Food Web Dynamics and Lahontan Cutthroat Trout (Onchorhynchus clarki henshawi) Energetics in

Pyramid Lake, Nevada

Tahoe Baikal Institute 2003

Anastasia Alexandrova, TBI 2003

Lesley Dampier, TBI 2003

Clara Long, TBI 2003

David Gilroy, Biologist, TBI 1997

Carri LeRoy, Project co-leader TBI 2000

Sudeep Chandra, Project co-leader, TBI 1997

ABSTRACT:

During the last century, the native trout of the western United States have been impacted by the introduction of nonnative species and alterations to landscape (Moyle 2002). Prior to expansive settlement by non-indigenous peoples in the 18^th century the Lahontan cutthroat trout (Onchorhynchus clarki henshawi) of Pyramid Lake, Nevada attained the largest size of any inland cutthroat trout (Coleman and Johnson 1988). After the extirpation of LCT from the Pyramid - Lake Tahoe watershed, a hatchery program was initiated by the Paiute tribe of Pyramid Lake to reestablish LCT into Pyramid Lake. Although tribal efforts have sustained a trophy fishery on Pyramid Lake, the fisheries program has noted a potential decline in the size of spawning fish returning to the hatchery. In this study we initiated a preliminary assessment of food web dynamics in Pyramid Lake, in order to evaluate the potential ecological (competition and predation) constraints regulating fish growth. Our preliminary results are that LCT are at the top of the food chain and do not appear to be limited by competition from other fish. We recommend further characterization of fish and benthic invertebrate communities in order to more completely understand the food web dynamics of LCT energetics.

INTRODUCTION:

Pyramid Lake is a large (183 mi²) mid-elevation terminal desert lake within the boundaries of the Pyramid Lake Paiute Tribe’s Reservation in western Nevada. The lake’s major tributary is the Truckee River which flows first from Lake Tahoe and through the Truckee meadows before terminating in the lake. Lake surface elevation is maintained by a balance between inflows from the river through releases from the Derby dam diversion located upstream on the Truckee River and through evaporation. It is an alkaline lake with a pH of 9.2 and is highly saline at 4.3 parts per thousand. The lake supports a population of Lahontan cutthroat trout (Onchorhynchus clarki henshawi), Sacramento Perch (Archoplites interuptus), Tui Chub (Gila bicolor obesus and G.b. pectinifer), Tahoe Sucker (Catostomus tahoensis), and Cui-ui (Chasmistes cujus).

Lahontan cutthroat trout (LCT) are unique in the Salmonid family in their ability to survive in the alkaline and saline waters of remnant great basin lakes like Pyramid (Sigler et al. 1983). Although LCT spend most of their lives in the lake, they are obligate stream spawners and historically migrated up the Truckee River into Lake Tahoe and surrounding lakes to spawn. The Pyramid Lake strain of LCT was the largest freshwater trout in western North America (Hickman and Behnke 1979). The largest recorded cutthroat was taken from Pyramid in 1925 weighing 18.6kg (41 lbs) (Sigler et al. 1978).

In 1905, the LCT spawning was interrupted when the Army Corps of Engineers built the Derby Dam on the Truckee River not far from its outlet in Pyramid Lake. The dam was part of the ambitious Newlands Restoration Project which allocated water resources to the creation of agricultural lands in the high desert south of Pyramid Lake. After the dam was built, trout spawning and rearing areas were reduced by 85% in the river and annual flow from the Truckee was reduced by 50% in wet years and nearly 100% in dry years (Coleman and Johnson 1988). The dam thus resulted in the reduction of lake level by 26m between 1905 and 1967 (Sigler et al. 1978) This, in conjunction with increasing pollution from new types of land use in the area, raised significant and legitimate concerns about water quality in the lake. Moreover, commercial operations supplied between 90,000 and 120,000 kg of LCT annually to markets in the region (Coleman and Johnson 1988). These pressures led to the extirpation of LCT from Pyramid Lake in 1944 (Coleman and Johnson 1988). The LCT was placed on the endangered species list in 1973 and subsequently redefined as threatened in 1975 to facilitate fisheries management (Hickman and Behnke 1979).

Another native species, the endemic cui-ui, experienced serious decline after the construction of the Derby Dam. It was the first fish to be placed on the endangered species list in the 1970’s and remains on the list today. Although cui-ui are also stream spawners, due to their long life cycles (up to 40 years) they were able to spawn in heavy rainfall years from Pyramid and survive under increasing anthropogenic pressures. Cui-ui are central to Pyramid Lake Paiute culture and as such are another population of concern although we do not specifically address these issues in this paper.

