Different life histories of brook trout populations invading mid-elevation and high-elevation cutthroat trout streams in Colorado

Western North Americn Nturlist Volume 63 Number 2 Article 7 4-30-2003 Different life histories of brook trout popultions invding mid-elevtion nd high-elevtion cutthrot trout strems in Colordo Benjmen M. Kennedy Colordo Stte University, Fort Collins Dougls P. Peterson Colordo Stte University, Fort Collins Kurt D. Fusch Colordo Stte University, Fort Collins Follow this nd dditionl works t: https://scholrsrchive.byu.edu/wnn Recommended Cittion Kennedy, Benjmen M.; Peterson, Dougls P.; nd Fusch, Kurt D. (2003) "Different life histories of brook trout popultions invding mid-elevtion nd high-elevtion cutthrot trout strems in Colordo," Western North Americn Nturlist: Vol. 63 : No. 2, Article 7. Avilble t: https://scholrsrchive.byu.edu/wnn/vol63/iss2/7 This Article is brought to you for free nd open ccess by the Western North Americn Nturlist Publictions t BYU ScholrsArchive. It hs been ccepted for inclusion in Western North Americn Nturlist by n uthorized editor of BYU ScholrsArchive. For more informtion, plese contct scholrsrchive@byu.edu, ellen_mtngelo@byu.edu.

Western North Americn Nturlist 63(2), 2003, pp. 215-223 DIFFERENT LIFE HISTORIES OF BROOK TROUT POPULATIONS INVADING MID-ELEVATION AND HIGH-ELEVATION CUTTHROAT TROUT STREAMS IN COLORADO Benjmen M. Kennedyl,2, Dougls P. Peterson 1, nd Kurt D. Fusch 1 ABsTRACf.-Brook trout (SlveUnus jontilul14s), ntive to estern North Americ, hve invded mny montne cold wter systems of western North Americ, nd these invsions re implicted in the decline of ntive cutthrot trout (Oncorhynchus clrki). Iffisheries biologists re to be effective in mnging brook trout invsions, demogrphic models tht predict invsion success will need to incorporte life history vrition in different environments. We tested whether brook trout popultions invding strems t 2 different elevtions vried in life history chrcteristics tht influence popultion dynmics nd potentil invsion success. In the high-elevtion strem (3195 m), wter tempertures were colder nd brook trout pprently grew more slowly (i.e., hd shorter lengths-t-ge), becme sexully mture 2 yers lter, nd hd life spns 2 to 3 times longer thn those in the mid-elevtion strem (2683 m). This flexibility in life history my llow brook trout to mximize their chnce of estblishment nd invsion success mong elevtions, We propose tht in mid-elevtion strems fst growth nd erly mturity mximize fitness nd cn led to rpid estblishment nd high popultion growth rtes. In high-elevtion strems, slow growth, lter mturity, nd long reproductive life spn my llow brook trout to successfully estblish popultions in these mrginl hbitts where recruitment is often poor. Key words: brook trout, Slvelinus fontiniis, cutthrot trout, Oncorhynchus clrki, invsion biology, life history, Rocky Mountin strems, otouths. Brook trout (Slvelinus fontinlis), ntive to estern North Americ, were introduced widely in western North Americ strting in the 1870s (McCrimmon nd Cmpbell 1969) nd hve invded mny montne cold-wter strems where cutthrot trout (Oncorhynchus clrki) were ntive (Fusch 1989, Adms 1999). These brook trout invsions, long with hbitt deslrnction nd hybridiztion with nonntive rinbow trout (0. mykiss), hve contributed to the extinction of 2 subspecies of cutthrot trout in the interior West nd tbe extirption of tbe other 12 from 95% or more of their originl rnges (Behnke 1992, Young 1995). To more effectively mnge brook trout invsions nd conserve cutthrot trout, it will be necessry to understnd the mechnisms of invsion t the popultion level. This will require detiled informtion on the life histories of brook trout in popultions invding different environments (Moyle nd Light 1996). Like ll slmonids, brook trout life history cn vry substntilly with ltitude nd elevtion (Reimers 1979, Power 1980, Adms 1999). Generlly, the wters ofsouthern ltitudes nd lower elevtions re wrmer, more productive, nd hve longer growing sesons. Consequently, brook trout grow fster, become sexully mture erlier, nd die t younger ges thn those in cold, unproductive wters ofnorthern ltitudes nd higher elevtions. This pttern my be ttributed to flexibility in life history chrcteristics in response to differences in juvenile growth nd dult survivl (Hutchings 1993). Although brook trout life history hs been documented in other geogrphic regions (McFdden 1961, Power 1980, Hutchings 1996), there re few published dt for mountin strems of the western u.s. Our objective ws to provide new informtion on differences