Age-0 Rainbow Research Articles

Introgression of hatchery rainbow trout in naturalized steelhead populations of western Lake Superior

Fisheries managers and anglers in Minnesota have long been concerned that reproduction in the wild by a hatchery strain of rainbow trout (Oncorhynchus mykiss), called kamloops, could reduce the fitness of naturalized steelhead (migratory rainbow trout) populations in Lake Superior. A previous study found no evidence of kamloops introgression, but a new evaluation is warranted in light of continued stocking and genetic advancements over the past 25 years. We used genotypes from 10 microsatellite DNA loci to assess kamloops ancestry in adult and age-0 rainbow trout samples from 40 streams in Minnesota, one stream in Wisconsin, and two adult broodstocks. We also evaluated genetic population structure for impacts of kamlooops introgression. Overall, the average estimated kamloops ancestry was 8% (range 2–44%) in age-0 juveniles and 4% (1–19%) in adults. The average percentages of kamloops descendants in age-0 and adults were 8% (0–54%) and 5% (0–40%), respectively. Kamloops descendants were found in many Minnesota streams and in Wisconsin’s Bois Brule River, but were most prevalent in streams near the source hatchery, near recent stocking locations, and in lower reaches of longitudinally sampled streams. Feral steelhead broodstock had 9% and 6% kamloops descendants in two years while captive broodstock had no kamloops descendants. Minnesota’s populations showed little spatial genetic structure but were distinct from the Bois Brule population and from kamloops, which indicated that kamloops introgression had not substantially altered structure. Kamloops introgression into Minnesota steelhead populations and impacts on other jurisdictions contributed to the decision to discontinue the kamloops stocking program.

The effects of severe flooding on native masu salmon and nonnative rainbow trout in the Atsuta River, Hokkaido, Japan

Debris flow has potentially pronounced effects on streams and fish populations. In August 2010, a severe flood, which was coupled with wide-ranging landslides and massive debris flows, occurred in the Atsuta River. In this study, we evaluated the effects of floods on salmonid populations by examining the abundance and density of masu salmon Oncorhynchus masou and rainbow trout O. mykiss in the Atsuta River and comparing these values in the post-flood period to those in the pre-flood period. The estimated abundance and density of masu salmon did not change between the pre- and post-flood periods. In contrast, the density of adult (age-1and older) rainbow trout decreased dramatically. Although the density of age-0 rainbow trout decreased after the floods, there was no significant difference between the pre- and post-flood values. These results suggest that the effects of severe floods differ among species and these differences may be attributed to differences between masu salmon and rainbow trout in the timing of life-history events, such as fry emergence and spawning.

Native rainbow smelt and nonnative alewife distribution related to temperature and light gradients in Lake Champlain

Alewife (Alosa pseudoharengus) recently became established in Lake Champlain and may compete with native rainbow smelt (Osmerus mordax) for food or consume larval rainbow smelt. The strength of this effect depends partly on the spatial and temporal overlap of different age groups of the two species; therefore, we need a better understanding of factors affecting alewife and rainbow smelt distributions in Lake Champlain. We used hydroacoustics, trawls, and gill nets to document vertical fish distribution, and recorded environmental data during 16day–night surveys over two years. Temperature, temperature change, and light were all predictors of adult and age-0 rainbow smelt distribution, and temperature and light were predictors of age-0 alewives' distribution (based on GAMM models evaluated with AIC). Adult alewives were 5–30m shallower and age-0 alewives were 2–15m shallower than their rainbow smelt counterparts. Adult rainbow smelt distribution overlapped with age-0 rainbow smelt and age-0 alewives near the thermocline (10–25m), whereas adult alewives were shallower (0–6m) and overlapped with age-0 alewives and rainbow smelt in the epilimnion. Adult rainbow smelt were in water <10–12°C, whereas age-0 rainbow smelt were in 10–20°C, and adult and age-0 alewives were in 15–22°C water. Predicting these species distributions is necessary for quantifying the strength of predatory and competitive interactions between alewife and rainbow smelt, as well as between alewife and other fish species in Lake Champlain.

Effects of flow fluctuations on habitat use and survival of age-0 rainbow trout (Oncorhynchus mykiss) in a large, regulated river1This article is a companion to Korman et al. 2011, published this issue.

