Trophic Economics of Lake Trout Management in Reservoirs of Differing Productivity

North American Journal of Fisheries Management 20:127±143, 2000 ᭧ Copyright by the American Fisheries Society 2000

BRETT M. JOHNSON Department of Fishery and Wildlife Biology, Colorado State University, Fort Collins, Colorado 80523, USA

PATRICK J. MARTINEZ* Colorado Division of Wildlife, 711 Independent Drive, Grand Junction, Colorado 81505, USA

Abstract.ÐBecause of their potential to reach large size, lake trout Salvelinus namaycush are managed as trophy ®sh in many reservoirs in the western United States. In Colorado, restrictive harvest regulations for lake trout were enacted in reservoirs exhibiting a range of productivities. Annual stockings of kokanee (lacustrine sockeye salmon Oncorhynchus nerka) and rainbow trout O. mykiss sustained sport ®sheries, but stocked ®sh also dominated the prey ®sh assemblages in these systems. Hydroacoustic surveys suggested that lake trout management allowed an imbalance to develop between prey ®sh biomass and the biomass of piscivorous lake trout. Piscivorous ®sh biomass was, on average, 60% of total pelagic ®sh biomass. Bioenergetics modeling con®rmed the imbalance: annual lake trout consumption was near or exceeded annual pelagic prey ®sh supply (standing stock plus production); the degree of imbalance was greater in less productive reservoirs. Annual subsidies to the food web, in the form of stocked sport ®sh, were necessary to allow the imbalance between predator and prey populations to persist, especially in the least-productive systems. Though highly sensitive to the size at which hatchery ®sh are consumed, the per capita costs to sustain lake trout growth at observed levels would total about US$200 per lake trout in the more productive reservoirs and $300±600 per lake trout in the less productive reservoirs. The cost of maintaining high lake trout biomass in these stocked systems may be a dif®cult management strategy to justify and sustain, either economically or socially.

Trophic economics is a term coined by Ney nomics concepts have been successfully applied to (1990) to describe the relationship between prey important management questions of carrying ca- resource supply and consumer demand. Ney's the- pacity and stocking policy on the Laurentian Great ses were that (1) sport ®sh production is often Lakes (Stewart et al. 1981), in limits to sport ®sh limited by food supply, (2) a variety of methods production in southeastern U.S. reservoirs (Plos- exist to quantify predator±prey relationships in key and Jenkins 1982), and to the effects of harvest ®sheries, and (3) ®shery managers' actions should restrictions on predator consumption demand maintain some balance between predator con- (Johnson et al. 1992; Luecke et al. 1994; Johnson sumption demand and prey supply. Failure to man- and Martinez 1995). Although predicting future age within the constraints dictated by ecosystem states of managed ®sheries is complicated by well- productivity and sport ®sh food supply threatens known recruitment stochasticity (Magnuson the ecological integrity of the ecosystem and re- 1991), quantifying existing relationships between sults in expensive and ultimately unsustainable predator demand and prey supply is important be- management programs. cause ultimately the persistance of predator and In practice, trophic economics has been an un- prey populations will depend in part on their rel- derlying tenet of ®shery management for decades. ative biomasses, or the balance between predation Swingle (1950) was perhaps the ®rst ®shery sci- rate and prey supply. entist to attempt to balance piscivorous sport ®sh and their prey in ponds for the purpose of manip- Balance may be an unfortunate term because it ulating growth rates and size structure of large- implies that managers should strive to achieve a mouth bass Micropterus salmoides and bluegills stable equilibrium between predator and prey pop- Lepomis macrochirus. More recently, trophic eco- ulations. Contemporary ecologists and managers are becoming more aware of the nonequilibrial dynamics of communities, particularly when systems * Corresponding author: [email protected] are subjected to anthropogenic disturbance (Pimm Received September 8, 1998; accepted September 14, 1999 1991; Meffe and Carroll 1994). Managers of west-

127 128 JOHNSON AND MARTINEZ ern reservoirs should be especially cognizant of TABLE 1.ÐSize, number, and cost of hatchery ®sh this variability because of the vagaries of man- stocked in study reservoirs during 1994. Subcatchable and aging human-made environments with nonnative catchable trout consisted mainly of rainbow trout but also included small numbers of cutthroat trout and brook trout. predator and forage species (Wydoski and Bennett 1981). In practice, balance is a dynamic system Taylor attribute, the value of which changes with recruit- Blue Mesa Lake Park Twin Measure Reservoir Granby Reservoir Lakes ment, growth, and survival patterns of managed populations. Hence, as Ney (1990) argues, man- Kokanee fry aging for a balance between predator demand and Length (mm) 51 38 76 Number/ha 380.0 334.5 264.4 prey supply is a dynamic, adaptive process re- Kg/ha 0.38 0.15 1.06 quiring periodic monitoring. Fishery managers can Total cost ($) 211,216 108,028 45,695 respond to unbalanced systems by applying stan- Subcatchable trout dard ®shery management tools: adjusting stocking Length (mm) 122 66 51 178 rates and manipulating harvest mortality by means Number/ha 226.7 16.8 5.1 50.4 of regulations. Kg/ha 5.2 0.04 0.005 2.0 Total cost ($) 298,251 9,380 628 29,351 In this paper, we assess trophic economics of Catchable trout four reservoir sport ®sheries by quantifying pe- Length (mm) 259 267 229 lagic predator and prey ®sh biomass, prey ®sh pro- Number/ha 27.6 90.8 75.4 duction, and the consumption demand (food con- Kg/ha 5.3 19.1 10.0 sumed by a predator population growing at phys- Total cost ($) 60,826 58,274 56,575 iological maximum) and realized consumption a 1994 US$. (biomass actually consumed by predator population) exerted by piscivorous lake trout Salvelinus namaycush on their prey. The four reservoirs ex- cur, coldwater reservoir ®sh assemblages in Col- hibited a wide range of productivities and, thus, orado are not species rich, in part due to zoogeo- different capacities to support top predators. Next, graphic constraints that resulted in no native, because most of the prey ®sh in our study reser- specialized lacustrine ®shes. The longnose sucker voirs were of hatchery origin, we evaluate the ®s- Catostomus catostomus and white sucker Catos- cal economics of sustaining the consumption by a tomus commersoni are the only nongame species lake trout population on a stocked forage base. common in these reservoirs; mottled sculpins Cot- Finally, we examine ®shery management strate- tus bairdi and johnny darters Etheostoma nigrum gies with respect to observed predator and prey are present in some reservoirs, but they are not biomasses and the economic and social costs as- abundant. sociated with maintaining the observed predator± Because natural reproduction of salmonids other prey systems. than lake trout is negligible in most Colorado reservoirs, large numbers of juvenile salmonids are Management Context stocked each year. In 1994, 3.8 ϫ 106 kokanee This study was conducted using data from four (lacustrine sockeye salmon Oncorhynchus nerka) of Colorado's largest and most important cold- and rainbow trout O. mykiss (as well as small num- water lentic ®shery resources: Blue Mesa Reser- bers of cutthroat trout O. clarki and brook trout voir (Gunnison County), Lake Granby (Grand Salvelinus fontinalis) were stocked in the four County), Taylor Park Reservoir (Gunnison Coun- study reservoirs alone (Table 1). Three sizes or ty), and Twin Lakes (Lake County). Each of these types of stocking are typically employed to man- reservoirs has been managed, in part, to produce age the state's coldwater lentic ®sheries: kokanee large lake trout. Lake trout are a popular nonnative fry (51 mm total length, TL), subcatchable rain- sport ®sh, and because of their long life span, many bow trout (120 mm TL), and catchable rainbow can attain large size (Ͼ20 kg) if prey ®sh resources trout (260 mm TL). Managers rely on the smallest are suf®cient and they are protected by harvest and, hence, least-expensive ®sh in productive sys- regulations (Luecke et al. 1994). Lake trout are tems where stocked ®sh can be expected to grow also the dominant apical predators in many Col- rapidly and recruit to the ®shery by exploiting a orado mountain reservoirs. Most of these systems no-cost reservoir food supply. Catchable-sized ®sh are oligotrophic to mesotrophic with a rather lim- are typically used where rapid and ef®cient angler ited capacity for sport ®sh production. Unlike oth- exploitation are expected and where the recipient er regions of North America where lake trout oc- system has insuf®cient food resources to allow fry TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 129

