Chris Luecke

Effects of Moonlight and Daylight on Hydroacoustic Estimates of Pelagic Fish Abundance

Abstract To determine how moonlight and daylight affect hydroacoustic estimates of fish abundance, we used a dual-beam transducer and echo integration to survey pelagic fish (primarily Bonneville ciscoes Prosopium gemmifer) in Bear Lake, Utah and Idaho. During the new moon, the fish were dispersed (not schooling) below the thermocline, chiefly at the depths of 10–20 m. At full moon, they were dispersed but much closer to the bottom, where they were difficult to detect. Acoustic estimates offish density and biomass during full moons were approximately 50% of values derived during new moons. A diel survey during a new moon indicated that fish were widely dispersed in the water column at night, but formed schools at dawn. Our study indicated that light conditions must be standardized to insure consistent and comparable population estimates of some pelagic fishes.

Effects of food web compensation after manipulation of rainbow trout in an oligotrophic lake

Stocking of the dominant planktivore of Castle Lake (rainbow trout) was discontinued to examine the impact of food web interactions on zooplankton communities and inter- and intra-annual dynamics of ecosystem properties (light penetration, primary productivity). Dynamics of zooplankton and ecosystem processes were examined for 3 yr following the manipulation and compared to 2-3 yr of premanipulation data. Sampling of vertebrate and invertebrate planktivores documented shifts in other members of the zoo-planktivore guild as rainbow trout declined. Reduction of rainbow trout densities led to compensatory responses in other components of the Castle Lake fish assemblage as brook trout and golden shiners increased in abundance. This compensation resulted in increased rates of vertebrate planktivory on daphnids within 2 yr after trout stocking was discontinued. Zooplankton shifts in response to discontinuance of trout stocking were more rapid, particularly an immediate increase in a previously rare invertebrate predator (Diacyclops thomasi). Other limnological parameters also responded rapidly following the manipulation : water transparency declined and primary productivity (PPr) increased. In addition, intra-annual patterns (i.e., seasonal development) and the vertical distribution (shallow vs. deep) of PPr appeared to be affected by the food web manipulation. Our results indicate that complexities of real food webs complicate the prediction of the outcome of food web perturbations. Reduction of the previously dominant planktivore (rainbow trout) led to increases in other zooplanktivores (Diacyclops, golden shiners, brook trout) that resulted in enhanced predation pressure on zooplankton herbivores. Our results also indicate that alterations in water quality parameters (transparency, PPr) in response to food web alterations need not necessarily be mediated through changes in the abundance of Daphnia, as strong limnological responses preceded reductions in Daphnia by a year. We hypothesize an alternative mechanism for food-web-induced changes in lake ecosystem dynamics : changes in water clarity and productivity can result when cyclopoid predation strongly affects micrograzers.

A New Estimate of Zooplankton Retention by Gill Rakers and Its Ecological Significance

Abstract The probability that prey will be retained by gill rakers of white crappies Pomoxis annularis (9–15 cm total length) was determined by comparisons of size-frequency distributions of prey in a laboratory pool and in stomach contents. White crappies trained to feed on large-bodied Daphnia magna were released individually into a pool containing that species and an assemblage of small-bodied species. Most attacks were directed at Daphnia magna, but several prey were ingested for every attack, and nonselective capture was assumed for the small-bodied among them. Estimated retention probabilities for species of Ceriodaphnia, Bosmina, and Cyclops were less than 10% for mean prey sizes smaller than 0.35 mm and increased linearly with mean prey size to 100% for prey larger than 0.55 mm. In contrast, retention probabilities calculated by traditional means from distances between gill rakers predicted that all prey larger than 0.23 mm would be retained. The new retention estimates, based on actual results of...

Simulated Growth and Production of Endangered Snake River Sockeye Salmon: Assessing Management Strategies for the Nursery Lakes

Abstract We examined the potential of employing a series of lake management strategies to enhance production of endangered Snake River sockeye salmon (Oncorhynchus nerka) in its historical nursery lakes in central Idaho. We used a combination of limnological sampling, experimentation, and simulation modeling to assess effects of lake fertilization and kokanee reduction on growth and survival of juvenile sockeye salmon. Juvenile sockeye salmon from a broodstock of this endangered species are being introduced into the lakes from 1995 to 1998. Results of our analyses indicated that several lakes were suitable for receiving broodstock progeny. Field experimentation and simulation modeling indicated that lake fertilization, coupled with a program of kokanee reduction, provided the management option most likely to enhance the survival of stocked juvenile sockeye salmon. Simulation models that encompass physiological requirements, ecological interactions, and life-history consequences could be used as templates ...

Lake Trout Consumption and Recent Changes in the Fish Assemblage of Flaming Gorge Reservoir

Abstract Bioenergetics modeling was used to quantify the consumption dynamics of lake trout Salvelinus namaycush in Flaming Gorge Reservoir, Utah–Wyoming. Analysis of diet and population estimates of different size-classes of lake trout indicated that kokanees Oncorhynchus nerka made up the greatest proportion of prey biomass. Examination of growth rates of forage fish and predator–prey size ratios indicated that Utah chub Gila atraria were more vulnerable than kokanees to lake trout predation. Utah chub grow slower than kokanees and thus were susceptible to piscivores over a longer age span. We conclude that kokanees will make up an even larger proportion of the pelagic fish assemblage of Flaming Gorge Reservoir in future years.

