Trout Density Research Articles

Declines of Juvenile Coastal Cutthroat Trout and Coho Salmon over Fifteen Years in a Salmon-Bearing Stream in the Salish Sea

Pacific salmon are ecologically, economically, and culturally important species indigenous to British Columbia, Canada. Unfortunately, some populations of Pacific salmon have been declining due to climate change, habitat loss, overfishing, and anthropogenic development. As such, considerable resources have been invested to study, restore, protect, and monitor Pacific salmon and their habitat. Since 2004, Lyall Creek, a salmon-bearing stream in the Salish Sea in British Columbia, Canada, has been monitored. Using 2-pass electrofishing, we investigated changes in observed densities over 15 years (2004–2019) for juvenile coho salmon (Oncorhynchus kisutch) and juvenile coastal cutthroat trout (Oncorhynchus clarkii clarkii) that inhabit Lyall Creek. A statistically insignificant but general decreasing trend was observed for juvenile coho salmon densities over 15 years in Lyall Creek. Juvenile coastal cutthroat trout densities in Lyall Creek exhibited a statistically significant decline. More research is required to understand what anthropogenic and environmental factors are negatively influencing Pacific salmon densities in Lyall Creek. Specifically, water availability may be an important stressor affecting these salmon populations.

Size, connectivity and edge effects of stream habitats explain spatio-temporal variation in brown trout (Salmo trutta) density.

Ecological theory postulates that the size and isolation of habitat patches impact the colonization/extinction dynamics that determine community species richness and population persistence. Given the key role of lotic habitats for life-history completion in rheophilic fish, evaluating how the distribution of swift-flowing habitats affects the abundance and dynamics of subpopulations is essential. Using extensive electrofishing data, we show that merging island biogeography with meta-population theory, where lotic habitats are considered as islands in a lentic matrix, can explain spatio-temporal variation in occurrence and density of brown trout (Salmo trutta). Subpopulations in larger and less isolated lotic habitat patches had higher average densities and smaller between-year density fluctuations. Larger lotic habitat patches also had a lower predicted risk of excessive zero-catches, indicative of lower extinction risk. Trout density further increased with distance from the edge of adjacent lentic habitats with predator (Esox lucius) presence, suggesting that edge- and matrix-related mortality contributes to the observed patterns. These results can inform the prioritization of sites for habitat restoration, dam removal and reintroduction by highlighting the role of suitable habitat size and connectivity in population abundance and stability for riverine fish populations.

Combining expert‐based and computational approaches to design protected river networks under climate change

AbstractAimEstimate the current and future distribution of brown trout and identify priority areas for conservation of the species.LocationRhône River basin and Mediterranean streams.MethodsWe first developed a spatially explicit species distribution model to estimate the current and future distribution of brown trout for three time horizons (2030, 2055 and 2080) and two climate change scenarios (RCP 4.5 and RCP 8.5). We then performed a prioritization analysis to identify priority areas for brown trout conservation, accounting for: (a) spatial dependencies along the riverine system, (b) several sources of uncertainty arising from climate‐related forecasts and (c) different protected area scenarios by comparing hypothetical, optimal protected networks to an actual protected network designed by regional fish experts.ResultsFuture projections of brown trout densities exhibited a general trend towards a gradual range contraction, with a significant risk of extirpation across mountainous regions of low to mid‐elevation. Overall, the projected current and future distributions were well‐covered by the existing protected network. In addition, up to 70% of the river reaches included in this expert‐based protection network were also priorities in the optimal priority set (e.g. the best set of areas to maximize biodiversity protection). Finally, a large proportion of these reaches were invariably identified regardless of climate change scenarios and uncertainties or spatial dependencies.Main conclusionsOur analytical approach highlighted priority areas for brown trout conservation which were robust to a set of climate and connectivity assumptions. This core priority network could be further refined by taking into account key fine‐scale processes like thermal refugia. Therefore, we advocate for combining computational and expert‐based approaches in conservation planning of riverine ecosystems to achieve a relevant consensus between regional‐scale management and fine‐grain ecological knowledge.

Habitat restoration for brown trout (Salmo trutta) has limited effects on macroinvertebrate communities in a historically metal-contaminated stream.

