Trout Stocks Research Articles

Evaluating the influence of stocking history and barriers to movement on the spatial extent of hybridization between westslope cutthroat trout and rainbow trout

In this study we examine how the stocking of rainbow trout (Oncorhynchus mykiss), as well as the presence of fish movement barriers, influences the probability of introgression with westslope cutthroat trout (Oncorhynchus clarkii lewisi). We measured the level of introgression in cutthroat trout sampled from 32 locations that occurred either above or below fish movement barriers along with the frequency and number of rainbow trout stocked in the watershed over a 43-year period. The occurrence and level of hybridization in cutthroat trout were not related to whether the site was above a movement barrier or not. In contrast, the level of introgression was related to the distance to the nearest stocking location, the number of stocking events within the nearest watershed, and the total number of rainbow trout stocked in that watershed. Our data indicate that westslope cutthroat trout located further from stocking have a lower risk of introgression with rainbow trout, but those isolated above movement barriers should not be considered free from introgression until a thorough genetic assessment of the location has been made.

Effects of Stocking With Brood Fish To Management With Resident Stream Dwelling Brown Trout Salmo cf. trutta L. Stock

Management with the indigenous stock of resident brown trout Salmo cf. trutta of the Danubi-an strain in the protected area of the River Gradac with the catch-and-release fishing regime since 2000 resulted in the prime fly fishing stream in Serbia. Stocking with the brown trout parr in 2007 in the density issued by fisheries management plan supplemented with the large-sized brood fish beyond the issued fisheries measures in the upper section of the River Gradac was followed by change in structure of fish community and population structure of brown trout there. The time of maturation of brown trout was delayed and their size at maturation increased in 2008, due to the abrupt decrease in the density of their stock. That change did not affect an increase of brown trout mean weight, but only an increase of their average age, with the de-crease in relative abundance, relative biomass and annual natural production, implying the pro-longed recovery period of brown trout stock in the upper section of the River Gradac. In con-trast to that, stocking with brown trout parr only in the density issued by fisheries management plan was beneficial on the second, downstream situated section. Another prominent feature that followed stocking as a management measure was the introgression of brown trout of the Atlan-tic lineage into the gene pool of brown trout of the native, Danubian lineage, as confirmed in wild-bred brown trout.

Assessing Effects of Stocked Trout on Nongame Fish Assemblages in Southern Appalachian Mountain Streams

AbstractFisheries managers are faced with the challenge of balancing the management of recreational fisheries with that of conserving native species and preserving ecological integrity. The negative effects that nonnative trout species exert on native trout are well documented and include alteration of competitive interactions, habitat use, and production. However, the effects that nonnative trout may exert on nongame fish assemblages are poorly understood. Our objectives were to quantify the effects of trout stocking on native nongame fish assemblages intensively on one newly stocked river, the North Toe River, North Carolina, and extensively on other southern Appalachian Mountain streams that are annually stocked with trout. In the intensive study, we adopted a before–after, control–impact (BACI) experimental design to detect short‐term effects on the nongame fish assemblage and found no significant differences in fish density, species richness, species diversity, or fish microhabitat use associated with trout stocking. We observed differences in fish microhabitat use between years, however, which suggests there is a response to environmental changes, such as the flow regime, which influence available habitat. In the extensive study, we sampled paired stocked and unstocked stream reaches to detect long‐term effects from trout stocking; however, we detected no differences in nongame fish density, species richness, species diversity, or population size structure between paired sites. Our results revealed high inherent system variation caused by natural and anthropogenic factors that appear to overwhelm any acute or chronic effect of stocked trout. Furthermore, hatchery‐reared trout may be poor competitors in a natural setting and exert a minimal or undetectable impact on native fish assemblages in these streams. These findings provide quantitative results necessary to assist agencies in strategic planning and decision making associated with trout fisheries, stream management, and conservation of native fishes.

