Published online: 06 Feb 2015.

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1 This rticle ws downloded by: [University of Idho] On: 09 Februry 2015, At: 13:08 Publisher: Tylor & Frncis Inform Ltd Registered in Englnd nd Wles Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Click for updtes North Americn Journl of Fisheries Mngement Publiction detils, including instructions for uthors nd subscription informtion: Trophic Ecology of Nonntive Northern Pike nd their Effect on Conservtion of Ntive Westslope Cutthrot Trout John D. Wlrth, Michel C. Quist b & Jon A. Firehmmer c Idho Coopertive Fish nd Wildlife Reserch Unit, Deprtment of Fish nd Wildlife Sciences, University of Idho, 875 Perimeter Drive, Mil Stop 1141, Moscow, Idho 83844, USA b U.S. Geologicl Survey, Idho Coopertive Fish nd Wildlife Reserch Unit, Deprtment of Fish nd Wildlife Sciences, University of Idho, 875 Perimeter Drive, Mil Stop 1141, Moscow, Idho 83844, USA c Coeur d Alene Tribe, 850 A Street, Plummer, Idho 83851, USA Published online: 06 Feb To cite this rticle: John D. Wlrth, Michel C. Quist & Jon A. Firehmmer (2015) Trophic Ecology of Nonntive Northern Pike nd their Effect on Conservtion of Ntive Westslope Cutthrot Trout, North Americn Journl of Fisheries Mngement, 35:1, , DOI: / To link to this rticle: PLEASE SCROLL DOWN FOR ARTICLE Tylor & Frncis mkes every effort to ensure the ccurcy of ll the informtion (the Content ) contined in the publictions on our pltform. However, Tylor & Frncis, our gents, nd our licensors mke no representtions or wrrnties whtsoever s to the ccurcy, completeness, or suitbility for ny purpose of the Content. Any opinions nd views expressed in this publiction re the opinions nd views of the uthors, nd re not the views of or endorsed by Tylor & Frncis. The ccurcy of the Content should not be relied upon nd should be independently verified with primry sources of informtion. Tylor nd Frncis shll not be lible for ny losses, ctions, clims, proceedings, demnds, costs, expenses, dmges, nd other libilities whtsoever or howsoever cused rising directly or indirectly in connection with, in reltion to or rising out of the use of the Content. This rticle my be used for reserch, teching, nd privte study purposes. Any substntil or systemtic reproduction, redistribution, reselling, lon, sub-licensing, systemtic supply, or distribution in ny form to nyone is expressly forbidden. Terms & Conditions of ccess nd use cn be found t

2 North Americn Journl of Fisheries Mngement 35: , 2015 Ó Americn Fisheries Society 2015 ISSN: print / online DOI: / ARTICLE Trophic Ecology of Nonntive Northern Pike nd their Effect on Conservtion of Ntive Westslope Cutthrot Trout John D. Wlrth* Idho Coopertive Fish nd Wildlife Reserch Unit, Deprtment of Fish nd Wildlife Sciences, University of Idho, 875 Perimeter Drive, Mil Stop 1141, Moscow, Idho 83844, USA Michel C. Quist U.S. Geologicl Survey, Idho Coopertive Fish nd Wildlife Reserch Unit, Deprtment of Fish nd Wildlife Sciences, University of Idho, 875 Perimeter Drive, Mil Stop 1141, Moscow, Idho 83844, USA Jon A. Firehmmer Coeur d Alene Tribe, 850 A Street, Plummer, Idho 83851, USA Abstrct Westslope Cutthrot Trout Oncorhynchus clrkii lewisi in Coeur d Alene Lke, Idho, hve declined in recent yers; predtion by Northern Pike Esox lucius, nonntive sport fish, is thought to be custive mechnism. The gol of this study ws to describe the sesonl food hbits of Northern Pike nd determine their influence on Westslope Cutthrot Trout in Coeur d Alene Lke by using bioenergetics modeling pproch. Fish were smpled monthly from Mrch to My 2013 using pulsed-dc electrofishing nd experimentl gillnetting in four bys. Northern Pike ctch rtes from electrofishing were generlly low but incresed slightly ech seson nd were highest in the southern portion of the lke; ctch rtes from gillnetting were pproximtely 50% higher during the two spring smpling periods compred with the summer nd fll. Sesonl growth nd food hbits of 695 Northern Pike (TL D cm; weight D 24 9,628 g) were nlyzed. Diets primrily consisted of koknee O. nerk, Westslope Cutthrot Trout, nd Yellow Perch Perc flvescens. Results of bioenergetics model estimted tht Westslope Cutthrot Trout represented pproximtely 2 30% of the biomss consumed by ge-1 4 Northern Pike. Totl Westslope Cutthrot Trout biomss consumed by Northern Pike ( yer-clsses) cross ll sesons smpled ws estimted to be 1,231 kg (95% CI D 723 2,396 kg), nd the totl number consumed ws 5,641 (95% CI D 3,311 10,979). The highest occurrence of Westslope Cutthrot Trout in Northern Pike diets ws observed during spring. Thus, reducing Northern Pike predtion on Westslope Cutthrot Trout would be one tool worth considering for conserving Westslope Cutthrot Trout popultions in Coeur d Alene Lke. The Cutthrot Trout Oncorhynchus clrkii historiclly hd one of the most widespred distributions of ny North Americn slmonid; however, Cutthrot Trout popultions hve been declining since the 19th century nd re now mjor focus of mngement nd conservtion (Gresswell 1988; Dunhm 2002). A primry fctor contributing to the decline of Cutthrot Trout is reduction in hbitt qulity nd quntity (Liknes nd Grhm 1988; Mrnell 1988; Sheprd et l. 2005; Gresswell 2011). The construction of dms hs creted movement brriers tht interfere with spwning nd other importnt life history events (Liknes nd Grhm 1988). Mny popultions lso exist in wtersheds with extensive griculture, where chnnel dewtering nd sedimenttion re common (Moeller 1981; Liknes nd Grhm 1988). Chnges in wter qulity nd dmge to riprin hbitt from livestock grzing hve been shown to exert negtive effects on Cutthrot Trout popultions 158 *Corresponding uthor: wlr7955@vndls.uidho.edu Received Mrch 7, 2014; ccepted September 22, 2014

