Tble 3). For this function, two-wy ANOVA found tht while the strins did not vry (F 2,498 = 2.08, P = 0.12), the presence of nonntive trout species significntly influenced cnonicl scores (F 1,498 = 80.6, P < 0.001), with strins inititing less ggression, consuming less food, growing less nd hving lower dominnce scores, but receiving more ggression in the presence rther thn bsence of the nonntive trout species. The interction between strin nd the presence of nonntives ws not significnt (F 1,498 = 0.39, P = 0.68). DISCUSSION Although we cnnot fully rule out the effects of body size mong species, our dt suggest tht the presence of nonntive slmonids ffects the ggressive nd forging behvior of juvenile Atlntic slmon. When juvenile Atlntic slmon were with conspecifics only, the level of ggression received by individul fish ws lowest nd the level of food consumption ws highest. Additionlly, Atlntic slmon were most ggressive in the conspecific tretment, s the presence of nonntive trout suppressed the mount of ggression Atlntic slmon initited. Specificlly, brown trout exerted stronger influence on Atlntic slmon thn rinbow trout. These dt mirror those of other reserchers including Strdmeyer et l. (2008), who found tht juvenile brown trout were lwys dominnt to Atlntic slmon. Using both strem chnnels nd field surveys, Hern nd Kynrd (1986) found tht wild rinbow trout nd juvenile Atlntic slmon compete nd Blnchet et l. (2009) found tht food consumption of juvenile Atlntic slmon ws lowered in the presence of rinbow trout. Collective evidence now suggests tht nonntive slmonids, prticulrly brown trout, cn hve strong behviorl effects on Atlntic slmon. Body size is n importnt fctor in determining the outcome of contests mong conspecifics. Mny studies hve shown tht dominnce in fish is directly linked to lrger body size (e.g., Abbott et l. 1985; Beugrnd et l. 1996). However, this reltionship between body size nd dominnce did not exist for Atlntic slmon in our study. Consistent with our dt, Huntingford et l. (1990) exmined dominnce competitions between pirs of juvenile Atlntic slmon in spring nd summer (when we conducted our trils) nd found no evidence tht dominnce tests were won by lrger fish, regrdless of the size difference between pir. Interestingly, when the experiment ws conducted in September, the reltionship did exist with 72% of the dominnce tests being won by the lrger fish of pir (Huntingford et l. 1990). The study found, however, tht ggression levels strongly influenced the socil dominnce of n individul. These dt suggest tht dominnce is function of behvior nd tht lrge body size my be consequence, not cuse, of dominnce, t lest in some slmonids. It is well known tht subordinte fish exhibit less growth s result of the behviors of dominnt fish. This pttern hs been shown in number of slmonids (e.g., Atlntic slmon nd brook trout; Gibson 1973). Consistent with these studies, we found tht the presence of brown trout suppressed the growth rte of Atlntic slmon, which were typiclly subordinte to the brown trout. We lso found tht in the brown trout nd Atlntic slmon tretment, subordinte fish grew t rtes much lower thn dominnt fish, nd food consumption of Atlntic slmon significntly declined s compred with when Atlntic slmon were lone. While dominnt brown trout re known to
914 VANZWOLETAL. 1.0 Incresing initited ggression, food consumption, growth rte, nd Dvid's score Decresing received ggression 0.5 LSJ lone LH lone SL lone DFA 2 0.0-0.5 SL NN Incresing initited nd received ggression Decresing Dvid's score -1.0-1.5-1.0-0.5 0.0 0.5 1.0 DFA 1 LHNN LSJ NN FIGURE 3. Cnonicl plot of the first two functions of the discriminnt function nlysis (DFA 1, DFA 2) exmining the vrition of ggression nd growth mesurements mong three strins of juvenile Atlntic slmon in four tretments in seminturl strem behviorl trils. Tretments with nonntive species present (NN) were grouped together nd compred