TABLE 2.ÐHarvest regulations for lake trout in the study reservoirs since 1970. Size restrictions are given in English units. Metric equivalents are as follows: 15 in ϭ 381 mm, 20 in ϭ 508 mm, 22 in ϭ 559 mm, 26 in ϭ 660 mm, 34 in ϭ 864 mm, and 36 in ϭ 914 mm.

Daily bag limit : size restriction for Regulation Blue Mesa Taylor Park period Reservoir Lake Granby Reservoir Twin Lakes 1970±1971 4:none 4:none 4:none 4:15 in minimum 1972±1973 4:none 4:none 4:none 2:20 in minimum 1974±1976 2:15 in minimum 2:15 in minimum 2:15 in minimum 2:15 in minimum 1977±1984 2:none 2:none 2:none 2:15 in minimum 1985±1987 1:20 in minimum 1:20 in minimum 1:20 in minimum 1:20 in minimum 1988±1989 1:20 in minimum 1:20±30 in 1:20 in minimum 1:20 in minimum protected slot 1990±1992 1:22±34 in 1:22±34 in 1:22±34 in 1:22±34 in protected slot protected slot protected slot protected slot 1993±1995 1:22±34 in 2:26±36 in 1:22±34 in 1:22±34 in protected slot protected slot, protected slot protected slot only 1 Ͼ36 in 1996±2000 8:none 4:26±36 in 3:only 1 Ͼ26 in 1:22±34 in protected slot,a protected slot only 1 Ͼ36 in a Protected slot relaxed during July±September due to high expected hooking mortality. or subcatchables to grow to a harvestable size. Yet, none of the regulation changes were enacted Thus, because the cost of hatchery ®sh increases with respect to productive capacity of the reser- with size at stocking, unproductive reservoirs re- voirs or the relationship between predator con- quire the most expensive hatchery products to sus- sumption demand and prey supply (Johnson and tain a ®shery. Martinez 1995). Further, no ®shery assessments The paucity of nongame ®shes and intensive aimed at evaluating trophic balance have occurred stocking in Colorado reservoirs have produced pe- until the present study. lagic prey ®sh assemblages that are dominated by stocked kokanee and rainbow trout. Not surpris- Methods ingly, preferred prey for lake trout in Colorado Reservoir productivity.ÐBecause basin morphom- include rainbow trout and kokanee, themselves etry, siltation, and water level ¯uctuations pre- popular sport ®shes. clude signi®cant primary and secondary produc- Lake trout biomass has been fostered in the four tion in the littoral zone, food webs in many res- reservoirs by stocking and restrictive harvest reg- ervoirs are sustained primarily by pelagic produc- ulations (Table 2). Over the past two decades, lake tion (Kimmel et al. 1990; Hayes et al. 1993; trout have been stocked sporadically in Blue Mesa Martinez and Wiltzius 1995). A wide variety of Reservoir, more regularly in Lake Granby, and variables have been used as indicators of lake pro- nearly annually in Twin Lakes. Restrictive bag ductivity (Downing and Plante 1993), here de®ned limits (two ®sh/d) and minimum size limits (most- as the inherent capacity of a system to produce ly 15 in [381 mm]) were in effect on all study ®sh biomass. We evaluated several abiotic and bi- reservoirs starting in 1974. In 1985, the bag limit otic indicators of reservoir ``productivity'': length was reduced to one ®sh/d, and the minimum size of growing season, morphoedaphic index (MEI; limit increased to 20 in (508 mm). During 1988± Ryder 1965; Henderson et al. 1973), midsummer 1995, a variety of closed slot regulations were im- Secchi disk depth, midsummer chlorophyll con- plemented, with the most restrictive regulations at centration, crustacean zooplankton density, den- Lake Granby. This trend raised concern with some sity of opossum shrimp Mysis relicta, and ®sh bio- biologists about the sustainability of the trophy mass density. lake trout management strategy and about the eco- Length of the growing season was de®ned as the nomic and recreational tradeoffs that were required number of days that reservoir surface temperature to produce trophy lake trout ®sheries at the ex- exceeded 10ЊC. The MEI was computed from pense of a sport ®sh forage base. If effective, these mean depth (at full pool) and conductivity (␮S/ regulations will have increased the biomass and, cm; total dissolved solids ϭ 0.62 ϫ conductivity; hence, realized consumption of large lake trout. APHA 1995) obtained from the National Park Ser- 130 JOHNSON AND MARTINEZ vice (M. Malick, Curecanti National Recreation fell in a size-class at the exact midpoint between Area, unpublished data), Weiler (1982), Ugland et two successive ages were divided equally into the al. (1994), and La Bounty and Sartoris (1993). two age-classes. Some ®sh were probably under- Midsummer chlorophyll-a concentrations were de- aged and others overaged in each age-class; how- termined for July±August from 0±10-m integrated ever, the bias should have been small if lengths of water column samples in 1994 in Blue Mesa Res- successive age-classes were distributed symmet- ervoir, Lake Granby, and Taylor Park Reservoir. rically. Although the error in assigned ages prob- Concentrations were measured by methanol ex- ably increased with ®sh size, abundance and an- traction (Holm-Hansen and Riemann 1977; Mark- nual growth increment decreased with age, making er et al. 1980) and a calibrated Turner model 450 errors in estimated ages less consequential to our ¯uorometer. Data for Twin Lakes were from 0± consumption estimates. Lake trout biomass was 15-m composite samples (S. Campbell, U.S. Geo- computed from reservoir-speci®c length±weight logical Survey, unpublished data). regressions (R2 Ն 0.97; Martinez 1995) that con- Crustacean zooplankton density was estimated verted hydroacoustic estimates of numbers of ®sh from oblique 0±10-m tows at three to ®ve locations in each length-class into biomass. in each reservoir with a Clarke-Bumpus metered Experimental gill nets were used to assess spe- plankton sampler with 153-␮m net. Crustacean cies composition and size structure of ®shes in zooplankters in two or three 1-mL subsamples each reservoir. Three standardized (Powell 1981) were identi®ed to species or genus, counted, and vertical nets measuring 3 ϫ 60 m, each composed measured. Reservoir densities were computed by of two mesh sizes (12.5, 19, 25, 32, 38, and 51 expanding subsample counts to the volume sam- mm, square measure), were ®shed overnight at pled (Martinez 1992). Because many planktivo- three locations in Blue Mesa Reservoir during rous ®shes strongly select for Daphnia spp. June, August, and October, and at a midlake station (O'Brien 1987) and kokanee select large Daphnia in Taylor Park Reservoir during June and August spp. (Martinez and Bergersen 1991; Stockwell and 1994. At Lake Granby, a combination of vertical Johnson 1997), we also computed density of (con®gured as above) and horizontal gill nets (2 daphnids that were more than 1.0 mm in length. m ϫ 46 m; 6 meshes, ranging from 19 to 76 mm) We measured density of opossum shrimp with a were ®shed overnight during May, July, and Oc- 1.0-m-diameter, 3-m-long net constructed of 500- tober 1994. Experimental horizontal gill nets (con- ␮m nitex and towed vertically at a rate of 0.3 ®gured as above) were ®shed overnight on 20±21 m/s. Sampling was conducted at least 30 min after July 1994, in Twin Lakes. sunset during the new moon in midsummer 1991± Hydroacoustic surveys were conducted with a 1994 at 10 stations per reservoir in two depth stra- Hydroacoustics Technology, Inc. (HTI) model 240 ta, with two replicate hauls per station (Martinez split-beam echosounder and bow-mounted trans- 1992). ducer (15Њ, 200 kHz). The acoustic system was Stock assessment.ÐLake trout length at age for fully calibrated. The transmitter was set to a pulse Blue Mesa Reservoir (P. J. Martinez, unpublished width of 0.3 ms, at 5 pings/s. Receiver gain was data), Taylor Park Reservoir (Weiler 1982), and set to Ϫ6 decibels (dB). The acoustic signal was Twin Lakes (Nesler et al. 1993) was determined monitored with a Hitachi V-212 oscilloscope, and from otoliths. Growth rates were derived by ®tting a dot matrix printer continuously recorded the ech- a von Bertalanffy growth function (Hilborn and ogram. Data were stored on digital audiotape for Walters 1992) to the mean lengths at age (R2 Ն subsequent analyses with HTI signal-processing 0.96 in each reservoir). Current age data were un- software. available for Lake Granby, so based on body con- Transects followed a zig-zag survey design dition and productivity indices, we assumed that (Gunderson 1993; Martinez 1994); the number and growth of Lake Granby lake trout was intermediate total length of transects surveyed was proportional between Blue Mesa and Taylor Park rates. For pop- to reservoir surface area; 15 transects and 24.4 km ulation level analyses, size and abundance data for were surveyed in Blue Mesa Reservoir, and 4 tran- large (Ͼ425 mm) ®sh from hydroacoustic surveys sects and 6.7 km were surveyed in Taylor Park were parsed into age-classes according to mean Reservoir. Hydroacoustic surveys were performed lengths at age determined from otoliths. We as- well after sunset during the new moon at Blue sumed lengths at age were normally distributed, a Mesa, Granby, Taylor Park, and Twin Lakes res- common and parsimonious assumption in length- ervoirs during August 1±3, September 28, August frequency analysis (MacDonald 1987). Fish that 4, and July 19, respectively, in 1994. TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 131

