Transactions of the American Fisheries Society Summer Survival and ...

This article was downloaded by: [Northeast Fisheries Science Center] On: 16 September 2013, At: 06:22 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Transactions of the American Fisheries Society Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/utaf20

Summer Survival and Growth of Brown Trout with and without Steelhead under Equal Total Salmonine Densities in an Artificial Stream a

John F. Kocik & William W. Taylor

a

a

Department of Fisheries and Wildlife, Michigan State University, East Lansing, Michigan, 48824, USA Published online: 09 Jan 2011.

To cite this article: John F. Kocik & William W. Taylor (1994) Summer Survival and Growth of Brown Trout with and without Steelhead under Equal Total Salmonine Densities in an Artificial Stream, Transactions of the American Fisheries Society, 123:6, 931-938, DOI: 10.1577/1548-8659(1994)1232.3.CO;2 To link to this article: http:// dx.doi.org/10.1577/1548-8659(1994)1232.3.CO;2

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden.

Downloaded by [Northeast Fisheries Science Center] at 06:22 16 September 2013

Terms & Conditions of access and use can be found at http://www.tandfonline.com/ page/terms-and-conditions

Transactions of the American Fisheries Society 123:931-938. 1994 © Copyright by the American Fisheries Society 1994

Summer Survival and Growth of Brown Trout with and without Steelhead under Equal Total Salmonine Densities in an Artificial Stream JOHN F. KociK1 AND WILLIAM W. TAYLOR


Department of Fisheries and Wildlife, Michigan State University East Lansing, Michigan 48824. USA Abstract.—We studied the survival and growth of age-0 brown trout Salmo trutta with and without age-0 steelhead Oncorhynchus mykiss, under conditions of equal salmonine density (numbers per unit area), to compare the relative effects of intraspecific and interspecific competition. We conducted these experiments from June through September in an artificial stream with recirculating water and used a replicated, completely randomized design. The experiment was initiated with four allopatric treatments of 14 brown trout each and four sympatric treatments of 7 brown trout and 7 steelhead each. Mortality during the experiment resulted in variable densities among replicates. Interactions with steelhead did not have negative effects on brown trout survival or growth. A model comparing the effects of total salmonine, steelhead, and brown trout densities on brown trout length suggested that intraspecific interactions had the strongest influence. Our results indicate that the steelhead did not influence the brown trout survival or growth during the summer growth period to the same extent as an equivalent number of brown trout. However, we recommend caution in combining these two species until the implications of interactions at other life history stages are examined. Competition between stream-dwelling salmonine species can be an important regulatory mechanism in their population dynamics (Chapman 1966; Hearn 1987; Fausch 1988). Recent research in the North American Great Lakes region suggests that stream-resident brown trout Salmo trutta and lake-run steelhead Oncorhynchus mykiss may compete for stream resources. Kruger et al. (1985) documented that brown trout abundance declined while steelhead abundance increased in the Pere Marquette River, Michigan. They also found that juvenile brown trout growth was below Michigan averages for age-groups co-occurring with steelhead parr of the same age-groups. After the out-migration of their associated cohort of steelhead, brown trout growth exceeded the Michigan average (Kruger et al. 1985). In a study of three tributaries to the Great Lakes, Ziegler (1988) found that age-0 fish of both species used similar habitat and food. Given the typical life history of these species in Great Lakes tributaries, it is characteristic for brown trout to emerge earlier from their redds than steelhead (Ziegler 1988; Seelbach 1993). This head start gives brown

1 Present address: National Oceanic and Atmospheric Administration, National Marine Fisheries Service, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, Massachusetts 02543, USA.

931

trout a size advantage during their first growing season. Typically, a larger body size is thought to be advantageous to competing salmonines (Hearn 1987). However, Rose (1986) observed that steelhead, despite a later emergence date, adversely affected the growth of brook trout Salvelinus fantinalis in Lake Superior tributaries. Because these field studies suggested that juvenile steelhead have the potential to be detrimental to resident brown trout through interspecies interactions, we felt that experimental studies were needed to gain a clearer understanding of the interactions involved. Our hypothesis was that interspecific interactions between brown trout and steelhead affect brown trout growth and survival more than an equivalent density of allopatric brown trout. In natural and artificial streams, salmonines form dominance hierarchies that allow dominant fish to use the most energetically profitable stream positions, realizing superior growth and survival (Kalleberg 1958; Fausch 1984; Titus and Mosegaard 1991). Thus, growth and mortality rates reflect the effects of behavioral interactions and dominance hierarchies, allowing a quantitative evaluation of salmonine competition. To test our hypothesis, we conducted a 100-d artificial stream experiment regulating fish diet, habitat, and initial densities. Artificial stream experiments allow true replication of food availability, habitat volume, and fish densities, facilitating statistical assess-