We evaluated effects of reduced hourly variation in flow from Glen Canyon Dam on survival of age-0 rainbow trout ( Oncorhynchus mykiss ) in the Colorado River, Arizona, USA, based on monthly abundance estimates. The proportion of the age-0 population in low-angle shorelines, which are potentially more sensitive to flow variability, declined from 70% in June to 20% in November as fish grew and made an ontogenetic habitat shift to deeper habitat. Average daily instantaneous mortality between August and September was 0.008 units lower in years when there was no change in the minimum flow compared with years when there was a sudden 50% reduction in the minimum flow. However, mortality was 0.006 units higher during the fall when there was no hourly variation in flow compared with years when flows fluctuated. As a result of these opposing patterns, 3-month age-0 survival across steady (0.31) and unsteady (0.28) flow regimes were very similar. While additional replication is required to strengthen inferences about effects of steadier flows, we demonstrate the utility of early life history monitoring for evaluating effects of flow management on fish populations in regulated rivers.

The Impacts of Atlantic Salmon Stocking on Rainbow Trout in Barnum House Creek, Lake Ontario

We compared the impacts of stocking age-0 Atlantic salmon ( Salmo salar) at high and low densities, and no stocking on abundance and growth of age-0 rainbow trout ( Oncorhyncus mykiss) in Barnum House Creek, Ontario during 1993 to 2005. A similar stream, Shelter Valley Creek, was chosen as an appropriate reference stream where age-0 Atlantic salmon were not stocked. The catches of age-0 rainbow trout in Barnum House and the reference stream were highly correlated (r = 0.96) during years when no stocking occurred; however, this relationship did not persist in years when Atlantic salmon were stocked. The catch of age-0 rainbow trout in Barnum House Creek was significantly lower under both high (P = 0.00026) and low (P = 0.011) density Atlantic salmon stocking treatments compared with the no stocking treatment. The catches of age-0 rainbow trout and age-0 Atlantic salmon were negatively correlated in Barnum House Creek (r = −0.63). The length of age-0 rainbow trout in Barnum House Creek was depressed significantly (P = 0.004), under the high intensity Atlantic salmon stocking treatment, but not under the low intensity treatment (P = 0.20). In contrast, the length of age-0 rainbow trout in Shelter Valley Creek was unchanged over the same period. Restoration stocking of Atlantic salmon in Lake Ontario tributaries may impact rainbow trout abundance and growth.

Fish Assemblage, Density, and Growth in Lateral Habitats within Natural and Regulated Sections of Washington's Elwha River Prior to Dam Removal

We characterized seasonal fish assemblage, relative density, and growth in river margins above and between two Elwha River dams scheduled for removal. Fish assemblage and relative density differed in the lateral habitats of the middle-regulated and upper-unregulated sections of the Elwha River. Rainbow trout was the numerically dominant salmonid in both sections, with bull trout present in low numbers. Sculpin were common in the middle section, but not detected in the upper section. In 2004, mean length and biomass of age-0 rainbow trout were significantly smaller in the middle section than in the upper section by the end of the growing season (September). In 2005, an earlier emergence of rainbow trout in the middle section (July) compared to the upper section (August) corresponded with warmer water temperatures in the middle section. Despite lower growth, the margins of mainstem units in the middle section supported higher mean areal densities and biomass of age-0 rainbow trout than the upper section. These results suggest that growth performance of age-0 rainbow trout was lower in the middle section than in the upper section, which could have been a density-dependent response, or a result of poor food production in the sediment-starved regulated section, or both. Based on our findings, we believe that seasonal sampling of river margins within reference reaches is a cost effective and repeatable method for detection of biologically important short- and long-term changes in emergence timing, density, and growth of rainbow trout before and after dam removals in the Elwha River.

