Trout Stocks Research Articles

Synthesis of recent research and attributes of recovered lean Lake Trout populations in Lake Superior, 1993–2022

AbstractObjectiveThis purpose of this paper is to describe the characteristics of recovered Lake Trout Salvelinus namaycush populations in Lake Superior by describing its population dynamics, ecology, and recent research and management activities since 1993, when Lake Trout were declared rehabilitated.MethodsData from commercial fisheries, recreational fisheries, agency stocking reports, and natural resource agency fishery‐independent survey data along with published research findings on lean Lake Trout in Lake Superior between 1993 and 2022 were synthesized and reported.ResultCurrently, Lake Trout populations are self‐sustaining and lightly exploited with only a few areas with elevated total mortality rates. The total annual mortality has been far below the target maximum range of 42–45%. Furthermore, stocking of hatchery Lake Trout is no longer necessary.ConclusionWe have learned from research and management experience that the regulatory role of Lake Trout in the Great Lakes is critical to proper ecosystem function. Thus, continued commitment from natural resource agencies to cooperate and implement effective management actions is required to preserve the accomplishments of lakewide recovery of Lake Trout populations.

Population-genetics analysis of the brown trout broodstock in the “Panjica” hatchery (Serbia) and its conservation applications

Artificial propagation and stocking of brown trout is a standard practice in recreational fishery management. In recent decades, the importance of maintaining intraspecific diversity and protecting locally adapted lineages has been recognized for the species' long-term survival. The first step in selecting donors for stocking involves distinguishing native trout from non-native and introgressed individuals. The established method for discerning Atlantic hatchery strains from the wild populations involves genetic screening of individual diagnostic SNPs and microsatellite assignment tests. This study, using Serbia's Panjica hatchery as an example, illustrates the proper conduct of routine genetic screening for identifying suitable donors for supportive stocking. The broodstock and reference populations were screened using mtDNA control region, LDH nuclear gene, and 12 microsatellite loci to assess the origin, diversity, and inbreeding levels. The analysis revealed only moderate contamination with Atlantic trout and showed the regional origin of the Danubian genes – over 50% of the broodstock was composed of non-introgressed Danubian individuals tracing their origin to the Zapadna Morava River system. Additionally, the study highlighted a considerable discordance between LDH locus and microsatellites in identifying introgressed individuals, raising concerns about the sole reliance on LDH locus for the identification of Atlantic genetic origin in nuclear DNA.

Habitat selection by lake trout at reefs near Drummond Island, Lake Huron: An evaluation of physical characteristics used for spawning

Continued dependence on stocking of hatchery-reared lake trout, Salvelinus namaycush, in the Laurentian Great Lakes indicates recruitment bottlenecks still exist between spawning and the juvenile life stage (age-1). Possible impediments include poor habitat quality and an inability of hatchery-origin adults to select substrates that support successful incubation. Understanding of characteristics of suitable lake trout spawning habitat is limited but has centered on the cobble-contour framework, which identifies sloped reefs with cobble substrate and deep interstices to entrain eggs as spawning habitat. A novel sampling design based on acoustic-telemetry detections was used to evaluate relative importance of cobble-contour habitat characteristics (substrate size, homogeneity, interstitial depth, and slope) for predicting egg presence on two spawning reefs in northern Lake Huron. Fixed effect logistic regression models were used to relate egg presence to all possible combinations of measured physical characteristics. Physical characteristics of sites with naturally spawned eggs present varied, and no single best model predicted egg presence. Model selection and multi-model inference revealed slope to be the only informative predictor. Substrate size, homogeneity, and interstitial depth were not important. Embryo survival-to-hatch was also compared among sampling sites using in situ habitat bioassays seeded with hatchery fertilized embryos. No difference in embryo survival was observed between sites where naturally spawned eggs were present vs. absent. Over half of sites evaluated exceeded 75% survival. Naturally spawned eggs were observed at 25 of 57 sites. We suspect other characteristics such as interstitial flow could be helpful for predicting selection of lake trout spawning habitats.

