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).