3 TROPHIC ECOLOGY OF NORTHERN PIKE 159 (Peterson et l. 2010). In ddition, the Cutthrot Trout ws mong the most common fish species encountered by Europen settlers in the 19th century nd s result ws importnt for subsistence nd commerce (Behnke 1988). The high ctchbility of Cutthrot Trout nd lck of hrvest regultions for this species cused mny popultions to be overexploited in less thn 100 yers (Behnke 1988). Nonntive fishes hve been shown to negtively ffect Cutthrot Trout popultions through competition, predtion, nd hybridiztion (Rich 1992; Sheprd et l. 2005; Tbor et l. 2007; Muhlfeld et l. 2008). As popultions of Cutthrot Trout hve become less bundnt, wter bodies hve often been stocked with nonntive species to crete or supplement fisheries. One of the gretest fctors contributing to the decline of Cutthrot Trout is their interction with nonntive Rinbow Trout O. mykiss, which compete nd hybridize with Cutthrot Trout (Mrnell 1988; Allendorf et l. 2004; Sheprd et l. 2005; Muhlfeld et l. 2009). Mny remining geneticlly pure popultions of Westslope Cutthrot Trout O. clrkii lewisi exist only in hedwter strems where movement brriers protect them from nonntive species (Rsmussen et l. 2010). In fct, Westslope Cutthrot Trout popultions known to be geneticlly pure currently inhbit less thn 10% of this subspecies historic distribution in the United Sttes nd less thn 20% of its historic distribution in Cnd (Sheprd et l. 2005). Other species, such s Brown Trout Slmo trutt in lrge strems, Brook Trout Slvelinus fontinlis in smll strems, nd Lke Trout Slvelinus nmycush in lke systems, hve lso replced Cutthrot Trout cross the species distribution (Behnke 2002; Quist nd Hubert 2004). In ddition to slmonids, vrious wrmwter nd coolwter species hve been introduced into systems contining Cutthrot Trout, primrily to diversify recretionl ngling opportunities. Some of these species include Smllmouth Bss Micropterus dolomieu, Lrgemouth Bss M. slmoides, Northern Pike Esox lucius, Wlleye Snder vitreus, nd Suger Snder cndensis. Nonntive top-level predtors not only hve n influence on ntive fishes but lso cn gretly lter prey popultion structure nd dynmics (Tbor et l. 2007; Muhlfeld et l. 2008). Introductions of nonntive species hve contributed to declines in Cutthrot Trout popultions, prticulrly Westslope Cutthrot Trout, cross much of the Pcific Northwest (Rich 1992; Nughton et l. 2004; Tbor et l. 2007; Muhlfeld et l. 2008). In Idho, Westslope Cutthrot Trout re ntive to the Kooteni, Pend Oreille, Spokne, Clerwter, nd Slmon River systems in the northern prt of the stte. Historiclly, Westslope Cutthrot Trout were bundnt in Idho nd s result were importnt for subsistence nd commerce (Wllce nd Zrobn 2013). In ddition, Westslope Cutthrot Trout hve culturl significnce to Ntive Americns. In the pst, the Coeur d Alene Tribe in northern Idho relied on Westslope Cutthrot Trout for subsistence, hrvesting roughly 42,000 fish per yer from Coeur d Alene Lke nd the St. Joe River (Firehmmer et l. ). However, Westslope Cutthrot Trout in Coeur d Alene Lke hve declined drsticlly, nd conservtion efforts hve been initited. Over the lst yers, the Coeur d Alene Tribe hs implemented restortion prctices in Lke nd Benewh creeks (i.e., two tributries to Coeur d Alene Lke) to recover popultions of dfluvil Westslope Cutthrot Trout. The Coeur d Alene Tribe is focused on restoring strem spwning nd rering hbitt by incresing sinuosity, creting deep pools, enhncing lrge woody debris, nd reconnecting strems to their floodplins (Firehmmer et l. ). Strem renovtions were initited to increse instrem survivl, but there is criticl knowledge gp ssocited with the survivl of dfluvil Westslope Cutthrot Trout once they out-migrte to Coeur d Alene Lke s juveniles nd return to spwn s dults. Recently, the Coeur d Alene Tribe embrked on n intensive PIT-tgging study to better understnd juvenile survivl nd dult return rtes. Of the 5,300 out-migrting juveniles tht were tgged during , only 1.7% returned s dults to Lke Creek nd 2.3% returned to Benewh Creek (Firehmmer et l. ). These juvenile-to-dult return rtes re 8 12 times lower thn estimtes obtined using similr techniques in comprble systems (Huston et l. 1984; Stpp nd Hywrd 2002; Muhlfeld et l. 2009). The mechnism responsible for the poor survivl of dfluvil Westslope Cutthrot Trout is unknown, but it is hypothesized to be the result of predtion by nonntive species, prticulrly Northern Pike, in Coeur d Alene Lke (Rich 1992; Nughton et l. 2004; Tbor et l. 2007; Muhlfeld et l. 2008). Northern Pike re top-level predtors with circumpolr distribution. Due to their populrity in recretionl fisheries, Northern Pike hve been introduced into systems cross North Americ (Crossmn 1978). In ddition to being stocked for sport fishery enhncement, Northern Pike hve lso been introduced to reduce densities of nuisnce species, such s the Common Crp Cyprinus crpio nd Gizzrd Shd Dorosom cepedinum (Pflieger 1997). Northern Pike re mbush predtors tht require littorl hbitt with bundnt vegettion for successful spwning (Crossmn 1978; Csselmn nd Lewis 1996). They re lso opportunistic predtors tht prefer softryed fishes (Ekl ov nd Hrrin 1989). In the Coeur d Alene River bsin of northern Idho, shllow, vegetted hbitt nd sloughs re common where tributries enter Coeur d Alene Lke. Juvenile dfluvil Westslope Cutthrot Trout out-migrte to Coeur d Alene Lke during spring nd must pss through hbitt tht is highly suitble for Northern Pike. Thus, there is the potentil for sptil nd temporl overlp between Westslope Cutthrot Trout nd Northern Pike in res where tributries enter the lke. Given the need to better understnd fctors influencing Westslope Cutthrot Trout in Coeur d Alene Lke, our objectives were to describe the sesonl food hbits of Northern Pike in the lke nd to estimte their consumption of Westslope Cutthrot Trout nd other prey tx.