932

KOCIK AND TAYLOR

brown trout


steelhead

FIGURE I. — Design of the artificial stream: (A) schematic overhead view detailing the layout and experimental design (fish symbol = 7 fish); (B) overhead view of an individual stream cell, indicating relative size and placement of habitat structures; (C) placement of artificial log in the lateral plane. All habitat structures are to scale, and stream cells were 1 m long and 0.6 m wide.

ments of competition (Kalleberg 1958; Li and Brocksen 1977; Ringler 1979; Fausch and White 1986). A long experiment was essential because, in the Great Lakes region, steelhead grow faster than brown trout (Ziegler 1988), which could result in shifts in competitive dominance that a shortterm experiment would not evaluate. We selected the summer growth period because preliminary work indicated that the ratio of steelhead size to brown trout size undergoes its most pronounced change at that time (Ziegler 1988; Kocik 1992). Methods Stream design, macrohabitat. and microhabitat. — We conducted this experiment in an artificial stream with recirculating water and eight 1 x 0.6 x 0.6 m experimental cells partitioned by 5-mm-mesh screens (Figure 1A). An electronic

timer maintained ambient light cycles by regulating artificial mercury-vapor and fluorescent lighting that provided peak intensities of 950 Ix. Using two refrigerator units, we regulated stream temperatures to reflect ambient summer temperatures measured in four coldwater streams in northern Michigan (Kocik, unpublished data). Experimental temperatures ranged from 16.6°C in July to 12.9°C in September, and exhibited diurnal fluctuations of 1.5°C; temperatures peaked in the afternoon because of increased heating by lights and changing air circulation in the building. Each experimental cell had replicate depth, velocity, substrate, and cover microhabitats that were analogous to those commonly used by both species in the wild (Ziegler 1988; Kocik 1992). We maintained water depth at 30.5 cm by adding dechlori-

nated tap water at each feeding. Two water pumps

933

BROWN TROUT AND STEELHEAD INTERACTIONS

TABLE 1.—Timing of salmonine introductions and data collections. Day of experiment

Date (1991)

Experimental operation

I

21 Jun

17

8Jul

Brown trout measurements

18

9Jul

Steelhead introduction

55

15 Aug

Experimental check 1

75

4Scp

Experimental check 2

100

29Sep

Experimental period

Brown trout introduction Before steel head

Before steelhead Early phase

Middle phase


Late phase

Experiment end

and a nitration system pump maintained average water velocities at 13.0 cm/s in open water and 11.4 cm/s in cover. All cells contained gravel substrate ranging in size from 4 to 8 mm. Each cell had five artificial cover structures: two Vallisneria, one Limnophila, and two logs (Figure IB). We constructed artificial logs from bisected 15-cm gray polyvinyl chloride pipe and placed them horizontally in the stream (Figure 1C). We fed fish an invertebrate diet at 5% of their body weight per feeding through tubes that released invertebrates in carrier water at the center of the upstream divider on the stream bottom. Fish were fed twice daily at approximately 0700 and 1600 hours. Frozen Artemiat Chironornus. and Euphausia, and freeze-dried Euphausia provided 90% of the diet by weight. Live Anemia, Culicidae, and Chironomus supplemented preserved foods as cultures became abundant enough for use. Laboratory experimental design. — We compared the survival and growth of brown trout when held only with its own species (allopatric treatment) and when held with steelhead (sympatric treatment) by means of a completely randomized design with equal treatment densities (number of fish/cell). Age-0 brown trout used in the experiment were collected from Gilchrist Creek, Michigan, by electrofishing. Steelhead were offspring of wild fish from the Little Manistee River, Michigan, and were reared to swim-up stage at the Michigan Department of Natural Resources* Wolf Lake Hatchery. Until introduced into the experimental stream, the two species were kept isolated from each other in separate artificial stream units for a minimum of 15 d to allow acclimation to their new environment. We selected fish randomly from holding tanks and placed them in experimental cells, after anesthetizing them in tricaine methanesulfonate (50