Stocked trout have minimal effects on littoral invertebrate assemblages of productive fish‐bearing lakes: a whole‐lake BACI study

Summary1. Rainbow Trout (Oncorhynchus mykiss [Walbaum]) is commonly stocked as a sport fish throughout the world but can have serious negative effects on native species, especially in headwater systems. Productive fish‐bearing lakes represent a frequently stocked yet infrequently studied system, and effects of trout in these systems may differ from those in headwater lakes.2. We used a Before‐After Control‐Impact (BACI) design to determine how stocked trout affected assemblage‐level and taxon‐level biomass, abundance and average length of littoral invertebrates in a stocked lake relative to three unstocked control lakes in the boreal foothills of Alberta, Canada. Lakes were studied 1 year before and for 2 years after stocking. Because characteristics of productive fish‐bearing lakes should buffer impacts of introduced fish, we predicted that trout would not affect assemblage‐level structure of littoral invertebrates but might reduce the abundance or average length of large‐bodied taxa frequently consumed by trout.3. Relative to the unstocked control lakes, biomass, but not abundance, of the littoral invertebrate assemblage was affected indirectly by trout through increases of some taxa after trout stocking. At the individual taxon‐level, trout stocking did not affect most (23 of the 27) taxa, with four taxa increasing in abundance or biomass after stocking. Only one taxon, Chironomidae, showed evidence of size‐selective predation by trout, being consumed frequently by trout and decreasing significantly in average length after stocking.4. Our results contrast with the strong negative effects of trout stocking on invertebrate assemblages commonly reported from headwater lakes. A combination of factors, including large and robust native populations of forage fish, the generalised diet of trout, overwinter aeration, relatively high productivity and dense macrophyte beds, likely works in concert to reduce potentially negative effects of stocked trout in these systems. As such, productive, fish‐bearing lakes may represent a suitable system for trout stocking, especially where native sport fish populations are lacking.

New Mitochondrial DNA Haplotype of Brown Trout Salmo trutta L. from Crni Timok Drainage Area in Serbia

Brown trout Salmo trutta wild stocks sustain a remarkable angling and hatchery-reared fish stocking pressure in waters of Serbia, where four drainage-specific and indigenous mitochondrial DNA haplotypes were reported for the drainage area of the Danube River basin. One of these mitochondrial DNA haplotypes, Da23b, was exclusive for brown trout in headwaters of the Crni Timok River (Grand Timok River system, Eastern Serbia). After its discovery in 2003, brown trout stocking was completely halted and abandoned, and a strict Catch-and-Release fishing regime was issued. However, failure of this regime due to enforcement resulted in decline of brown trout fishery. On checking the aboriginality of brood fish sampled in three headwater forks for artificial brown trout propagation, a novel, drainage-specific mtDNA haplotype was recorded in 54% of brown trout. It was assigned Da23c, due to its similarity (substitutions at the two variable sites) to the hitherto known haplotypes of Da23 group. Its finding and absence of Da23b haplotype imply an oversight in earlier haplotype determination. Incorporation of the non-indigenous haplotypes Da2 and Da-s1, introduced in the area by stocking before 2003, into the gene pool of native brown trout and declining of the latter's stock imposes the need for the more stringently enforced conservational activities (genotyping of each brood fish involved in artificial propagation) and improved fisheries management (Catch-and-Release) in the area.

Chromosome rearrangements, recombination suppression, and limited segregation distortion in hybrids between Yellowstone cutthroat trout (Oncorhynchus clarkii bouvieri) and rainbow trout (O. mykiss)