Hydroacoustic Assessment of Abundance and Diel Distribution of Sockeye Salmon and Kokanee in the Sawtooth Valley Lakes, Idaho

Abstract We used dual-beam hydroacoustics and echo integration techniques, combined with midwater trawling and gillnetting, to assess the abundance and distribution of the endangered Snake River juvenile sockeye salmon and resident kokanee (both Oncorhynchus nerka) in Sawtooth Valley lakes of Idaho during September 1991 and 1992. Abundance of O. nerka varied among the four lakes containing this species (12,500–257,000) and varied between years in Redfish Lake (86,400 in 1994 and 241,000 in 1992) and Alturas Lake (230,000 in 1991 and 257,000 in 1992). In Alturas Lake, where piscivore densities were high and zooplankton densities were low, small acoustic targets (≤18 cm long) were nearly absent from the limnetic zone during daylight, and high densities remained in colder intermediate depths (15–30 m) during crepuscular and nocturnal periods. In Redfish Lake, where predator density was much lower and zooplankton density was higher, targets concentrated in schools at 25–30 m during daylight, dispersed into th...

Effects of planktivorous fish mass mortality on the plankton community of Lake Mendota, Wisconsin: implications for biomanipulation

Massive mortality of planktivorous fish had a dramatic impact on plankton community dynamics of Lake Mendota, Wisconsin, USA. After fish mortality, the largerDaphnia pulicaria replaced the smallerDaphnia galeata mendotae, resulting in greater grazing pressure on phytoplankton. This was accompanied by a much longer spring clear-water period and lower summer phytoplankton biomass compared to years before the fish mortality. Analysis of historical data (from the mid-1970s) showed that previous fluctuations in planktivorous abundance had similar effects onDaphnia abundance and species composition, and on spring phytoplankton biomass. However, the mid-1970s fish fluctuations had no detectable effect on summer phytoplankton. Concentrations of phosphorus were much higher in the 1970s (spring P 80–135 µg 1−1) than in the 1980s (spring P 19–36 µg 1−1) and it is possible that high P concentrations may reduce trophic cascade effects on summer phytoplankton communities. This suggests that the success of biomanipulation programs may be dependent on lake nutrient status.

Notes: Estimating the Energy Density of Daphnid Prey for Use with Rainbow Trout Bioenergetics Models

Abstract Energy densities and percent dry matter of zooplankton were estimated from both field collections and from stomach contents of rainbow trout Oncorhynchus mykiss. Dry-mass energy densities were similar for zooplankton collected from the field and from fish stomachs, averaging 22.7 kJ/g. The percent dry matter of zooplankton from fish stomachs was approximately twice that of zooplankton from field collections, although species composition was similar. Energy density of wet zooplankton was consequently twice as high in fish stomachs as it was in field collections. These results indicate that the wet-mass energy density of zooplankton prey used in fish bioenergetics models should be increased over values that have been reported in the literature.

Salmonine Consumption and Competition for Endemic Prey Fishes in Bear Lake, Utah–Idaho

Abstract Two principal sport fish—the indigenous Bonneville cutthroat trout Oncorhynchus clarki utah and the introduced lake trout Salvelinus namaycush—are the dominant piscivores in Bear Lake, a 282-km2 oligotrophic system. These piscivores rely predominantly on four endemic prey fish species that make up a major portion of the unique Bear Lake fish assemblage. We estimated the annual biomass of pelagic and benthic prey fish by using hydroacoustic and trawling techniques. We also estimated the lakewide abundance of piscivores with a multiple mark–recapture survey and used a bioenergetics model to compare the population-level consumption of prey fish with prey fish production. Prey fish biomass declined to a minimum during 1991 and 1992 but subsequently recovered to reach maximum levels during 1994 and 1995. The proportion of maximum ration estimates from model simulations indicated that the piscivores were consuming well below maximum rations during a period when predation exceeded prey fish production, ...

The Role of Anadromous Sockeye Salmon in the Nutrient Loading and Productivity of Redfish Lake, Idaho

Abstract We constructed a simulation model for Redfish Lake, Idaho, using water budget and nutrient loading measurements, to predict the dependence of lake production on nutrients from the watershed, precipitation, lake fertilization, and marine-derived nutrients from sockeye salmon Oncorhynchus nerka which historically have reared in the lake. We also used the model to simulate different management scenarios to help restore the endangered Snake River sockeye salmon. The model and other empirical evidence indicated that even before hydropower dams were present in the migration corridor, marine-derived nutrients were not of major importance to lake production, contributing only about 3% of the annual phosphorus load of the lake. This contribution was partially removed by the quick flushing rate (3 years) of the lake and phosphorus export by smolts. The model predicted annual adult sockeye salmon returns to be 3,800 fish under predam conditions, 370 fish under modern conditions, 780 fish when watershed nutr...