with the Atlntic slmon lone tretment for the LHve (LH), Lc Sint-Jen (LSJ), nd Sebgo Lke (SL) strins. The symbols represent strin centroids (with 95% confidence intervls). monopolize feeding res, reducing feeding opportunities of subordintes (Höjesjö et l. 2005), Metclfe (1986) hs postulted tht regrdless of the ctions of the dominnt fish, it is better for subordinte fish to minimize energetic costs, rther thn mximize food intke. Although we did not directly quntify the behviorl tctics used to cquire food, our results re consistent with Metclfe (1986). For exmple, Sebgo Lke slmon ppered to choose growth strtegy tht minimized energy expenditure, opting out of the competition nd consequently consuming the lest mount of food nd losing the most TABLE 3. Summry of discriminnt function nlysis (DFA) of gonistic nd growth mesurements of three strins of juvenile Atlntic slmon in four tretments in seminturl strem behviorl trils. The DFA ws performed on five gonistic nd growth mesurements of the LHve, Lc Sint-Jen, nd Sebgo Lke strins. Tretments with nonntive species present were grouped together nd compred with the Atlntic slmon lone tretment for ech strin. Vlues represent pooled within-group correltions of cnonicl roots nd stndrdized cnonicl discriminnt function coefficients. Correltion of vribles with discriminnt functions Stndrdized cnonicl discriminnt function coefficients Vrible DFA 1 DFA 2 DFA 3 DFA 4 DFA 1 DFA 2 DFA 3 DFA 4 Initited ggression/h 0.641 0.632 0.05 0.395 1.26 0.706 0.460 0.679 Dvid s score 0.161 0.564 0.463 0.587 0.797 0.81 0.169 1.26 Received ggression/h 0.640 0.397 0.462 0.461 0.157 0.792 0.344 0.962 Food consumption (items/h) 0.067 0.580 0.786 0.198 0.493 0.565 0.788 0.271 Stndrd growth rte (%/d) 0.022 0.263 0.490 0.137 0.16 0.164 0.247 0.220
EFFECTS OF NON-NATIVE SALMONIDS ON ATLANTIC SALMON 915 mss of the three strins in the tretments with nonntive trout. Regrdless of the ctul feeding tctics used by Atlntic slmon, our dt clerly show tht Atlntic slmon feed less in the presence of dominnt brown trout nd consequently disply reduced growth. Community ecology studies hve long shown tht competition mong ecologiclly similr species cn led to sptil seprtion or shifts in resource use if the species continue to live in symptry (e.g., Werner nd Hll 1977; Lngelnd et l. 1991). Brown trout nd Atlntic slmon hve historiclly coexisted in rivers in Europe (Höjesjö et l. 2005) but tend to sptilly seprte in strems, lrgely driven by the ggressive behvior of brown trout (Armstrong et l. 2003). Our study confirmed the dominnce of brown trout over Atlntic slmon s hs been shown by Strdmeyer et l. (2008). Additionlly, we found tht the food consumption nd growth of Atlntic slmon declined in the presence of brown trout. Rinbow trout nd Atlntic slmon, however, hve not historiclly coexisted, yet studies hve shown there is degree of niche overlp (Gibson 1981; Hern nd Kynrd 1986), which we expected would influence the gonistic interctions nd growth of Atlntic slmon in our study. Similr to reserch by Blnchet et l. (2008), the presence of rinbow trout did not ffect the food consumption or growth rte of Atlntic slmon. Atlntic slmon received no more ggression in the presence of rinbow trout thn they did in the conspecifics tretment. These dt support study by Volpe et l. (2001) tht found tht lthough rinbow trout were much more ggressive thn Atlntic slmon, gonistic interctions were lrgely between rinbow trout conspecifics nd not Atlntic slmon. Hence, it is conceivble tht lthough there is niche overlp between these two species, rinbow trout lrgely ignore Atlntic slmon, t lest during gonistic interctions. Thus, density issues side, these dt suggest tht brown trout, more thn rinbow trout, influence Atlntic slmon gonistic nd feeding behviors, nd unless sptil seprtion is possible for brown trout nd Atlntic slmon, competition between these two species poses thret to Atlntic