4 160 WALRATH ET AL. METHODS Coeur d Alene Lke is the second-lrgest nturl lke in Idho, with surfce re of 12,700 h (Figure 1). The lke hs men depth of pproximtely 24 m nd mximum depth of 61 m (Rich 1992; Vitle et l. 2004). Primry tributries to Coeur d Alene Lke re the Coeur d Alene nd St. Joe rivers; mny smll strems lso contribute to the system. Post Flls Dm ws constructed on the outlet of Coeur d Alene Lke in 1906 nd rised the wter level of the lke by 2.5 m, creting n bundnce of shllow, vegetted hbitts (Rich 1992). The lke hs been clssified s mesotrophic bsed on nutrient concentrtions; however, hevy metls from 100 yers of mining nd ore processing in the wtershed limit biologicl production (Ntionl Reserch Council 2005). Fisheries in Coeur d Alene Lke re co-mnged by the Coeur d Alene Tribe nd the Idho Deprtment of Fish nd Gme. Ntive sport fish species in Coeur d Alene Lke include Westslope Cutthrot Trout, Bull Trout Slvelinus confluentus, nd Mountin Whitefish Prosopium willimsoni. Currently, sport fishes re primrily nonntive species, such s koknee FIGURE 1. Coeur d Alene Lke, locted in northern Idho. The Idho Deprtment of Fish nd Gme mnges the lke north of the Coeur d Alene River mouth; the Coeur d Alene Tribe mnges the lke south of the river mouth s well s the Lke Creek wtershed. Smpling sites were locted in Cougr By, Wolf Lodge By, Windy By, nd Benewh Lke. O. nerk, Chinook Slmon O. tshwytsch, Rinbow Trout, Brook Trout, Lrgemouth Bss, Smllmouth Bss, Blck Crppie Pomoxis nigromcultus, Pumpkinseed Lepomis gibbosus, Yellow Perch, Brown Bullhed Ameiurus nebulosus, Blck Bullhed Ameiurus mels, nd Northern Pike. Other notble ntive species in the bsin include Northern Pikeminnow Ptychocheilus oregonensis nd Longnose Sucker Ctostomus ctostomus. TenchTinc tinc, nonntive speciesinnorthameric,islsocommonincoeurd Alene Lke. Four mjor bys (i.e., Wolf Lodge By, Cougr By, Windy By, nd Benewh Lke) in Coeur d Alene Lke were selected for this study becuse they re the primry res where Northern Pike re common or becuse they represent importnt res for sport fish mngement (Figure 1; Rich 1992; Firehmmer et l. ). Strtified rndom smpling ws used to select smpling sites by dividing the shoreline of ech by into 300-m sections nd rndomly ssigning ger type to section. A smpling event consisted of smpling 18 nonoverlpping sections composed of 12 gillnetting sites nd 6 electrofishing sites. Smpling occurred once per month in Cougr nd Wolf Lodge bys from Mrch to My Windy By nd Benewh Lke were smpled once per month during June November nd twice per month from Mrch to My in both nd Spring biweekly smpling ws performed to increse the resolution in Windy By nd Benewh Lke, where the Coeur d Alene Tribe is intensely monitoring Westslope Cutthrot Trout in tributries (i.e., Lke nd Benewh creeks). Hzrdous lke conditions prevented us from smpling during winter (December Februry 2013). Northern Pike were smpled using pulsed-dc electrofishing nd experimentl gill nets. Electrofishing ws conducted using 5,000-W genertor mounted in n luminum bot with Smith-Root equipment (Smith-Root, Inc., Vncouver, Wshington). Power output ws stndrdized to 2,750 3,250 W bsed on mbient wter conductivity (ms/cm; Mirnd nd Boxrucker 2009). In n effort to minimize mortlity nd prey digestion, gill nets ( m, with pnels of 2.5-, 3.2-, 3.8-, 4.4-, nd 5.0-cm br-mesure mesh) were fished for h. Kobler et l. (2008) found tht Northern Pike movement ws more homogeneous during winter months thn in other months, with slightly higher movement occurring during the dy. Those uthors lso reported tht Northern Pike were most ctive t dwn nd dusk during the summer. Therefore, we conducted smpling t dusk in My September nd during the dy in October April. Additionlly, opertion of bot t night with low wter levels nd ice becme hzrdous during fll nd winter. All Northern Pike were mesured for TL to the nerest millimeter nd were weighed to the nerest grm. Ech Northern Pike ws mrked by completely removing the left pelvic fin (Nielson 1992; Guy et l. 1997). Age structures (i.e., left pelvic fin ry) were collected from 10 fish per centimeter