Attempts to reestablish LCT in Pyramid Lake began in the 1950s. However, it was not until 1974 that the LCT and cui-ui populations were supported by the Paiute tribe’s yearly hatchery program, with LCT strain taken from Summit Lake. This program introduces one million LCT and 1-2 million cui-ui into Pyramid Lake each year (Nancy Vucinich 2003, personal communication). The tribe remains concerned that the fish have not been as large as they were historically. Although this could be due to both genetic and ecological issues, we focus on the latter in order to determine if Pyramid Lake’s LCT are limited in size through their position in the food web. Recent research suggest both pelagic and benthic invertebrate communities are important energy sources for fish production. We investigate these communities in order to understand the food web structure in Pyramid Lake (Vander Zanden et al. 2003).

PREVIOUS RESEARCH:

Research at Pyramid Lake has focused mostly on water quality issues over the past 30 years, as concern mounted over the lake’s declining surface elevation. An understanding of the physical, chemical, and biological aspects of the lake and river was needed to develop management plans that would provide adequate protection for the lake and facilitate recovery of the cui-ui and LCT. Major research projects were undertaken by Sigler et al. (1978), Lockheed Ocean Science Laboratories (1982), Galat (1981, 1982) and the University of California, Davis (1994).

During 1976 and 1977, Sigler et al. (1978), in addition to limnological studies, documented the life history and ecology of Pyramid Lake’s five fish species. Their examination of life histories included morphology, age and growth, food and feeding habits, reproduction, habitat, and suggestions for fishery management. Lockheed Ocean Science Laboratories (1982) conducted bioassay experiments on 17 species of algae, zooplankton, invertebrates, and fish inhabiting Pyramid Lake, to investigate the effect of rising total dissolved solids (TDS) levels on survival and growth. At the time of their study, TDS was about 5,800 mg/L. The authors concluded that substantial increases in TDS to around 8,500 mg/L could cause significant degradation of species composition, diversity and biomass. Galat (1981, 1982) researched primary productivity and other limnological processes in Pyramid Lake, in an effort to determine the amounts of “food” available to fish. In 1989, the University of California- Davis (UC Davis 1994) was contracted by the tribe to conduct a four year study to: (1) develop water quality standards for Pyramid Lake and the lower Truckee River, (2) research limnological processes, with emphasis on internal and external nutrient cycling, and (3) train tribal personnel to conduct water quality tests and institute a water quality monitoring program.

Thus, most contemporary research has identified the sources of nutrient inputs and losses, primary productivity, and other limnological processes in Pyramid Lake. A thorough investigation of the food web dynamics has been conducted after LCT restoration through hatchery introductions (Sigler et al. 1978) Recently, contemporary “snap shot” of the lake’s fishery was undertaken by Monda (1999) with the goal of determining the status of fish populations in the lake. In a comprehensive report, he outlines basic information for most of the species of fish in the lake looking at length-weight relationships, age at maturity, growth rate and diet information. Much of the diet data suggests potential competition and limitation of food resources of pelagic origin for Pyramid Lake fish. Competition occurs particularly between LCT, Tui Chub, and Sacramento perch. Furthermore, recommendations were made to establish a regular monitoring program for fishes of Pyramid Lake to assess long-term changes to these fish populations. This research is necessary to determine the status of current LCT populations, their growth rates, and in the future provide tribal managers with information and stocking rates in the lake.

METHODS:

During the investigation of food web structure at Pyramid Lake we surveyed both benthic invertebrate communities and vertebrate populations.

Benthic Invertebrates

We collected benthic invertebrates both along the shoreline and through a depth transect. Qualitative shoreline collection occurred at four sites (Warrior Point, Anaho Island, Pelican and Popcorn points) by handpicking specimens from cobbles and through dip net collections at sandy or silty sites. For a more quantitative analysis, Ponar and Eckman bottom dredges were conducted along a depth transect out to midlake (5m, 10m, 15m, 20m, 25m, 35m, 40m, 60m, 75m, 80m and 90m) from Pelican Point. Additionally, minnow traps at a depth of 10m, although not placed to collect invertebrates, returned some Odonata specimens. Invertebrates were hand sorted live using sugar flotation method preserved in isopropyl alcohol, counted and, identified to the lowest taxonomic level possible.