BackgroundIntrogressive hybridization is an important evolutionary process that can lead to the creation of novel genome structures and thus potentially new genetic variation for selection to act upon. On the other hand, hybridization with introduced species can threaten native species, such as cutthroat trout (Oncorhynchus clarkii) following the introduction of rainbow trout (O. mykiss). Neither the evolutionary consequences nor conservation implications of rainbow trout introgression in cutthroat trout is well understood. Therefore, we generated a genetic linkage map for rainbow-Yellowstone cutthroat trout (O. clarkii bouvieri) hybrids to evaluate genome processes that may help explain how introgression affects hybrid genome evolution.ResultsThe hybrid map closely aligned with the rainbow trout map (a cutthroat trout map does not exist), sharing all but one linkage group. This linkage group (RYHyb20) represented a fusion between an acrocentric (Omy28) and a metacentric chromosome (Omy20) in rainbow trout. Additional mapping in Yellowstone cutthroat trout indicated the two rainbow trout homologues were fused in the Yellowstone genome. Variation in the number of hybrid linkage groups (28 or 29) likely depended on a Robertsonian rearrangement polymorphism within the rainbow trout stock. Comparison between the female-merged F1 map and a female consensus rainbow trout map revealed that introgression suppressed recombination across large genomic regions in 5 hybrid linkage groups. Two of these linkage groups (RYHyb20 and RYHyb25_29) contained confirmed chromosome rearrangements between rainbow and Yellowstone cutthroat trout indicating that rearrangements may suppress recombination. The frequency of allelic and genotypic segregation distortion varied among parents and families, suggesting few incompatibilities exist between rainbow and Yellowstone cutthroat trout genomes.ConclusionsChromosome rearrangements suppressed recombination in the hybrids. This result supports several previous findings demonstrating that recombination suppression restricts gene flow between chromosomes that differ by arrangement. Conservation of synteny and map order between the hybrid and rainbow trout maps and minimal segregation distortion in the hybrids suggest rainbow and Yellowstone cutthroat trout genomes freely introgress across chromosomes with similar arrangement. Taken together, these results suggest that rearrangements impede introgression. Recombination suppression across rearrangements could enable large portions of non-recombined chromosomes to persist within admixed populations.

Modeling Prey Consumption by Native and Nonnative Piscivorous Fishes: Implications for Competition and Impacts on Shared Prey in an Ultraoligotrophic Lake in Patagonia

AbstractWe examined trophic interactions of the nonnative salmonids Rainbow Trout Oncorhynchus mykiss, Brown Trout Salmo trutta, and Brook Trout Salvelinus fontinalisand the main native predator Creole Perch Percichthys trucha in Lake Nahuel Huapi (Patagonia, Argentina) to determine the relative impact of each predator on their forage base and to evaluate the potential vulnerability of each predator to competitive impacts by the others. Using bioenergetics simulations, we demonstrated the overall importance of galaxiids and decapods to the energy budgets of nonnative salmonids and Creole Perch. Introduced salmonids, especially Rainbow Trout, exerted considerably heavier predatory demands on shared resources than did the native Creole Perch on both a per capita basis and in terms of relative population impacts. Rainbow Trout consumed higher quantities and a wider size range of Small Puyen (also known as Inanga) Galaxias maculatus than the other predators, including early pelagic life stages of that prey; as such, this represents an additional source of mortality for the vulnerable early life stages of Small Puyen before and during their transition from pelagic to benthic habitats. All predators were generally feeding at high feeding rates (above 40% of their maximum physiological rates), suggesting that competition for prey does not currently limit either Creole Perch or the salmonids in this lake. This study highlights the importance of keystone prey for the coexistence of native species with nonnative top predators. It provides new quantitative and qualitative evidence of the high predation pressure exerted on Small Puyen, the keystone prey species, during the larval to juvenile transition from pelagic to littoral‐benthic habitat in Patagonian lakes. This study also emphasizes the importance of monitoring salmonid and Creole Perch population dynamics in order to detect signs of potential impacts through competition and shows the need to carefully consider the rationale behind any additional trout stocking.