slmon estblishment in Lke Ontrio strems. Behviorl differences mong popultions or strins within species hve been observed cross mny tx (e.g., Jones 1977; Rex et l. 1996; Moretz et l. 2007), nd compring these differences cn provide n understnding of phenotypic ttributes tht will strengthen efforts of ntive species reintroduction (Curio 1996). One importnt ttribute for successful estblishment nd persistence is ggression (s reviewed by Holwy nd Surez 1999). We hve shown differences mong Atlntic slmon strins in both ggressive nd feeding behviors. Indeed, the Lc Sint-Jen strin initited the most ggression nd lost the lest mss of the three strins, suggesting they re better competitors ginst brown trout nd rinbow trout, two nonntive species prevlent in Lke Ontrio tributries. The DFA confirmed these strin differences by showing tht the presence of nonntive trout species influenced the LHve nd Sebgo Lke strins the most but hd less of n impct on the Lc Sint-Jen strin. Differences observed here suggest tht stocking the Lc Sint-Jen strin, the strin believed to be the closest geogrphiclly nd geneticlly of the three strins to the originl Lke Ontrio popultion (Dimond nd Smitk 2005), will chieve greter restortion success s they re better competitors ginst brown trout nd rinbow trout. Indeed, the fct tht LHve strin ws the most significntly ffected by the presence of the nonntive trout my explin the previous filed ttempts of restoring Atlntic slmon with this strin. High species richness cn led to competition for resources, resulting in declines in growth rtes of the competing species. This effect hs been shown in, for exmple, sunfish (Centrrchide; Mittelbch 1988), Dphni spp. (Bengtsson 1993), nd desert nnuls, where competition mong the plnts leds to decreses in growth, biomss, nd fecundity (Inouye et l. 1980). We found tht the presence of multiple slmonid species led to increses in ggression received nd, in the cse of Atlntic slmon, reductions in food consumption nd growth. Such interctions often led to prtitioning of hbitt nd resources mong the competing species llowing the individuls to coexist (e.g., Robertson nd Gines 1986; Young 2001). Becuse of our experimentl setup, we could not esily ssess potentil hbitt or resource prtitioning. Nevertheless, we found no evidence tht Atlntic slmon shifted hbitt use cross the pool nd riffle sections when lone versus with either or both of the nonntive species. Regrdless, our dt suggest tht high slmonid species richness could be detrimentl for Atlntic slmon during the strem stge of life. Assessing the species community of trgeted strems nd rivers for Atlntic slmon restortion my lso help to llevite competition for Atlntic slmon. In conclusion, our dt point to some considertions tht my help to direct restortion of Atlntic slmon mong the nturlized popultions of brown trout nd rinbow trout in the Lurentin Gret Lkes wtershed. First, the three strins re predicted to hve differentil poststocking ecologicl success in tributry environments, with Atlntic slmon originting from Lc Sint-Jen outperforming the LHve nd Sebgo Lke strins. Whether these differences would similrly extend to incresed performnce in Lke Ontrio in terms of growth, survivl, nd dult returns still needs to be determined. Second, the successful estblishment of juvenile Atlntic slmon my be gretly impeded by the presence of brown trout. Lke Ontrio rinbow trout pper to hve less of n influence on Atlntic slmon, lbeit high species richness did impede the performnce of Atlntic slmon. As such, we suggest voiding stocking juvenile Atlntic slmon in Lke Ontrio strems with high densities of brown trout or in strems with multiple estblished slmonid species. ACKNOWLEDGMENTS We grtefully cknowledge the support of World Wildlife Fund Cnd nd the Ontrio Ministry of Nturl Resources. The reserch ws lso supported by funding from the Nturl
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