Quantifying the success of stream remediation or restoration projects that are designed to improve water quality or habitat, respectively, is often challenging because of insufficient posttreatment monitoring, poorly defined restoration goals, and failure to consider fundamental aspects of ecological theory. We measured the effects of habitat restoration on aquatic and terrestrial prey resources in a system recovering from the long-term effects of mining pollution. The study was conducted in the Upper Arkansas River, a Rocky Mountain stream located in central Colorado, USA. Remediation of California Gulch, a United States Environmental Protection Agency (USEPA) Superfund Site that discharged metals from past mining operations into the stream, was completed in 2000, resulting in significant improvements in water quality, benthic macroinvertebrate communities, and brown trout (Salmo trutta) populations. A large-scale restoration project designed to improve habitat and increase the density and biomass of brown trout was completed in 2014. To assess the effectiveness of these habitat improvements on invertebrate communities in this system, we sampled sites for 9 years before (2010-2014) and after (2015-2018) restoration was completed. In contrast to our expectations, we observed few changes in the abundance of aquatic or terrestrial invertebrates after restoration. The most common response was an overall reduction in abundance resulting from significant instream disturbances during and immediately after restoration, followed by a gradual return to pretreatment conditions. Despite reductions in prey abundance, the number of prey items in the diet of brown trout increased significantly after restoration. We discuss several explanations for these responses, including the effects of residual metals, increased predation by brown trout, and the recalcitrance of novel communities dominated by metal-tolerant species. Our results suggest that the effectiveness of remediation and restoration differed between macroinvertebrates and fish. Benthic macroinvertebrates were more dependent on water quality improvements at the watershed scale, whereas brown trout populations responded to both improvements in water quality and reach-scale improvements in habitat.Integr Environ Assess Manag 2022;18:1047-1055. © 2021 SETAC.

Influence of the Landscape Template on Chemical and Physical Habitat for Brown Trout Within a Boreal Stream Network

We used the distribution of stream-dwelling brown trout (Salmo trutta) in a 67 km2 boreal catchment to explore the importance of environmental organizing factors at a range of spatial scales, including whole-catchment characteristics derived from map data, and stream reach chemical and physical characteristics. Brown trout were not observed at any sites characterized by pH < 5.0 during the spring snowmelt episode, matching published toxicity thresholds. Brown trout distributions were patchy even in less acidic regions of the stream network, positively associated with glaciofluvial substrate and negatively associated with fine sand/silty sediments. A multivariate model including only whole-catchment characteristics explained 43% of the variation in brown trout densities, while models with local site physical habitat characteristics or local stream chemistry explained 33 and 25%, respectively. At the stream reach scale, physical habitat apparently played a primary role in organizing brown trout distributions in this stream network, with acidity placing an additional restriction by excluding brown trout from acidic headwater streams. Much of the strength of the catchment characteristics-fish association could be explained by the correlation of catchment-scale landscape characteristics with local stream chemistry and site physical characteristics. These results, consistent with the concept of multiple hierarchical environmental filters regulating the distribution of this fish species, underline the importance of considering a range of spatial scales and both physical and chemical environments when attempting to manage or restore streams for brown trout.

Modeling spatial correlation that grows on trees, with a stream network application

Spatial data on a network, like spatial data on a Euclidean domain, may exhibit nonstationarity. This article develops two classes of nonstationary models for continuously indexed data on directed tree networks, such as stream networks, that are adaptations of models used previously for nonstationary temporal or spatial data on Euclidean domains. These classes, called elastic models and spatially varying moving average models, allow the spatial dependence between observations at sites any fixed distance apart to grow monotonically as one moves either up or down the network. The process variance, or components thereof, may also be allowed to grow monotonically. An example of trout density data from a stream network in Wyoming, USA indicates that the proposed nonstationary models fit those data much better than their existing stationary or quasi-stationary counterparts.