Influence of Diet Changes on the Condition and Physiological State of Juvenile Sea Trout (Salmo trutta)

Abstract The aim of the study was to determine the influence of diets (factor D) and the time period (factor T) during which they were applied on the growth performance and physiological condition (blood plasma hematological and biochemical indicators of stress and immunity) in juvenile sea trout (Salmo trutta; initial body weight approximately 73 g). The diet of the fish that was used prior to the experiment (formulated feed; initial fish sample) was modified as follows: a different formulated feed (group B), mixed feed (feed B + prey fish; group B/N), prey fish exclusively (group N). The fish from group A were given the feed that was used prior to the beginning of the experiment. During the 28-day trial neither factors D nor T influenced absolute or relative fish growth rates. Factor D significantly influenced hematological indicators and leukograms, while the phagocytic index and cidal ability were determined by the time test (factor T). Myeloperoxidase (AMPO) was related significantly with the influence of factors D and T and also with the interaction of D×T. A significant decrease in AMPO was noted after two weeks of the test in groups B, N, and B/N, but after four weeks this indicator did not differ from that confirmed in the initial fish sample. The factors tested influenced stress indicators, i.e., cortisol (D and T) and glucose (D). Significant increases in cortisol (group B) and glucose (groups A, B, and B/N) concentrations were noted after two weeks of the test. After the subsequent two weeks (four weeks of the test) these indicators also stabilized. Sea trout from aquaculture readily accept prey fish; however, changes in diet cause stress reactions such as temporarily reduced immunity. Thus, the procedure for preparing sea trout stocking material for release, which is to change the feed from formulated to natural (live fish), should last approximately four weeks.

Influence of angling methods and terminal tackle on survival of salmon and steelhead caught and released in the Cowlitz River, Washington

Efforts to recover depressed stocks of salmon and steelhead trout in North America include implementation of mark-selective recreational fisheries, whereby anglers are allowed to harvest hatchery-origin fish but must release natural-origin fish. Catch and release angling (C&R) is generally thought to be an effective tool for conservation relative to traditional retention fisheries due to high survival of released adult salmon and steelhead in freshwater. Studies designed to estimate C&R mortality have produced highly variable results among species and size classes of fish, gear types, and environmental conditions. Therefore, crude approximations of C&R mortality are commonly used to quantify impacts to natural-origin salmon and steelhead. In addition, managers often restrict use of certain angling methods and terminal tackle that are assumed to result in higher mortality, leading to a multiplicity of different regulatory requirements with limited empirical support. We conducted a novel three-year mark-recapture study in the Cowlitz River, Washington to estimate effects of a variety of factors hypothesized to influence salmon and steelhead C&R survival using a control-treatment design. Three species of anadromous salmonids were captured and released as treatments using various angling techniques and terminal tackle. Fight time, handling time, and water temperature were recorded during each capture event. Non-angled fish were captured in a trap and released back into the fishery to serve as controls. Recovery rates of Coho Salmon differed less than a percent between angled and non-angled fish across multiple gear types, indicating negligible effects of C&R. Angled Spring Chinook Salmon experienced 3.6–10.2 % C&R mortality relative to non-angled control fish, depending on terminal tackle. Barbless hooks were associated with higher survival than barbed hooks for both Chinook and Coho Salmon, although differences were small for Chinook and negligible for Coho. In contrast, steelhead trout angled on barbed hooks were recovered at slightly higher rates than those caught on barbless hooks. We also found evidence for a reduction in landing rates when angling using barbless hooks. Finally, use of bait increased the probability that salmon would be hooked in a critical location such as the esophagus or stomach. Our findings are useful for assessing trade-offs between conservation measures and harvest opportunity when defining fishing regulations in mark-selective salmon and steelhead fisheries.