Over-winter lipid depletion and mortality of age-0 rainbow trout (Oncorhynchusmykiss)

In this study we identify the size-dependent risk of winter starvation mortality as a strong selective pressure on age-0 rainbow trout (Oncorhynchus mykiss) that could promote the risk-taking behaviour and allocation of energy to lipids previously observed in young trout cohorts. Age-0 trout subjected to simulated winter starvation conditions gradually depleted lipid reserves to a critical minimum lipid content below which death occurred. Small fish with lower lipid content exhausted lipid reserves earlier, and experienced high mortality rates sooner, than larger fish with greater lipid content. Consequently, winter starvation endurance was dependent upon size-dependent lipid reserves and winter duration. To validate the laboratory findings in the field, we stocked several size classes of hatchery-raised trout with known lipid content at the start of winter into two experimental lakes, and estimated survival and lipid depletion at winter's end. Larger age-0 trout had greater initial lipid reserves than smaller trout. Individuals depleted most of their lipid reserves over the winter, and experienced mortality that ranged from just under 60% for the largest individuals to just over 90% of the smallest individuals. Many survivors had lipid contents near, but none were below, the minimum lipid content determined in the laboratory.

FROM INDIVIDUALS TO POPULATIONS: PREY FISH RISK-TAKING MEDIATES MORTALITY IN WHOLE-SYSTEM EXPERIMENTS

Recent research suggests that the behavior of individuals under risk of predation could be a key link between individual behavior and population and community dynamics. Yet existing theory remains largely untested at large spatial and temporal scales. We manipulated food available to age-0 rainbow trout while at risk of cannibalism, in a replicated factorial whole-lake experiment, to test whether the trade-off between growth and mortality rates is mediated by foraging activity by young fish under predation risk. We found that this trade-off exists for young fish at the whole-system scale, and that food-dependent behavioral variation has large mortality consequences. In high-food lakes, age-0 trout spent less time moving, fewer individuals swam continuously, and those swimming continuously swam at slower speeds relative to those in low-food lakes. Age-0 trout also used deep, risky habitats less when food was abundant. This lower activity, combined with avoidance of risky habitats, coincided with 68% higher survival in high-food lakes. If general, this trade-off may be a key mechanism linking individual behavior to population-level processes in size-structured populations.

POPULATION CONSEQUENCES OF A PREDATOR-INDUCED HABITAT SHIFT BY TROUT IN WHOLE-LAKE EXPERIMENTS

In a replicated whole-lake experiment, we (a) tested for the existence of a flexible habitat shift in response to predator presence in age-0 rainbow trout (Oncorhynchus mykiss) at risk of cannibalism and (b) evaluated the population-level consequences of habitat shifts in terms of growth and survival over their first growing season. Daphnid food and adult trout predators were substantially more abundant in pelagic than in littoral habitats. Age-0 trout used all habitats in populations without adult trout predators, whereas age-0 trout were observed only in the less profitable littoral habitat in populations with adult trout. Consequently, mean fall mass of age-0 trout in the presence of predators was almost half that observed in populations without adult trout. Despite the shift in habitat use, age-0 trout experienced 90% mortality when adult trout predators were present, in comparison to only 36% mortality when absent. We conclude that the commonly observed habitat shifts by fish at risk of predation, observed at smaller scales, do in fact occur at the whole-system scale over long time intervals. These results suggest that fish are able to perceive risk at large spatial scales and thus take advantage of profitable (but normally risky) habitats when predators are absent, or move to less profitable refuge habitats when predators are present. Corresponding Editor: K. O. Winemiller

The relation between age-0 rainbow trout (Oncorhynchus mykiss) abundance and winter discharge in a regulated river

We identified and experimentally tested a discharge–abundance relation that predicted, based on the mean river discharge in the second half of winter (15 January – 31 March), the spring abundance of age-0 rainbow trout (Oncorhynchus mykiss) in a section of the Henrys Fork of the Snake River, Idaho, with complex bank habitat. We also considered a competing hypothesis in which autumn abundance determined spring abundance. We established that large abundances of age-0 trout were present in autumn (34 000 – 81 000) and lower abundances remained in spring (8000 – 15 000). Winter loss of age-0 trout was initiated in January. Spring abundance in 1996–1998 was related to autumn abundance (r2 &gt; 0.99) and mean discharge in the second half of winter (17.1–22.8 m3·s–1; r2 &gt; 0.99) but not mean discharge in the first half of winter (15.1–21.1 m3·s–1; r2 = 0.11). We experimentally maintained a high discharge (20–21 m3·s–1) in the second half of winter in 1999 to test model predictions. Autumn abundance failed to predict spring abundance (observed = 11 109; predicted = 6822; 95% prediction interval = 4669–8975). However, the discharge–abundance model accurately predicted spring abundance (predicted = 11 980; 95% prediction interval = 10 728 – 13 231). Higher discharge in the second half of winter may have provided more bank habitat at a critical time for survival.

