Klamath Tribes Research Report: KT-93-01
Laboratory Studies of Fathead Minnow Predation on Catostomid Larvae
by
Larry Dunsmoor
December 1993
Natural Resources Department The Klamath Tribes P.O. Box 436 Chiloquin, OR 97624 2
Abstract
Since their introduction in the mid-1970's, fathead minnows Pimephales promelas rose to numerical dominance of the fish community in Upper Klamath Lake, Oregon, occupying nursery habitats of the endangered Lost River and shortnose suckers (Deltistes luxatus and Chasmistes brevirostris) in great numbers. This study, in which laboratory experiments assessed the predatory capability of fathead minnows on sucker larvae, is a first attempt at assessing potential predatory impacts of introduced fathead minnows on the endangered suckers. Despite lack of literature accounts of fathead minnow piscivory, fathead minnows readily consumed sucker larvae in this study. Fathead minnow piscivory appeared to be related to group behavior, no predation on sucker larvae occurred unless at least three fathead minnows were present. Fathead minnows were not gape-limited when consuming sucker larvae because groups of minnows tore larvae apart and ate them piecemeal. Presence of Daphnia and artificial dry food as alternate prey reduced but did not eliminate predation on sucker larvae. Provision of cover in the form of Scirpus stems reduced mean predation rates to 63% relative to 97% in tanks without cover. Potential significance of these results is discussed relative to sucker year class formation and water management practices in Upper Klamath Lake. 3
Recent declines in populations of shortnose suckers Chasmistes brevirostris and Lost River suckers Deltistes luxatus, native to the Upper Klamath River system in California and Oregon, resulted in their addition to the federal endangered species list in 1988 (Williams 1988). Their declines have been attributed to a wide array of factors, including the presumed negative impacts of exotic species (USFWS 1992). Hypothesized negative impacts of non-native fish on endangered suckers include (among other things) predation, competition for food and space, and introduction of exotic diseases or parasites, although no research has assessed these hypotheses. The most recently established exotic fish in the Upper IClarnath River system is the fathead minnow Pimephales promelas, which probably became established in Upper Klamath Lake between 1974 and 1979 (Andre-asen 1975; Ziller 1991). Fathead minnows rapidly increased in numbers in Upper Klamath Lake thereafter, they comprised 18% of the catch by number in a trap net in 1983 (Ziller 1991). In 1992, fathead minnows numerically dominated fish caught in trap nets set in Upper Klamath and Agency Lakes (directly connected to Upper Klamath Lake) by a wide margin (Logan and Markle 1993). High densities of adult fathead minnows occupy the same shoreline nursery habitats as larval suckers in Upper Klamath Lake (Buettner and Scoppettone 1990), prompting concern about their potential predatory impacts. While food habits of fathead minnows in Upper Klamath Lake are unknown, elsewhere their diets are reported to include algae, aquatic insect larvae, zooplankton, detritus, and mud (Carlander 1969; Scott and Crossman 1973; Held and Peterka 1974). The only record I found of piscivory was in Franzin and Harbicht (1992), who found walleye Stizostedion vitreum larvae in stomachs of fathead minnows captured in trap nets. However, they were uncertain whether the predation occurred before capture or resulted from concentrating predators and prey in the trap net. Despite the absence of piscivory in literature accounts of fathead minnow diets, a pre-experimental trial for this study in which five fathead minnows ate 39 of 40 Lost River sucker larvae showed that fathead minnows were willing and able to consume sucker larvae. This demonstration of piscivory by fathead minnows coupled with their high densities in nursery habitats of endangered suckers in Upper Klamath Lake prompted study of the predatory interactions of fathead minnows with sucker larvae under laboratory conditions. In this study I assessed predation rates of fathead minnows on sucker larvae in tanks as influenced by water depth, the presence and absence of cover, the presence of alternate prey, and fathead minnow mouth size relative to body size of sucker larvae. 4
Methods and Results