Minimum target strength threshhold was set to S ϭ 57%/year). The higher mortality estimate was Ϫ55 dB to avoid counting mysids. Fish distribu- considered the nominal rate in simulations and tions were suf®ciently dispersed to analyze for probably produced a conservative underestimate abundance by echo counting. Fish densities and of the potential prey biomass consumed by lake target strengths were analyzed separately in three trout. depth strata: 2±10 m, 11±20 m, and Ͼ20 m. Fish Temperature and dissolved oxygen pro®les were density (®sh/m3) was computed by totaling tracked obtained periodically with a Yellow Springs In- targets detected in each depth stratum and dividing struments model 58 meter as part of routine lim- this by the volume of that stratum that was acous- nological sampling (Lee 1994; Johnson et al. tically sampled. Elevation±capacity relationships 1995a; Martinez 1995). We assumed lake trout oc- (U.S. Department of Interior, Bureau of Recla- cupied water of 4ЊC under ice cover and that during mation, unpublished data) were used to compute the strati®ed, ice-free season, they behaviorally the total reservoir volume in each stratum, which thermoregulated to the temperature nearest 10ЊC, was multiplied by ®sh density to obtain lakewide provided that dissolved oxygen concentrations ex- abundance of pelagic ®shes. In situ target strength ceeded 3 mg/L. Temperatures at which lake trout measurements were range-weighted and converted were captured in vertical gill nets during summer to ®sh length with the equation in Love (1977). 1994±1997 in Blue Mesa Reservoir (Johnson et No lake trout were caught shallower than 20 m in al. 1997) averaged 9.8ЊC(N ϭ 40), supporting our gill-net sampling during 1994±1996, so we as- assumption of behavioral thermoregulation. Re- sumed all lake trout in Blue Mesa Reservoir oc- sulting thermal histories were comparable with curred in the Ͼ20-m depth stratum during the hy- those for lake trout in other waters (Stewart et al. droacoustic survey. Biomass of prey-sized ®sh was 1983; Yule and Luecke 1993). computed from abundance using length estimated Stomach contents of lake trout sampled from from target strength and length±weight regressions anglers' creels and in gill nets during April±Oc- for each species and reservoir. Prey ®sh production tober 1994±1997 were preserved in 10% formalin (kg´haϪ1´yearϪ1) was computed from maximum in- until they could be examined in the laboratory. dividual prey body weight and prey population The proportion of the stomach contents (by vol- biomass using the approach of Downing and Plante ume) consisting of ®sh and macroinvertebrates (1993). was estimated by eye. Prey organisms were iden- Consumption estimates.ÐWe used the bioener- ti®ed to the lowest taxonomic level possible; how- getics modeling package of Hanson et al. (1997) ever, results varied with the state of digestion. to estimate consumption by piscivorous lake trout Mean diet composition was computed for four by size-class. Model assumptions are detailed in size-classes: Ͻ425 mm, 425±600 mm, 601±900 the software's documentation (Hanson et al. 1997); mm, and Ͼ900 mm. The latter three groups cor- details about the lake trout component can be responded to important life stanzas for lake trout found in Stewart et al. (1983). Simulations were in these ®sheries: the onset of piscivory up to the run for 365 d, and consumption was ®t to the es- lower end of a typical closed slot limit, the size- timated annual growth increment while accounting class that is protected by a commonly applied for energy loss each fall due to spawning (average closed slot limit, and the size beyond the upper for both sexes: 6.8% of individual mass per year; slot limit. Stewart et al. 1983). In addition to demographic In simulations, we entered the diet composition data described earlier, the following information estimated from stomach samples and assumed wet was required to perform these simulations: mor- weight energy content of prey to be 3,641 J/g for tality rate, thermal history, diet composition and invertebrates (Rudstam 1989) and 7,528 J/g for prey energy content, and lake trout energy content. ®sh prey (Yule and Luecke 1993). Lake trout en- Mortality rates were not available so we performed ergy content was modeled as a function of body simulations using two instantaneous annual mor- weight (Hanson et al. 1997). The bioenergetics tality schedules: Z ϭ 0.693 (S [survival] ϭ 50%/ model computed the proportion of the maximum year) and Z ϭ 0.288 (S ϭ 75%/year). Although possible consumption rate that was actually few studies exist, this range encompasses survival achieved (pCmax) to match the observed growth in rates of lake trout reported in the literature (Fa- each population, accounting for allometric effects brizio et al. 1997) and was supported by a catch of body size on consumption rate, the thermal re- curve analysis performed on data from Blue Mesa gime, and diet composition. The proportion of Reservoir (where missing age-classes were fewest; maximum consumption that a ®sh achieves is an 132 JOHNSON AND MARTINEZ