in life bistories of brook trout popultions invding contrsting environments by compring lengtb-t-ge, length-t-mturity, nd longevity ofbrook trout in 2 Rocky Mountin hedwter. cutthrot trout strems of different elevtions. We predicted tht the mid-elevtion strem would be wrmer, resulting in brook trout with longer lengths-t-ge, erlier sexul mturtion, nd shorter life spns. In contrst, the bighelevtion strem ws predicted to be colder, resulting in shorter lengths-t-ge, lter mturtion, nd longer life spns. IDeprtmentofFishel)' nd Wildlife Biology, Colordo Stte University, Fort Collins. CO 80523. l!pj:erent ddress' Deprtment of Aqutic, Wter$hed, nd Erth Resources, Ecology Center, Uth Stte University, Logn, UT 84322. 215

216 WESTERN NORTH AMERICAN NATURALIST [Volume 63 METHODS Brook trout were smpled in 2 strems s prt of n ongoing, lrge-scle field experiment to determine effects of invding brook trout on ntive Colordo River cutthrot trout (0. c. pleuriticus). Both re hedwter strems in the centrl Rocky Mountins in Colordo of similr width (Little Muddy Creek 3.8 m, Willow Creek 2.6 m) nd grdient (4.4%, 4.0%). Ech receives little or no ngling (Peterson nd Fusch unpublished dt). Long segments of ech strem were smpled (Little Muddy Creek 877 m, Willow Creek 1200 m). Elevtion t the midpoints of the mid-elevtion strem (Little Muddy Creek, 2683 m, rnge 2659-2695 m) nd the high-elevtion strem (Willow Creek, 3195 m, rnge 3189-3220 m) differed by 512 m. Brook trout popultions were ctively invding nd becoming nturlized in both study reches. In the high-elevtion strem, brook trout incresed from bout 30% of the popultion in 1988 (Fusch 1989) to 98% in 1998, pprently minly by upstrem movement (Peterson nd Fusch unpublished dt). In the mid-elevtion strem, brook trout rpidly reinvded the study segment from which they hd been removed (Peterson 2000). In lte summer (8 August to 20 September) 1998, 1999, nd 2000, we removed brook trout using 2-pss electrofishing removl (see Riley et l. 1992 for methods). Cptured brook trout were euthnized in n overdose of MS-222, mesured (fork length, nerest mm), nd weighed (nerest g). Sgittl otoliths were removed from fish >100 mm fork length (FL; n = 160) in the field nd stored dry in envelopes, wheres fish < 100 mm FL (n = 24) were preserved in 70% ethnol nd otoliths were lter removed in the lbortory. In 2000 we collected dditionl otoliths from brook trout <178 mm FL (n = 32) in the high-elevtion strem becuse insufficient numbers of smll fish were collected in 1998 nd 1999. Sgittl otoliths re more ccurte nd precise thn scles for determining ges of brook trout (Dutil nd Power 1977, Reimers 1979, Power 1980). Excised otoliths were clened, mounted on microscope slides with thermoplstic cement, nd ground long the sgittl plne (horizontl cross section) with 400-grit wet sndpper to clrify nnuli. Hll (1991) found no significnt difference between ges determined from verticl versus horizontl cross sections. The senior uthor counted otolith nnuli using compound microscope t 100X mgnifiction with trnsmitted light. Otoliths were red without knowledge of strem loction or fish length. Annulr bnds, lso referred to s «winter bnds," indicte periods of slow winter growth (light trnslucent bnds) between periods of fst summer growth (drk opque bnds; Dutil nd Power 1977, Reimers 1979). Fish in their 1st yer of life did not hve nnuli nd were clssed s ge O. Age estimtes were verified by the 2nd uthor who ged subset of otoliths rndomly selected from ech ge clss, gin without knowledge of strem or fish length. An ge bis plot nd coefficients of vrition were used to ssess systemtic reder bis nd precision mong ge clsses s recommended by Cmpn et l. (1995). All brook trout cptured in the 2 strems were removed to mintin tretments in the lrger field experiment tht ws being conducted simultneously. Therefore, we could not tg fish nd recpture them in subsequent yers to vlidte nnuli. Insted, we vlidted ge estimtes for brook trout in the mid-elevtion strem by ging 13 fish of known ge in control strem of similr elevtion (Est Fork Prchute Creek, 2530 m) where brook trout were not removed. Twelve fish hd been mrked t ge 0 in 1998 nd were recptured in 2000 t ge 2. One ws mrked t ge 0 in 1999 nd recptured in 2000 t ge 1. Age estimtes for fish in the high-elevtion strem could not be vlidted due to smll smple size of mrked fish recptured in similr high-elevtion control strem. Thus, ge estimtes of brook trout from the high-elevtion strem were vlidted for the first 3 yers of life by compring them to ge estimtes derived from distinguishble modes in length-frequency histogrm. Brook trout ge estimtes for the mid-elevtion strem were lso compred with length-frequency histogrm for the first 2 yers oflife. Differences in men length-t-ge between strems were nlyzed by 2-wy ANOVA on log-trnsformed dt (min effects were strem nd ge) using PROC GLM (SAS version 8.0, SAS Institute Inc., Cry, NC). Lengths of fish preserved in ethnol were converted to fresh lengths using 1.9% shrinkge correction fctor (fresh length = preserved length * 0.019 + preserved length,,2 = 0.995, P < 0.0001) for