Sperm Quality of Hatchery‐Reared Lake Trout Throughout the Spawning Season

AbstractThe objective of this study was to investigate variation in sperm quality metrics (motility, velocity, linearity, longevity, and density) of hatchery‐reared Lake Trout Salvelinus namaycush throughout the spawning season. Seasonal variation in sperm quality was investigated using both a regression and repeated‐measures approach. Sperm was collected from the same 16 individuals over four sampling periods, separated by 3‐week intervals. Regression analyses showed that 7–27% of the variation in sperm traits could be explained by seasonal variation, indicating that seasonality can have a significant impact on the quality of sperm. Significant positive linear relationships were found for percent motility and linearity at 5 s postactivation. Significant negative quadratic relationships were found for velocity at 5 s postactivation, longevity, and density, whereas a positive quadratic relationship was found for linearity at 10 s postactivation. Repeated measures ANOVAs showed a significant effect of season for percent motility and linearity at 5 and 10 s postactivation, velocity at 10 s postactivation, and longevity. Our findings are important for optimizing fertilization protocols for hatchery production and can also be used to understand reproductive biology and ecology of wild Lake Trout stocks.

Habitat Use, Survival, and Site Fidelity of Rainbow Trout Stocked into an Appalachian River

Rainbow trout from private culture facilities were stocked into Bluestone River, West Virginia, to evaluate biological feasibility of stocking to enhance local angling opportunities. Between fall 2007 and spring 2009, over 2,400 fish were stocked. A subset of fish was tagged using radio transmitters (ATS model 1580), each bearing a unique frequency (48.000 to 49.999 Mhz) to monitor survival and availability of fish to anglers along a 1.6 km river reach. Tagged fish were 366 mm TL (33 mm SD) and 617g (128g SD) on average. Summarized seasonally, 73%–96% of fish remained within the study reach. Survival varied between years. Between fall 2007 and spring 2008 seasonal survival was 62%–89%, but from fall 2008 through spring 2009 it was 32%–66%. Mean availability (product of survival and site fidelity) ranged from 48%–78% over the study except it declined to 23% in winter 2008 due to low survival. This case study shows that cultured rainbow trout from private producers can be stocked into seasonally suitable streams to enhance local angling.

Historical stocking data and 19th century DNA reveal human‐induced changes to native diversity and distribution of cutthroat trout

Many species are threatened with extinction and efforts are underway worldwide to restore imperilled species to their native ranges. Restoration requires knowledge of species' historical diversity and distribution. For some species, many populations were extirpated or individuals moved beyond their native range before native diversity and distribution were documented, resulting in a lack of accurate information for establishing restoration goals. Moreover, traditional taxonomic assessments often failed to accurately capture phylogenetic diversity. We illustrate a general approach for estimating regional native diversity and distribution for cutthroat trout in the Southern Rocky Mountains. We assembled a large archive of historical records documenting human-mediated change in the distribution of cutthroat trout (Oncorhynchus clarkii) and combined these data with phylogenetic analysis of 19th century samples from museums collected prior to trout stocking activities and contemporary DNA samples. Our study of the trout in the Southern Rocky Mountains uncovered six divergent lineages, two of which went extinct, probably in the early 20th century. A third lineage, previously declared extinct, was discovered surviving in a single stream outside of its native range. Comparison of the historical and modern distributions with stocking records revealed that the current distribution of trout largely reflects intensive stocking early in the late 19th and early 20th century from two phylogenetically and geographically distinct sources. Our documentation of recent extinctions, undescribed lineages, errors in taxonomy and dramatic range changes induced by human movement of fish underscores the importance of the historical record when developing and implementing conservation plans for threatened and endangered species.