Histomorphometric evaluation of melanomacrophage centers (MMCs) and CD3+ T cells of two morphs of brown trout (Salmo trutta) fed diets with immunostimulants

The brown trout (Salmo trutta) is a salmonid residing in riverine and coastal waters throughout the Northern Hemisphere, whose various populations evolved into distinct ecological morphs, differing in their migratory tendencies and preferred habitats. Unfortunately, due to progressing degradation of natural environment, the conservation of these populations is of growing importance and is undoubtedly a challenging task. Therefore, various means to refine the preparatory protocols for restocking using hatchery-reared fish are being pursued, some of which involve the administration of immunity-boosting substances. The current study assessed the effects of two dietary immunostimulants: Bioimmuno (4% inosine pranobex and 96% β-glucan) and Focus Plus (commercial preparation by Biomar, Denmark) on two morphs of the brown trout – the river trout (S. trutta morpha fario) and the sea trout (S. trutta morpha trutta). Tissue samples were obtained from ∼75 to 100g fish after 0, 2 and 4 weeks of experimental feeding. Multi-factorial analysis of conducted histological measurements of melanomacrophage centers (MMCs) revealed no changes of their parameters within spleens, but showed a decrease of the occupied tissue area and MMC counts in the livers, progressing with time regardless of the applied diet. Immunohistochemical analysis of CD3+ T cells showed their increased recruitment into mucosal folds of pyloric caeca in the 2-week sampling of trouts fed with the diet with 2% Bioimmuno addition, but this effect was not present in the 4-week sampling. When studying all groups jointly within each morph, there was a significant difference in terms of maintained CD3+ T cells levels, as sea trouts showed significantly higher tissue areas occupied by these cells than river trouts, both in the pyloric caeca and hepatic parenchyma. The study revealed that feeding with a diet enriched with Bioimmuno for 2 weeks may be a favorable enhancement of rearing protocols of brown trout stocks prior to their release, but more studies need to be conducted to test the possibility of an even shorter feeding period.

Mechanical Removal of Minnows (Gila bicolor) to Improve Water Quality in a Hydropower Impoundment, Lemolo Lake, Oregon, USA

A project to mechanically remove an over-abundant population of cyprinids (Gila bicolor) was tested in a hydropower impoundment, Lemolo Lake (USA). The netting program removed 29.7 tons of fish (163 kg/ha) over a seven-year period that resulted in a reduction in the intensity and duration of cyanobacteria blooms. However, the suppression of the tui chub population apparently allowed for a residual population of kokanee (Oncorhynchus nerka) to proliferate. The resurgence of the kokanee, combined with the initiation of a rainbow trout stocking program, offset some of the gains in water quality achieved with the reduction of tui chub biomass. The dominant cyanobacterium present in Lemolo Lake transitioned to Gloeotrichia echinulata in 2013, but the majority of these colonies were apparently derived from an upstream lake. The changes in water quality from 2005–2013 were simulated for Lemolo Lake using the two-dimensional, hydrodynamic model, CE-QUAL-W2. The model reproduced many aspects of the water quality in Lemolo Lake and was used, in combination with a simple Bayesian model, to evaluate a number of potential scenarios for further improving water quality in the impoundment. Promoting a deeper drawdown of the hydroelectric impoundment during the winter to selectively increase entrainment of tui chub and kokanee, combined with continued netting targeted at tui chub, was judged to offer the greatest opportunity to manage the facility for water quality, fisheries and hydropower objectives.

The Evolution of Trout Stream Management in the Black Hills, 1883–2023, as Evidenced Through Hatchery Activities and Stocking