mg/L) to reduce stress and to facilitate handling and measuring each for total length (mm) and weight (0.001 g). On 21 June 1991 (day 1), we introduced 14 brown trout to each allopatric cell and 7 brown trout to each sympatric cell (Table 1). An 18-d acclimation period before steelhead introduction allowed brown trout to establish hierarchies within cells. This timing is similar to what occurs in Great Lakes tributaries, where wild brown trout typically emerge earlier than steelhead (Kocik 1992). On 9 July 1991 (day 18), we introduced seven steelhead into each sympatric cell (Table 1). To periodically assess growth and survival, we collected the salmonines in each experimental cell by dip net. We then anesthetized them in tricaine methanesulfonate (50 mg/L) and measured each for total length (mm) and weight (0.001 g), returning them immediately to their experimental cells after recovery from the anesthesia. We measured these biological variables at the end of the early phase (day 55), middle phase (day 75), and late phase (day 100) of the experiment (Table 1). Beyond these procedures, we observed general fish behavior and location daily during feeding periods and recorded general observations. At least twice a week, we also observed fish during resting periods to record deaths, behavior, and location. Rate calculations and statistical analysis. — We calculated mortality, growth, and condition over four periods: the period before steelhead introduction and the three experimental phases (Table 1). This periodic analysis allowed us to assess growth and survival differences as the ratio of steelhead to brown trout sizes changed. We calculated mean total length, weight, growth, and mortality on a per-cell basis, then averaged the means by treatment. We used salmonine density at each sampling to calculate instantaneous mor-

934

KOCIK AND TAYLOR

-•— Brown trout allopatric o- Brown trout sympathc • Steelhead

14-

g 12-


s.

0) O)

10-

4-

i S

confounding effects of density on fish growth (Snedecor and Cochran 1982). We conducted statistical analyses partitioned by sampling date for point estimates, partitioned by experimental phase for rate variables, and for the entire experiment. Using an analysis of variance (ANOVA), we evaluated differences in brown trout total length between sympatric and allopatric sections (Snedecor and Cochran 1982). Instantaneous mortality and growth rates were not normally distributed, which is typical for ratio data. Thus, we used the ratio transformation RT-1 to normalize the data and then performed ANOVA for statistical comparisons (Conover and Iman 1981;

Snedecor and Cochran 1982). The critical level of significance for all statistical tests was P < 0.05. 0

20

40

60

80

100

Day of experiment FIGURE 2. —Mean (±SE) numbers of juvenile brown trout in allopatric and sympatric treatments during the 100-d artificial-stream experiment. Data points for days 1 and 18 have no error bars because no fish died until after these dates. Points for sympalric and allopatric treatments are offset horizontally for clarity.

tality (Z) for each period and for the entire duration of the experiment (Ricker 1975): where Nt\ and N& are numbers of fish at time 1 (t\) and time 2 (/2>. To assess growth, we calculated instantaneous daily growth (/u) for each period and overall (Ricker 1975): log 0.05). Brown trout lengths and weights (mean ± SE) were not significantly different between sympatric 6 70 O) and allopatric treatments (57.6 ± 2.95 mm, 1.91 c ± 0.243 g versus 60.0 ± 2.02 mm, 2.16 ± 0.222 O g; P = 0.177, 0.164) at the onset of the experiment (Figure 3). Analysis of instantaneous growth rates indicated that brown trout growth differed be50tween treatments (P = 0,017); brown trout in the sympatric sections grew faster (M = 0.0050) than 40those in the allopatric sections (M = 0.0039). Growth rate comparisons for each experimental 0 20 40 60 80 100 phase indicated a significant difference only for the early experimental phase, when growth in the Day of experiment sympatric section was higher than in the allopatric FIGURE 3. —Mean (±2 SE) total lengths of brown trout section (M = 0.0013 versus 0.0009; P = 0.027). At the end of that phase (day 55), the size of sym- and Steelhead in sympatric and allopatric treatments during the 100-d artificial-stream experiment. Points for patric-section brown trout (74.7 mm, 4.22 g) had sympatric and allopatric treatments are offset horizonsurpassed that of allopatric-cell fish (72.3 mm, 3.84 tally for clarity. g) but differences were not significant (P = 0.39). Despite an increasing difference in brown trout length between the two treatments, total length Salmonine Abundance. Biomass. and Density was not significantly different by the end of the Effects on Brown Trout Total Length experiment (P = 0.147). Over the course of the Total salmonine numbers were equal (N = 14) experiment, brown trout condition factor in- at the start of the experimental period in each creased from 2.94 at day 1 to 3.04 at day 100. sympatric and allopatric cell. Because of size difBrown trout condition was not significantly differences between species, initial mean biomass in ferent between treatments for any comparisons (P the sympatric cells (16.4 g) was 55% of that of > 0.05). allopatric cells (29.0 g). At the conclusion of the experiment, the average number of salmonines was 10 per sympatric cell and 8 per allopatric cell. Steelhead Biological Statistics Mean total biomass in sympatric cells exceeded Steelhead mortality was low (Z = 0.1501) and that in allopatric cells at day 55 and, by the end growth was rapid in all treatment cells. Mortality, of the experiment, the biomass in sympatric cells growth, and condition were not significantly dif- (72.4 g) was 33% higher than in allopatric cells ferent between cells (P > 0.05). At the start of the (54.5 g; Figure 4). experiment, Steelhead averaged (± SE) 38.7 ± 0.93 Linear models indicated that brown trout length mm and 0.45 ± 0.041 g, and by the end of the in sympatric sections was not related to overall investigation, they averaged 82.5 ± 4.38 mm and salmonine abundance (P = 0.73) or to Steelhead 6.14 ± 0.918 g (Figure 3). For Steelhead, M over density (P = 0.22). However, brown trout length the course of the experiment averaged 0.0093 ± was inversely related to brown trout abundance 0.0001, significantly higher (P = 0.005) than brown in both sympatric and allopatric sections (P < trout growth rates. This difference is further re- 0.01), and the slopes of these models were not flected by the fact that at day 18, Steelhead were significantly different (P > 0.05). only 66% the length of brown trout. By the end of Discussion the experiment, Steelhead had grown to 90% of brown trout length. Steelhead condition averaged Our results suggested that, during their first 3.39 at their introduction and 2.87 by day 100. summer, the mortality and growth of brown trout