2003] LIFE HISTORIES OF INVADING BROOK TROUT 217 juvenile sockeye slmon from Shields nd Crlson (1996). To estimte length t mturity for brook trout mles nd femles, we exmined fish collected in 1998 nd 1999 tht were preserved in 10% formlin. Femles were considered mture when their ovries contined visible ornge eggs nd extended ventrlly, filling the body cvity. If their ovries contined smll grnulr white oocytes tht were dorslly restricted, we considered the fish immture. Mles were considered mture when their testes were lrge nd white nd immture when their testes were thredlike nd restricted to the dorsl surfce ofthe body cvity (see Lgler 1978, Downs et l. 1997, nd Hutchings et l. 1999 for detiled descriptions), Preserved lengths were converted to fresh lengths using 4.8% shrinkge correction fctor (fresh length = preserved length *0,048 + preserved length, r 2 = 0,994, P < 0.0001) derived from smple of25 brook trout from the mid-elevtion strem where fresh nd preserved lengths were mesured, We used logistic regression (PROC LOGISTIC) to nlyze how mturity depended on fish length nd strem. Seprte nlyses were performed for mles nd femles. All sttisticl tests were considered significnt t ex = 0.05. Wter temperture ws mesured in 1999 nd 2000 using TidBit thermogrphs (Onset Computer Corp., Inc., Pocssett, MA), Thermogrphs set to record temperture ech 24 72 minutes were plced in the deepest pool in ech study strem during summer 1999 nd retrieved in summer 2000. Wter temperture differences between the strems were ssessed by compring men nnul dily temperture, men July temperture, nd wrmest men dily temperture of the summer. To compre length ofthe growing seson between strems, we determined the number of dys on which men dily wter temperture equled or exceeded 4 C. This is useful mesure becuse feeding nd growth ofbrook trout in quculture hve been shown to virtully cese below 4 C (Power 1980, Dwyer et l. 1983). RESULTS Vlidtion In the mid-elevtion control strem, 12 brook trout mrked s ge 0 in 1998 nd recptured in 2000 s ge 2 (men 157 mm FL, rnge 145-166) were determined from otoliths to be ge 2, The single ge-o fish mrked in 1999 nd recptured in 2000 s ge 1 (123 mm FL) ws ged to be ge 1. In the mid-elevtion strem, otolith ges for fish ged 0 nd 1 mtched ges estimted from distinguishble modes in the length-frequency histogrm for 1998 nd 1999 combined (Fig. 1). In the highelevtion strem, ges estimted by otoliths for fish ges 0, 1, nd 2 lso mtched ges estimted from distinguishble modes in the lengthfrequency histogrm for 1998 through 2000 combined. Older ges could be neither distinguished from the length-frequency histogrms nor vlidted, Verifiction Visul inspection of the ge bis plot of reder 1 versus reder 2 reveled no bis in ge estimtes for ges 0 through 7 (i.e., miniml devition from the 1:1 rtio reference line), However, for fish ged 8 nd older, ges estimted by reder 2 were lower thn those by reder 1. 10 resolve this discrepncy, the 8 otoliths with the lrgest difference in ge estimtes were red gin by ech reder without knowledge ofprevious ge estimtes or strem of origin. The ge bis plot reveled no bis until ge 9, nd estimtes of older fish were closer between reders (Fig. 2). Coefficients of vrition (CV) of ge estimtes for ges 0 to 8 were low (men CV 4.5%, rnge 0-14). Six of 9 ge clsses hd CV <4.0%. Coefficients of vrition for ges 9 nd older were not clculted due to the bis ssocited with those ge clsses. Length-t-Age nd Longevity Men length-i-ge ofbrook trout from the high-elevtion strem (Willow Creek) ws significntly less thn brook trout from the midelevtion strem (Little Muddy Creek) t ges 0, 1, nd 2 (P < 0.001 by t tests; Tble 1). Age 3 brook trout were lso smller in the highelevtion strem, but the difference ws not significnt (P = 0.51), Tho-wy ANOVA showed significnt interction of fish ge nd strem (P < 0.001), so t tests were used to compre simple effects of strem t ech ge. Brook trout length reched n symptote fter ge 5 in the high-elevtion strem but did not rech n symptote in the mid-elevtion strem (Fig. 1).