5 TROPHIC ECOLOGY OF NORTHERN PIKE 161 length-group (Line et l. 1991; Quist et l. ). Fin rys were plced into coin envelopes nd were llowed to ir dry before processing (Koch nd Quist 2007). Cleithr from Northern Pike were collected to corroborte ges from pelvic fin rys. Agreement between cleithrum ges nd pelvic fin ry ges ws 100%. Hlf of the cptured Northern Pike were tgged by using n individully numbered, nonrewrd FD-94 T-br nchor tg (7.6 cm; Floy Tg, Inc., Settle) inserted ner the posterior end of the dorsl fin. All other Northern Pike were tgged with n individully numbered, nonrewrd Crlin dngler tg ( cm; Floy Tg; Quist et l. 2010) in the cudl peduncle. Individully numbered tgs were used to obtin individul recpture histories, which were used to estimte Northern Pike popultion bundnce in Progrm MARK (Cooch nd White 2010). Stomch contents were obtined vi gstric lvge from 5 fish per centimeter length-group during ech smpling event in ech by. A 12-V, 14.4-L/min pump (Fimco, North Sioux City, South Dkot) equipped with pressure guge, chngeble hose fittings, nd pressure-relese vlve ws used to flush stomchs (Light et l. 1983; Bowen 1996; Venturelli nd Tonn 2006). Lrge prey items tht were not flushed from the stomch were removed using forceps. Filtered wter held in n on-bord continer ws used for the lvge process to ensure tht smples were not contminted with orgnisms from the lke. Before fish ws relesed, gstroscope ws inserted through the esophgus nd into the stomch to ensure the removl of ll prey items, wter, nd ir. If prey items were observed, the lvge process ws repeted until the stomch ws empty. Stomch contents were fixed with 10% buffered formlin (Grvey nd Chipps ). The efficiency of removing ll prey items from stomchs with the pulsed gstric lvge technique ws evluted by dissecting mortlities nd exmining their stomchs for ny remining content; efficiency ws 98%. Stomch contents were lso scnned for PIT tgs by using n Allflex ISO compct reder (Allflex, Sn Antonio, Texs), s dfluvil Westslope Cutthrot Trout in Lke nd Benewh creeks re PIT-tgged. In the lbortory, vertebrte diet items were counted nd identified to species, nd invertebrte diet items were counted nd identified to order. Lengths of prey items were mesured to the nerest cm by using cliper (Mitutoyo, Auror, Illinois). Wet nd dry weights were mesured to the nerest milligrm. For prtilly digested items, totl lengths nd weights were estimted from hrd structures (e.g., vertebre nd hed cpsules) by using published length weight equtions (Duke nd Crossley 1975; Dumont et l. 1975; Smock 1980; Evenson nd Kruse 1982; Rust 1991; Grvey nd Stein 1993; Gnihr 1997; Altindg et l. 1998; Behnke et l. 1999; Sbo et l. 2002; Anders et l. 2003; Bumgrtner nd Rothhupt 2003; Wigley et l. 2003). Reltive weight (W r ) ws used to evlute body condition of Northern Pike: W r D W 100; W s where W is the weight of n individul nd W s is the stndrd weight from species-specific length weight regression (Neumnn et l. ). A W r vlue greter thn 100 indictes bove-verge body condition. Food hbits dt were pooled by seson bsed on wter temperture: spring (Mrch My), summer (June August), nd fll (September November). All dt were summrized by Northern Pike yer-clss for those yer-clsses tht were represented by t lest five individuls in ech seson (i.e., the yer-clsses). Frequency of occurrence, percent by number, percent energy contribution, nd prey-specific energy contribution were used to summrize the diet dt (Grvey nd Chipps ). Percent energy contribution ws estimted by multiplying the weight of txon by its cloric vlue nd then dividing the totl energy of tht txon by the totl energy of ll prey items. Prey-specific energy contribution ws the percentge of energy contributed by prey txon to totl energy (ll tx) for only those stomchs in which the prey txon occurred (Amundsen et l. 1996). Only Northern Pike with identifible prey items in their stomchs were used in the food hbits nlysis. Unidentifible prey items were rre (<1%) nd therefore removed from further nlysis. To gin insight on prey importnce, feeding strtegy, nd components of diet niche width for Northern Pike in Coeur d Alene Lke, we used modifiction of the Costello method, which plots the prey-specific energy contribution of prey txon ginst its frequency of occurrence (Amundsen et l. 1996). Feeding strtegies cn be defined s follows: (1) rre tx occur t low frequencies, contribute little energy, nd re typicl of generlist diet; (2) prey tx tht occur t high frequencies nd tht contribute substntil mounts of energy indicte speciliztion t the popultion level; nd (3) prey tx with low frequencies of occurrence nd high prey-specific energy contributions indicte speciliztion by individuls (Amundsen et l. 1996). Bioenergetics modeling conducted with Fish Bioenergetics 3.0 (Hnson et l. 1997) ws used to estimte the weights of vrious tx consumed by Northern Pike. Bioenergetics models re populr for understnding the growth nd trophic ecology of fishes nd re bsed on the generlized eqution C D.R C A C S/ C.F C U/ C.DB C G/; where C D consumption, R D respirtion, A D ctive metbolism, S D specific dynmic ction, F D egestion, U D excretion, DB D somtic growth, nd G D gond production (Hnson et l. 1997). The simultion covered 440 d (from Mrch 15,, to My 31, 2013) with dily time step nd

6 162 WALRATH ET AL. TABLE 1. Biomss (g) of individul prey types consumed by Northern Pike in Coeur d Alene Lke, Idho, s estimted with bioenergetics models. Estimtes re provided for ech Northern Pike yer-clss ( ) nd ech seson. Months were grouped together bsed on wter temperture: spring (Mrch My), summer (June August), nd fll (September November) yer-clss 2010 yer-clss Txon Spring Summer Fll Spring 2013 Spring Summer Fll Spring 2013 Invertebrtes Annelid Coleopter Decpod Hymenopter Odont Fish Ctostomide Lrgescle Sucker Centrrchide Blck Crppie Bluegill Lrgemouth Bss Unknown species White Crppie Pomoxis nnulris Clupeide Pcific Herring Clupe pllsii Cottide Sculpin Cyprinide Northern Pikeminnow Tench Esocide Northern Pike Ictluride Brown Bullhed Percide Yellow Perch < 15.0 cm Yellow Perch 15.0 cm Slmonide Koknee Unknown species Westslope Cutthrot Trout Other Idho gint slmnder Dicmptodon terrimus Detritus ws divided into four periods (i.e., spring, summer, nd fll ; nd spring 2013) to better represent sesonl trends in consumption nd growth for ech Northern Pike yer-clss. Bioenergetics models were not developed for winter (i.e., December 1, Mrch 14, 2013) becuse no smpling occurred during tht period. The two most common uses of bioenergetics models re for estimting how environmentl conditions ffect growth nd (Continued on next pge) estimting the weight of prey consumption by predtors (Hrtmn nd Kitchell 2008). The models require wter temperture dt, prey energy densities, predtor energy densities, nd cohort-specific informtion on diet proportions, initil weights, finl weights, nd physiologicl prmeters for the focl species (e.g., Hnson et l. 1997; Muhlfeld et l. 2008). Wter temperture ws recorded by three Onset temperture loggers (Model H ; Onset, Cpe Cod, Msschusetts)