Re‐building brown trout populations in dredged boreal forest streams: in‐stream restoration combined with stocking of young trout

Summary1. Rivers in boreal forested areas were often dredged to facilitate the transport of timber resulting in channels with simplified bed structure and flow fields and reduced habitat suitability for stream organisms, especially lotic fishes. Currently, many streams are being restored to improve their physical habitat, by replacing boulders and gravel and removing constraining embankments. The most compelling justification behind stream restoration of former floatways has been the enhancement of native fish populations, specifically salmonids.2. We examined the success of a stream management programme aimed at re‐building diminished brown trout (Salmo trutta) populations by monitoring densities of young‐of‐year and older trout in 18 managed and three reference streams during 2000–2005. Rehabilitation included in‐stream restoration combined with a 5‐year post‐restoration period of stocking young brown trout. Our space‐for‐time substitution design comprised four pre‐management, four under‐management, 10 post‐management and three reference streams.3. Densities of young‐of‐year brown trout, indicating population establishment, were significantly higher in post‐ compared with pre‐management streams. However, density of young‐of‐year brown trout in post‐management streams was significantly lower compared with near‐pristine reference streams. Furthermore, success of managed brown trout population re‐building varied, indicating stream‐specific responses to management measures. Density of burbot (Lota lota), a native generalist predator, was associated with low recruitment of brown trout.4. Stream‐specific responses imply that rehabilitation of brown trout populations cannot be precisely predicted thereby limiting application. Our findings support the importance of adaptive stream restoration and management, with focus on identifying factor(s) limiting the establishment of target fish populations.

Coastal migrations, temporary use of neighbouring rivers, and growth of sea trout (Salmo trutta) from nine northern Baltic Sea rivers

Abstract Degerman, E., Leonardsson, K., and Lundqvist, H. 2012. Coastal migrations, temporary use of neighbouring rivers, and growth of Sea trout (Salmo trutta) from nine northern Baltic Sea rivers – ICES Journal of Marine Science, 69: 971–980. The wild migratory trout (Salmo trutta) stocks in the northern Baltic Sea are threatened, and the fishery is thought to play a significant role in the population decline. Therefore, knowledge about the migration patterns of these stocks is needed to develop appropriate management plans. For this reason, we analysed the movement pattern from recaptures of mandatory releases of tagged hatchery-reared trout smolt from nine rivers in the region (1998–2007). The median time from release until recapture was 366 days, and the median migration distance at recapture was 27 km, with a dominating southward direction for northern stocks. Most of the recaptured fish were immature (65%), and recaptures in rivers (55%) dominated over recaptures along the coast (44%). Riverine recaptures were most frequent during autumn–spring. A total of 16% of all recaptures in rivers occurred in non-natal rivers. Straying was about twice as frequent among fish from small rivers as from large rivers. The results from the river Gideälven indicate that using broodstock from other rivers in stocking programmes may lead to a high proportion of strayers. In addition, temporary use of neighbouring rivers was more frequent in large rivers during winter. Growth, in terms of length increment, was high, especially in fish from southern parts of the Baltic, and higher than in neighbouring warmer and more saline sea areas.

Characteristics of ovarian follicle steroidogenesis during vitellogenesis in an asynchronously ovulating stock of rainbow trout Oncorhynchus mykiss

This study explored several physiological criteria that could be used to assess the steroidogenic condition of the ovarian follicles of individual fish of an asynchronously ovulating captive rainbow trout Oncorhynchus mykiss stock. In these fish, the date of sampling, morphological variables such as gonado-somatic index or ovarian follicle mass and visual assessment of the ovary provided accurate indications of the maturational condition of an individual. The physiological variables measured included the in vitro basal and cyclic adenosine monophosphate (cAMP)-stimulated synthesis by ovarian follicles of 17β-oestradiol (E(2)) and testosterone (T); in addition, quantitative reverse-transcription (RT)-PCR was used to measure the relative expression of star and p450scc genes by ovarian follicles. The ratios of cAMP-stimulated E(2) and T synthesis to basal E(2) and T synthesis provided a reliable indication of differences in the steroidogenic status of the follicles of individual animals. On the basis of these criteria, together with the use of gene expression profiles, it was possible to classify individual fish as being at an early, mid or late-vitellogenic stage.