218 WESTERN NORTH AMERICAN NATURALIST [Volume 63 300 275 + Mid..levtion strem High-elevtion strem 250 225 g g + ~ 0 200 + 8 i - E- 0.0+ J: E 175 ot 8+ 8 ~ ~ 0 0 0 g $ : 0+ g (l en 150 p+ 150 <: '" + ~ 0*... '40... 0 ~ 0 ll 125 pi 8+ 130 ':ro - 110 100 + 0 E.. 100 I ~ - 0 '" c ~ 00 eo 75... 3 70 50 25 0 0 ~ t1. eo 50 40 30 :ro High-elevtion MidMelevtion 0 10 :ro 30 o 20 40 eo 80 100120140160 Frequency 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Number of Annuli Fig. L Fork length-t-ge nd length frequency histogrms for brook trout from the high elevtion strem (Willow Creek) nd mid-elevtion strem (Little Muddy Creek). Ages re bsed on otoliths red by reder 1 (senior uthor) from 216 fish removed in 1998, 1999, nd 2000. Dt points re offset for clrity. The length-frequency histogrm for the high-elevtion strem includes ll 6sh <146 mm fork length (FL) cptured during popultion SUlVeys in August nd September 1998, 1999, nd 2000, nd the histogrm for the mid-elevtion strem includes ll fish <141 mm FL cptured during popultion surveys in September 1998 nd 1999. Brook trout from the high-elevtion strem hd mximum life spn 2 to 3 times longer thn trout collected from the mid-elevtiou strem (Tble 1, Fig. 1). Mny brook trout removed from the high-elevtion strem were ges 8 to 10 nd few were ged to be 11 to 14. Ages of brook trout lrger thn 200 mm were highly vrible, rnging from ge 4 to ge 14. Most brook trout from the mid-elevtion strem reched only ge 2 or ge 3, nd the oldest fish were ges 4 nd 5. Mturity Brook trout mles ud femles from the mid-elevtion strem mtured t shorter length nd youuger correspouding ge thn those from the high-elevtion strem (Fig. 3, Tble 2). Both strem (femles X 2 = 26.7, P < 0.0001; mles X 2 = 16.9, P < 0.0001) nd FL (femles X 2 = 106.3, P < 0.0001; mles X 2 = 62.0, P < 0.0001) were significnt fctors in predicting the mturity sttus of brook trout.