TABLE 3.ÐBiological productivity indicators in four Colorado montane reservoirs (Martinez 1995). Chlorophyll a is reported as the mean of integrated 0±10-m (0±15-m in Twin Lakes) samples taken in midsummer 1994. Mean densities of Daphnia spp. and other zooplankton (mainly Diacyclops bicuspidatus thomasi) were computed from sampling at three to ®ve stations per lake (with 153-␮ net) during mid-July and mid-August 1991±1994. Mean mysid densities were computed from sampling at 10 stations per lake during mid-July and mid-August 1991±1994. Total ®sh biomass densities estimated from hydroacoustic surveys conducted during August 1994 (variability estimates were not available; numbers in parentheses indicate total lake volume [m3] sampled in each survey).

Blue Mesa Taylor Park Reservoir Lake Granby Reservoir Twin Lakes Attribute Mean SD N Mean SD N Mean SD N Mean SD N Chlorophyll a (␮g/L) 2.66 0.56 3 5.76 0.43 5 1.66 0.14 5 0.55 0.14 2 Daphnia density (number/L) 13.2 1.20 3 6.52 4.52 4 3.03 2.95 3 1.35 1.85 3 Other zooplankton (number/L) 33.3 3.82 3 28.9 6.81 4 31.5 19.2 3 9.90 4.52 3 Mysid density (number/m 2) 278 178 4 307 161 4 97.0 19.0 4 Fish biomass (kg/ha) 43.5a 30.7b 12.0c 3.70d a Lake volume sampled ϭ 5.264 ϫ 106 m3. c Lake volume sampled ϭ 5.973 ϫ 105 m3. b Lake volume sampled ϭ 1.194 ϫ 106 m3. d Lake volume sampled ϭ 6.990 ϫ 105 m3. indicator of the ratio of predator demand and prey rophyll concentration, zooplankton and ®sh den- supply, or relative food availability for piscivores. sities, and lake trout growth rate and condition. We used length±weight regressions for kokanee Chlorophyll-a concentration was highest in Lake and rainbow trout (Martinez 1995) to estimate the Granby followed by Blue Mesa Reservoir (Table weight of various-sized hatchery ®sh consumed by 3). Although chlorophyll concentration was higher lake trout. The per capita consumption necessary in Granby than Blue Mesa, we believe this was for a lake trout to grow through three piscivorous due to food web rather than productivity differ- size-classes at observed rates in each reservoir was ences. In Granby, high mysid density probably re- computed. This biomass was then apportioned into duced zooplankton density (Martinez and Berger- the number of prey consumed by assuming a lake sen 1991), limiting herbivory in the system and trout consumed prey 33% of its length (similar to allowing primary production to accumulate. Zoo- the mean ®sh prey: lake trout size ratio observed plankton density was highest in Blue Mesa despite by Yule and Luecke (1993) in Flaming Gorge Res- the highest planktivorous ®sh density. Zooplank- ervoir). ton and mysid densities were comparable in Lake The cost of hatchery ®sh was computed by using Granby and Taylor Park Reservoir. Granby was a linear cost function derived by the Hatchery Sec- probably more productive than Taylor Park be- tion, Colorado Division of Wildlife: cost ϭ cause the growing season is about a month longer $0.00373 ϫ SS, where SS is total length (mm) at (Table 4) and chlorophyll concentration and ®sh stocking (Martinez 1996). These costs did not in- density were much higher than at Taylor Park (Ta- clude administrative and support services or cap- ble 3). Judging from the length of the growing ital replacement costs (Johnson et al. 1995b). season, conductivity, zooplankton density, total Three sizes and types of stocking were evaluated: ®sh biomass density (Tables 3, 4), and kokanee kokanee fry (51 mm TL, $0.19/®sh), subcatchable growth rates (Martinez 1994, 1995, 1996), Blue rainbow trout (120 mm TL, $0.45/®sh) and catch- Mesa was the most productive system in upper able rainbow trout (254 mm TL, $0.95/®sh). To trophic levels. estimate predation costs, the number of prey con- Most abiotic indices appeared to be weakly re- sumed by lake trout was multiplied by the cost of lated at best to biotic characteristics of the reser- each hatchery ®sh based on its size at stocking. voirs. Elevation and length of the growing season did not distinguish the more productive Taylor Results Park Reservoir from the least productive system, Reservoir Productivity Twin Lakes. Reservoir morphometry appeared to Based on biotic measures, Twin Lakes was the have little in¯uence on trophic status: Blue Mesa most oligotrophic reservoir, with the lowest chlo- was the largest reservoir in terms of mean and TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 133

TABLE 4.ÐPhysical and chemical characteristics of four Colorado trophy lake trout reservoirs. Lake elevation is maximum surface elevation (m) above sea level (ASL). Growing season is expressed as number of days with water temperatures 10ЊC or higher (data from Cudlip et al. 1987; Lee 1994; Martinez 1986, 1992, 1994; LaBounty and Sartoris 1993). Secchi depth is mean for June±August (Blue Mesa, Granby, and Taylor Park) or July only (Twin Lakes). Conductivity was obtained for Blue Mesa from M. Malick (National Park Service, unpublished data), for Granby from Ugland et al. (1994), for Taylor Park from Weiler (1982), and for Twin Lakes from LaBounty and Sartoris (1993).