All experimental trials were conducted in a recirculating water system at the Khunath Tribe Native Fish Hatchery in Chiloquin, Oregon. Experimental fish tanks were light blue, rectangular, 68 L Rubbermaid tubs (45 cm long, 40 cm wide, and 41 cm tall). Water flowed into each tank at about 2 L/min, and flowed out via a centrally located standpipe which was screened to prevent escape of larval fish. Water temperature remained a fairly constant 17°C, which is within the range of ambient temperatures reported in shoreline areas of Upper Klamath Lake by Buettner and Scoppettone (1990). Trial's one through five each ran for 14-18 hours, while trial six ran for 40 hours. Water depths were maintained at about 27 cm except in trial six when it was decreased to 15 cm in low water treatments. Sucker larvae used in the experiments were spawned from adults captured in the Sprague and Williamson Rivers in May, 1992. Fathead minnows were beach seined from Upper IClamath Lake near Modoc Point in early June, 1992. Fathead minnows were treated for a colurnnaris infection with 20 ppm nitrofurazone upon arrival at the hatchery. Many fish died during the seining process in the lake, indicating that the infection was well advanced. The nitrofurazone treatment successfully controlled the infection and all remaining fathead minnows were healthy when used in the experiments two weeks later. Trials one through five focused in part on assessing the influence of fathead minnow gape width (i e maximum mouth width at full gape) on their ability to prey on various sizes of sucker larvae. I measured gape widths to the nearest 0.1 mm on 50 randomly selected, live fathead minnows (48-68 mm TL) using a hand ruler and magnifying glass. Using the resulting frequency distribution, I stratified gape widths into three groups: small (2.0-2.3 mm), medium (2.5-2.8 mm), and large (3.1-3.8 mm). Fathead minnow gape width classes were then used as the three treatments in trials one through five. Tanks (experimental units) were randomly assigned to treatments (fathead minnow gape width class in trials one-five) until there were three replicate tanks per treatment. The percentage of larvae eaten in each tank was transformed to normality (arcsine transformation) and used in one-way ANOVA tests to assess differences among treatment means. Statistical tests were performed using SYSTAT 5.0 (Wilkinson 1990). Statistical power was determined post hoc when the null hypothesis was not rejected following methods of Cohen (1988). A severe outlier in trial 6 prompted use of a nonparametric two-way ANOVA (Zar 1984) followed by multiple comparisons (Conover 1980). I followed the admonitions and suggestions of Yoccoz (1991) concerning 5 appropriate use of significance tests and balancing biological and statistical significance in stating conclusions. In trial one, fifteen shortnose sucker larvae (15-21 mm standard length) were stocked in each tank with one fathead minnow from the appropriate gape width class. Trial two was similar to trial one but used three fathead minnows per tank. No predation occurred during trials one or two (Table 1). In trial three, a wider array of prey sizes was presented to fathead minnows by adding 5 Lost River sucker larvae (13.5-14.0 mm) to the 15 shortnose sucker larvae remaining from earlier trials, bringing the total number of sucker larvae to 20 per tank. Predation occurred in the medium and large gape width treatments, although no significant differences were found in predation rates among fathead minnow gape width classes (Table 1). However, the low power of the test (-0.08) precludes concluding with any certainty that predation rate was unrelated to gape width. Kolmogorov-Smirnov tests comparing length frequency distributions of larvae in individual tanks before and after the trial showed no significant differences in length frequencies, suggesting that fathead minnows were not selecting particular larval sizes. In trial four, six fathead minnows from the appropriate gape width classes and ten Lost River sucker larvae (14-18 mm) were randomly assigned to each tank. At the end of the trial larvae remained in only three of the nine tanks, and no significant differences in predation rates were found among fathead minnow gape width classes (Table 1). Again, statistical power of the ANOVA was low (-.0.09). Only two tanks had large numbers of larvae remaining, and upon close inspection I noticed the standpipes (which controlled water depth) in these tanks were slightly taller than in other tanks. As a result, a ledge (a reinforcing structure for the tank) about 1 cm wide was 1 cm below the water surface, and the surviving larvae were clearly using these ledges to avoid predators. These ledges likely influenced predation rates in these two tanks in trial three as well, because they were the only tanks in which no predation occurred in the medium and large gape width treatments. Both standpipes were shortened before subsequent trials. Trial five was similar to trial four (six fathead minnows and ten Lost River sucker larvae in each tank), but fathead minnows were provided with alternate prey in the form of live Daphnia and dry feed. Large numbers of Daphnia were concentrated in a small water volume, and 150 ml of this mixture was added to each tank, along with 1 g of dry food. At the trial's end, predation on sucker larvae had occurred in five of the nine tanks (Table 1). Again, there were no significant differences in feeding rates on sucker larvae among treatments, and power of the ANOVA was low (0.17). Lack of an appropriate control treatment precluded statistical comparison of predation rates with and without 6