7 TROPHIC ECOLOGY OF NORTHERN PIKE 163 TABLE 1. Extended. TABLE 1. Continued yer-clss 2008 yer-clss Txon Spring Summer Fll Spring 2013 Spring Summer Fll Spring 2013 Invertebrtes Annelid Coleopter Decpod Hymenopter Odont Fish Ctostomide Lrgescle Sucker Centrrchide Blck Crppie Bluegill Lrgemouth Bss Unknown species White Crppie Pomoxis nnulris Clupeide Pcific Herring Clupe pllsii Cottide Sculpin Cyprinide Northern Pikeminnow Tench Esocide Northern Pike Ictluride Brown Bullhed Percide Yellow Perch < 15.0 cm Yellow Perch 15.0 cm Slmonide Koknee Unknown species Westslope Cutthrot Trout Other Idho gint slmnder Dicmptodon terrimus Detritus in ech of the four bys. Temperture loggers were plced t depths vrying from 0.9 to 10.7 m cross bys nd recorded the tempertures tht were likely to be experienced by Northern Pike. The loggers recorded wter temperture ( C) every 6 h to generte men dily temperture. Cloric densities for prey items nd predtors were obtined from the published literture (Cummins nd Wuycheck 1971; Yule nd Luecke 1993; Bryn et l. 1996; Hnson et l. 1997; Lio et l. 2004; Antolos et l. 2005; Muhlfeld et l. 2008). Dietry informtion for ech smpling dy ws ggregted cross ll individul Northern Pike by yer-clss nd ws input into the model s the proportion by weight of prey tx consumed. Initil nd finl weights of Northern Pike for ech period nd yer-clss were estimted using the medin weights of individuls from ech yer-clss. If n initil or finl weight ws less thn the previous vlue, the weight ws ssumed to be the sme s tht recorded for the previous period. Physiologicl prmeters from Bevelhimer et l. (1985) were developed for

8 164 WALRATH ET AL. TABLE 2. Totl biomss (g) of individul prey types consumed by Northern Pike in Coeur d Alene Lke, s estimted with bioenergetics models. Estimtes re presented for ech Northern Pike yer-clss ( ) s well s summed cross yer-clsses. Yer-clss Txon Totl Invertebrtes Annelid Coleopter Decpod Hymenopter Odont Fish Ctostomide Lrgescle Sucker Centrrchide Blck Crppie Bluegill Lrgemouth Bss Unknown species White Crppie Clupeide Pcific Herring Cottide Sculpin Cyprinide Northern Pikeminnow Tench Esocide Northern Pike Ictluride Brown Bullhed Percide Yellow Perch < 15.0 cm , Yellow Perch 15.0 cm , Slmonide Koknee , , Unknown species Westslope Cutthrot Trout , Other Idho gint slmnder Detritus All prey 1, , , , Northern Pike vrying from 12.8 to 22.7 cm TL nd from 9.5 to 53.2 g. Ben (2010) demonstrted tht there is risk of overestimting consumption when the prmeters developed by Bevelhimer et l. (1985) re pplied to lrger individuls (i.e., >22.7 cm). Therefore, to correct inccurcies for lrger individuls, Ben (2010) developed prmeters for Northern Pike vrying from 25.0 to 71.8 cm TL nd from 86 to 2,146 g. Thus, physiologicl prmeters from Fish Bioenergetics 3.0 (i.e., Bevelhimer et l. 1985) were used for the 2011 yerclss, wheres prmeters from Ben (2010) were used for the yer-clsses. Bioenergetics models were not developed for the yer-clsses, s fewer thn five individuls were cptured during ech seson. After ll species- nd site-specific dt were entered, the proportion of mximum consumption (P c ) ws clculted s C P c D ðc mx r c Þ ; where C is the estimted consumption, C mx is the mximum consumption of specific rtion t given temperture, nd r c

9 TROPHIC ECOLOGY OF NORTHERN PIKE 165 TABLE 3. Estimtes of totl Westslope Cutthrot Trout (WCT) biomss consumed by Northern Pike ( yer-clsses) in Coeur d Alene Lke from Mrch 15,, to My 31, 2013 (excluding December 1, Mrch 15, 2013). Northern Pike ge composition percentges were derived from n ge length key. The 95% CIs re shown in prentheses for Northern Pike bundnce (N) nd totl WCT biomss. Yer-clss Age composition (%) Northern Pike N Totl WCT biomss (kg) (358 1,240) 17.1 ( ) ,026 (576 1,997) ( ,315.8) ,007 (565 1,959) ( ) ( ) ( ) Totl ,056 (1,717 5,947) 1,231.3 ( ,396.4) is temperture-dependent proportionl djustment of the consumption rte (Hnson et l. 1997). In the present model, P c ws estimted by solving the eqution with observed growth nd temperture dt. Progrm MARK (Cooch nd White 2010) ws used to estimte the popultion bundnce of Northern Pike in ech of the study bys in Coeur d Alene Lke by using closed-popultion cpture recpture models. Closed cpture models include single mixture, so only two prmeters re used: the cpture probbility (p i ) nd the recpture probbility (c i ). We used this method to estimte popultion bundnce with four models: M 0,M b,m t, nd M tb. Model M 0 ws the null model, with constnt detection probbilities. Model M b ssumed tht p i ws equl to c i. Model M t lso ssumed tht c i nd p i were equl, but the vlues were llowed to vry through time. In model M tb, p i nd c i were modeled s constnt offsets of one nother. The four cndidte models were compred in n informtion theoretic frmework by using Akike s informtion criterion corrected for smll smple size (Burnhm nd Anderson 2002). The bundnce of Northern Pike in ech by ws ggregted to estimte the totl popultion bundnce. Totl bundnce of Northern Pike in ech yer-clss ws clculted by multiplying the estimted totl popultion bundnce by the percent ge composition derived from n ge length key. The totl weight of Westslope Cutthrot Trout consumed sesonlly by Northern Pike ws estimted s follows: (popultion bundnce of Northern Pike [ yer-clsses]) (the corresponding estimte of Westslope Cutthrot Trout biomss consumed by n individul Northern Pike). The estimted totl number of Westslope Cutthrot Trout consumed by Northern Pike ws derived from the Westslope Cutthrot Trout s (1) estimted totl consumed biomss, (2) length weight reltionship, nd (3) frequency of consumption. RESULTS Smpling ws performed on 138 d, nd 15,645 individul fish representing 24 species were cptured. Electrofishing effort totled 62.4 h, nd 638 gill nets were fished for 1,166.0 h. In totl, 58 Northern Pike were smpled with electrofishing nd 678 were smpled with gillnetting. Although electrofishing ctch rtes vried gretly mong seson nd mong bys, Northern Pike ctch rtes were consistently higher in Benewh Lke thn in the other bys (Figure 2). Northern Pike ctch rtes from gillnetting during the two spring smpling periods were twice s high s the ctch rtes observed during summer nd fll. The dt suggest tht body condition (W r ) of Northern Pike decresed between summer nd fll nd incresed the following spring (Figure 3). Additionlly, Northern Pike in Windy By tended to be in better condition thn those in the other bys cross ll sesons. Among the 736 Northern Pike cptured, 573 were mrked, 98 were recptured, nd 73 were mortlities. The recpture rte of Northern Pike ws highest (38%) in Windy By, wheres the recpture rte in the other bys ws roughly 9%. Sesonl growth nd food hbits were nlyzed from 695 Northern Pike (including recptures) vrying from 16.2 to cm TL nd from 24 to 9,628 g. Age of Northern Pike vried from 1 to 7 yers; pproximtely 95% of individuls were ges 1 4. In generl, the mjority of growth occurred between fll nd spring for most of the Northern Pike yerclsses (Figure 4). The proportion of empty stomchs vried mong yer-clsses nd sesons but ws highest for most yer-clsses during spring (Figure 5; Appendix Tble A.1). For Northern Pike belonging to the 2011 yer-clss, the diet ws dominted by wrmwter species (i.e., Yellow Perch, Bluegills Lepomis mcrochirus, nd Brown Bullheds). Slmonids becme n importnt prey txon for the 2011 yer-clss during the spring of 2013 nd ccounted for pproximtely 40% of the totl energy consumed by tht yer-clss. For older Northern Pike ( yer-clsses), the diets were dominted by slmonids (i.e., koknee nd Westslope Cutthrot Trout). Throughout the yer, koknee represented the highest frequency of occurrence, percent by number, nd energy contribution to the diet. Koknee were consumed t the highest rte during summer, ccounting for 87% of the totl energy consumed. Northern Pike consumption of Westslope Cutthrot Trout ws highly vrible mong sesons. During spring, Westslope Cutthrot Trout occurred in pproximtely 25% of Northern Pike stomchs but contributed roughly 75% of the totl energy consumed (Figure 5). During summer nd fll,