Fish community and abundance response to improved connectivity and more natural hydromorphology in a post-industrial subcatchment

Barrier removal and fish pass construction are increasingly used as tools to restore river connectivity and improve habitat quality, but the effectiveness of subcatchment-scale connectivity restoration on recovery of fish communities is poorly understood. We used a before-after-downstream-upstream methodology to determine the effects of subcatchment-scale connectivity restoration on fishes in a fragmented tributary of the River Wear, Northeast England, between 2013 and 2019. Following restoration (three barriers removed, five barriers fitted with fish passes, two barriers unaltered), riffle habitat increased, fine sediment decreased, and most fish species benefitted. Total fish abundance, comprising seven native species, increased 3 years after the restoration and remained elevated to the end of the study. Mean brown trout (Salmo trutta) density increased from 20.9 ± 6.3 to 33.8 ± 16.8 per 100m2 from 2013 to 2019, with Young-of-Year trout increasing from 10.6 ± 4.6 to 19.8 ± 11.8 per 100m2. Connectivity restoration reduced the mean age of trout, suggesting a change to an increased migratory component of the population. Density of bullhead (Cottus perifretum), a species with poor dispersal ability, increased from 4.6 ± 2.7 to 32.6 ± 17.9 per 100m2 over 2013 to 2019. Stone loach (Barbatula barbatula), also a less mobile species but tolerant to fine sediment, decreased in abundance where barriers were removed. Atlantic salmon (Salmo salar) were absent over the study timescale, despite being common in the Wear, and despite suitable habitat and water quality in the restored subcatchment, suggesting a hysteresis effect. Our findings indicate that, where good water quality exists, restoring river connectivity and hydromorphology at a subcatchment scale is beneficial for most native resident and migratory fishes. However, the ecological benefits of connectivity restoration, especially in rivers with many barriers, may take several years to develop. We encourage well-controlled long-term studies reporting the outcomes of large-scale connectivity restoration.

Long-term effects and cost-benefit analysis of eight spawning gravel augmentations for Atlantic salmon and Brown trout in Norway

River regulation alters flow and sediment regime, habitat availability, and ultimately the ecological functioning of rivers. Various restoration and mitigation measures have been developed to improve ecological function, and among them is the addition of gravel to enhance the reproduction of gravel bed spawning fishes. However, information on long-term efficiency, costs, and maintenance needs of gravel additions are scarce. Here, we study the functioning of gravel additions at eight sites in three rivers in western Norway for up to 18 years. Gravel was added between 2002 and 2010 to enhance spawning of Atlantic salmon and anadromous Brown trout. We monitored changes in the size of the gravel-covered areas, sediment composition, interstitial oxygen, egg survival, and juvenile fish densities. Additionally, we report monetary costs, identify potential maintenance needs, and calculate a cost-benefit ratio. Juvenile densities of Atlantic salmon and Brown trout increased significantly after the gravel augmentations. After 10–18 years, the median egg survival was still high (> 90%) and sediment conditions were still suitable for salmonid fish reproduction. The areas were, however, shrinking across time (median area reduction 26%), mostly caused by scouring of gravel in the steep, supply-limited, and partly regulated rivers. The average construction costs of spawning gravel augmentations were 11.2 NOK (1.12 €) m−2 year−1. Compared to similar measures elsewhere, the measures have had a long life span (up to 18 years) at relatively low costs. Gravel augmentation was concluded to be a successful management measure that contributed to significantly increased Atlantic salmon and Brown trout reproduction.

Season‐specific survival rates and densities of coastal cutthroat trout across stream sizes in southwestern British Columbia

AbstractVariation in seasonal survival rates, densities and growth rates of coastal cutthroat trout (Oncorhynchus clarkii clarkii) were assessed across a size gradient of small, forested streams in the Pacific Northwest. We used a robust, mark‐recapture study, stratified seasonally to estimate monthly survival rates of trout in coastal British Columbia (not including young‐of‐the‐year). Survival estimates showed that the summer season had the lowest monthly survival rates (0.907) across all streams in our study (0.927 remainder of year). Within the size range of the seven small streams studied, low‐flow habitat availability (defined by residual pool depth in summer) was the best predictor of mean monthly survival rates, supporting the hypothesis that trout survival increases with the quantity of aquatic habitat, particularly depths of residual pools. In addition, there was an asymptotic relation between water depth and survival rates, where beyond ~20 cm of residual pool depth, greater depth did not confer greater rates of trout survival. Growth rates in all but the largest stream were also lowest during summer. While densities tended to be higher in streams with greater residual depth, this was not significant. Body mass in a given season was a good predictor of survival to the next sampling period. The distribution and success of resident cutthroat trout populations in small streams appear to be constrained by summer low‐flow periods and specific geomorphologies that support deeper pools.