alternate prey. However, mean predation rates in the presence of alternate prey (5-43%) were markedly lower than those in trial 4(75-98%), which offered predators no alternate prey (Table 1). In trial six, three tanks were randomly assigned to each of four treatments (three more tanks were used than in trials one-five): high water with cover, high water without cover, low water with cover, and low water without cover. Water depth in 'high water' tanks was 27 cm, and 15 cm in 'low water' tanks. Cover provided by emergent vegetation in natural shoreline habitats was simulated in 'with cover' treatments. Scirpus stems were collected from Upper Klamath Lake, cut to approximately 60 cm, and wedged into holes in 1/4" hardware cloth to achieve a stem density of approximately 565 stems/m2 (stems were spaced approximately 4 cm apart). These assemblies were lowered into the appropriate tanks with the Scirpus stems hanging down from the hardware cloth so that half of a tank had Scirpus stems in it and the other half was open water. Scirpus stems were held in place on the tank bottom by sheets of black, 1/2" mesh plastic netting. Each tank was stocked with 20 randomly selected shortnose sucker larvae (14-21 mm standard length), followed by 10 randomly selected fathead minnows without regard to gape width. Trial six ran for 40 hours. A severe outlier (only 2 larvae eaten in a tank in the high water/cover absent treatment) prompted use of a nonparametric two factor ANOVA, performed on ranked percentages of larvae eaten per tank. Null hypotheses of no effect of water depth and cover presence/absence were not rejected (water depth P>0.75; cover 0.10
Table 1. Mean percentages (±1 SD) of sucker larvae consumed by fathead minnows in experimental trials. Treatments (each with three replicates) consisted of fathead minnow gape width classes (S=small, M=mediutn, and L=large). One-way ANOVA tests on arcsine-transformed percentages were used to detect significant differences among treatment means. Mean percentages and their standard deviations presented here were calculated using transformed data, and were then converted to percentages for presentation. Statistical power was estimated following Cohen (1988).
Mean percentage Starting number of larvae eaten Trial Fatheads Larvae S M L ANOVA Power
1 1 15 0 0 0
2 3 15 0 0 0
3 3 20 0 13±15a 22±21a P>0.10 0.08
4 6 10 98±7 75±62a 87±36a P>0.10 0.09
5 6 10 10±0 5±15 43±36 P>0.10 0.17 a Means were likely under-estimated and standard deviations over-estimated as a result of experimental bias. One tank in each of the M and L treatments had a slightly longer standpipe than the other tanks, resulting in immersion of a small ledge which was used as escape cover by larvae. 8
Table 2. Ranges in percentages of sucker larvae eaten by fathead minnows in various combinations of cover and water depth in tanks (trial 6). Each of three tanks in each cover/water depth combination contained 10 fathead minnows and 20 shortnose sucker larvae. Percentages were ranked from low to high, and ranks were used in subsequent nonparametric two factor ANOVA and multiple comparison procedures. Bold letters next to rank sums show results of multiple comparisons; factor combinations with letters in common were not significantly different (4x .10).
Range of % Factor combination mortality Sum of ranks
low water, cover present 15-45 10.0a
high water, cover absent 10-100 18.5ab
high water, cover present 45-100 22.5 ab
low water cover absent 65-100 27.0 b 9
Discussion
Piscivory is not documented in literature reviews of fathead minnow food habits (Carlander 1969; Scott and Crossman 1973; Held and Peterlat 1974). While Franzin and Harbicht (1992) found walleye larvae in stomachs of fathead minnows captured in trap nets, they were uncertain whether the predation was a natural event or an artifact of confinement of prey and predator in the trap net. I have shown fathead minnows to be effective predators on sucker larvae in tanks. It is possible that the observed predation resulted solely from artificial confinement in tanks (e.g. La Bolle 1981). However, coincident use of shoreline habitats by larval suckers and very large numbers of fathead minnows in Upper Klamath Lake (Buettner and Scoppettone 1990; Dunsmoor, unpublished data) makes the possibility of such predation an important management question in view of the endangered status of the suckers. Fathead minnows displayed some specific predatory characteristics in this study. First, fathead minnows appeared to be effective as predators only when in a group of at least three fish; when alone, no fathead minnows consumed sucker larvae. Second, there was no