2003] LIFE HISTORIES OF INVADING BROOK TROUT 219 12 11 10 9 8 N 7 ~ 6 j : 3 2 1 o 4 _1 o 1 2 3 4 5 6 7 8 9 10 11 12 Fig. 2. Age bis plot compring ge estimtes of reder 1 (senior uthor) nd reder 2 (second uthor) for smple of61 brook trout otoliths. Error brs represent 2 stndrd errors bout the men ge ssigned by reder 2 for ll brook trout ssigned given ge by reder 1. Numbers bove dt points re smple sizes for ech comprison. The 1:1 equivlence is lso shown. The effect of FL did not vry by strem (i.e., no FL * strem interction: femles X 2 = 2.70, P = 0.10; mles X 2 = 2.54, P = 0.11), so the interction term ws removed to produce the finl equtions: probbility ofmturity (femles) = [exp (-20.0091 + 5.0592' strem + 0.1107' FL)] I [1 + exp (-20.0091 + 5.0592 '" strem + 0.1107 '" FL)] probbility ofmturity (mles) = [exp (-12.3065 + 3.7229' strem + 0.0735' FL)J I [I + exp (-12.30 5 + 3.7229' strem + 0.0735' FL)J where strem = 1 for the mid-elevtion strem nd strem = 0 for the high-elevtion strem. In the mid-elevtion strem, logistic regression predicted femles 135 mm FL (128-142 mm, 95% fiducil confidence limits) to hve 0.5 (i.e., 50%) probbility ofbeing mtnre versus 181 mm FL (171-187 mm) in the high-elevtion strem. Mles 117 mm FL (111-123 mm) were predicted to hve 0.5 probbility of being mture in the mid-elevtion strem versus 168 mm FL (148-179 mm) in the highelevtion strem. Men length-t-ge reltionships indicted tht the mjority of femles from the mid-elevtion strem mtured t ge 2 versus ge 4 in the high-elevtion strem 1 (Tble 1). The mjority of mle brook trout mtured t ge 1 in the mid-elevtion strem nd ges 3 nd 4 in the high-elevtion strem. Temperture The high-elevtion strem ws substntilly colder nd the growing seson much shorter thn the mid-elevtion strem during summer 1999 to 2000 (27 August 1999 to 26 August 2000). Men dily wter temperture for this yer ws 1.9 C in the high-elevtion strem nd 3.8 C in the mid-elevtion strem. Men wter tempertures in July were 7.1 C in the high-elevtion strem versus 12.5 C in the mid-elevtion strem, nd men dily wter tempertures for the wnnest dy were 9.2 C nd 13.5 C, respectively. In the high-elevtion strem, wter temperture verged 4 C or wrmer on only 87 dys, wheres in the midelevtion strem 134 dys were t lest this wrm, difference in growingseson of47dys. DISGUSSION Brook trout in the mid-elevtion strem hd very different life history from those in the high-elevtion strem. As predicted, temperture dt showed tht the high-elevtion strem ws much colder thn the mid-elevtion strem nd the growing seson ws 47 dys shorter. In n quculture setting, brook trout hve lower growth efficiency t colder wter tempertures (4-7 C) nd chieve mximum growth efficiency t 1O-13 C (Dwyer et l. 1983). Brook trout in the mid-elevtion strem were longer t ges 0, 1, nd 2 reltive to brook trout in the high-elevtion strem. This suggests tht brook trout grew fster s juveniles in the mid-elevtion strem thn in the high-elevtion strem. However, ge-3 brook trout were similr in length in both strems, which could hve been cused by t lest 3 fctors. First, growth of brook trout in the mid-elevtion strem my hve been slowed fter first reproduction, which occurred t n erly ge. Second, brook trout my hve been immigrting into the high-elevtion study segment from res downstrem where growth is fster. Third, the fster-growing brook trout in the mid-elevtion strem my hve been dying t younger ges, leving only the slower-growing fish in the popultion for our comprison. Overll, we predicted tht brook trout in the high-elevtion strem would mture lter nd