In this paper, we review and synthesize trout management philosophy and policies for Black Hills streams in the states of South Dakota and Wyoming through the roles of hatcheries, fish stocking, and natural production from 1883 to 2023. We identified three specific fisheries management time periods (eras) in the Black Hills: (1) the early management–fry stocking era (pre‐1950), (2) the catchable‐trout stocking era (1950s to mid‐1990s), and (3) the integrated management era (mid‐1990s to present). The first era emphasized juvenile trout stocking, with little evaluation of effectiveness in terms of return to creel. The second era was characterized by greater emphasis on stocking catchable‐sized trout and a substantial increase in hatchery rearing capabilities. In the first two eras, instead of fish stocking being one of many potentially useful tools for managers of Black Hills streams, stocking was the dominant activity—leading and directing fisheries management rather than supporting it. By the third era, trout management began to embody a more encompassing and complex national ideology and philosophy that more fully considered ecology, including habitat improvement, natural reproduction of trout, defining the role of fish stocking, and understanding the relationship of trout to the depauperate native (nongame) fish fauna. We also discuss the three eras in the Black Hills in relation to national trends in trout management. In contrast to many other localities, the lack of any native salmonids and the lack of native game fish species have facilitated the development of valuable, self‐sustaining recreational fisheries in streams that support trout, with less concern for native species than in many areas with harvestable native fish species. The cultural ideology that brought western expansion of settlers to the Black Hills in search of gold, land, freedom, and prosperity was emboldened by a science‐based belief and confidence in technology and industry to rapidly shape lands and waters to improve the human condition. In part, the sculpting and shaping of Mount Rushmore symbolize that ideology. In the context of fisheries management in the Black Hills, the introduction of trout, development of stream fisheries, and investment in hatchery technology to produce catchable trout for an expanding tourist economy also exemplify that prevailing ideology, modulated by improved ecological understanding and an accompanying shift toward a more ecological management philosophy.

Lake Trout population dynamics in the Northern Refuge of Lake Michigan: Importance of stocking rate

AbstractObjectiveThe Northern Refuge of Lake Michigan was established under the auspices of the Great Lakes Fishery Commission in 1985 as an additional step toward restoring a self‐sustaining Lake Trout Salvelinus namaycush population to the lake. The overall goal of our study was to assess progress toward Lake Trout rehabilitation in the Northern Refuge through 2021.MethodsWe conducted annual (1992–2021) gill‐net surveys in the fall to assess the adult population and beam trawl surveys in the spring to assess naturally reproduced age‐0 Lake Trout.ResultSpawner abundance averaged 45 fish • 305 m of gill net−1 • day−1 during 1992–1999, just 4 fish • 305 m of gill net−1 • day−1 during 2000–2014, and then 67 fish • 305 m of gill net−1 • day−1 during 2018–2021. Sea Lamprey Petromyzon marinus wounding rate decreased by nearly 90% between the 2000–2014 and 2015–2021 periods. The average annual mortality rate of adult Lake Trout decreased from 43% during the 1990s and 2000s to 22% during 2015–2021. All of these population dynamics were partly driven by changes in Lake Trout stocking rates, which declined by roughly 50% during 1994–1996 but then increased roughly twofold during 2006–2010 and remained at this higher level during 2010–2018. In 2021, early signs of natural recruitment of Lake Trout became evident. Beam trawling during June 2021 yielded a wild age‐0 Lake Trout, the first ever caught since the surveys began in 1992. In addition, 15 (4% of total catch) of the Lake Trout caught in the fall 2021 gill‐net survey were unclipped (wild) fish. Estimated ages of these wild fish indicated that natural recruitment had begun sometime around 2015.ConclusionWith Alewife Alosa pseudoharengus abundance in Lake Michigan at record low levels, Sea Lamprey wounding rate greatly reduced, and Lake Trout spawner abundance at relatively high levels, the outlook for Lake Trout rehabilitation in the Northern Refuge holds promise.

The impact of parasitic sea lice on harvest quantities and sizes of farmed salmon

Sec lice infections are recognized as a primary challenge for both salmon farming and adjacent stocks of wild salmon and sea trout, triggering strict regulations at farm sites and in larger coastal production areas. In 2017 the Norwegian government implemented the Traffic Light System (TLS), where green, yellow, and red lights imply that salmon farmers can raise, maintain, or must reduce production quantities depending on estimated sea lice-induced mortality of wild salmonids. Past research has explored the impacts of sea lice on the growth rate of salmon, indicating a possible link between sea lice numbers and harvest practices. This study evaluates the impact of sea lice on salmon farmers' harvest behavior focusing on production quantities and fish sizes while controlling for market prices of salmon and fish meal. We also investigate whether and to what extent the TLS implementation affects harvest behavior. Our empirical results indicate that farmers tend to harvest marginally faster in response to increasing levels of sea lice and slower during delousing operations. The implementation of the TLS strengthened the negative impacts of delousing operations on harvest quantities. Fish sizes at harvest are negatively associated with sea lice levels and delousing operations, regardless of the implementation of the TLS. Control variables, such as seawater temperature, salmon prices, and fish meal prices, also influence harvest quantities and fish sizes.