discernable relationship between body size of sucker larvae consumed and mouth size of fathead minnows (i.e. fathead minnows do not appear to be gape-limited when foraging on soft-bodied sucker larvae). The typical gape-limitation paradigm applies to predators that consume prey whole, a practice for which the size of some cross-sectional body dimension of the prey fish (commonly body depth) relative to some cross-sectional dimension of the predator's mouth or pharyngeal apparatus limits the maximum size of prey fish that can be ingested whole by a predator of a given size (e.g. Lawrence 1958). Such relationships can have important management implications, as exemplified by the utility of the Available Prey to Predator Ratio (Jenkins and Morais 1978) and the insights generated into size-structured mechanisms related to year class formation in largemouth bass Micropterus salmoides (Gutreuter and Anderson 1985; Keast and Eadie 1985). In this study, fathead minnows ingested some sucker larvae headfirst and whole, while others were torn apart and eaten piecemeal by a group of minnows. Therefore, the maximum size of sucker larvae that could be consumed by fathead minnows will be determined by something other than body dimension relationships. Sucker larvae used in this study ranged in size from 13-21 mm. This study did not define the upper limit in larval body size that fathead minnows could consume, and until field studies show otherwise it must be considered possible that sucker larvae larger than 21 mm are susceptible to fathead minnow predation in natural habitats. 10
Sucker larvae used in this study ranged in age from 40-52 days (about 26-32 days after swimup) and were in the late mesolarval to mid metalarval developmental stages [terminology after Snyder (1981)]. Wild suckers are generally <40 days old when they reach the lower Williamson River and Upper Klamath Lake as protolarvae or early mesolarvae (Dunsmoor, unpublished data). Miler et al. (1988) demonstrated an inverse relation between larval developmental stage or size and susceptibility to predation, primarily resulting from ontogenetic increases in predator detection (vision and lateral line development) and avoidance (fm and muscle development) capabilities. Therefore, due to age-related developmental differences, wild sucker larvae just entering their rearing habitats in Upper Klamath Lake may be even more susceptible to predation by fathead minnows than the larvae used in these experiments. Availability of sucker larvae as prey for fathead minnows in the wild is most likely determined by factors which influence their encounter frequency and/or ability to evade fathead minnows. If sucker larvae change habitat use patterns through ontogeny, encounter rates with specific predators may also change. When prey and predators coexist in the same habitat, complex habitats provided by macrophytes generally reduce foraging efficiency of fish predators (Boyle 1979; Cooper and Crowder 1979; Savino and Stein 1982; Heck and Crowder 1991). Consequently, the typical result of water drawdowns in reservoirs is an increase in predation as forage fish are forced to abandon refugia in littoral areas, becoming concentrated and more available to predators (Ploskey 1986). Results of trial six follow this general pattern, in which efficiency of fathead minnow predation on larval suckers was reduced in the presence of cover. The statistical analysis was complicated somewhat by an outlier, but the nonparametric statistical analysis showed significantly lower predation rates in the low water treatment when cover was present. Parametric ANOVA with the oudier excluded showed a significantly lower mean predation rate in the presence of cover regardless of water depth (P<0.06). Specifically, excluding the outlier, mean predation rates were 96.6% (11.3% SD) with cover absent, and 63.3% (18.4% SD) with cover present. The observed 33% decrease in mean predation rate resulting from the addition of cover to tanks is biologically significant because an influence of this magnitude on survival of the larval life stage could be influential in year class formation for the endangered suckers in Upper Klamath Lake. Another factor that can influence prey selection by a predator is the presence of alternative prey items (Hodgson and Kitchell 1987). In trial five of this study fathead minnows had access to sucker larvae, Daphnia, and dry food. Presence of alternate prey 11 more accurately represented field conditions in which an array of potential prey items would be available to fathead minnows. While resulting predation rates on sucker larvae were lower than in previous trials, fathead minnows continued to consume sucker larvae despite the presence of readily accessible alternate prey items. This result suggests that fathead minnows in the wild may consume sucker larvae despite the availability of alternate prey.