936

KOCIK AND TAYLOR 80

the influence of density-dependent factors upon brown trout growth and survival have been noted by other researchers in artificial systems (Fausch and White 1986; Titus and Mosegaard 1991) and in natural systems (Elliott 1984, 1986). Our study design was developed to understand interactions under equal total salmonine densities during the summer growth period. We chose this approach because we wanted to examine interactions over a period when rapid steelhead growth substantially changes the size ratio between the species. We felt that equal densities of the two species were necessary to compare them on an equivalent plane for this initial investigation. Thus,

70-

CO

E 50-

o 3

0) 40-

tc


I 30^ (0

20-

our experiment simulates a condition where 50% 10-

O — test cell biomass- brown trout only

20

40

60

80

100

Day of experiment FIGURE 4. —Mean (±SE) total salmonine biomasses in sympatric and allopatric treatments during the 100-d artificial-stream experiment. Points for sympatric and allopatric treatments are offset horizontally for clarity.

was not affected as strongly by sympatry with steelhead as it was by allopatry with an equivalent density of brown trout (14 fish per treatment cell). The only significant difference in mortality we observed was higher mortality in allopatric cells than in sympatric cells during the early-experimental phase. This result suggests that higher initial brown trout densities in allopatric cells fostered increased hierarchical interactions that were severe enough to cause mortality (Kalleberg 1958). In natural systems, these interactions typically result in either mortality or emigration of subordinate fish displaced from a discrete stream reach by dominant fish (Chapman 1962; Elliott 1986; Hearn 1987; Titus and Mosegaard 1991). Because no emigration could occur in our design, mortality was the only mechanism that regulated densities to the carrying capacity of the experimental cells. Despite the mode of population regulation, the resultant carrying capacities represented hierarchical processes in a discrete stream section. Length-density regression models and growth rate comparisons indicated that brown trout densities appeared to significantly affect brown trout size. The role of intraspecific competition appeared strongest in the early-experimental period, inhibiting brown trout growth in the allopatric cells with higher fish densities. Similar observations on