Distribution and Abundance of California (Zalophus californianus) and Steller (Eumetopias jubatus) Sea Lions in the Inshore Waters of Washington, 2013-2016

Two species of sea lions occur in the in387land waters of Washington State: the California sea lion (Zalophus californianus) and the Steller sea lion (Eumetopias jubatus). Both species breed elsewhere, but they typically move into Puget Sound and adjacent waters of the Salish Sea from autumn through spring. There is a need for information on their current abundance and seasonal use patterns as both species prey heavily on threatened/endangered stocks of salmon and steelhead trout (Oncorhynchus spp.), and empirical abundance estimates of these species are lacking for inland Washington waters. From 2013 to 2016, we conducted 39,399 km of aerial surveys for marine mammals in this area, sighting 255 groups of sea lions. We used a subset of 7,841 km of effort and 165 sea lion sightings made during surveys in good sighting conditions to estimate in-water abundance using line-transect methods. Historical tagging data collected in Pacific Northwest waters were used to evaluate the proportions of time that each species spent on land and conducting dives, and then to develop correction factors to derive total abundance for both sea lion species, providing the first empirical abundance estimates for these waters. We estimated that between 33 and 442 California sea lions were found in Puget Sound/Hood Canal in different seasons, with nearly 3,000 being found in the broader inland Washington waters in the peak season (spring). Steller sea lions occurred in much smaller numbers, with a peak of 219 animals in Puget Sound/Hood Canal/Strait of Juan de Fuca in autumn (and possibly as many as 600 to 700 in the entire study area). While some estimates suffer from low precision, this study demonstrates that substantial numbers of sea lions use waters of the study area throughout much of the year. Our results provide an important step toward a better understanding of these two species in the inland waters of Washington, as well as their potential effects on protected salmonid prey species.

A review on the threatened species of snow trout Schizothorax richardsonii Gray, 1832 (Cypriniformes: Cyprinidae): From the climate change and conservation perspectives

Snow trout (Schizothorax richardsonii Gray, 1832) occupies the Himalayan domain from the foothills to 3000 m and inhabits all types of water bodies, like rivers, rivulets, lakes, and reservoirs. It is a rheophilic fish species that can thrive in a wide range of ecological and climatic conditions. Globally snow trout is now a 'threatened' species. Invasion of its natural habitats by exotic trouts, river valley hydropower projects as well as destructive fishing in the Himalayan region have long been attributed to the vulnerability of this species. The intrinsic characteristics of snow trout also make it a weak competitor in the Himalayan rivers. Climate change coupling with above mentioned threats poses a new challenge for Himalayan fish, especially snow trout, as it is confined to the Himalaya. Given these threats, an intense drive for snow trout conservation is urgently required. This contribution is an attempt to find out solution to this fact. In line with the opinions of ecologists and conservationists, we suggest a few measures like setting up free-flowing and undammed rivers as 'fish conservation zones', environmental flow to downstream areas of existing dams, prohibition of stocking of exotic fish species in natural habitats and use of hatcheries for stocking and restocking of snow trout in the rivers for the conservation of the species. Keywords: Climate change, Conservation, Himalaya, Hatcheries, Snow trout, Threats, Vulnerable

Origin and History of Introduction of Rainbow Trout, Oncorhynchus mykiss (Walbaum, 1792) Stocks in Southern India As Inferred From Y-linked Marker

Rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), is a popular cold-water fish widely distributed and farmed globally. In the nineteenth century, rainbow trout were introduced into India, and since then, it has gradually spread and established itself as one of the most prevalent non-native fish. In Southern India, rainbow trout were introduced from New Zealand in 1909 in the Ooty region of the Western Ghats, followed by introductions in other coldwater regions in the Peninsular uplands such as Munnar and Kodaikanal. Continuous introductions were done from various geographical locations at different periods to increase O. mykiss stocks in the Peninsular upland regions. Despite being regularly introduced in the streams of Munnar, Ooty, and Kodaikanal of Southern India, they still have not become self-sustaining, and genetic diversity has been suggested as a potential underlying factor. Therefore, this study aimed to resolve these questions and explore the structure and origin of Southern Indian stocks by conducting a population genetic study. The Y-linked marker of the trout stocks sampled from Munnar, Ooty, and Kodaikanal was compared with that of the native populations from North America. The results showed less proportion of inter-population genetic variation, suggesting that Southern Indian stocks were derived from multiple origins of population, with a great majority of parental populations belonging to the coastal rainbow trout from North America. This study revealed no considerable genetic differences among the Southern Indian stocks and reported the major ecotype as the Steelhead trout, Oncorhynchus mykiss irideus (Gibbons, 1955).

Limited contribution of hatchery-produced individuals to the sustainment of wild marble trout (Salmo marmoratus Cuvier, 1829) in an Alpine basin

Fish stocking constitutes a common management practice in freshwaters all over the world, to enhance fisheries or to support threatened fish populations. Pervasive detrimental effects may affect the real effectiveness of stocking programs. However, studies assessing the real impacts and relative contribution of stocked trout in wild populations are surprisingly few.The marble trout, Salmo marmoratus (Cuvier 1829), is a critically endangered sub-endemic salmonid in Northern Italy, and an iconic species for recreational fishing and conservation, also representing an emblematic case of species negatively affected by restocking. For instance, marble trout inhabiting the Toce River, the second largest tributary of Lake Maggiore, has been stocked with different hatchery congener trout belonging to the Salmo trutta complex (putative marble trout, Atlantic trout Salmo trutta Linnaeus 1758 and putative Mediterranean trout Salmo ghigii Pomini 1941) over the last decades. Using mitochondrial (D-loop) and nuclear (12 microsatellites and LDH-C1*) markers, we characterised the genetic variability and gene flow among the wild and hatchery individuals of marble trout of this basin, to investigate the effectiveness of stocking activities on the native residual population.Despite extensive hybridization of marble trout with non-native brown trout stocks was shown, the presence of individuals belonging to pure native stock has been detected as well. However, concerns could be advanced regarding its future persistence, due to climatic and hydraulic instabilities or loss of environmental heterogeneity. Moreover, despite ongoing yearly massive stocking activities, a negligible contribution of reared putative marble trout in the wild sample has been documented, suggesting that natural recruitment represents the greatest source of this wild population sustainment. Important adaptive differences between wild and domestic trout are present, likely due to the deleterious long-term effects of the close breeding system of hatcheries. Finally, possible implications for stocking management improvement have been discussed.

Determination of Hatching Performance, Yolk-Sac Absorption, and Larval Growth Rates in Abant Trout (Salmo trutta abanticus), Brook Trout (Salvelinus fontinalis), and their Hybrids

In this study, hatching performance, yolk-sac absorption, and growth rates of Abant trout (Salmo trutta abanticus), brook trout (Salvelinus fontinalis), and their hybrids were investigated. In this contex, relationships between duration (days) and total larvae length, total larvae wet weight, dry larvae body weight and yolk-sac weights, and larval growth were determined. At the end of the study, eye pigmentation, hatching, and larval survival rates were defined as 99.50%, 97.68%, 98.97% for Abant trout, 87.35%, 68.97%, 71.54% for brook trout, 96.12%, 64.18%, 56.82% for S.t.a. ♀ x S.f. ♂ hybrid and for S.f. ♀ x S.t.a. ♂ hybrid 14.98%, 0.45%, respectively. The results indicated there were significant linear relationships between degree-day and length, total wet weight, dry body weight, and dry yolk-sac weight in Abant trout, brook trout, and their hybrids. This relationship has been demonstrated as decreasing in the yolk-sac and as increasing in others. As a result of crossing, it was observed that the hatching efficiency of hybrid (S.t.a.♀ x S.f.♂) was similar to brook trout, and growth performance was similar to Abant trout. The results will help further studies and will contribute to the management practices of the stocks of Abant trout, brook trout, and their hybrids.