10 166 WALRATH ET AL. FIGURE 2. Men (CSE) Northern Pike ctch per unit effort (fish/h) with (A) electrofishing nd (B) gillnetting by seson in Coeur d Alene Lke from Mrch to My Months were grouped together bsed on wter temperture: spring (Mrch My), summer (June August), nd fll (September November). percent occurrence nd energy contribution of Westslope Cutthrot Trout in the diet decresed by bout 50%. In spring 2013, the occurrence of Westslope Cutthrot Trout in Northern Pike diets incresed gin reltive to summer nd fll (Figure 5). The bioenergetics models estimted tht Westslope Cutthrot Trout contributed pproximtely 2 30% of the biomss consumed by ge-1 4 Northern Pike (Tbles 1, 2). Sesonl P c vlues for Northern Pike in Coeur d Alene Lke were generlly highest during spring nd lowest during summer (Figure 6). The estimte of Northern Pike bundnce generted by the best cndidte model ws 3,268 fish (95% CI D 2,000 6,361) cross the four study bys. For the Northern Pike yer-clsses used in the bioenergetics model (i.e., the yer-clsses), bundnce ws estimted t 3,056 fish (95% CI D 1,793 5,947; Tble 3). The TL of Westslope Cutthrot Trout consumed by Northern Pike vried from 8.7 to 43.7 cm nd verged 25.0 cm (Figure 7). The TLs of Westslope Cutthrot Trout consumed by Northern Pike were highly vrible cross sesons nd generlly incresed with Northern Pike TL

11 TROPHIC ECOLOGY OF NORTHERN PIKE 167 FIGURE 3. Men (CSE) reltive weights of Northern Pike cptured in four bys of Coeur d Alene Lke from Mrch to My Mens were clculted for ech seson s well s for ech site cross ll sesons. Months were grouped together bsed on wter temperture: spring (Mrch My), summer (June August), nd fll (September November). (Figure 8). The totl Westslope Cutthrot Trout biomss consumed sesonlly by Northern Pike ( yer-clsses) in the four bys ws estimted to be 1,231 kg (95% CI D 723 2,396 kg), nd the totl number of Westslope Cutthrot Trout consumed ws estimted t pproximtely 5,641 fish (95% CI D 3,311 10,979). DISCUSSION Northern Pike hve been introduced into mny wtersheds to crete recretionl fishing opportunities throughout North Americ, including Coeur d Alene Lke. Unfortuntely, mny studies hve found tht Northern Pike cn hve detrimentl effects on ntive fishes (Muhlfeld et l. 2008; Sepulved et l. 2013). Therefore, n understnding of the effects of Northern Pike on ntive fishes is criticl for developing mngement strtegies to blnce recretionl sport fisheries with efforts to conserve ntive fishes, prticulrly species like the Westslope Cutthrot Trout. The food hbits of Northern Pike hve been extensively studied throughout the species distribution; lthough Northern Pike re generlly piscivorous, they re highly opportunistic (Frost 1954; Soupir et l. 2000; Venturelli nd Tonn 2006). For exmple, Soupir et l. (2000) reported tht invertebrtes were common in Northern Pike diets when the vilbility nd bundnce of fish were low in six lkes within Voygeurs Ntionl Prk, Minnesot. Similrly, in three eutrophic lkes in northest Albert tht lcked prey fishes, introduced Northern Pike consumed leeches nd other invertebrtes (Venturelli nd Tonn 2006). Northern Pike in the current study consumed diversity of food items, including invertebrtes, fish, nd slmnders. Invertebrtes were consumed spordiclly throughout the yer but contributed little to the overll mount of energy consumed by Northern Pike in Coeur d Alene Lke. Rther, koknee hd the gretest energy contribution to the diets in ech seson. Westslope Cutthrot Trout were consumed t the highest frequency during spring. Northern Pike lso preyed on spiny-ryed fishes (e.g., Yellow Perch nd Blck Crppies) throughout the yer, with the highest occurrence in fll nd spring 2013, likely result of prey vilbility. Ekl ov nd Hrrin (1989) reported tht Northern Pike preferred soft-ryed fishes nd tht Northern Pike switched to spiny-ryed fishes or cnniblism when preferred prey types were unvilble. Ontogenetic chnges in the diet re common for Northern Pike (Frost 1954; Miller nd Krmer 1971). The only exception ppers to be in systems with simple fish ssemblges (Soupir et l. 2000). In Coeur d Alene Lke, ontogenetic shifts in the food hbits of Northern Pike were pprent, prticulrly between ges 1 nd 2. For the 2011 yer-clss, diets primrily consisted of Yellow Perch less thn 15.0 cm, Brown Bullheds, nd centrrchids during fll. In spring 2013, the diets of the 2011 yer-clss shifted towrd lrge Yellow Perch (i.e., 15.0 cm) nd slmonids. Although the dt suggest tht Northern Pike undergo n ontogenetic shift in feeding hbits towrd slmonids t young ge, fctors such s prey vilbility, hbitt, nd gpe size lso likely ply role in the shift (Nilsson nd Bronmrk 2000). Growth of Northern Pike in Coeur d Alene Lke vried mong yer-clsses nd mong sesons. Most yer-clsses