220 WESTERN NORTH AMERICAN NATURALIST [Volume 63 TABLE 1. Lengthwt-ge (mm) of brook trout removed from the high-elevtion strem (Willow Creek) nd mid-elevtion strem (Little Muddy Creek) in lte summers of1998, 1999, nd 2000. Ages re bsed on otoliths red by reder 1. 1\vosmple t tests were used to test simple effects ofstrem t ech ge. High-elevtion strem Mid-elevtion strem Age n Men Rnge 95%CI n Men Rnge 95%CI P 0 10 45 38-58 40-50 10 69 63-74 67-72 <0.001 1 11 77 70-88 74-81 41 117 99-134 114-119 <0.001 2 22 131 103-170 124-138 35 154 115-195 147-161 <0.001 3 6 158 134-177 141-176 8 166 138-187 149-182 0.51 4 7 175 135-210 154-197 1 190 5 5 198 189-209 186-210 1 203 6 10 201 182-222 193-209 7 5 197 194-203 193-202 8 11 209 194-231 200-218 9 18 211 190-236 205-217 10 8 216 200-222 210-222 11 2 212 208-216 161-263 12 4 213 187-226 184-242 13 14 1 246 hve longer life spns thn those in the midelevtion strem. Our dt support this prediction, showing tht, on verge, mle nd femle brook trout in the high-elevtion strem were sexully mture 2 yers lter nd lived 2 to 3 times longer thn fish in the mid-elevtion strem. Brook trout invding cutthrot trout strems in Colordo could be ccurtely ged using otoliths. Ages derived from otoliths mtched known ges from mrked fish, indicting tht brook trout otoliths formed single distinguishble nnulus ech winter. Age estimtes for brook trout up to ge 8 were precise nd highly reproducible s indicted by the miniml devition between reders nd low coefficients of vrition. Precision of ge estimtes for older fish depended on the redbility of nnuli. For exmple, one otolith with obscure, closely spced nnuli ws ged 11 by reder 1 nd ged 8 by reder 2. In contrst, nother otolith with clerly distinguishble nnuli ws ged 12 by both reders. Reimers (1979) vlidted otolith ge estimtes for old brook trout in stunted popultions (up to 24 yers old) in unproductive high-elevtion lkes by compring otolith ge estimtes with known ges bsed on stocking records nd found tht the estimtes mtched. Hll (1991) mrked fish with oxytetrcycline to vlidte the formtiou ofnnuli on otoliths of older brook trout (5-10 yers old) in stunted popultions nd found tht one distinguishble nnulus ws formed 1 o o 50. f.'.... Fig. 3. Probbility of sexul mturity s function of length for mle nd femle brook trout removed from the high-elevtion strem (Willow Creek) nd mid-elevtion strem (Little Muddy Creek) estimted by logistic regression. The left 2 curves re for the mid-elevtion strem, the right 2 curves for the high~elevtion strem. Arrows indicte lengths offemles with 0.5 probbility ofbeing mture (134 mm FL in the mid-elevtion strem, 181 mm FL in the high.elevtion strem). Probbilities re bsed on smple of402 brook trout removed in erly fll 1998 nd 1999. ech winter. Kruse et l. (1997) found otolith ge estimtes for Yellowstone cutthrot trout to be very precise when compring ge estimtes from 3 reders who red ech otolith 3 different times, Ptterns tht we found in brook trout life histories between strems t 2 elevtions hve been documented in the Rocky Mountins of the northern u.s. nd elsewhere in North " Mid-elevtion High-elevtion strem i strem.' "....,...,..., ~Femles...... Mres //.J 100 150 200 250 Fork length (mm) 300

2003J LIFE HISTORIES OF INVADING BROOK TROUT 221 TABLE 2. Percent ofmle nd femle brook trout tht were sexully mture by size clss in the high-elevtion (Willow Creek) nd mid-elevtion (Little Muddy Creek) strems (smple sizes re in prentheses). Dt re bsed on 402 fish removed during August nd September 1998 nd 1999 from the high-elevtion strem nd during September 1999 from the mid-elevtion strem. High-elevtion strem Mid-elevtion strem Fork length clss (mm) Mles Femles Mles Femles <101 0(2) 18.2 (II) 0(14) 101-110 0(1) 0(1) 33.3 (24) 0(19) 111-120 o(1) - 65.2 (23) 22.2 (9) 121-130 100 (I) 0(2) 66.7 (6) 60.0 (5) 131-140 0(3) 0(1) 100 (2) 87.5 (8) 141-150 0(1) 100 (5) 66.7 (6) 151-160 0(1) 0(1) 100 (6) 100 (6) 161-170 66.7 (3) 33.3 (3) 100 (6) 100 (6) 171-180 88.9 (9) 50.0 (6) 100 (5) 100 (2) 181-190 91.7 (12) 88.5 (26) 100(1) 191-200 90.5 (21) 95.0 (40) 201-210 93.9 (33) 96.0 (25) 100 (I) 211-220 100 (20) 100 (16) 221-230 100 (5) 100 (1) >230 100 (I) 0(1) Americ. Power (1980) described life history of brook trout in productive southern-ltitude strems, where 2-yer-old femles provide most of the egg production but rrely rech their 4th yer of life (McFdden 1961). Power (1980) lso described life history in unproductive northern-ltitude nd lpine wters, where brook trout grow slowly, mles mture t ge 3 or older nd femles t ge 4 or older, nd trout live to be 8 to 12 yers old. In the Rocky Mountins ofthe northwestern U.S., Mulln et I. (1992) reported tht brook trout in wrm, mid-elevtion strems grow quickly, mture between ges 2 nd 3, nd live to