Comparison of Brook Trout Abundance and Distribution in Vermont's Streams between the 1950s and the 2000s

Salvelinus fontinalis (Mitchill) (Brook Trout) abundance and distribution have declined in much of their native range in the eastern United States, with most intact habitats concentrated in the northern states. We used stream electrofishing data from the 1950s (1952 to 1960) and the 2000s (2005 to 2016) to quantify changes in the abundance and distribution of Brook Trout in Vermont over the last half century. We also investigated whether Brook Trout's co-occurrence with Salmo trutta (Brown Trout) and Oncorhynchus mykiss (Rainbow Trout) in the 1950s influenced their abundance in the 2000s and, further, whether or not the distribution or abundance of non-native trout has changed appreciably since the 1950s. Overall, Brook Trout abundance was higher in the 2000s than in the 1950s. Increased abundance was driven by increases in young of year (YOY), which was not related to the occurrence of non-native trout. Although present at fewer sites in the 2000s, non-native trout densities remained unchanged. Brook Trout distribution at the HUC-12 watershed scale was essentially identical during both time periods. This study confirms previous modeling efforts, which indicated that Brook Trout have a higher probability of persisting in higher latitude, higher elevation, and less-developed regions.

Rapid response of fish and aquatic habitat to removal of a tidal barrier

Abstract River barrier removal is used increasingly as a conservation tool to restore lotic habitat and river connectivity, but evidence of its efficacy is incomplete. This study used a before–after methodology to determine the effects of removing a tidal‐limit barrier on the fishes, macroinvertebrates, and habitats of an English coastal stream. Following barrier removal, habitat diversity increased immediately upstream and remained similar downstream. Mobilized silt altered the substrate composition immediately downstream, but this was temporary as silt was flushed out the following winter. Changes to macroinvertebrate communities occurred upstream and downstream of the former barrier but these were transient. A dramatic and sustained increase in fish density occurred immediately upstream of the barrier after its removal, but effects downstream were minor. The fish community upstream changed, largely due to rapid recruitment and dispersal of endangered European eel (Anguilla anguilla). Eel density in the formerly impounded zone increased from 0.5 per 100 m2 before barrier removal to 32.5 per 100 m2 5 months after removal. By 17 months after barrier removal there was no difference in eel density across the six sections sampled. Although resident stream fishes such as bullhead (Cottus gobio species complex, protected under the European Habitats Directive) were abundant in middle and upper‐stream sections, brown trout (Salmo trutta, a listed species for biodiversity conservation in England and Wales) density remained low during the study and recruitment was poor. This suggests that although colonization access for anadromous trout was available, habitat upstream may have been unsuitable for reproduction, indicating that wider catchment management is required to complement the restoration of connectivity. These findings suggest that tidal barrier removal is an effective method of restoring lotic habitats and connectivity, and can be beneficial for resident and migratory fishes including those of conservation importance (e.g. European eel) in coastal streams.

Current and projected impacts of the parasite Tetracapsuloides bryosalmonae (causative to proliferative kidney disease) on Central European salmonid populations under predicted climate change

Abstract Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is an emerging salmonid disease implicated in recent declines in salmonid populations. Laboratory experiments have shown that the clinical symptoms of PKD exacerbate with increasing temperature. However, empirical evidence for a relationship between climate change and PKD driven declines in wild salmonid stocks is scarce. The current study uses both empirical data and ecological niche modelling to extrapolate future changes in temperature and precipitation on the spread of PKD in relation to changes in suitability of rivers for its primary bryozoan and secondary salmonid hosts. A 20‐year dataset on brown trout density using standardised method shows a decline of more than 50% in the population of the river Wutach, south‐western Germany at 536–755 m elevation. The decline coincides with higher prevalence of PKD. This decline was temperature related and driven by reduced local survival of parr to yearlings. Kidney hyperplasia was highest at low elevations and correlated negatively with trout density. Niche models based on state‐wide data show strong overlap in the areas suitable for T. bryosalmonae and its primary bryozoan hosts, and a strong link with temperature. Projections based on moderate to high emission‐case climate change scenarios predict a 50%–82% decrease in the area suitable for salmonids within this century, linked to PKD. The empirical data identify temperature‐mediated PKD as the underlying cause of the population decline. This relationship was corroborated by the niche modelling results. The highest losses are expected in projected salmonid suitable areas where salmonid habitats currently overlap with areas suitable for T. bryosalmonae. With ongoing climate change, both current and future populations are at severe risk to T. bryosalmonae and its associated disease PKD. Conservation efforts of salmonid stocks are dependent on climate mitigation and measures to help salmonids adapt to the disease.