Seasonal Survival, Movement, and Habitat Use of Age-0 Rainbow Trout in the Henrys Fork of the Snake River, Idaho

We quantified seasonal abundances, apparent survival rates, movements, and habitat use of age-0 rainbow trout Oncorhynchus mykiss in a 25-km reach of the Henrys Fork of the Snake River, Idaho, to determine what factors limit recruitment to the population. Natural production of rainbow trout occurred in each year of the study (1995-1997) and ranged from 158,000 to 306,000 age-0 fish each summer. No significant loss of age-0 rainbow trout occurred between summer and autumn; suitable habitat was present throughout the channel in all reaches and supported large abundances of age-0 rainbow trout through this period. The greatest losses (77-100%) occurred during winter and primarily involved fish from center-channel macrophyte beds. Most river sections, which had only simple bank habitat, did not support any age-0 rainbow trout through the entire winter. Overwinter survival was greatest (18-23%) in Box Canyon, a river section characterized by complex bank habitat, high gradient, and large substrate. Fish that were marked in river sections with simple bank habitat in autumn had moved to sections with complex bank habitat by the following spring. Winter habitat limits rainbow trout recruitment in the Henrys Fork.

Estimating Abundances of Age-0 Rainbow Trout by Mark–Recapture in a Medium-Sized River

Abstract We developed and evaluated a sampling methodology to obtain mark–recapture data to estimate abundances of age-0 rainbow trout Oncorhynchus mykiss in 70–125-m-wide reaches of the Henrys Fork of the Snake River, Idaho. Sampling by electrofishing was concentrated in sample areas that were 100 m long and extended from bank to bank; these areas were electrofished 3–5 times within periods of 3–17 d. Adjacent 50-m-long areas upstream and downstream were sampled to quantify movements out of the 100-m sample areas. We evaluated assumptions—closed population and equal catchability—using the field data, and we used simulation to identify the most appropriate abundance estimator for sparse data. Both closed and open population abundance estimators were evaluated. Most trout (84%) were recaptured in the area where they were marked, but about 10% had moved downstream and about 6% were recaptured upstream. Multistrata model analyses confirmed that apparent mortality rates, and hence movement rates, were low. Th...

Size-structured interactions between native and introduced species : can intraguild predation facilitate invasion by stream salmonids?

Dynamics of biological invasions may be complicated in size-structured animal populations. Differences in timing of life history events such as juvenile emergence create complex interaction webs where different life stages of native and non-native species act as predators, competitors, and prey. Stream salmonids are an ideal group for studying these phenomena because they display competition and predation in size-structured populations and have been introduced worldwide. For example, introduced rainbow trout (Oncorhynchus mykiss) are invading streams of Hokkaido Island, Japan and have caused declines in native masu salmon (O. masou) populations. However, age-0 rainbow trout emerge later than age-0 masu salmon and are smaller, which raises the question of why they are able to recruit and therefore invade in the face of a larger competitor. We conducted experiments in laboratory stream channels to test effects of increasing density of age-0 and age-1 rainbow trout on age-0 masu salmon. Age-1 rainbow trout dominated age-0 masu salmon by aggressive interference, relegating them to less favorable foraging positions downstream and reducing their foraging frequency and growth. The age-1 trout also reduced masu salmon survival by predation of about 40% of the individuals overall. In contrast, age-0 rainbow trout had little effect on age-0 masu salmon. Instead, the salmon dominated the age-0 trout by interference competition and reduced their survival by predation of 60% of the individuals. In each case, biotic interactions by the larger species on the smaller were strongly negative due to a combination of interspecific competition and intraguild predation. We predict that together these produce a positive indirect effect in the interaction chain that will allow the recruitment of rainbow trout in the face of competition and predation from age-0 masu salmon, and thereby facilitate their invasion in northern Japan.