Trap design for catching fish unharmed and the implications for estimates of sea lice (Lepeophtheirus salmonis) on anadromous brown trout (Salmo trutta)

The decline in stocks of sea trout, i.e. anadromous brown trout (Salmo trutta L.), on the west coast of Norway has given rise to a series of conservation measures and highlighted the need for a non-destructive capturing method. For this reason, a trap was developed for use in marine environments, with the overall goal of capturing sea trout unharmed and obtaining accurate assessments of sea lice (Lepeophtheirus salmonis) infection levels. The trap design captured all size classes of sea trout, and a variety of other coastal fish species, none of which suffered net-induced mortality. The trap was used to compare the abundance and intensity of sea lice on sea trout captured in that particular trap, as opposed to gillnets. The abundance of adult sea lice on trout caught in the trap was significantly higher than on trout caught in gillnets, but not significant for larval sea lice. The intensity of sea lice was only significant for the total number of lice (adult and larvae). These results indicate that estimated infection levels for trout captured by gillnets, which are frequently used in assessment programmes for sea lice, are likely to be underestimated. Consequently, the use of this particular trap can be recommended for the capture of sea trout for sea lice infection assessments. In general, the trap is an appropriate tool for studies where live, unharmed, coastal fish are required for research or conservation purposes.

Evidence for sea lice-induced marine mortality of Atlantic salmon (Salmo salar) in western Ireland from experimental releases of ranched smolts treated with emamectin benzoate

Sea trout ( Salmo trutta ) stock collapses in coastal areas of western Ireland subject to salmon aquaculture were contemporaneous with high abundances of larval sea lice ( Lepeophtheirus salmonis ) on juvenile sea trout. Whereas sea trout remain in near-shore waters throughout their marine migration, Atlantic salmon ( Salmo salar ) smolts typically move quickly offshore into oceanic waters. It might therefore be predicted that salmon smolts would be less vulnerable to coastal stressors and less likely to be negatively affected by infestations of sea lice early in their marine phase. Groups of microtagged, hatchery-reared Atlantic salmon smolts were fed either untreated pellets or pellets incorporating the in-feed sea louse treatment SLICE (emamectin benzoate) prior to eight experimental releases in three marine locations over a 3-year period. In total, 74 324 smolts were released and analysis of tag recaptures from returning adult salmon showed that emamectin-treated smolts experienced increased survivorship and were 1.8 times more likely to return compared with control fish. These results suggest that sea lice-induced mortality on adult Atlantic salmon returns in Ireland can be significant, and that sea lice larvae emanating from farmed salmon may influence individual survivorship and population conservation status of wild salmon in these river systems.

Paenibacillus thiaminolyticus is not the cause of thiamine deficiency impeding lake trout (Salvelinus namaycush) recruitment in the Great Lakes

Thiamine (vitamin B1) deficiency is a global concern affecting wildlife, livestock, and humans. In Great Lakes salmonines, thiamine deficiency causes embryo mortality and is an impediment to restoration of native lake trout ( Salvelinus namaycush ) stocks. Thiamine deficiency in fish may result from a diet of prey with high levels of thiaminase I. The discoveries that the bacterial species Paenibacillus thiaminolyticus produces thiaminase I, is found in viscera of thiaminase-containing prey fish, and causes mortality when fed to lake trout in the laboratory provided circumstantial evidence implicating P. thiaminolyticus. This study quantified the contribution of P. thiaminolyticus to the total thiaminase I activity in multiple trophic levels of Great Lakes food webs. Unexpectedly, no relationship between thiaminase activity and either the amount of P. thiaminolyticus thiaminase I protein or the abundance of P. thiaminolyticus cells was found. These results demonstrate that P. thiaminolyticus is not the primary source of thiaminase activity affecting Great Lakes salmonines and calls into question the long-standing assumption that P. thiaminolyticus is the source of thiaminase in other wild and domestic animals.