What are the best upland river characteristics for glass eel restocking practice?

The fitness of restocked European eel (Anguilla anguilla), an endangered fish species, was studied in relation to the environmental variables of habitats in six upland rivers that are typologically different in terms of their hydromorphological and physicochemical characteristics, food resources and fish communities. These rivers received a total of 76,370 imported glass eels in 2017. During a three-year period, we monitored eels with respect to total length, annual growth rate, condition factor and density using capture-mark-recapture experiments to understand the effects of the characteristics of receiving rivers on restocking success levels. Our results showed the survival of the restocked eels in the six rivers and revealed significant differences between them in terms of total length, condition factor and density. Better performance in eel yield variables was observed in a eutrophic alkaline river with greater roughness of riverbed substrates, dominant pool- and riffle-type flow facies and lower brown trout density. The variables conductivity and total hardness had higher explanatory power and were strongly associated with increased eel density. This study suggests that a well-selected habitat/river in a restocking programme can be beneficial for the species and recommends restocking practice as a management tool to achieve eel conservation goals.

Factors Associated with Detection and Distribution of Native Brook Trout and Introduced Brown Trout in the Driftless Area of Iowa

AbstractFish populations at the fringe of their geographic range often inhabit marginal habitats and are vulnerable to ecological disturbances, such as species invasion, land cover conversion, and climate change. The Driftless Area of northeastern Iowa, USA represents the southwestern edge of the native range of Brook Trout Salvelinus fontinalis, where endemic populations have been greatly reduced due to habitat degradation and introduced Brown Trout Salmo trutta. Therefore, documenting Brook Trout occurrence patterns at the southern tip of their range may prove useful for the conservation of peripheral populations. We fitted single‐species occupancy models at multiple spatial scales to estimate the relative effects of biotic (e.g., Brown Trout density) and habitat (instream, riparian, and watershed) variables on Brook and Brown Trout occupancy to predict their current distribution in the Iowa Driftless Area. Species occurrence and physical habitat data were collected at 138 stream segments of the Upper Iowa, Yellow, and Little Maquoketa River watersheds (8‐digit hydrologic unit codes) during May–September in 2018 and 2019. Brook Trout occupancy was low (19 of 138 sites) and affected by local (instream and riparian) habitat covariates, particularly August–September stream temperatures, but no adverse effects of Brown Trout density were detected. Brown Trout occupied 54% of streams (74 of 138 sites), and occupancy was influenced by a combination of local‐scale (mean stream velocity, percentage of run macrohabitat, and August–September stream temperatures) and catchment‐scale (percent forested land cover and upstream catchment area) habitat variables. Our results provide evidence that instream thermal conditions are a critical determinant of Brook Trout distribution, while Brown Trout exhibit plasticity in habitat suitability and increased colonization in areas where they are introduced. Given the dependence of Brook Trout on cold stream temperatures, the projected loss of thermally suitable habitats due to climate change may facilitate their replacement by naturalized Brown Trout at the southwestern extent of their range.