Interactions between Young-of-the-Year Exotic Rainbow Smelt and Native Yellow Perch in a Northern Temperate Lake

Yellow perch Perca flavescens recruitment in Crystal Lake, Vilas County, Wisconsin, has declined since the exotic rainbow smelt Osmerus mordax increased in abundance. The yellow perch population is in decline because of low juvenile survival. However, diet analyses indicate that predation by adult smelt does not appear to be the cause. We investigated interactions between age-0 yellow perch and age-0 rainbow smelt by comparing changes in abundance and distributions, maximum mouth gapes, and the amount and type of prey eaten. During 1995 and 1996, age-0 smelt and perch hatched at similar times, had similar spatial distributions, and preferred similar prey. Diet overlap exceeded 45% on all sampling dates during the spring and summer of 1995 and 1996 and exceeded 60% on all but the first and last sampling dates in 1996. Two species of Diaptomus were the preferred prey of both smelt and perch, although smelt ate more of the larger size-classes than did perch throughout 1996. Consumption by smelt may have led to a decline in Diaptomus during the early summer of 1996. Estimates of temperature, hatch date, and growth derived from a long-term data set were used as input for bioenergetics simulations that estimated the proportion of maximum attainable consumption realized by the average age-0 perch in each year, when they occurred, between 1981 and 1996. During 1995 and 1996, years where age-0 smelt and perch were documented together, the proportion of maximum attainable consumption was significantly below that of any successful year-class observed during the long-term record. Age-0 perch did not survive in measurable numbers through the entire summer in either 1995 or 1996. Exploitation competition between age-0 perch and smelt may reduce the likelihood of strong year-classes of yellow perch when year-classes of rainbow smelt occur.

ENERGY ALLOCATION STRATEGY IN YOUNG FISH: ALLOMETRY AND SURVIVAL

We observed substantial variation in seasonal growth rates, autumn body size, and growing-season mortality among eight experimental cohorts of age-0 rainbow trout, Oncorhynchus mykiss. Wet mass, water, lipids (storage), and lipid-free dry mass (structure) had biphasic allometries with inflexions at ∼10 cm in length. Dry:wet mass and storage:structure ratios were positively related to fish length, indicating that the relative quantities of these constituents change with body size. Lipid concentration varied according to a sigmoid relationship with wet mass which also had a growth-rate dependence. Independent assessments of the allometry of growing-season survival and winter metabolism facilitated assessment of the costs and benefits of two alternate energy allocation strategies of young fish. For cohorts with low growth rates and small autumn body size, a somatic growth rate maximization strategy is optimum, producing a 5% net survival advantage over an energy storage maximization strategy. For cohorts with intermediate growth rates and autumn mass, somatic growth and energy storage strategies lead to similar first-year survival. The fastest growing cohorts are estimated to have a net survival advantage of 7%, by adopting an energy storage maximization strategy over a growth rate maximization strategy.

Multiple Electrofishing Removals for Eliminating Rainbow Trout in a Small Southern Appalachian Stream

Abstract We evaluated multiple electrofishing removals of rainbow trout Oncorhynchus mykiss as a management tool for the restoration of native brook trout Salvelinus fontinalis in a small southern Appalachian stream. Six, three-pass depletion removal efforts were conducted between Jun 1996 and Oct 1997. Four removals successfully eliminated rainbow trout reproduction. Five removals were required to successfully eliminate rainbow trout from Mannis Branch. During the study, 428 rainbow trout were removed from the treatment area. The initial removal collected 296 (70%) of the total rainbow trout removed and was dominated by adults. Subsequent removals were dominated by age-0 rainbow trout (57–83%). The initial two removals successfully eliminated 96% of the reproductively mature adults as well as 86% of the age-0 fish. In Oct 1996, 105 native southern Appalachian brook trout of various age-classes were reintroduced into Mannis Branch. Surveys conducted in May 1997 indicated adult brook trout successfully spa...

