John M. Erhardt

Prey Availability, Consumption, and Quality Contribute to Variation in Growth of Subyearling Chinook Salmon Rearing in Riverine and Reservoir Habitats

AbstractWe examined prey availability, prey consumed, and diet energy content as sources of variation in growth of natural fall Chinook Salmon Oncorhynchus tshawytscha subyearlings rearing in riverine and reservoir habitats in the Snake River. Subyearlings in riverine habitat primarily consumed aquatic insects (e.g., Diptera, Ephemeroptera, Trichoptera), of which a high proportion was represented by adult, terrestrial forms. In the reservoir, subyearlings also consumed aquatic insects but also preyed heavily at times on nonnative lentic amphipods Corophium spp. and the mysid Neomysis mercedis, which were absent in riverine habitats. The availability of prey was typically much higher in the reservoir due to N. mercedis often composing over 90% of the biomass, but when this taxon was removed from consideration, biomass estimates were more often higher in the riverine habitat. Subyearling diets during 2009–2011were generally 17–40% higher in energy in the riverine habitat than in the reservoir. Observed grow...

Precision and Accuracy of Age and Growth Estimates Based on Fin Rays, Scales, and Mark-Recapture Information for Migratory Bull Trout

Abstract Accurate quantitative descriptions of bull trout (Salvelinus confluentus) growth are important for understanding life history and developing reliable stock assessments. In the absence of age validation, important steps are to review the precision of age estimation methods and evaluate whether age estimates yield growth rates consistent with known fish growth based on tag recoveries. We assessed the precision of age estimates using pelvic fin rays and scales for migratory bull trout (297–605 mm total length) from the North Fork Clearwater River, and then compared growth estimates derived from both structures with growth based on tag recoveries. Fin rays produced a lower coefficient of variation (CV = 5.84) than scales (CV = 12.56). Ages estimated from scales were higher for fish aged < 5 with fin rays and lower for fish aged ≥ 5. Comparisons of growth estimates derived from 70 tagged bull trout at large from 0.35 to 3.02 years with age-length equations based on fin ray and scale annuli indicated that ages estimated from fin rays (N = 189, predicted length of an age 3 fish = 310mm) were closely related to the apparent ages estimated from the mark-recapture model (apparent age of a 310mm fish = 2.9) whereas scales (N = 65, predicted length of an age 3 fish = 408mm) were not. This is the first study to assess the precision of structures for modeling growth of larger migratory bull trout. However validation of annuli formation from the recapture of known-age fish is recommended.

Ecology of the Opossum Shrimp (Neomysis mercedis) in a Lower Snake River Reservoir, Washington

Abstract The opossum shrimp Neomysis mercedis has expanded its range from the lower Columbia River upstream 695 kilometers into Lower Granite Reservoir where it is now very abundant. We studied Neomysis ecology in the reservoir during 2011–2015 to better understand the physical and biological factors that shape their distribution as well as their potential role in the food web. Benthic densities in offshore habitats ranged from 19 to 145 mysids m-2 in shallow (2–12 m) water and from 3 to 48 mysids m-2 in deep (> 12 m) water. Water velocity, depth, substrate, and seasonal interactions were important variables for explaining variation in Neomysis densities in offshore habitats. During spring, daytime densities in shoreline habitats (< 2 m deep) were variable, but nighttime densities generally decreased in summer following reproduction and as temperatures approached 23 °C. Neomysis were mainly collected from the water column during nighttime vertical tows in the downstream end of the reservoir when water velocities were low during summer and autumn. Reproduction occurred mainly in spring and early summer, but a second, smaller reproductive event was observed during autumn. The diet of Neomysis consisted primarily of detritus, rotifers, and copepods, but cladocerans were more prominent during summer and autumn. Physical factors like water velocity may have limited vertical migrations of Neomysis to feed in the water column and influenced use of different habitats in the reservoir. Neomysis are prey for a number of species, including juvenile salmon, but their relations are still largely unknown, and continued monitoring and research is warranted.

Ecology of the Sand Roller (Percopsis transmontana) In A Lower Snake River Reservoir, Washington

Abstract The Sand Roller (Percopsis transmontana), has not been abundant in the Snake River since it was first found in the system in the 1950s, but its population has apparently increased in recent years. As a result, we initiated a study to better understand its ecology in habitats of Lower Granite Reservoir. From November 2014 to October 2015, Sand Rollers were present along shorelines, with peak abundance being observed during spring months. Logistic regression analyses showed that Sand Rollers were more likely to be present in shoreline habitats at temperatures ≤18.4°C. Fish were found over a range of substrates, with the lowest odds of fish presence being associated with riprap, which is common in hydropower reservoirs. From length-frequency analysis, we suggest that Sand Roller spawning occurs primarily in May and early June. Assessment of Sand Roller diets found dipteran (chironomid) larvae and pupae were the most important prey consumed by all sizes of Sand Rollers, but Opossum Shrimp (Neomysis mercedis) were also prominent in diets of larger fish in shoreline and offshore habitats. At a time when the populations of so many native species are in decline, the increase of the Sand Roller population in the lower Snake River represents a positive, yet curious occurrence.

Growth Model Selection and its Application for Characterizing Life History of a Migratory Bull Trout (Salvelinus confluentus) Population

Abstract Five growth models were evaluated for their effectiveness in characterizing growth of an isolated population of bull trout, Salvelinus confluentus, undergoing a long term cessation of harvest (1994–2005) in the North Fork Clearwater River, Idaho, above Dworshak Dam. To explore the dynamics of growth in migratory bull trout, one of the models, the von Bertalanffy model, was also fitted based on age estimates derived from annuli and growth increments on pelvic fin ray sections for three juvenile outmigration age groups. Among five growth models evaluated, four models fitted to the age-length data described the fish growth for ages 3 to 11 comparably well. Only a model that exhibited positive growth acceleration as age progressed fitted the data less effectively. Growth was significantly (P < 0.001) related to the age of outmigration from natal streams (1–3 years). Migratory bull trout were found to reach maturation at ages 4–6; the common age-4 maturation was younger than most comparable studies. All mature fish studied reared for 2–3 years in downriver or reservoir habitat. As the adult population size has continued to increase since harvest closure in 1994, further research should be directed towards understanding the trade-offs between these increasing numbers of adult fish and growth, survival, migration, and maturation schedules.