We used the Shannon-Wiener index of species diversity to characterize invertebrate populations at different depths.

Vertebrates

We sampled fish with trap nets, gill nets, and minnow traps. We placed trap nets between 15 and 25m at two locations in the lake, Windless Bay and Pelican Point. Monofilament-multimesh gill nets were set three times at two depths, shallow (15 to 23m) and deep (50 to 52m) at three locations, Blockhouse, Windless Bay and Pelican Point. Minnow traps were also set along a transect at Pelican Point at the following depths: 3m, 10m, 20m, 30m, 40m and 50m.

To determine size and age ratios, we collected weight and length (standard, fork and total) of fish. We removed opercules for future analysis for age by D. Gilroy. To determine feeding patterns, dorsal muscle tissue was taken for stable isotope analysis and stomachs were preserved in formalin for future content analysis. When we found whole fish in the stomach they were removed and weighed. We completed sex identification and eggs were weighed when present. During the sampling, we noted anomalies in the body cavity. Since the Pyramid Lake fisheries department tags about 20% of its hatchery release, we checked for the presence of microtags on LCT without adipose fins. To differentiate between benthic and pelagic Tui chubs, gill rakers from all chubs were removed and counted. Tui chubs with fine gill rakers (10-16 per gill) were identified as pelagic, whereas those with coarse gill rakers (25-37 per gill) were classified as benthic. Between these ranges chubs were considered hybrid feeders.

Isotope Analysis

Stable carbon and nitrogen isotope ratios are increasingly used to provide time integrated information about feeding relationships and energy flow through food webs (Vander Zanden et al. 1999). Tissue samples and invertebrates are dried at 60 °C for at least 24 hours and ground into a fine powder using a mortar and pestle. After being massed and packed into tin capsules (8 x 5 mm), a continuous flow isotope ratio mass spectrometer (IRMS) (20-20, PDZEuropa Scientific Sandbach, United Kingdom) analyzes the samples for carbon and nitrogen. Sample combustion to CO² and N² occurs at 1000 °C in an inline elemental analyzer (PDZEuropa Scientific, ANCA-GSL). A Carbosieve G column (Supelco, Bellefonte, PA, USA) separated the gas before introduction to the IRMS. Standard gases (Pee Dee Belemnite for δ¹³C and N₂ gas for δ¹⁵N ) are injected directly into the IRMS before and after the sample peaks.

Isotopic ratio is expressed as a per mil (‰) notation. Using δ¹³C as an example, it was defined by the following equation:

δ¹³C = [(¹³C/¹²C)_sample / (¹³C/¹²C)_standard – 1] * 1000

A more positive δ¹³C indicated isotopic enrichment, or contained proportionally higher concentrations of heavier ¹³C isotope. After every twenty samples a replicate and a standard were added to the analysis sequence. Replicate variation was less than 3% and machine analytical variation was within 0.2 ‰.

The trophic niche of each species is determined by measuring their pelagic carbon reliance and feeding level. The dependence of individual fish on pelagic energy is determined by the following equation:

% Pelagic = [(δ¹³C_fish- δ¹³C_littoral)/ (δ¹³C_pelagic- δ¹³C_littoral)] * 100,

Where δ¹³C _fish is the individual value for fish. The littoral endpoint, δ¹³C_littoral, represents the benthic primary production signal. The pelagic endpoint, δ¹³C_pelagic, represents the pelagic primary production signal.

Fish trophic position, or how high in the food chain an organism is feeding, is estimated from fish δ¹⁵N values. This isotope is used because it predictably discriminates through the food web. Individual fish signatures are corrected for baseline variation using invertebrate primary consumer δ¹⁵N similar to Vander Zanden and Rasmussen (1999). Trophic position is calculated by the equation:

TP= ((δ ¹⁵N _fish- δ ¹⁵N_baseline)/3.4) + 2

where 3.4 is the trophic level enrichment factor (Minagawa and Wada 1984; Vander Zanden and Rasmussen 2001). This factor means that each trophic level (1, 2, 3, 4) in the food web is separated by 3.4 units of δ ¹⁵N. Thus, if an invertebrate feeding solely on algae, which represents a trophic level of 1 had a measured δ ¹⁵N value of 2‰, an invertebrate in the next trophic level would be 3.4 units higher and have a measured value of 5.4‰. An organism feeding purely on these invertebrates should also be 3.4 units greater than invertebrates and thus would be measured as 8.8‰. This pattern continues predictably through each trophic level. The trophic position calculation quantifies fish position in trophic levels in a food web. Thus a fish feeding at a trophic position of 3.5 relies partly on invertebrates and partly on fish for food sources.