12 168 WALRATH ET AL. FIGURE 4. Men ( SE) TLs nd weights of Northern Pike from four yerclsses (blck shded circles D 2008; open circles D 2009; blck shded tringles D 2010; open tringles D 2011) smpled in Coeur d Alene Lke from Mrch to My Months were grouped together bsed on wter temperture: spring (Mrch My), summer (June August), nd fll (September November). incresed in weight from spring to summer, but weight then decresed in the fll. Interestingly, bout 50% of the nnul growth in weight ws chieved between fll nd the beginning of the next spring. Hedrick nd Crline (1993) reported similr results, with Northern Pike losing weight from My to October nd then gining weight from October to Mrch. Other coolwter species hve lso been observed to exhibit the mjority of their growth in the fll. For exmple, Quist et l. (2002) reported tht for Wlleyes in Glen Elder Reservoir, Knss, pproximtely 80% of length nd weight ws chieved between August nd October. Percentge of C mx reflects the intensity of predtion nd prey vilbility (Rice et l. 1983). Sesonl P c vlues were consistently highest for the 2011 yer-clss of Northern Pike in Coeur d Alene Lke, likely due to the incresed metbolic demnd in juveniles (Ben 2010). We lso observed sesonl pttern in which estimtes of P c were generlly highest during spring for ll cohorts. The high estimtes of P c for spring likely reflect the vilbility of slmonids s prey nd the postspwn feeding intensity of Northern Pike. Low P c vlues for Northern Pike in the summer nd fll re probbly ttributble to decrese in prey vilbility nd the lower metbolic rtes chieved by Northern Pike s they move to cooler wter during those sesons (Bevelhimer et l. 1985). As concerns over nonntive species increse, mny such species hve been the focus of removl or suppression efforts (Mueller 2005; Spens nd Bll 2008; Kolr et l. 2010). However, high density of other nonntive species my ctully ssist in the recovery of ntive fish popultions. When predtor s preferred prey is depleted, predtors often switch to nother prey type, thereby llowing the preferred prey type to recover (Sinclir et l. 2006). The current study suggests tht some nonntive species my ct s predtion buffer for Westslope Cutthrot Trout throughout much of the yer. Specificlly, koknee nd Yellow Perch (nonntive species) ech ccounted for 30% of the totl nnul biomss consumed by Northern Pike. The occurrence of predtion buffer hs lso been reported for other qutic systems. Stpnin nd Mdenjin (2007) determined tht when Se Lmpreys Petromyzon mrinus begn preying on Lke Trout in Lke Erie, Burbot Lot lot incresed in bundnce. Nonntive prey species my crete predtion buffer for Westslope Cutthrot Trout, but numerous studies in the Pcific Northwest hve shown tht Northern Pike consume lrge quntities of Westslope Cutthrot Trout when present. For exmple, Muhlfeld et l. (2008) estimted tht Northern Pike in the upper Flthed River system, Montn, consumed pproximtely 13,000 Westslope Cutthrot Trout nnully. Similrly, Rich (1992) reported tht Westslope Cutthrot Trout contributed bout 45% of the prey weight consumed by Northern Pike in Killrney Lke, Idho. More importntly, the bility of Northern Pike to consume lrge quntities of Westslope Cutthrot Trout suggests tht high densities of Westslope Cutthrot Trout my not be fesible in system tht contins Northern Pike. An exception ws provided by Sepulved et l. (2013), who reported tht slmonid escpement objectives were met in Wood River Lke, Alsk, despite high level of predtion by Northern Pike. Sepulved et l. (2013) hypothesized tht slmonid nd Northern Pike hbitts were sptilly segregted. Northern Pike predtion on Westslope Cutthrot Trout in Coeur d Alene Lke decresed in the summer nd fll,

13 TROPHIC ECOLOGY OF NORTHERN PIKE 169 FIGURE 5. Frequency of occurrence nd prey-specific energy contribution of prey types in the diets of Northern Pike smpled from Coeur d Alene Lke during spring, summer, nd fll nd spring The sesonl frequency of empty stomchs nd the smple size (n; the number of Northern Pike stomchs with diet contents) re lso provided (INV D invertebrtes; LSS D Lrgescle Sucker; BCR D Blck Crppie; BLG D Bluegill; LMB D Lrgemouth Bss; WCR D White Crppie; CEN D Centrrchide; HER D Pcific Herring; SCP D sculpin; NPM D Northern Pikeminnow; TNC D Tench; NPK D Northern Pike; BBH D Brown Bullhed; YLP-A D Yellow Perch 15.0 cm; YLP-J D Yellow Perch < 15.0 cm; KOK D koknee; WCT D Westslope Cutthrot Trout; SAL D Slmonide; DET D detritus; TAD D tdpole; SMB D Smllmouth Bss). suggesting hbitt segregtion during those periods. Hbitt segregtion between slmonids nd Northern Pike is possible becuse slmonids typiclly spend miniml time in the shllow, vegetted res commonly occupied by Northern Pike (D Angelo nd Muhlfeld 2013). Unfortuntely, the incresed occurrence of Westslope Cutthrot Trout in Northern Pike diets during spring my negte ny benefits obtined from hbitt segregtion during other time periods. Although the period of sptil overlp ppers to be reltively short (i.e., April nd My) bsed on Northern Pike diets, previous reserch hs shown tht Northern Pike cn consume lrge quntities of prey over short time period. In study of the Dnish River, Denmrk, Jepsen et l. (1998) found tht Northern Pike predtion over 3-week period ws responsible for 56% of Atlntic Slmon Slmo slr smolt mortlities. In Coeur d Alene Lke, pproximtely 80% of the predtion on Westslope Cutthrot Trout in occurred during spring. However, the potentil effects of Northern Pike predtion on Westslope Cutthrot Trout vried depending on the study loction. Although only 29% of the Northern Pike were cptured in