Management Implications Larval Lost River and shortnose suckers enter shoreline nursery habitats in the lower Williamson River and Upper Klamath Lake during May and June soon after swimup and emigration from spawning areas (Buettner and Scoppettone 1990). Historically, these nursery habitats were structure-rich. Aerial photographs taken in 1941 show 9.7 km of lower Williamson River channel meandering through 1,930 ha of emergent marsh, with dense growths of willows Salix sp. and cottonwoods Populus sp. lining the riverbanks. Shorelines in Upper Klamath Lake for many kilometers on both sides of the river mouth were uninterrupted edges of emergent marsh before conversion of the entire marsh to agricultural use. Both river and lake shorelines of the Williamson River delta are now dikes constructed with materials dredged from the river and lake bottoms. These dikes provide shoreline habitats ranging from narrow strips of emergent macrophytes (Scirpus, Sparganium, or 7ypha), to bare expanses of sand or riprap. In addition to the vegetative changes resulting from dredging and diking, 4.5 km of the lower Williamson River were lost through channelization. The net result of human activity has been to greatly reduce the quality and quantity of available nursery habitat for larval suckers in the lower Williamson River and Upper Klamath Lake. Availability of any existing emergent cover to larval suckers in Upper Klamath Lake is determined by fluctuations in lake surface elevations, which are regulated by a dam constructed at the outlet in 1921. As water is removed from the lake for irrigation throughout the summer, emergent vegetation along much of the lake shoreline can be left above the waterline during the period when sucker larvae are entering and using shoreline nursery areas. Measurements taken at the mouth of the Williamson River (from a single location) indicate that when water elevations decrease to about 0.48 m below maximum water elevation, emergent vegetation will no longer be in the water. During the previous two decades (1970-1990) water elevations dropped 0.48 m below full pool before mid-July in 9 out of 20 years. While resulting impacts on interactions between larval suckers and their predators have not been measured in the field, my results indicate 12 that eliminating cover components from nursery areas can increase intensity of predation on sucker larvae, which could significantly influence sucker year class strength. Results of this laboratory study raise serious concerns about the potential impact of predation by fathead minnows on Lost River and shortnose sucker larvae in the wild. However, before predation by fathead minnows can be positively identified as a serious problem for the endangered suckers in Upper IClarnath Lake, results of this study must be validated in the field and the predatory impacts of fathead minnows quantified. Pending completion of field validation studies, it may be prudent to begin managing sucker nursery habitats for structural complexity. In the short term, maintaining water levels , that inundate emergent vegetation in shoreline areas through at least mid-July would retain cover components in nursery areas. Long term efforts should focus on restoring the lower Williamson River and the associated shorelines on Upper Klamath Lake to some semblance of their original conditions.
Acknowledgements
I thank Dr. Doug Markle for suggesting that this work be done, and for his editorial comments. I also thank Craig Bienz and Jake Kann for their suggestions and extensive editorial comments. Bill Ehinger, Bud Ullman and Emily Dyke provided editorial comments, and Dominic Herrera, Mike Moore, and Loren Schonchin helped with fish collection, and fish and tank maintenance. 13
References
Andreasen, J. K. 1975. Occurrence of the fathead minnow, Pimephales promelas, in Oregon. California Fish and Game 61:155-156.
Boyle, T. P. 1979. Responses of experimental lentic aquatic ecosystems to suppression of rooted macrophytes. Pages 269-283 in, J. E. Bmck, R. T. Prentki, and 0. L. Loucks, editors. Aquatic Plants, Lake Management, and Ecosystem Consequences , of Lake Harvesting. Center for Biotic Systems, Institute for Environmental Studies, University of Wisconsin, Madison.
Buettner, M., and G. Scoppettone. 1990. Life history and status of catostomids in Upper Klamath Lake, Oregon. United States Fish and Wildlife Service Completion Report, National Fisheries Research Center, Reno, Nevada.
Carlander, K. D. 1969. Handbook of freshwater fish biology. Volume one. Iowa State University Press, Ames.
Cohen, J. 1988. Statistical power analysis for the behavioral sciences. Second edition. Lawrence Erlbaum Associates, Hillsdale, New Jersey.
Conover, W. J. 1980. Practical nonparametric statistics, second edition. John Wiley and Sons, New York.
Cooper, W. E, and L B. Crowder. 1979. Patterns of predation in simple and complex environments. Pages 257-267 in, R. H. Stroud and H. Clepper, editors. Predator- prey systems in fisheries management. Sport Fishing Institute, Washington, District of Columbia.
Franzin, W. G., and S. M. Harbicht. 1992. Tests of drift samplers for estimating abundance of recently hatched walleye larvae in small rivers. North American Journal of Fisheries Management 12:396-405.