of a brown trout population was replaced by steelhead, as could occur when redd interference by steelhead limits brown trout production (Hayes 1989; Kocik 1992). Because brown trout and steelhead mortality and growth rates in this study paralleled those of wild age-0 populations (Ziegler 1988; Kocik 1992), we feel that our study was an accurate representation of situations that can occur in Great Lakes tributaries. As such, our study provides insight into mechanisms that may have influenced brown trout and steelhead population dynamics in this experiment and thus, potentially, in natural populations. We hypothesize that vertical stratification was an important partitioning mechanism between these species during the summer growth period. Our observations of brown trout's close association with the bottom and steelhead's tendency to be suspended are consistent with behavior noted in other populations (Fausch 1984; Hearn and Kynard 1986; Ziegler 1988; Hayes 1989). As such, vertical segregation may have reduced interactions between these species during their first growing season. Hayes (1989) noted the compatibility of these species, suggesting that age-0 brown trout and rainbow trout (lacustrine Oncorhynchus mykiss) were adapted to complementary niches and social roles. The vertical partitioning of habitat also appears to allow for an increased total salmonine carrying capacity in the sympatric populations since the total salmonine biomass averaged 33% higher in terms of biomass and 25% higher in terms of numbers offish. Ziegler (1988) presented abundance indices for allopatric and sympatric brown trout and steelhead populations that suggest a similar phenomenon may occur in some natural river systems. In our experiment, we examined interactions from approximately 3 weeks after steelhead emer-


BROWN TROUT AND STEELHEAD INTERACTIONS

937

gence through the first growing season because in equal starting densities. Although our results previous research suggested that this may be an suggest interspecific compatibility at this life hisimportant time for competition (Kruger et al. 1985; tory stage, fishery scientists must consider all life Fausch and White 1986; Hayes 1989). However, history stages before deciding upon a management these two species may interact before this period scheme that combines these two species. The full because steelhead spawning occurs approximately spectrum of competition between these species will 5-7 months after brown trout spawning (Fausch be unknown until fishery scientists more fully exand White 1986; Ziegler 1988). Although the amine other life history stages where habitat use spawning requirements of these fish are known to by these species overlaps, such as spawner-alevin overlap (Raleigh et al. 1984, 1986), quantitative and age-1-parr interactions (where steelhead may studies of these interactions have not been con- have a size advantage). Thus, we recommend cauducted. Brown trout alevins are still in their redds tion in introducing steelhead into waters that supwhen steelhead spawn, and we hypothesize that port a self-sustaining brown trout population bephysical damage to the redds may occur at this cause the cumulative effect of such an introduction time, limiting brown trout production. This is is still uncertain. highly probable in many tributaries to the Great Lakes, where steelhead spawner biomass is high Acknowledgments and spawning area limited (Seelbach 1993). In addition, the different life histories of these This study is a result of work sponsored by the salmonines result in steelhead typically emerging Federal Aid in Sport Fish Restoration (project: 5-9 weeks after brown trout emergence. This dif- Michigan F-35-R, number 641). Additional supference results in two factors that could be im- port was provided by the Michigan Agricultural portant to competition between these species: first, Experiment Station; Michigan Council of Trout a size and prior-residence advantage for brown Unlimited; the Challenge, Oakbrook, Paul Young, trout; and, second, a numerical advantage for and West Michigan chapters of Trout Unlimited; steelhead. Our experiment addressed the size and the Michigan Polar Equator Club; and the Michprior-residence components of the sequential igan Salmon and Steelhead Fishing Association. emergence of brown trout and steelhead. The nu- We thank L. Kocik, K. Newman, T. Nuttle, and merical advantage of steelhead has two modes of H. Sysak for their assistance in maintaining the action: destruction of brown trout alevins by su- artificial stream and the fish during this study. We perimposition, and the greater overall fecundity also thank K. Friedland, R. Haas, R. G. Titus, A. of steelhead due to their migratory life history. If J. Gatz, Jr., and an anonymous reviewer for their both modes of action were to occur, the results of comments on an earlier version of this manuour study (equal densities of the two species) would script. be conservative. This scenario is likely in tributaries where spawning area is limited, and it is References possible that the very large biomass of emergent steelhead would cause a greater strain on food re- Chapman. D. W. 1962. Aggressive behavior in juvenile coho salmon as a cause of emigration. Journal sources and brown trout would have to spend more of the Fisheries Research Board of Canada 19:1047time defending territories (Titus and Mosegaard 1080. 1991; Seelbach 1993). We feel that the result of Chapman, D. W. 1966. Food and space as regulators of salmonid populations in streams. American Natan increased ratio of steelhead to brown trout could uralist 100:345-357. be an increased frequency of interactions between these species, which may be detrimental to brown Cone, R. S. 1989. The need to reconsider the use of condition indices in fishery science. Transactions of trout. the American Fisheries Society 118:510-514. Conclusions and Management Implications Our study indicates that during their summer growing season, steelhead do not negatively affect brown trout relative to an equivalent density of allopatric brown trout. This finding suggests that these juveniles can coexist in a riverine environment. Our objective was to investigate only the first growing season and conditions that resulted