ge 4, wheres brook trout in cold, high-elevtion strems grow slowly, mture t ge 3, nd live to ge 9. Within single Montn strem, Adms (1999) found longer lengths-t-ge nd erlier mturity in wrmer, mid-elevtion rech thn in high-elevtion rech, but found little difference in life spns. Life history theory of teleosts nd brook trout presented by Power (1980), Roff (1984), nd Hutchings (1993, 1994, 1996) cn help explin observed differences in life histories between strems in our study. Vrying environmentl conditions tht result in different juvenile growth nd dult survivl rtes cn lter the life history strtegy required to mximize fitness. Hutchings (1996) found tht brook trout with the fstest growth rtes will mximize fitness by mturing erly with gret reproductive effort (e.g., in the mid-elevtion strem). This mximizes the chnce of successful reproduction before deth, but energy spent during reproduction t this size cuses high post-reproductive mortlity, resulting in shorter life spns (Roff 1984). In contrst, brook trout with the slowest growth rtes will mximize fitness by delying mturity to lrger sizes nd older ges (e.g., the high-elevtion strem). This dely will decrese post-reproductive mortlity (Hutchings 1994), resulting in longer life spns, nd enbles fish to spwn in multiple yers with incresed totl fecundity. If fisheries biologists re to be effective in mnging brook trout invsions in Rocky Mountin strems, demogrphic models tht predict invsion success will need to be developed t lndscpe scles (Fusch et!' 2002). However, such models will be sensitive to life history vrition ssocited with different environments, mking empiricl dt such s we pre~ sent necessry to increse their robustness (Adms 1999). Differences in pprent growth, sexul mturity, nd longevity between strems suggest tht flexibility in life history is one mechnism whereby brook trout mximize the chnce of estblishment nd promote successful 'invsions cross wide rnge of environments in the western u.s. In mid-elevtion strems, fst growth nd erly mturity will fvor rpid estblishment nd high popultion growth rtes (Vermeij 1996). In turn, this my led to high dispersl nd high coloniztion rtes elsewhere, often frther upstrem (Gown

222 WESTERN NORTH AMERICAN NATURALIST [Volume 63 nd Fusch 1996). In contrst, in cold, unproductive, high-elevtion strems, lter mturity nd long reproductive life spns cused by slow growth my llow brook trout to successfully estblish popultions even in these mrginl hbitts, where probbility of successful recruitment is reltively low mong yers. Thus, lndscpe-scle models predicting brook trout invsion rtes, distribution, nd popultion growth rtes should incorporte dt from multiple elevtions to ccount for differences in life history chrcteristics. ACKNOWLEDGMENTS We thnk the lrge number of technicins nd volunteers, too mny to nme, who helped smple trout in the study strems during 1998 through 2000. Robert Behnke, Amy Schrnk, Ed Weber, nd 2 nonymous reviewers mde helpful comments on the mnuscript. We thnk Rlph Swnson (U.S. Deprtment of Interior); Dve Lnglois nd Tom Nesler (Colordo Division of Wildlife); Mike Young, Dve Winters, nd Pul Guenther-Gloss (US. Forest Service); Whit Fosburgh (Trout Unlimited); Jy Thompson nd Mike Kinser (Bureu of Lnd Mngement); nd Chris Kennedy nd Bruce Rosenlund (US. Fish nd Wildlife Service) for help with reserch nd dministering funding. This reserch ws funded by the US. Deprtment of Interior Centrl Uth Project Completion Act, Colordo Division ofwildlife, US. Forest Service, nd Trout Unlimited. In ddition, reserch funding ws provided by Colordo Stte University Student Chpter Americn Fisheries Society Undergrdute Reserch Grnt to B. Kennedy, nd West Denver Trout Unlimited Scholrship to D. Peterson. LITERATURE CITED ADAMS, S.B. 1999. Mechnisms limiting vertebrte invsion: brook trout in mountin strems of the northwestern USA. Doctorl disserttion, University of Montn, Missoul. BEHNKE, RJ. 1992. Ntive trout ofwestern North Americ. Americn Fisheries Society Monogrph 6. Bethesd, MD. 275 pp. CAMPANA, S.E., M.e. ANNAND, AND ].I. McMILLAN. 1995. Grphicl nd sttisticl methods for determining the consistency of ge determintions. Trnsctions of the Americn Fisheries Society 124:131-138. DOWNS, e.c., R.C. WHITE, AND B.B. SHEPARD. 