Blue Mesa Taylor Park Characteristic Reservoir Lake Granby Reservoir Twin Lakes Elevation (m ASL) 2,292 2,524 2,844 2,804 Surface area (ha) 3,706 2,936 823 1,120 Mean depth (m) 31 23 16 15 Maximum depth (m) 101 66 44 31 Volume (m3) 1.16 ϫ 109 6.65 ϫ 108 1.31 ϫ 108 1.73 ϫ 108 Residence time (years) 0.97 1.77 0.73 0.53 Growing season (d) 129 122 93 112 Secchi depth (m) 4.6 2.5 7.6 5.2 Conductivity (␮S/cm) 175 55 126 66 Morphoedaphic index 3.5 1.5 4.9 2.8 maximum depth and volume (Table 4) but was the Stock Assessment most productive. Conversely, Twin Lakes was the Horizontal and vertical experimental gill nets in shallowest reservoir but was the least productive. Blue Mesa Reservoir (Johnson et al. 1995a), Lake Secchi disk depth was not correlated with chlo- Granby, and Taylor Park Reservoir (Martinez rophyll concentration (P ϭ 0.26) or zooplankton 1995) showed that hydroacoustic targets Ϫ33dB (P ϭ 0.77) or ®sh biomass density (P ϭ 0.51). A and greater (425 mm) were exclusively piscivores, commonly used chemical indicator, conductivity, most of which were lake trout. In Twin Lakes (G. was poorly linked to system productivity (Tables Policky, Colorado Division of Wildlife, unpub- 3, 4). Even MEI, an index of nutrient status cor- lished data) some white suckers exceeded the 425- rected for basin morphometry that has been shown mm threshold size; however, large suckers did not to predict ®sh yields in natural lakes, was not con- appear in the pelagic zone. Kokanee and rainbow sistent with the other indicators of system pro- trout made up the great majority of the non-lake ductivity (Table 3) and was not correlated (P Ն trout gill-net catch (Figure 1) in all reservoirs, ex- 0.45) with chlorophyll, zooplankton, or ®sh bio- cept in Twin Lakes where rainbow trout consti- mass in our study reservoirs. tuted 39% of the non-lake trout catch and catostomids made up the remainder. Catches in Twin Lakes probably overestimated relative abundance of the more benthic catostomids because only horizontal gill nets were used there. Consistent with trends in reservoir productivity, lake trout were heaviest for a given length and grew fastest in Blue Mesa Reservoir; lake trout were leanest and slowest growing in Twin Lakes (Figure 2a). Individual weight at maximum age was about 68% lower in the slowest growing population (Twin Lakes) compared with Blue Mesa, the fastest growing population (Figure 2b). Growth rates of lake trout in Granby and Taylor Park were about midway between the maximum and mini- FIGURE 1.ÐSpecies composition of the catch in ver- mum growth rates. Horizontal gill-net sampling at tical gill nets (Blue Mesa Reservoir ϭ Blue, Taylor Park Blue Mesa in 1997 (B. M. Johnson, unpublished Reservoir ϭ Taylor), horizontal gill nets (Twin Lakes data) captured a broad size distribution of lake ϭ Twin), or both (Lake Granby ϭ Granby). Total number trout, consistent with the size structure estimated of ®sh sampled is shown above each bar. Nets were experimental, with six mesh sizes ranging from 12 to by hydroacoustics. Older lake trout (Ͼ13 years 76 mm bar. Catostomids were longnose and white suck- old) were most abundant in Lake Granby. ers. Blue Mesa Reservoir and Lake Granby showed 134 JOHNSON AND MARTINEZ

Mesa, Granby, Taylor Park, and Twin Lakes, respectively.

Consumption Estimates Only in Lake Granby were lake trout unable to behaviorally thermoregulate to their preferred temperature of 10ЊC during late summer due to hyp- oxic conditions in the hypolimnion. There, lake trout were assumed to have inhabited 10.5±11.5ЊC water from late August through mid-October. The number of days per year when lake trout could inhabit water near their preferred temperature was highest at Blue Mesa (129 d) and Granby (122 d), about 2 weeks less at Twin Lakes, and about a month less at Taylor Park, the highest-elevation reservoir. The species composition in gill-net samples showed that sport ®sh should make up a large proportion of the ®sh encountered by piscivorous lake trout in all reservoirs. This was corroborated by diet information (N ϭ 188 nonempty stomachs) that showed that sport ®sh made up the majority of the diet of lake trout greater than 425 mm TL during May±October (Figure 4). Identi®able ®sh in the stomachs of 425±600-mm lake trout were mostly salmonids, of which 33% were identi®able as rainbow or cutthroat trout and 33% were kokanee. Diets of large lake trout (Ͼ600 mm) consisted of about 95% salmonids and 5% catostomids. Juvenile lake trout (Ͻ425 mm) fed heavily on mysids, cray®sh, and other macroinvertebrates un- FIGURE 2.ÐLake trout (a) length±weight and (b) weight-at-age relationships in four Colorado reservoirs til they reached a size large enough to begin ex- based on otolith analyses (Martinez 1995). Reservoir ploiting ®sh prey. abbreviations are de®ned in Figure 1. A von Bertalanffy Lake trout were feeding nearest their maximum growth function was ®tted to each data set to ®ll in physiological limit at Blue Mesa Reservoir, where missing ages and produce the smooth curves shown here. pCmax averaged 68% for all sizes. Lake trout in Taylor Park Reservoir exhibited an intermediate similar lakewide abundance estimates of all sizes average pCmax (60%), and lake trout at Lake Gran- of ®sh (7.47 ϫ 105 versus 7.42 ϫ 105 ®sh; Table by and Twin Lakes showed the lowest (53% and 5). However, prey-sized ®sh densities were lower 49%, respectively), suggesting that food avail- on an areal basis in Blue Mesa (182 ®sh/ha; Figure ability, scaled for temperature, was lower in those 3) than at Granby (246 ®sh/ha). Alternatively, the reservoirs. density of predator ®sh was higher at Blue Mesa Annual population consumption (kg/ha) by pi- (12 ®sh/ha) than at Granby (7 ®sh/ha). Total ®sh scivorous age-classes of lake trout increased with abundance was lower at Taylor Park Reservoir (82 reservoir productivity. The Twin Lakes lake trout ®sh/ha; Figure 3) and much lower at Twin Lakes population consumed the least ®sh prey, and the (14 ®sh/ha). It was not known what proportion of Blue Mesa population consumed the most (Figure prey-sized ®sh were actually young lake trout. Bio- 5). Prey ®sh production varied by more than an mass density of predators was greater than prey order of magnitude across reservoirs. Prey ®sh biomass density in the low-productivity reservoirs production was similar at Blue Mesa and Granby (Taylor Park and Twin Lakes), and predator and (11.4 and 10.6 kg´haϪ1ýearϪ1, respectively), was prey biomasses were similar in Blue Mesa and much lower at Taylor Park (2.5 kg´haϪ1ýearϪ1), Granby (Figure 3). Predator ®sh constituted 50, and lowest at Twin Lakes (0.6 kg´haϪ1ýearϪ1; Fig- 41, 68, and 78% of the total ®sh biomass in Blue ure 5). Estimated total annual consumption by lake TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 135

TABLE 5.ÐHydroacoustic survey results in three depth strata showing total volume sampled (N ϭ 15 transects at Blue Mesa, N ϭ 10 transects at Granby, N ϭ 4 transects at Taylor Park, and N ϭ 6 transects at Twin Lakes), total number of ®sh targets detected (by size), and estimated lakewide abundance of ®shes in study reservoirs during summer 1994.