14 170 WALRATH ET AL. FIGURE 6. Proportion of mximum consumption (P c ) from the bioenergetics model used to estimte consumption nd growth of four yer-clsses of Northern Pike in Coeur d Alene Lke from Mrch to My Months were grouped together bsed on wter temperture: spring (Mrch My), summer (June August), nd fll (September November). Windy By, those fish ccounted for 75% of the Westslope Cutthrot Trout tht were consumed. Bsed on our estimtes of bundnce nd consumption, Northern Pike consumed pproximtely 335 dult Westslope Cutthrot Trout (30.0 cm) in Windy By during spring. In Lke Creek, the tributry tht enters Windy By, the estimted bundnce of spwning dult Westslope Cutthrot Trout ws 410 fish (SE D 85; Firehmmer et l. ). Similr estimtes re not vilble for 2013 or for tributries of the other bys. Estimtes of Westslope Cutthrot Trout consumption by Northern Pike re lso conservtive since no smpling occurred during winter. Nevertheless, the observed predtion by Northern Pike is of concern nd my explin the low juvenile-to-dult return rtes FIGURE 7. Length frequency histogrm of dfluvil Westslope Cutthrot Trout consumed by Northern Pike in Coeur d Alene Lke from Mrch to My Dshed lines delinete the TL (cm) clsses corresponding to juvenile, subdult, nd dult Westslope Cutthrot Trout. FIGURE 8. Reltionship between Northern Pike TL (cm) nd the TLs of Westslope Cutthrot Trout consumed sesonlly in Coeur d Alene Lke from Mrch 15,, to My 31, of Westslope Cutthrot Trout (Firehmmer et l. ). Fortuntely, intense sesonl predtion on Westslope Cutthrot Trout might be llevited by reducing Northern Pike densities ner the tributries used by spwning Westslope Cutthrot Trout. Vrious mechnicl removl methods hve been used or recommended to reduce densities of nonntive predtors (Broughton nd Fisher 1981; Mnn 1985; Kulp nd Moore 2000; Mueller 2005). Suppressing nonntive predtor such s Northern Pike my be importnt for conserving slmonids nd other ntive fish species. However, desired effects from suppression efforts re usully diminished becuse the remining individuls of the trget species typiclly disply compenstory increses in recruitment, survivl, nd growth (Kolr et l. 2010). Additionlly, the mount of resources needed to reduce piscivore biomss in lrger systems is generlly prohibitive (Goemn et l. 1993). In some systems, complete erdiction of nonntive piscivores is required for chieving vibility of slmonid popultions (Spens nd Bll 2008); however, erdiction of Northern Pike from lrge systems hs been unsuccessful (Aguilr et l. 2005). Additionlly, the Northern Pike is n importnt sport fish in Coeur d Alene Lke, nd nglers re likely to show substntil opposition to removl pln. Future reserch should focus on mngement strtegies (i.e., hrvest regultions) tht might be used in Coeur d Alene Lke to reduce Northern Pike densities t smll sptil nd/or temporl scles. Results of the present study hve importnt implictions for the mngement of Northern Pike nd the conservtion of Westslope Cutthrot Trout. The primry limittion of our study ws the inbility to estimte the lkewide bundnce of Westslope Cutthrot Trout. Unfortuntely, given the size of the lke nd its mjor tributries, ttempts to estimte bsolute bundnce of Westslope Cutthrot Trout re unlikely.

15 TROPHIC ECOLOGY OF NORTHERN PIKE 171 Nevertheless, we determined tht Northern Pike consumption of Westslope Cutthrot Trout vries cross sesons nd tht Westslope Cutthrot Trout could ccount for up to 30% of the totl biomss consumed by Northern Pike. High sptil nd temporl overlp between these two species during spring resulted in reltively lrge quntities of Westslope Cutthrot Trout being consumed in some res. Thus, the Coeur d Alene Tribe s mngement objective to restore Westslope Cutthrot Trout to level tht llows for subsistence hrvest, mintins genetic diversity, nd increses the probbility of persistence despite nthropogenic influences might be chieved if predtion by Northern Pike ner tributries used by dfluvil Westslope Cutthrot Trout cn be reduced during the spring. ACKNOWLEDGMENTS We thnk C. Brown, M. Cstned, W. Field, B. Hrper, E. Hendrickson, J. Fredericks, D. Jolibois, I. Lee, N. Porter, J. Snchez, T. Schill, C. Smith, J. Smith, M. Stnger, C. Wtkins, S. Whitlock, nd T. Wilson for ssistnce with field reserch. We lso thnk A. Vitle (Coeur d Alene Tribe) nd J. Fredricks (Idho Deprtment of Fish nd Gme) for ssistnce in plnning nd implementing this study. We re grteful to N. Ben (Colville Tribe) for insights on the physiologicl prmeters he developed for bioenergetics modeling of lrger Northern Pike. C. Muhlfeld, M. Wiest, F. Wilhelm, nd two nonymous reviewers provided helpful comments on n erlier version of this mnuscript. Funding for the project ws provided by the Coeur d Alene Tribe. Additionl support ws provided by the Idho Coopertive Fish nd Wildlife Reserch Unit. The unit is jointly sponsored by the U.S. Geologicl Survey, University of Idho, Idho Deprtment of Fish nd Gme, Coeur d Alene Tribe, nd Wildlife Mngement Institute. This project ws conducted under the University of Idho s Institutionl Animl Cre nd Use Committee Protocol The use of trde, firm, or product nmes is for descriptive purposes only nd does not imply endorsement by the U.S. Government. 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