RESULTS AND DISCUSSION:

Vertebrates:

Isotope data revealed energy source and trophic positions for Tui chubs, Tahoe suckers, Sacramento perch and Lahontan cutthroat trout (Figure 1).

Figure 1: Fish food web in Pyramid Lake, Nevada.

Tui chubs

Isotope data agreed with the gill raker classification system separating benthic and pelagic Tui chubs. Benthic chubs were only feeding 20% pelagic, primarily consuming benthic organisms. However, pelagic chubs were feeding 55-60% pelagic thus relying more heavily on open water organisms. Benthic chub samples were measured at a trophic level of 3.4, eating predacious invertebrates or fish. In contrast, pelagic samples were determined at a trophic level of 3.1, consuming almost exclusively primary consumer invertebrates.

Tahoe suckers

The stable isotope signal measured from the Tahoe suckers was 5-10% pelagic. This primarily benthic signal agrees with its morphology and behavior. Tahoe suckers were measured to be feeding at 3.1 trophic level, eating mostly primary consumer invertebrates.

Cui-ui

The cui-ui were found to be feeding 40% pelagic at a trophic level of 3. Although the cui-ui sample of this study did not offer information about a range of sizes of fish, it seems that the cui-ui might be more benthically oriented than previously thought. This deserves more attention and a more diverse sample would be needed for more legitimate conclusions. As cui-ui are an endangered species it is difficult to do studies that involve killing them although they are an important object of study.

Sacramento perch

Only two Sacramento perch were caught in this study thus our characterization of the food web interactions of this species are very preliminary. The perch were about 10% pelagic while eating at a trophic level of 3.5. The relatively high observed trophic level in the perch samples indicates that they are probably eating secondary consumers both vertebrate and invertebrate. Although we tried to increase our sample size for perch we were unable to collect more specimens. Moyle (2002) comments that perch were introduced into Nevada and potentially into Pyramid lake starting in 1877. The difficulty we experienced in catching perch could be because they are having little impact on the food web or that our sampling methodology was not sufficient to accurately sample the lake’s population.

Lahontan cutthroat trout

LCT was measured as feeding 40% pelagic at a trophic level of about 3.7. Preliminary gut content observations confirms the pisciverous feeding habits of LCT. The top predator in the Pyramid Lake system, the LCT’s energy sources are as pelagic as the Cui-ui’s. Rather than preying on Cui-ui, the LCT’s percent pelagic is the result of feeding on a mix of benthic and pelagic chubs (Monda 2000). Previous research shows that LCT In Lake Tahoe in the 1870s and in Cascade Lake were feeding at a trophic level of 3.3-3.8 but 75% pelagic (Chandra 2000). The spatial and temporal variation in LCT feeding habits could be ecological or genetic.

Invertebrates:

Twelve taxa of invertebrates from nine different orders were identified: Hyalella azteca, Ambrysus mormon, Telebasis sp., Erpetogomphus compositus, Macromia magnifica, Limnodrilus hoffmeisteri, seed shrimp, Helobdella stagnalis, Chironomidae (larva, pupae and adult), Nectopsyche sp., Hydroptilidae pupae, Petrophila confusalis (Merrit and Cummins 1996).

Diversity is made up of two important components: 1) number of different species and 2) distribution of population between each species. The Shannon-Wiener value assigns a number between 0 and 1 to taxa diversity, with one showing the greatest diversity. For example, at a value of one the population has many different species, with approximately equal numbers of individuals in each taxa. Figure 2 shows clearly that taxa diversity decreases with depth. This trend makes sense because habitat is much harsher at greater depths (less sunlight, less food availability).

Figure 2: Invertebrate taxa diversity in Pyramid Lake, Nevada.

Figures 3a and 3b show the relationship between pelagic carbon and depth for bottom invertebrates. In figure 3a, both oligocheates and chironomids were used but no clear relationship was found (r² = 0.43). When the oligochaetes were treated separately a clear relationship exists between the percent of pelagic carbon and depth, where pelagic carbon increases with depth (r² =0.82). This may perhaps be expected because of the increasing influence the water column has on energy sources and the decreasing availability of light at greater depths. This graph makes the point that food sources can change with depth for the same invertebrate species.