Depth Volume Fish targets Lakewide abundance stratum sampled (m) (m3) Ͻ425 mm Ն425 mm Ͻ425 mm Ն425 mm Blue Mesa Reservoir 2±10 134,097 57 0 56,116 0 10±20 405,329 1,193 86 311,394 26,514a Ͼ20 4,724,364 4,195 602 299,218 53,653 Lake Granby 2±10 111,715 172 1 336,657 6,951 10±20 321,146 713 33 358,669 18,661 Ͼ20 761,128 110 17 18,467 3,172 Taylor Park Reservoir 2±10 85,757 60 1 40,630 725 10±20 205,197 77 3 11,456 502 Ͼ20 306,328 192 47 11,444 2,951 Twin Lakes 2±10 126,229 4 0 2,565 0 10±20 373,573 15 2 2,354 164 Ͼ20 199,157 94 6 9,861 735 a Based on gillnetting in 1994±1996. Large targets measured in water shallower than 20 m were not considered to be lake trout. trout exceeded the sum of prey standing stock in ductivity systems. Consumption estimates using August plus annual production in the two low- the lower mortality rate (Z ϭ 0.288) were about productivity reservoirs and was somewhat less 19% higher than those computed using the nominal than prey stock plus production in the higher-pro- mortality rate, which suggests that our assump-

FIGURE 3.ÐAbundance (open bars) and biomass (black bars) of pelagic ®shes estimated from nighttime hydroacoustic surveys conducted during July, August, or September 1994. Biomass (kg/ha) was estimated from size- speci®c abundances and length±weight regressions for ®sh 425 mm total length [TL] or less (prey, shaded) and ®sh greater than 425 mm TL (predators; see text for equations). Note that y-axes differ among lakes. 136 JOHNSON AND MARTINEZ

FIGURE 5.ÐAnnual population consumption of salmonid prey by three size-classes of lake trout estimated with a bioenergetics model, and estimates of prey supply FIGURE 4.ÐAverage proportions of salmonids (black shading; mainly stocked rainbow trout and kokanee), (standing stock in August plus annual production of pe- other ®sh (gray shading; mainly catostomids), and in- lagic prey ®sh) in four study reservoirs (abbreviations vertebrates (no shading; mainly chironomids, decapods, de®ned in Figure 1). and mysids) in diets of four size-classes of lake trout. Number of nonempty stomachs is shown above each bar. about seven times higher for a 600±900-mm slot limit (about $700), and about 15 times higher for tions about mortality were not extremely important a 900-mm maximum size limit ($1,500). Because to the outcome of analyses. kokanee fry are only available for a short period, Per capita costs of lake trout predation were these costs would not accrue over the entire year. highest in the lower-productivity systems (Figure Hence, these estimates represent upper limits to 6). The cost of stocked prey consumed by a lake predation costs. At a given prey size, catchable trout growing through the three piscivorous size- trout stocking was the most expensive way to sat- classes was about $200/®sh in Blue Mesa and isfy lake trout consumption (Figure 7). Granby, where lake trout were assumed to con- Discussion sume kokanee that were stocked as fry but grew to a length 33% of that of the lake trout. Costs Abiotic productivity indices that are commonly were about $450/®sh in Taylor Park and Twin applied in natural lakes did not correspond well Lakes, which are stocked with catchable rainbow trout that did not grow before being consumed. Predation costs were sensitive to the size at which the prey was stocked and the size at which it was consumed. Taking Lake Granby as an example, cost per lake trout growing through a particular size-class was highest ($100±830/®sh) if lake trout fed on the smallest-sized prey (kokanee fry) shortly after stocking (Figure 7). At all lake trout sizes, predation cost declined rapidly as prey size at the time of consumption increased because the number of ®sh prey eaten declined according to the expo- nential relationship of length and weight of the prey and the assumed ®xed cost of the stocking FIGURE 6.ÐCost per predator ($US, 1995) of prey event. Because of their much higher absolute per consumed by lake trout growing through three pisciv- capita consumption, the largest lake trout gener- orous size-classes in four reservoirs (abbreviations de- ated the highest cost of prey consumed per lake ®ned in Figure 1), assuming lake trout consumed prey approximately 33% of the predator's length. Stocked ko- trout. Harvest regulations that protect lake trout to kanee fry were the prey base in high-productivity res- 600 mm would have a per capita cost as high as ervoirs (Blue and Granby), and catchable rainbow trout $100/®sh if their diet consisted entirely of newly were the prey base in low-productivity reservoirs (Taylor stocked kokanee fry. Per capita costs would be Park and Twin Lakes). TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 137