Comparing dominance relationships and movement of native marble trout (Salmo marmoratus) and introduced rainbow trout (Oncorhynchus mykiss).

Behaviour observations of the endangered native marble trout (Salmo marmoratus, Cuvier, 1829) and introduced rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) in the laboratory and in a tributary to the Idrijca River in Slovenia were combined to study the movements and dominance relationships between individuals of the two species in an open field test. Under laboratory conditions, no difference between the species was detected for neither time spent actively or distance moved. In species paired tests, rainbow trout initiated more aggressive behaviours towards marble trout than vice versa, and rainbow trout were clearly the dominant individuals. After simultaneous release in the river, marble trout immediately left the release area and spent twice as long time as rainbow trout until they settled in an area of the river; therefore, the release site was immediately occupied exclusively by rainbow trout. Thus, the dominant and aggressive behaviour of rainbow trout seen in the laboratory before release might have influenced marble trout's subsequent behaviour in the river, by marble trout leaving the areas occupied by rainbow trout and moving to locations further away from the release site. In the field, the marble trout occupied sites individually, whereas rainbow aggregated at a few locations. Rainbow trout showed higher movement activity in the morning compared to marble trout. There was a positive correlation between swimming speed in the laboratory and movement in the field for marble trout but not for rainbow trout. In conclusion, the results in this study support the need to end stocking of rainbow trout in rivers with native marble trout. To better understand the interaction between the species, and to develop efficient management plans to protect the native marble trout, reference behaviours should first be understood, and future research in sites where the two species do not co-exist is needed. This is especially important for marble trout for which behavioural research and data are lacking.

Post-release effects of catch and release angling for sea trout: Mortality, growth and wound healing

Sea trout (anadromous brown trout, Salmo trutta L.) is a popular recreational angling target species in the Baltic Sea region and beyond. Across countries, a substantial part of angled sea trout is released after capture either due to management regulations or due to voluntary decisions by the anglers. However, information about post-release impacts of catch and release (C&R) on survival and growth in saline waters is largely missing. We used a flow-through seawater raceway (4–10 °C) to investigate impacts on survival and growth of angled and released sea trout (< 40 cm) up to 29 days post release. Bleeding was common among angled sea trout, but differed between angling treatments, i.e., lure fishing with treble hook (size 4), lure fishing with single hook (size 1/0) and fly fishing with single hook (size 12). However, no mortality and no significant differences in growth were found after a 26–29 days monitoring period among a control group and the three treatment groups, but a small subset (6%) of the angled fish caught on lure had unhealed/infected hooking wounds at the end of the experiment. The results infer that adverse effects of C&R on coastal sea trout due to high post-release mortality or reduced growth can be limited and may not pose a significant problem for sea trout stocks. Nevertheless, further studies are required to corroborate these results under more natural field conditions and at higher water temperatures. In addition, further studies on long-term sublethal impacts of C&R on sea trout are needed.

Factors Influencing Early Growth of Juvenile Tiger Trout Stocked into Subalpine Lakes as Biocontrol and to Enhance Recreational Angling