Successional Change in the Lake Superior Fish Community: Population Trends in Ciscoes, Rainbow Smelt, and Lake Trout, 1958--2008

The Lake Superior fi sh community underwent massive changes in the second half of the 20 century. Those changes are largely refl ected in changes in abundance of the adults of principal prey species, the ciscoes (Coregonus spp.), the invasive rainbow smelt (Osmerus mordax), and the principal predator, lake trout (Salvelinus namaycush). To better understand changes in species abundances, a comprehensive series of gillnet and bottom trawl data collected from 1958 to 2008 were examined. In the late 1950s/early 1960s, smelt abundance was at its maximum, wild lake trout was at its minimum, and an abundance of hatchery lake trout was increasing rapidly. The bloater (Coregonus hoyi) was the prevalent cisco in the lake; abundance was more than 300% greater than the next most abundant cisco, shortjaw cisco (C. zenithicus), followed by kiyi (C. kiyi) and lake cisco (C. artedi). By the mid-1960s, abundance of hatchery lake trout was nearing maximum, smelt abundance was beginning to decline, and abundances of all ciscoes declined, but especially that of shortjaw cisco and kiyi. By the late 1970s, recovery of wild lake trout stocks was well underway and abundances of hatchery lake trout and smelt were declining and the ciscoes were reaching their nadir. During 1980–1990, the fi sh community underwent a dramatic shift in organization and structure. The rapid increase in abundance of wild lake trout, concurrent with a rapid decline in hatchery lake trout, signaled the impending recovery. Rainbow smelt abundance dropped precipitously and within four years, lake cisco and bloater populations rebounded on the heels of a series of strong recruitment events. Kiyi populations showed signs of recovery by 1989, and shortjaw by 2000, though well below historic maximum abundances. High abundance of adult smelt prior to 1980 appears to be the only factor linked to recruitment failure in the ciscoes. Life history traits of the cisco species were examined to better understand their different responses to conditions of low and high predator levels, i.e., late 1950s–early 1960s vs. post 1980. Bloaters are most likely to become the predominant cisco in the absence of strong predation and the least abundant under prolonged predation; smelt share this pattern. Conversely, the lake cisco and shortjaw cisco fare better when predator abundance is high. The recovery of lake trout in Lake Superior reestablished a strong top-down infl uence on the fi sh community and its present structure and organization appears to be approaching an equilibrium that refl ects a more natural state. If lake trout recovery is sustained, shortjaw cisco abundance is expected to increase and join lake cisco and kiyi as dominant cisco species, and bloater and smelt will oscillate at lower abundances.

Patterns of Hybridization of Nonnative Cutthroat Trout and Hatchery Rainbow Trout with Native Redband Trout in the Boise River, Idaho

AbstractHybridization is one of the greatest threats to native fishes. Threats from hybridization are particularly important for native trout species as stocking of nonnative trout has been widespread within the ranges of native species, thus increasing the potential for hybridization. While many studies have documented hybridization between native cutthroat trout Oncorhynchus clarkii and nonnative rainbow trout O. mykiss, fewer have focused on this issue in native rainbow trout despite widespread threats from introductions of both nonnative cutthroat trout and hatchery rainbow trout. Here, we describe the current genetic (i.e., hybridization) status of native redband trout O. mykiss gairdneri populations in the upper Boise River, Idaho. Interspecific hybridization was widespread (detected at 14 of the 41 sampled locations), but high levels of hybridization between nonnative cutthroat trout and redband trout were detected in only a few streams. Intraspecific hybridization was considerably more widespread (almost 40% of sampled locations), and several local populations of native redband trout have been almost completely replaced with hatchery coastal rainbow trout O. mykiss irideus; other populations exist as hybrid swarms, some are in the process of being actively invaded, and some are maintaining genetic characteristics of native populations. The persistence of some redband trout populations with high genetic integrity provides some opportunity to conserve native genomes, but our findings also highlight the complex decisions facing managers today. Effective management strategies in this system may include analysis of the specific attributes of each site and population to evaluate the relative risks posed by isolation versus maintaining connectivity, identifying potential sites for control or eradication of nonnative trout, and long‐term monitoring of the genetic integrity of remaining redband trout populations to track changes in their status.Received February 24, 2011; accepted August 4, 2011