ductivity controls number of trophic levels con- tinues to be debated (Hairston and Hairston 1993), it stands to reason that, given equivalent prey exploitation ef®ciency by consumers, higher biomasses of prey ®sh and apical predators could be sustained in more productive systems (Lindeman 1942; Oglesby 1977; Pauly and Christensen 1995). Despite the trophic implications of the wide range of productivity in our study reservoirs, management actions and ®sh community structure in- dicated that high biomass of large lake trout has been a management goal in all four systems. Stock- ing and restrictive harvest regulations fostered the development of high lake trout biomass and size structure, and estimated piscivorous trout biomass exceeded estimated prey biomass in two of the four reservoirs. Inverted Eltonian pyramids of biomass are not uncommon in pelagic freshwater ecosystems (Diana 1995), where zooplanktivore standing stock may exceed that of zooplankton, which exceeds standing stock of phytoplankton. High turnover rates in lower trophic levels account for this pattern. However, it is unusual to see this pattern continue to top piscivores, as was the case in our study reservoirs. Because of the much greater sim- ilarity in body size and, therefore, turnover rates of planktivorous and piscivorous ®sh (Downing and Plante 1993), we would not expect such a pattern to be stable. A probable interpretation of this apparent paradox is that annual subsidies to the food web, in the form of stocked sport ®sh, maintain an otherwise unsustainable trophic structure. FIGURE 7.ÐCost per predator ($US, 1995) of three Management actions and reservoir productivity types of stocked prey (kokanee [KOK] fry, subcatchable also appeared to interact to affect food web struc- rainbow trout [RBT Subc], and catchable rainbow trout ture and dynamics. The proportion of total ®sh [RBT Catc]) consumed at various sizes after stocking biomass represented by piscivores was generally by three size-classes of lake trout in Lake Granby. higher and lake trout growth rate was lower in the reservoirs with the lowest productivity, indicating with observed biomasses of various trophic levels that protective harvest regulations may have cre- or the predator±prey relationships in the study res- ated the greatest imbalance in predator and prey ervoirs. This is perhaps due to the overriding in- biomasses in those reservoirs. Further, the differ- ¯uences of hydraulic residence time and elevation, ence between realized consumption and consump- which were inversely related to productivity. Pro- tion demand was greatest in the least productive ductivity can in¯uence food web structure and ecosystems. Therefore, lake trout biomass was more system dynamics (Persson et al. 1996), and en- limited by food availability in these reservoirs. It ergetic arguments have often been invoked to ex- would be necessary to stock considerably more plain how food chain length can increase with in- prey ®sh in the low-productivity systems to increasing productivity (Elton 1927; Slobodkin crease lake trout growth and condition to the level 1960; Persson et al. 1992). Thus, low-productivity seen in the more productive systems. Because lake systems may not be capable of sustaining signif- trout biomass, growth rate, and condition in- icant piscivore biomass under natural conditions. creased with reservoir productivity, the biomass Although the generality of the argument that pro- consumed by each lake trout and by the population 138 JOHNSON AND MARTINEZ growing through various length intervals also in- exponentially related to temperature, this period creased with increasing reservoir productivity. should encompass the majority of the annual con- Although size limits are usually set on the basis sumption. Lake trout may in fact be feeding on of ®sh length, most trophy anglers probably gauge nongame benthic prey ®sh during colder months, their success by the weight of the ®sh they catch. but their contribution to annual consumption If one computes the ®sh biomass required to grow should be small. a lake trout from the onset of piscivory to a ``tro- Managing for trophy lake trout has important phy'' weight, the pattern of biomass consumed economic costs when the prey base includes hatch- (and costs) versus productivity is quite different ery-derived sport ®shes. Hatchery ®sh serve many because it takes much longer for a lake trout in an purposes, and their value can be quanti®ed in many unproductive system to attain trophy weight than ways (Stroud 1986; Wiley et al. 1993; Johnson et it does in a productive system. For example, a 7- al. 1995b; Schramm and Piper 1995; Loomis and kg lake trout in Blue Mesa Reservoir would be Fix 1999). We made a conservative estimate of the about 10 years old and would have consumed 39 hatchery cost to produce a stocked kokanee or kg of ®sh to reach that size. In Twin Lakes, a lake rainbow trout because the cost function we used trout of the same weight would probably be 17 was a linear function of ®sh length. This function years old and would have consumed 50% more undoubtedly underestimated production costs of biomass (60 kg total), with a proportional increase larger stocked ®shes, and hatchery administrative in cost, to attain trophy weight. Because lake trout and capital replacement costs were not included in our study lakes and elsewhere (Elrod 1983; Trip- (Johnson et al. 1995b). We then used the produc- pel and Beamish 1989; Rieman and Myers 1991) tion cost to compute the cost of sustaining lake undergo a characteristic trophic ontogeny, regu- trout populations on a stocked forage base. The lations that protect lake trout to trophy sizes also cost of a kokanee or rainbow trout recruited to the protect the most piscivorous segment of the lake ®shery would be much higher because only a small trout population. Work at Flaming Gorge Reser- fraction of the ®sh stocked survive to harvestable voir, Utah±Wyoming (Luecke et al. 1994), showed size. An alternative approach that included the op- that the harvest regulation that maximized catches portunity cost of ®sh lost to the sport ®shery would of trophy lake trout also produced the greatest in- have yielded higher estimates of the cost to sustain crease in consumption of prey ®sh populations. lake trout consumption with hatchery sport ®sh. Although the biomass of prey required to sustain Total predation costs were higher in the more lake trout in Taylor Park and Twin Lakes reservoirs productive reservoirs because these systems sup- was less than the biomass of sport ®shes stocked port higher lake trout biomass at higher growth each year, hydroacoustic density estimates sug- rates. However, the actual cost of individual hatch- gested that few stocked ®sh survived to fall. The ery ®sh consumed in more productive waters is prey ®sh biomass required to maintain observed lower than in unproductive waters. Stocked ®shes lake trout growth rates at Blue Mesa Reservoir and grow more rapidly in more productive systems, Lake Granby was near or exceeded the biomass of and hence the cost incurred when these ®sh are pelagic prey ®sh, yet sport ®sheries for kokanee consumed by lake trout will decline more rapidly and rainbow trout persist there. The discrepancy because the prey biomass is determined more by between predator demand and prey supply could in-lake tissue elaboration that comes at no cost to be accounted for by several factors: bias in our the hatchery system. hydroacoustic data that either overestimated lake Predation cost was sensitive to assumptions trout biomass or underestimated prey biomass, about the size of the prey ®sh when consumed overestimates of lake trout growth rate, or under- relative to its size at stocking. Cost was highest estimates of the importance of benthic ®shes as when lake trout fed on the smallest stocked prey. prey for lake trout. Our hydroacoustic estimates Despite their lower per capita cost to produce, of lake trout density should be less than or equal small prey (e.g., kokanee fry) have exponentially to true density because lake trout within about 1 less biomass per ®sh than longer prey, and hence m of the bottom would be acoustically undetect- predators must consume many more of these small able. Growth rates of lake trout in our study lakes stocked ®sh to satisfy their energetic requirements. are similar to those found in other western lakes The rapid decline in per capita predation cost with and reservoirs (Yule and Luecke 1993; Ruzycki increasing prey size emphasizes the importance of and Beauchamp 1998). Diet data came only from poststocking production of the stocked cohort. April±October, but because consumption rates are However, in unproductive waters, growth of TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 139