Tiger trout (Salmo trutta × Salvelinus fontinalis) are sterile hybrids often stocked as a biocontrol agent for undesirable fishes and to enhance recreational angling. Yet, how different ecological processes affect their post-stocking performance remain poorly understood. Rapid growth early in life can foster rapid transitions to piscivory, and improve survival. Identifying factors that benefit early growth can help managers optimize tiger trout stocking for meeting multiple fisheries management objectives. Here, we characterized the trophic ecology and growth of tiger trout stocked at varying densities into seven lentic subalpine systems in Colorado, USA. Study systems supported different species of undesirable fish (e.g., minnows or suckers). We used stable isotopes of carbon and nitrogen to quantify trophic relationships among tiger trout and other fishes in each system. We then evaluated several system-specific attributes as predictors for the size of tiger trout at age-1 using a Random Forest model. Stable isotopes demonstrated the potential for resource competition among tiger trout and other fishes, but potential varied by system. Indices of resource competition ranked highest in the Random Forest model, but the stocking density of tiger trout was most important, suggesting that intraspecific competition outweighed interspecific competition in driving early growth. These processes were mediated by system productivity. Thus, stocking density in combination with the realized carrying capacities of systems should be considered when making management decisions for tiger trout.

What you see isn’t always what you get: On how anglers’ fish stock perceptions are influenced by motivations, satisfaction and engagement

Recreational anglers collectively spend a great amount of time on fishing activities. While being out fishing, these anglers perceive various components of aquatic ecosystems, allowing them to accumulate knowledge of the ecological status of the system and the fish they are targeting in it. Such fishers’ ecological knowledge (FEK) is increasingly being recognized as a valuable source of information in fisheries research and management, but it has also been criticized for a lack of accuracy. People’s perceptions of their environment are shaped by a variety of psychological and environmental factors, and the same is expected for anglers’ perceptions of fish stocks and the resulting FEK. This study assessed anglers’ perceived change in salmon and sea trout stocks at the Mörrum river in Sweden, comparing the year that they first fished there to the present (2019). In addition, effects on these perceptions of the anglers’ motivations to fish, satisfaction with catches, and their engagement with this fishery were examined. The majority of the respondents perceived a negative trend for both species, even when the reported total catches during the year they first fished at Mörrum were lower than the present catches. Binomial regression models indicated significant influences of engagement at Mörrum on the anglers’ stock trend perceptions. The results of this study emphasize the importance of understanding different individual angler characteristics when collecting and interpreting FEK. Moreover, the fact that anglers with a longer history at Mörrum had a more negative perception overall, regardless of actual stock status in their first year in this fishery, suggests the occurrence of the shifting baseline syndrome in recreational fisheries. Therefore, the prevailing narrative on stock trends and its effect on angler perceptions should be considered when using FEK.

Effect of Bag Limits on Angler Harvest, Catch Rates, and Satisfaction at Put‐and‐Take Community Pond Fisheries

AbstractCommunity pond fisheries are valuable recreational resources for anglers because of their ease of access and often‐regular stocking frequency. Effective management of these fisheries requires an understanding of how bag limits, stocking frequency, stocking density, and angler effort affect angler satisfaction and catch rates. We released tagged, catchable‐sized Rainbow Trout Oncorhynchus mykiss and conducted creel surveys at Idaho ponds with either two‐ or six‐trout daily bag limits (hereafter, “two‐trout ponds” and “six‐trout ponds”) to evaluate whether the bag limit affected catch rates between stocking events. Angler catch rates averaged 0.53 fish/h at two‐trout ponds and 0.76 fish/h at six‐trout ponds. Catch rates declined in the days after trout stocking but did so at similar rates in ponds with different bag limits. Catch rates were negatively affected by air temperatures throughout the study. Fifty‐four percent of anglers reported being somewhat satisfied or very satisfied with their fishing experience at two‐trout ponds compared to 64% of anglers at six‐trout ponds. This measure of satisfaction was positively related to an individual's catch rates. Despite the threefold difference in daily bag limits, 57% of anglers harvested two or fewer fish at six‐trout ponds. Angler‐reported tags indicated that stocked trout remained at large longer (mean = 29 d) in two‐trout ponds compared to six‐trout ponds (mean = 17 d). Tag returns suggested that on average, total use (harvested trout plus caught‐and‐released trout) was much higher at six‐trout ponds (57%) compared to two‐trout ponds (34%). Although the recruitment potential and accessibility of community ponds are well known, our results suggest that catch‐related outcomes of community pond management are a complex result of environmental factors, fisheries regulations, and fish stocking variables.