Spatial ontogeny of lentic age-0 rainbow trout, Oncorhynchus mykiss: whole-lake manipulations of population size structure

We assessed the influence of the presence of larger-bodied conspecifics on the spatial ontogeny of age-0 rainbow trout, Oncorhynchus mykiss. We created populations with three different size structures in experimental lakes: age-0 and larger-bodied conspecifics, age-0 only, and an initially age-0 only population into which we added larger-bodied conspecifics midexperiment. In the continuous presence of larger conspecifics, age-0 rainbow trout spatial distribution was skewed toward littoral and benthic habitats which had low abundances of their primary prey (zooplankton). The opposite pattern was observed in the experimental lake devoid of larger-bodied conspecifics where the largest proportion of the age-0 class was found in the open-water limnetic habitat where their prey was most abundant. The change in spatial distribution of age-0 rainbow trout in the lake with a midseason addition of larger-bodied conspecifics was consistent with a treatment effect. This addition effect was weakened by a significant ontogenetic change in spatial distribution from littoral to benthic to pelagic habitats as the age-0 rainbow trout grew, and the strength of the change was inversely related to the intensity of interactions with conspecifics. This study provides experimental evidence that the risk of interactions with larger-bodied conspecifics is the process that controls the ontogeny of habitat use by young fish in lakes.

Spatial ontogeny of lentic age-0 rainbow trout, &lt;i&gt;Oncorhynchus mykiss&lt;/i&gt;: whole-lake manipulations of population size structure

We assessed the influence of the presence of larger-bodied conspecifics on the spatial ontogeny of age-0 rainbow trout, Oncorhynchus mykiss. We created populations with three different size structures in experimental lakes: age-0 and larger-bodied conspecifics, age-0 only, and an initially age-0 only population into which we added larger-bodied conspecifics midexperiment. In the continuous presence of larger conspecifics, age-0 rainbow trout spatial distribution was skewed toward littoral and benthic habitats which had low abundances of their primary prey (zooplankton). The opposite pattern was observed in the experimental lake devoid of larger-bodied conspecifics where the largest proportion of the age-0 class was found in the open-water limnetic habitat where their prey was most abundant. The change in spatial distribution of age-0 rainbow trout in the lake with a midseason addition of larger-bodied conspecifics was consistent with a treatment effect. This addition effect was weakened by a significant ontogenetic change in spatial distribution from littoral to benthic to pelagic habitats as the age-0 rainbow trout grew, and the strength of the change was inversely related to the intensity of interactions with conspecifics. This study provides experimental evidence that the risk of interactions with larger-bodied conspecifics is the process that controls the ontogeny of habitat use by young fish in lakes.

Spatial and Temporal Variation in Risk to Piscivory of Age-0 Rainbow Trout: Patterns and Population Level Consequences

Abstract We used tethering to measure spatial and temporal patterns of relative risk of predation to age-0 rainbow trout Oncorhynchus mykiss over 2 years in five lakes that varied widely in the abundance of piscivorous rainbow trout. During the day, the most risky habitats were just above the thermocline near the substrate and in open water at the same depth. Risk was intermediate at the surface in the littoral zone and at the surface over the intersection of the thermocline and the lake bottom. The least risky habitat was at the surface in the open water. The intensity and spatial pattern of relative predation risk changed at dusk. The most risky habitat was still at the thermocline–lake bottom intersection, but it was more than twice as risky as during the day. At dusk, relative predation risk increased in the littoral zone and at the surface above the thermocline–lake bottom intersection. The open-water habitats, both at the surface and at the thermocline, had a low relative predation risk. Age-0 and o...

Microhabitat Selection by Native Brook Trout and Introduced Rainbow Trout in a Small Pennsylvania Stream

ABSTRACT Due to species introductions, brook trout (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) occur together in many North American streams. Some have suggested that the two species do not compete because they select different habitats or are adapted to different environmental conditions. We assessed whether native brook trout and introduced rainbow trout selected different microhabitats in a small Pennsylvania stream. Underwater observations of brook and rainbow trout showed adult fish (≥ 90 mm total length) of both species were found significantly more often in deep water microhabitats than would be expected based on habitat availability. Total depth was the most important microhabitat variable in discriminating between the two species, irrespective of fish size. Adult rainbow trout were found in significantly deeper water than adult brook trout. Adult brook trout also were found significantly farther from cover and closer to the stream bottom than adult rainbow trout. Age-0 brook trout were found in significantly deeper water than age-0 rainbow trout. In small streams during low flow, water depth and distance to nearest cover are likely to be major factors in discriminating between brook and rainbow trout.