Gutreuter, S. J., and R. 0. Anderson. 1985. Importance of body size to the recruitment process in largemouth bass populations. Transactions of the American Fisheries Society 114:317-327. 14
Heck, K. L, Jr., and L B. Crowder. 1991. Habitat structure and predator-prey interactions in vegetated aquatic systems. Pages 281-299 in, S. S. Bell, E. D. McCoy, and H. R. Mushinsky, editors. Habitat structure: the physical arrangement of objects in space. Chapman and Hall, New York.
Held, J. W., and J. J. PeterIca. 1974. Age, growth, and food habits of the fathead minnow, Pimephales promelas, in North Dakota saline lakes. Transactions of the American Fisheries Society 103:743-756.
Hodgson, J. R., and J. F. Kitchell. 1987. Opportunistic foraging by largemouth bass (Micropterus salmoides). American Midland Naturalist 118:323-336.
Jenkins, R. M., and D. L Morais. 1978. Prey predator relations in the predator-stocking evaluation reservoirs. Proceedings of the Annual Conference of the Southeastern Association of Fish and Wildlife Agencies 30:141-157.
Keast, A., and J. McA. Eadie. 1985. Growth depensation in year-0 largemouth bass: the influence of diet. Transactions of the American Fisheries Society 114:204-213.
La Bolle, L D., Jr.. 1981. Constraints of the laboratory environment on predator-prey systems in fishes. Pages 24-32 in, G. M. Cailliet and C. A. Simenstad, editors. Gutshop '81: Fish Food Habits Studies, Proceedings of the Third Pacific Workshop. Washington Sea Grant, University of Washington, Seattle.
Lawrence, J. M. 1958. Estimated sizes of various forage fishes largemouth bass can swallow. Proceedings of the Annual Conference of the Southeastern Association of Game and Fish Commissioners 11:220-225.
Logan, D. J., and D. F. Markle. 1993. Fish faunal survey of Agency Lake and northern Upper Klamath Lake, Oregon. Chapter 7 in, S. G. Campbell, editor. Environmental research in the Klamath Basin, Oregon: 1992 annual report. United States Department of the Interior, Bureau of Reclamation, Denver Office, Colorado. 15
Miller, T. J., L. B. Crowder, J. A. Rice, and E. A. Marschall. 1988. Larval size and recruitment mechanisms in fishes: toward a conceptual framework. Canadian Journal of Fisheries and Aquatic Sciences 45:1657-1670.
Ploskey, G. R. 1986. Effects of water-level changes on reservoir ecosystems, with implications for fisheries management. Pages 86-97 in, G. E. Hall and M. J. Van Den Avyle, editors. Reservoir Fisheries Management: Strategies for the 80's. Reservoir Committee, Southern Division American Fisheries Society, Bethesda, Maryland.
Savino, J. F., and R. A. Stein. 1982. Predator-prey interaction between largemouth bass and bluegills as influenced by simulated, submersed vegetation. Transactions of the American Fisheries Society 111:255-266.
Scott, W. B., and E. J. Crossman. 1973. Freshwater fishes of Canada. Bulletin 184, Fisheries Research Board of Canada, Ottawa.
Snyder, D. E. 1981. Contributions to a guide to the cypriniforrn fish larvae of the Upper Colorado River system. Final Report, US Bureau of Land Management, Denver, Colorado.
USFWS (United States Fish and Wildlife Service). 1992. Formal consultation on the effects of the long-term operation of the Klamath Project on the Lost River sucker, shortnose sucker, bald eagle, and American peregrine falcon. Memorandum to the Regional Director, Bureau of Reclamation, Sacramento, California from the Regional Director, Fish and Wildlife Service, Region 1, Portland, Oregon.
Wilkinson, L. 1990. SYSTAT: the system for statistics. SYSTAT, Inc., Evanston, Illinois.
Williams, J. E. 1988. Endangered and threatened wildlife and plants; determination of endangered status for the shortnose sucker and Lost River sucker. Federal Register 53(137):27,130-27,134.
Yoccoz, N. G. 1991. Use, overuse, and misuse of significance tests in evolutionary biology and ecology. Bulletin of the Ecological Society of America 72:106-111. 16
Zar, J. H. 1984. Biostatistical analysis, second edition. Prentice-Hall, Englewood Cliffs, New Jersey.
Ziller, J. S. 1991. Factors that limit survival and production of largemouth bass in Upper Klamath and Agency Lakes, Oregon. Information Report No. 91-6, Fish Division, Oregon Department of Fish and Wildlife, Portland.