The presence of rainbow trout reduces drift of aquatic insects in Yungas streams (Tucumán, Argentina)

The drift or transport of individuals downstream is an important phenomenon of rivers and streams, since most aquatic organisms participate in it at some period of their life cycle. This work describes the assemblage of macroinvertebrate drift from ten sites in eight mountain streams of Tucumán, and evaluates the effect of the presence of rainbow trout (Oncorhynchus mykiss Walbaum), an exotic fish and visual predator, on this fraction of the community. Thirty-one (31) families of drifting aquatic invertebrates were identified, mainly immature insects and to a lesser extent water mites (Acari: Hydrachnidia), oligochaetes (Annelida: Oligochaeta) and nematodes (Nematoda). The richness and diversity of the sites with trout did not differ from the others, except when comparing lower and upper reaches of a same stream. Samples from rivers without trout or with low density of trout did not show significant changes in drift density of macroinvertebrates. In contrast, in rivers with high trout densities, a notable decrease in insect taxa was observed (those with large sized individuals): Baetidae (Ephemeroptera), Gripopterygidae (Plecoptera) and Leptoceridae (Trichoptera). On the contrary, the density of small Diptera, water mites, oligochets and nematodes increased in trout-streams

Controls on Somatic Growth and Population Dynamics of Rainbow Trout

Photo 1: A rainbow trout holding immediately downstream of Glen Canyon Dam in the clearwater Glen Canyon reach of the Colorado River. Photo credit: David Herasimtschuk, Freshwaters Illustrated. Photo 2: Rainbow trout holding over a cobble bar in the clearwater Glen Canyon reach of the Colorado River and feeding on drifting aquatic invertebrates. Photo credit: David Herasimtschuk, Freshwaters Illustrated. Photo 3: Nighttime boat electrofishing was used to capture rainbow trout for tagging. Time-lapse photography captures the path of electrofishing boats on left and right shores. Captured fish were brought to a centrally located processing boat (foreground) to be measured, weighed, tagged, and scanned, and were then returned to their original capture site. Photo credit: Dustin Patar. Photo 4: The Marble Canyon reach of the Colorado River can be very turbid during the summer monsoon season. High turbidity reduced feeding efficiency and growth rates of rainbow trout, which, combined with higher water temperatures and reduced prey production, led to a severe collapse of the population in this reach. Photo credit: Dustin Patar. Photo 5: The confluence of the Little Colorado River (LCR, entering from top of photograph) and the Colorado River. The LCR is an important spawning and rearing area for endangered Humpback Chub. Juvenile Humpback Chub disperse from the LCR into the main stem Colorado River where their survival and growth rates are reduced due to cooler water temperature and competition and predation from non-native rainbow trout. Our sampling reaches IVa and IVb are located in the Colorado River upstream (left side of photograph) and downstream (right side of photograph) of the confluence. Occasional floods from the LCR result in high levels of turbidity downstream of the confluence which reduces growth rates and densities of rainbow trout. Photo credit: Ted Grussing, tedgrussing photography. These photographs illustrate the article “Changes in prey, turbidity, and competition reduce somatic growth and cause the collapse of a fish population” by J. Korman, M.D. Yard, M.C. Dzul, C.B. Yackulic, M.J. Dodrill, B.R. Deemer, and T.A. Kennedy published in Ecological Monographs. https://doi:10.1002/ecm.1427

Remarkable response of native fishes to invasive trout suppression varies with trout density, temperature, and annual hydrology

Recovery of imperiled fishes can be achieved through suppression of invasives, but outcomes may vary with environmental conditions. We studied the response of imperiled desert fishes to an invasive brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) suppression program in a Colorado River tributary, with natural flow and longitudinal variation in thermal characteristics. We investigated trends in fish populations related to suppression and tested hypotheses about the impacts of salmonid densities, hydrologic variation, and spatial–thermal gradients on the distribution and abundance of native fish species using zero-inflated generalized linear mixed effects models. Between 2012 and 2018, salmonids declined 89%, and native fishes increased dramatically (∼480%) once trout suppression surpassed ∼60%. Temperature and trout density were consistently retained in the top models predicting the abundance and distribution of native fishes. The greatest increases occurred in warmer reaches and in years with spring flooding. Surprisingly, given the evolution of native fishes in disturbance-prone systems, intense, monsoon-driven flooding limited native fish recruitment. Applied concertedly, invasive species suppression and efforts to mimic natural flow and thermal regimes may allow rapid and widespread native fish recovery.