Conover, W. J., and R. L. Iman. 1981. Rank transformations as a bridge between'parametric and nonparametric statistics. American Statistician 35:124129. Elliott, J. M. 1984. Numerical changes and population regulation in young migratory trout Salmo truita in a Lake District stream, 1966-83. Journal of Animal Ecology 53:327-350. Elliott, J. M. 1986. Spatial distribution and behav-

ioural movements of migratory trout Salmo trutta


938

KOCIK AND TAYLOR

in a Lake District stream, 1966-83. Journal of Animal Ecology 55:907-922. Fausch, K. D. 1984. Profitable stream positions for salmonids: relating specific growth rate to net energy gain. Canadian Journal of Zoology 62:441-451. Fausch, K. D. 1988. Tests of competition between native and introduced salmonids in streams: what have we learned? Canadian Journal of Fisheries and Aquatic Sciences 45:2238-2246. Fausch, 1C D., and R. J. White. 1986. Competition among juveniles of coho salmon, brook trout, and brown trout in a laboratory stream, and implications for Great Lakes tributaries. Transactions of the American Fisheries Society 115:363-381. Hayes, J. W. 1989. Social interactions between 0+ brown and rainbow trout in experimental stream troughs. New Zealand Journal of Marine and Freshwater Research 23:163-170. Hearn, W. E. 1987. Interspecific competition and habitat segregation among stream-dwelling trout and salmon: a review. Fisheries (Bethesda) 12(5):24-31. Hearn, W. E., and B. E. Kynard. 1986. Habitat utilization and behavioral interaction of juvenile Atlantic salmon (Salmo salar) and rainbow trout (S. gairdneri) in tributaries of the White River of Vermont. Canadian Journal of Fisheries and Aquatic Sciences 43:1988-1998. Kalleberg, H. 1958. Observations in a stream tank of territoriality and competition in juvenile salmon and trout (Salmo salar L. and S. trutta L.). Institute of Freshwater Research Drottningholm Report 39:5598.

Kocik, J. F. 1992. Evaluation of juvenile brown trout and steelhead competition in Great Lakes tributaries. Doctoral dissertation. Michigan State University, East Lansing. Kruger, K. M., W. W. Taylor, and J. R. Ryckman. 1985. Angler use and harvest in the Pere Marquette River near Baldwin. Michigan. Michigan Academician 317-330. Li, H. W., and R. W. Brocksen. 1977. Approaches to

the analysis of energetic costs of imraspecific competition for space by rainbow trout (Salmo gairdneri). Journal of Fish Biology 11:329-341. Raleigh, R. F., T. Hickman, R. C. Solomon, and P. C. Nelson. 1984. Habitat suitability information: rainbow trout. U.S. Fish and Wildlife Service Biological Report 82(10.60). Raleigh, R. F., L. D. Zuckerman, and P. C. Nelson. 1986. Habitat suitability index models and instream flow suitability curves: brown trout. U.S. Fish and Wildlife Service Biological Report 82(10.124). Ricker, W. E. 1975. Computation and interpretation of biological statistics of fish populations. Bulletin of the Fisheries Research Board of Canada 191. Ringler, N. 1979. Selective predation by drift-feeding brown trout (Salmo trutta). Journal of the Fisheries Research Board of Canada 36:392-403. Rose, G. A. 1986. Growth decline in subyearling brook trout (Salvelinus fontinalis) after the emergence of rainbow trout (Salmo gairdneri). Canadian Journal of Fisheries and Aquatic Sciences 43:187-193. Seelbach, P. W. 1993. Population biology of steelhead in a stable-flow, low-gradient tributary of Lake Michigan. Transactions of the American Fisheries Society 122:179-198. Snedecor, G. W., and W. G. Cochran. 1982. Statistical methods. Iowa State University Press, Ames. Titus, R. G.. and H. Mosegaard. 1991. Selection for growth potential among migratory brown trout (Salmo trutta) fry competing for territories: evidence from otoliths. Canadian Journal of Fisheries and Aquatic Sciences 48:19-27. Ziegler, R. L. 1988. Stream resource utilization of sympatric and allopatric juvenile brown (Salmo trutta) and steelhead trout (Salmo gairdneri). Master's thesis. Michigan State University, East Lansing. Received June 25. 1993 Accepted April 25, 1994