1997. Age t sexul mturity, sex rtio, fecundity, nd longevity of isolted hedwter popultions of westslope cutthrot trout. North Americn Journl of Fisheries Mngement 17:85-92. DUTIL, J.D., AND C. POWER. 1977. Vlidity of reding otoliths compred to reding scles for determintion of ge of brook trout (Slvelinus fontinlis). Nturliste Cndien 104:361-367. DWYER, w.p, C.E. SMITH, AND R.C. PIPER. 1983. Brook trout growth efficiency s ffected by temperture. Progressive Fish-Culturist 45: 161-163. FAuseR, K.D. 1989. Do grdient nd temperture ffect distributions o nd interctions between, brook cho' (Slvelinus fontinlis) nd other resident slmonids in strems? Physiology nd Ecology Jpn, specil volume 1:303-322. FAuseR, K.D., c.e. TORGERSEN, c.v. BAXTER, AND H.W. LI. 2002. Lndscpes to riverscpes: bridging the gp between reserch nd conservtion of strem fishes. BioScience 52:483-498. COWAN, G., AND K.D. FAUSCH. 1996. Mobile brook trout in two high-elevtion Colordo strems: re-evluting the concept of restricted movement. Cndin Jouml of Fisheries nd Aqutic Sciences 53,1370-1381. HALL, D.L. 1991. Age vlidtion nd ging methods for stunted brook trout. Trnsctions of the Americn Fisheries Society 120:644-649. HUTCHINGS, J.A. 1993. Adptive life histories effected by ge-specific survivl nd growth rte. Ecology 74: 673-684.. 1994, Age- nd size-specific costs of reproduction --within popultions of brook trout, Slvelinm fon-unlis. Oikos 70:12-20. ---0::.' 1996. Adptive phenotypic plsticity in brook trout, Slvelinus fonti-nzis, life histories. Ecoscience 3,25-32. HUTCHINGS, J.A., A. PICKLE, C.R. MCGREGOR-SHAW, AND L. POIRIER. 1999. Influence ofsex, body size, nd reproduction on overwinter lipid depletion in brook trout. Journl offish Biology 55:1020-1028. KRUSE, C.G., W.A. HUBERT, AND RJ. RAHEL. 1997, Using otoliths nd scles to describe ge nd growth ofyellowstone cutthrot trout in high-elevtion strem system, Wyoming. Northwest Science 71:30-38. LACLER, K.R 1978. Cpture, smpling nd exmintion of fishes. Pges 7-47 in T. Bgenl, editor, Methods for ssessment of fish production in fresh wters. 3rd edition. Blckwell Scientific Publictions, Oxford, London, MACCRIMMON, H.R.,.AND J.S. CAMPBELL. 1969. World dis tribution ofbrook trout, Slvelinusfonttnlis. Journl of the Fisheries Reserch Bord of Cnd 26: 1699-1725. McFADDEN, J.T, 1961. A popultion study of the brook trout, Sw"'"",,fontinnlis. Wildlife Monogrph 7. 73 pp. MOYLE, EB., AND T. LIGHT. 1996. Biologicl invsions of fresh wter: empiricl rules nd ssembly theory. Biologicl Conservtion 78:149-161. MULLAN, J.w., K.R. WILLIAMS, G. RHODUS, T.W HILLMAN, AND J.D. MCINTYRE. 1992. Production nd hbitt of slmonids in mid-columbi River tributry strems. U.S. Fish nd Wildlife Service, Monogrph 1. Leven~ worth, WA. PETERSON, D.E 2002. Popultion ecology of n invsion: demogrphy, dispersl, nd effects ofnonntive brook trout on ntive cutthrot trout. Doctorl disserttion, Colordo Stte University, Fort Collins, CO. 213 pp.

2003J LIFE HISTORIES OF INVADING BROOK TROUT 223 POWER, G. 1980. The brook chrr, SlveUnus fontinlis. Pges 141-203 in E.K. Blon, editor, Chm: slmonid fishes of the genus Slvelinus. Dr. W. Junk, The Hgue, The Netherlnds. ' REIMERS, N. 1979, A history of stunted brook trout popultion in n lpine lke: lifespn of24 yers. Cliforni Fish nd Gme 65:196-215. RILEY, S.C., K.D. FAUSCH, AND C. GOWAN. 1992. Movement of brook trout (Slvelinus fontinlis) in four smll sublpine strems in northern Colordo. Ecology of Freshwter Fish 1:112-122. ROFF, D.A. 1984. The evolution of life history prmeters in teleosts. Cndin Journl offisheries nd Aqutic Sciences 41:989-1000. SHIELDS, P.A., AND S.B. CARLSON. 1996. Effects of formlin nd lcohol preservtion on lengths nd weights ofjuvenile sockeye slmon. Alsk Fishery Reserch Bulletin 3:81-93. VERMEIl, G.J. 1996. An gend for invsion biology. Biologicl Conservtion 78:3-9. YOUNG, M.K. 1995. Conservtion ssessment for inlnd cutthrot trout. Generl Technicl Report RM-GTR 256. USDA Forest Service, Rocky Mountin Forest nd Rnge Experiment Sttion, Fort Collins, CO. Received 12 June 2001 Aceeprod 25 April 2002