FIGURE 8.Ð(a) Relative weight of three size-classes (mm, total length) of lake trout in Lake Granby during 1986±1997 and (b) kokanee angler catch rate and ®shing effort at Blue Mesa Reservoir from May through October during 1993±1997. Numbers beside data points indicate last two digits of year. stocked trout and kokanee is poor, larger stocked Creel surveys in Oregon and Idaho (Rieman and sizes are required to enhance ®sheries for these Maiolie 1995) and in Colorado show a strong re- species, and the size-speci®c predation cost is lation between kokanee density and total ®shing high. effort. In Blue Mesa Reservoir, ®shing effort has There are also important social costs and man- declined each year from about 520,000 angler- agement tradeoffs associated with managing for hours/year in 1993 to about 230,000 angler-hours/ trophy lake trout on a sport ®sh prey base. Recent year in 1997 (Figure 8b), concurrent with a decline survey data suggest that kokanee populations are in the kokanee population. When the kokanee pop- declining in Blue Mesa Reservoir and Lake Gran- ulation was strong in Lake Granby, angler effort by (Martinez 1994, 1995, 1996). Data do not exist averaged 150,000 h during the summers of 1975± to conclusively link kokanee declines to lake trout 1985 (Martinez and Bergersen 1991) but dropped predation, and other factors, such as competition to about 90,000 h for the entire year after the de- with Mysis relicta (but see Bowles et al. 1991 and cline of the kokanee population in 1986 (Martinez Beauchamp 1996) or entrainment in dam releases, and Bergersen 1991; M. Jones, Colorado Division could play a role. However, total lake trout con- of Wildlife, unpublished data). Reductions in ®sh- sumption exceeded our estimates of pelagic ®sh ing pressure occurred in both systems in spite of biomass in these reservoirs, strongly implicating an apparently burgeoning lake trout population in lake trout predation as a contributing factor in Blue Mesa Reservoir and several widely publi- these kokanee declines. Lake trout predation has cized ®sh over 15 kg that were caught by anglers been implicated as a threat to prey ®sh populations from both reservoirs in 1995. In considering the (including kokanee) in several other western lakes tradeoffs of managing for a trophy piscivore ®sh- and reservoirs (Bowles et al. 1991; Rieman and ery at the expense of a put-grow-and-take ®shery, Myers 1991; Yule and Luecke 1993; Beauchamp it does not appear that ®shing effort lost from ko- 1996; Carty et al. 1997; Ruzycki and Beauchamp kanee ®sheries is compensated for by increases in 1998). In Lake Granby, relative weights (Piccolo lake trout angling. Unless lake trout anglers spend et al. 1993) of large lake trout have decreased by inordinately more money per angler-day than ko- about 2% per year since 1986, concomitant with kanee and rainbow trout anglers, managing for the decline of a major kokanee ®shery (Figure 8a). lake trout at the expense of kokanee and rainbow Kokanee population declines are disturbing be- trout ®sheries is not economically justi®able. cause of the implications for ®shing participation, Socially, this management strategy is question- kokanee propagation programs, and the sustaina- able because the state's limited hatchery produc- bility of lake trout and kokanee ®sheries. tion is used to support a piscivore ®shery in which 140 JOHNSON AND MARTINEZ a minority of anglers participate but that all license egg supply in 1996 was so limited that the number buyers must support. Anglers and angler groups of Colorado kokanee ®sheries stocked in 1997 petitioning for restrictive harvest regulations in dropped from 27 to 10, focusing primarily on those Colorado in effect command a highly dispropor- waters where stocking had a chance to enhance tionate allocation of the state's hatchery produc- future egg takes. If kokanee populations in our tion by increasing predation demand as piscivore study lakes crash, it may be impossible to reha- numbers and size structure are enhanced (Martinez bilitate them, which would be a misfortune to the 1995). Further, recent recommendations to con- anglers and lake trout that exploit them. sider user-pay scenarios for hatchery-sustained ®sheries (Bennett et al. 1996) should also include Conclusions an analogous evaluation of the predation cost on Balancing predator demand with prey supply hatchery salmonid prey bases in ®sheries where has historically been an implicit, but challenging numbers or size structure of piscivores increase as goal of ®shery management. We found that system a result of piscivore management. This kind of productivity indices alone may not be good indi- evaluation would provide a basis for discussion of cators of the status of predator and prey ®sh pop- ®shery trade-offs and user-pay considerations for ulations in highly managed, montane reservoirs. trophy piscivore management. Yet, ®shery attributes relevant to trophic econom- We believe efforts by ®sh managers to portray ics, such as the balance between biomass and pro- trophy-sized piscivores as the most valuable com- duction of prey and the biomass and consumption ponent in reservoir ®sheries has led to the mis- of predators, are dif®cult to measure. Thus, growth conception of many anglers that restricted harvest is often used by managers as an indicator of bal- or catch and release is the only way to manage ance of the predator±prey system in the ®sheries piscivores (Martinez 1995). Although it is true, under their stewardship. If growth rates are low, based on our analyses, that trophy piscivores are then stocking of that species may be reduced or the most costly ®sh in the state's coldwater res- harvest restrictions may be relaxed in the hopes ervoirs, their actual value relative to the reduction of bringing the population back below the system's or loss of other valued ®shery components (put- carrying capacity. However, more direct and im- grow-and-take species and kokanee egg supplies) mediate measures of balance allow managers to has largely been ignored in past angler education respond to unsustainable trophic conditions before efforts. We foresee confusion and con¯ict among radical declines in prey population abundance and anglers as managers who promoted catch and re- predator growth and condition occur. Such mea- lease of large piscivores as ``conservation'' even- sures can be obtained by quantifying the biomass tually succumb to the necessity of liberalizing lake of predator and prey ®shes; a wide variety of tech- trout regulations to retain other valued ®shery niques are available. Production is the best mea- components (Martinez 1995). sure of prey supply and should be computed to In Colorado, the debate over the appropriateness assess the predation rate that the prey population and sustainability of trophy piscivore management can sustain. Prey production can be estimated from using hatchery salmonids as prey has intensi®ed P/B ratios or as a function of prey size and pop- as hatchery production of prey ®sh and body con- ulation biomass. Coupled with growth and diet in- dition of lake trout have declined. Continuing de- formation, predator abundance data can be input clines in kokanee populations at Blue Mesa Res- to bioenergetics models to quantify the predation ervoir and Lake Granby are especially alarming pressure exerted on prey ®sh populations. Sus- because the spawning runs in these populations tainability of the ®shery management policy can are the primary egg sources for the statewide then be evaluated by comparing the balance be- hatchery program, and kokanee are semelparous tween prey production and predator consumption. and short-lived. Chronic shortages in the kokanee If successful, trophy harvest regulations and egg supply in the western United States (Martinez other management actions aimed at enhancing 1995) preclude satisfying hatchery needs with predator biomass and size structure will increase eggs from other sources. Further, lake trout pre- consumption demand by the target population. dation has frustrated efforts to reestablish kokanee When predator consumption exceeds prey produc- in Flathead Lake through stocking (Carty et al. tion, declines in prey population abundance and 1997). Although harvest regulations for lake trout predator growth are inevitable. To sustain such un- in Blue Mesa and Granby were liberalized in 1996, balanced systems, the food web must be subsidized the changes may have come too late. The kokanee (e.g., by stocking prey ®sh). This can be an ex- TROPHIC ECONOMICS OF LAKE TROUT MANAGEMENT 141 pensive enterprise. When the prey populations of limnology, 1965±1985. U.S. Bureau of Reclama- piscivorous sport ®sh must be regularly stocked, tion, Technical Report 85-9, Denver. ®scal economics become an important manage- Diana, J. S. 1995. Biology and ecology of ®shes. Bi- ological Sciences Press, Carmel, Indiana. ment consideration, and managers must evaluate Downing, J. A., and C. Plante. 1993. Production of ®sh the costs borne by all license buyers to sustain a populations in lakes. Canadian Journal of Fisheries piscivore ®shery enjoyed by a relatively small seg- and Aquatic Sciences 50:110±120. ment of the angling public. Elrod, J. H. 1983. Seasonal food of juvenile lake trout in U.S. waters of Lake Ontario. Journal of Great Acknowledgments Lakes Research 9:396±402. Cory Counard, Glenn Szerlong and Mike Wise Elton, C. 1927. Animal ecology. Sidgwick and Jackson, London. collected and analyzed zooplankton samples from Fabrizio, M. C., M. E. Holey, P. C. McKee, and M. L. Blue Mesa Reservoir. Cory Counard performed all Toneys. 1997. Survival rates of adult lake trout in chlorophyll analyses. Steve Johnson led hydroa- northwestern Lake Michigan, 1983±1993. North coustic surveys and analyses. We thank Patricia American Journal of Fisheries Management 17: Soranno for assistance with limnological proce- 413±428. dures. Robert Behnke, Jason Stockwell, Charles Gunderson, D. R. 1993. Surveys of ®sheries resources. Wiley, New York. Bronte, Dave Beauchamp and two anonymous re- Hairston, N. G., Jr., and N. G. Hairston, Sr. 1993. Cause- viewers provided helpful reviews of earlier drafts. effect relationships in energy ¯ow, trophic structure Bill Weiler, Colorado Division of Wildlife, pro- and interspeci®c interactions. American Naturalist vided stocking records and hatchery ®sh produc- 142:379±411. tion costs. Support for this research was provided Hanson, P. C., T. B. Johnson, D. E. Schindler, and J. F. by grants from the U.S. Bureau of Reclamation Kitchell. 1997. Fish bioenergetics 3.0. University Grand Junction Projects Of®ce, Grand Junction, of Wisconsin, Sea Grant Publication WISCU-T- 97-001, Madison. Colorado and the Federal Aid in Sport Fish Res- Hayes, D. B., W. W. Taylor, and E. L. Mills. 1993. Nat- toration, projects F-85 and F-242, and the Colo- ural lakes and impoundments. 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