Evaluation of Offshore Stocking of Lake Trout in Lake Ontario

AbstractRestoration stocking of hatchery‐reared lake trout Salvelinus namaycush has occurred in Lake Ontario since 1973. In U.S. waters, fish stocked through 1990 survived well and built a large adult population. Survival of yearlings stocked from shore declined during 1990–1995, and adult numbers fell during 1998–2005. Offshore stocking of lake trout was initiated in the late 1990s in response to its successful mitigation of predation losses to double‐crested cormorants Phalacrocorax auritus and the results of earlier studies that suggested it would enhance survival in some cases. The current study was designed to test the relative effectiveness of three stocking methods at a time when poststocking survival for lake trout was quite low and losses due to fish predators was a suspected factor. The stocking methods tested during 2000–2002 included May offshore, May onshore, and June onshore. Visual observations during nearshore stockings and hydroacoustic observations of offshore stockings indicated that release methods were not a direct cause of fish mortality. Experimental stockings were replicated for 3 years at one site in the southwest and for 2 years at one site in the southeast. Offshore releases used a landing craft to transport hatchery trucks from 3 to 6 km offshore out to 55–60‐m‐deep water. For the southwest site, offshore stocking significantly enhanced poststocking survival. Among the three methods, survival ratios were 1.74 : 1.00 : 1.02 (May offshore : May onshore : June onshore). Although not statistically significant owing to the small samples, the trends were similar for the southeast site, with survival ratios of 1.67 : 1.00 : 0.72. Consistent trends across years and sites indicated that offshore stocking of yearling lake trout during 2000–2002 provided nearly a twofold enhancement in survival; however, this increase does not appear to be great enough to achieve the 12‐fold enhancement necessary to return population abundance to restoration targets.Received November 29, 2010; accepted May 18, 2011

Relative Performance of Diploid and Triploid Catchable Rainbow Trout Stocked in Idaho Lakes and Reservoirs

AbstractIdaho Department of Fish and Game hatcheries stock predominantly sterile triploid (3n) rainbow trout Oncorhynchus mykiss to provide sportfishing opportunities while minimizing the genetic risks to wild stocks. Triploid catchable‐sized rainbow trout are stocked in over 500 water bodies across Idaho annually, but there remains some uncertainty regarding the performance of triploid rainbow trout relative to their diploid (2n) counterparts. We examined the relative survival, growth, and returns of diploid and triploid all‐female catchable rainbow trout across 13 lakes and reservoirs. Most reservoirs showed higher returns of 2n rainbow trout to anglers. In 2008, 3n rainbow trout returned on average at only 72% and 81% of the rates of 2n trout in gill nets and snout collection boxes, respectively, and the difference for both methods was statistically significant. Carryover of marked rainbow trout from 2008 was low or zero in most reservoirs. Where there was carryover, snout collection boxes suggested that 3n rainbow trout returned to anglers at 71% of the rate of 2n rainbow trout in the second year after planting, but the difference was not statistically significant. Triploid rainbow trout did not show any growth advantages over 2n rainbow trout but were similar in length, weight, and dressed weight. The disparity in returns between 2n and 3n trout varied across reservoirs but was more pronounced in locations subjected to greater drawdown and with greater species diversity. While 2n rainbow trout may grow and survive better in reservoirs subject to low water levels, triploid rainbow trout may perform equally well under good habitat conditions while not having genetic impacts on native stocks. These findings are rather fortuitous for fisheries managers, as triploids probably perform better in higher‐quality habitats where native trout often exist, whereas diploids are better suited to reservoirs with degraded habitats where native stocks have usually been extirpated.Received September 14, 2010; accepted May 18, 2011