Figure 3: Algal sources for (a) bottom invertebrates and (b) oligochates.

At the near shore collection sites (Pelican Point, Warrior Point, Anaho Island, and Popcorn point) we found variable invertebrate community structures (Figure 4). Anaho Island showed particular diversity. This island is an important nesting area for many different species of birds which may be loading the area around it with nitrogen. Isotope results of invertebrates from this location have yet to be analyzed but they may provide insight into the nutrient loading hypothesis.

Figure 4: Diversity of near shore benthic communities in Pyramid Lake, Nevada.

FUTURE DIRECTIONS:

This research spawned many important avenues of further study. Although isotope data is a powerful means of understanding energetic exchanges in a system it cannot be understood in the absence of other ecological data. More analysis of the non-isotope data we collected will allow us to refine our analysis of this system. In particular, using the sampled opercules to age the fish and analyzing gut contents will improve our characterization of the food web. Beyond this unanalyzed data, one unanswered question is the place of perch and cui-ui in the lake’s food web. More energy should be spent to understand where and on what these fish are feeding. In addition, the spatial effects of variable nitrogen enrichment from bird species (the nutrient loading hypothesis) could be characterized to more fully understand the Pyramid lake food web. This kind of effort could happen in conjunction with a more general project to understand benthic food quality and quantity in the lake. Since it seems that LCT feed very benthically, understanding benthic energy sources is essential to good management of the fisheries. We also observed many LCT to be infected with an unidentified parasite. Although we did not note this trend quantitatively, parasites might have a significant effect on fecundity and growth rates and deserve further attention since they are potentially depriving the LCT of energy. Moreover, this study has not touched on the effect of which LCT strain has been introduced by the hatcheries. Pen incubation studies and historical samples might help us to understand the genetic factors in LCT energy strategies and how these could limit growth.

ACKNOWLEDGEMENTS

We would like to thank the Paiute Tribe Fisheries Program and the Tahoe Research Group for the use of their equipment and laboratory space. In particular Eric, Nancy, and Beverly at the Pyramid Lake Fisheries Department were very helpful. Sudeep Chandra, Carri LeRoy and David Gilroy were inspiring and infectiously enthusiastic people to work with. We learned a lot from them.

BIBLIOGRAPHY

Allen, B. C., S. Chandra, et al. (2003). An evaluation of the re-introduction of native Lahontan cutthroat trout, Oncorhynchus clarki henshawi, in Fallen Leaf Lake, California, Submitted to the U.S. Fish and Wildlife Service, Nevada Office.

Behnke, R. J. (1992). Native Trout of Western North America. Bethesda, MD, American Fisheries Society Monograph 6.

Chandra, S., D. Gilroy, et al. (2000). Cascade Lake, CA-- Potential for Lahontan Cutthroat Trout Studies, Tahoe Baikal Institute.

Coleman, M. E., V. K. Johnson. (1988). "Summary of Trout Management at Pyramid Lake, Nevada, with Emphasis on Lahontan Cutthroat Trout, 1954-1987." American Fisheries Society Symposium 4: 107-115.

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Galat, D. L. and N. Vucinich (1983). "Food of Larval Tui Chubs, Gila Bicolor, in Pyramid Lake, Nevada." The Great Basin Naturalist 43(1).

Galat, D. L. and N. Vucinich (1983). "Food Partitioning between Young of the Year of Two Sympatric Tui Chub Morphs." Transactions of the American Fisheries Society 112: 486-497.

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Hickman, T. J. and R. J. Bechnke (1979). "Probable Discovery of the Original Pyramid Lake Cutthroat Trout." Progressive Fish Culturist 41(3): 135-137.

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Vander Zanden, J., S. Chandra, et al. (2003). "Historical Food Web Structure and Restoration of Native Aquatic Communities in the Lake Tahoe (California-Nevada) Basin." Ecosystems 6: 274-288.

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PROJECT EVALUATION

Strengths of the project:

-great mentoring

-useful to somebody else besides us, it is really important to us that the data is going to be used for something.

-Pyramid lake is a really amazing area

-we had fun with each other and our supervisors

-we didn’t see any rattlesnakes

Challenges:

-very little time for analysis

-project time was broken up with other TBI activities (much traveling)

-dirty rotten bastards that cut our net