Contingent trade-off decisions with feedbacks in cyclical environments: testing alternative theories

Abstract Many animals make contingent decisions, such as when and where to feed, as trade-offs between growth and risk when these vary not only with activity and location but also 1) in cycles such as the daily light cycle and 2) with feedbacks due to competition. Theory can assume an individual decides whether and where to feed, at any point in the light cycle and under any new conditions, by predicting future conditions and maximizing an approximate measure of future fitness. We develop four such theories for stream trout and evaluate them by their ability to reproduce, in an individual-based model, seven patterns observed in real trout. The patterns concern how feeding in four circadian phases—dawn, day, dusk, and night—varies with predation risk, food availability, temperature, trout density, physical habitat, day length, and circadian cycles in food availability. We found that theory must consider the full circadian cycle: decisions at one phase must consider what happens in other phases. Three theories that do so could reproduce almost all the patterns, and their ability to let individuals adapt decisions over time produced higher average fitness than any fixed behavior cycle. Because individuals could adapt by selecting among habitat patches as well as activity, multiple behaviors produced similar fitness. Our most successful theories base selection of habitat and activity at each phase on memory of survival probabilities and growth rates experienced 1) in the three previous phases of the current day or 2) in each phase of several previous days.

Increasing connectivity of Great Lakes tributaries: Interspecific and intraspecific effects on resident brook trout and brown trout populations

AbstractUnderstanding resident fish population responses to restored connectivity would enhance decision‐making on dam removal and fish passage. Since such evaluations are limited in the Great Lakes region of North America, we compared abundance, survival, and growth of resident brook trout and brown trout between sets of Michigan streams where populations were or were not interacting with salmonid species that might be present if connectivity existed. We analysed data from 34 electrofishing index sites to compare resident trout populations between streams without versus with Great Lakes access (and migratory Pacific salmonids), and brook trout populations in Great Lakes inaccessible (land‐locked) streams where brown trout were present versus absent. Great Lakes accessibility effects on fish density became increasingly positive for older age groups of brown trout while generally negative for all age classes of brook trout. Brown trout had consistently negative effects on brook trout density in land‐locked streams. Increased connectivity had significant effects on annual survival for only one of seven trout age classes modelled, while intraspecific density‐dependent effects on survival were significant in six models. Significant intraspecific effects on resident trout growth occurred for seven of eleven age classes examined. Negative interspecific effects of Great Lakes access on resident trout growth were most noticeable for age‐0 and age‐1 resident trout, age classes that likely compete with juvenile Pacific salmonids. Our findings provide a more robust understanding of how Great Lakes connectivity affects resident trout populations, highlighting negative influences of brown trout on brook trout and intraspecific density‐dependent effects.

Population status and ecology of the Salmo trutta complex in an Italian river basin under multiple anthropogenic pressures.

Salmonids inhabiting Mediterranean rivers are of particular concern for biodiversity conservation, as they are threatened by various stressors, including habitat alterations, overfishing, climate change, and introgressive hybridization with alien species. In the Tiber River basin (Central Italy), genetic introgression phenomena of the native Salmo cettii with the non‐native Salmo trutta hinder the separate analysis of the two species, which are both included in the S. trutta complex. Little is known about the factors currently limiting the trout populations in this area, particularly with respect to climate change. With the intention of filling this gap, the aims of the current study were to (a) quantify changes in the climate and (b) analyze the distribution, status, and ecology of trout populations, in the context of changing abiotic conditions over the last decades. Fish stock assessments were carried out by electrofishing during three census periods (1998–2004, 2005–2011, and 2012–2018) at 129 sites. The trend over time of meteorological parameters provided evidence for increased air temperature and decreased rainfall. Multivariate analysis of trout densities and environmental data highlighted the close direct correlation of trout abundance with water quality, altitude, and current speed. Climate‐induced effects observed over time in the sites where trout were sampled have not yet led to local extinctions or distribution shifts, indicating a marked resilience of trout, probably due to the buffering effect of intrinsic population dynamics. Decreasing body conditions over time and unbalanced age structures support the hypothesis that variations in hydraulic regime and water temperature could overcome these compensatory effects, which may lead to a severe decline in trout populations in the near future. In a climate change context, habitat availability plays a key role in the distribution of cold‐water species, which often do not have the possibility to move upstream to reach their thermal optimum because of water scarcity in the upper river stretches.