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Walleye (Sander vitreus) are an ecologically important species managed for recreational, tribal, and commercial harvest. Walleye prefer cool water and low light conditions, and therefore changing water temperature and clarity potentially impacts walleye habitat and populations across the landscape. Using survey data collected from 1993 to 2018 from 312 lakes in Minnesota, we evaluated the relationship between thermal-optical habitat and the relative abundance of small (0-300 mm), medium (300450 mm), and large (450 + mm) walleye. Thermal-optical habitat was positively correlated with the relative abundance of small and medium walleye but not large walleye. Walleye were more abundant in larger, naturally reproducing lakes opposed to smaller, stocked lakes. Thermal-optical habitat changed in 59% of lakes since 1980 (26% increasing and 33% decreasing) and appears to be driven primarily by changes in water clarity and thus optical habitat area. Our study provides important insights into local and regional drivers that influence walleye populations that can be used to assist fisheries managers in setting population goals and managing harvest. Key words: hierarchical models, walleye, thermal-optical habitat, safe operating space, water clarity, temperature, 1. Introduction In dynamic ecosystems, management regimes that account for fluctuations in habitat availability can increase resilience to disturbance (Hansen et al. 2015). However, managed aquatic resources often encompass broad [...]

Recent advances in the ability to quantify longitudinal connectivity of riverine systems is enabling a better understanding of how connectivity affects fish assemblages. However, the role of connectivity relative to other factors, such as land use, in structuring biological assemblages is just emerging. We assessed the relevance of a structural connectivity index to stream fish communities at a relatively large scale (across five watersheds of Lake Ontario) while controlling for confounding habitat variables, such as land use, elevation, and stream topology. The results were assessed to determine whether species' sensitivities to connectivity are in accordance with expectations of life history. Our results indicated that at large scales, structural connectivity explains significant amounts of variation in community structure (1% to 5.4% as measured by Bray-Curtis similarity), but remains secondary to other habitat components. Connectivity also was significantly related to abundance in three of the seven species assessed. The lower explanatory power of our models compared with studies done at smaller scales suggests that the relevance of connectivity to fish communities is scale-dependent and diminishes relative to other environmental factors at larger spatial extents. Des avancees recentes permettant de mieux quantifier la connectivite longitudinale de systemes fluviaux permettent une meilleure comprehension de l'incidence de la connectivite sur les assemblages de poissons. Toutefois, la comprehension du role de la connectivite dans la structuration des assemblages biologiques comparativement a celui d'autres facteurs comme l'utilisation du sol demeure limitee. Nous avons evalue la pertinence d'un indice de connectivite structurale pour les communautes de poissons de cours d'eau a une echelle relativement grande (couvrant cinq bassins versants du lac Ontario), en tenant compte de variables de confusion associees a l'habitat telles que l'utilisation du sol, l'altitude et la topologie du cours d'eau. Les resultats ont ete evalues afin de determiner si la sensibilite des especes a la connectivite concorde avec les previsions relativement au cycle biologique. Nos resultats indiquent que, a grande echelle, la connectivite structurale explique une portion significative des variations de la structure de la communaute (de 1% a 5,4%, mesuree avec l'indice de similarite de Bray-Curtis), mais que son influence demeure secondaire comparativement a celle d'autres composantes de l'habitat. La connectivite etait aussi significativement reliee a l'abondance pour trois des sept especes evaluees. La puissance explicative plus faible de nos modeles comparativement a des etudes realisees a plus petite echelle donne a penser que l'importance de la connectivite pour les communautes de poissons depend de l'echelle et que, a des echelles plus grandes, elle diminue relativement a celle d'autres facteurs du milieu. [Traduit par la Redaction], Introduction The increased awareness of the effects of anthropogenic structures that may act as barriers on aquatic ecosystems has prompted new research to understand, quantify, and mitigate fragmentation impacts (Fullerton [...]

The production of Atlantic salmon in aquaculture has grown substantially over the last 40 years. The unintentional release of domesticated salmon poses a significant risk to the long‐term persistence of wild Atlantic salmon populations through ecological interactions and genetic introgression. Our ability to link aquaculture production to farmed escaped salmon in rivers is still limited and hinders identifying the appropriate production capacity of salmon aquaculture to reduce unwanted interactions between wild and escaped Atlantic salmon.Here we use a 14‐year dataset of farmed escapee abundance in rivers to model how the a priori selected covariables of wild salmon abundance, aquaculture intensity, river discharge, hydropower and fjord placement of the river affects escapee abundance across 54 rivers in western Norway. Then, we evaluate the predictive strength of the model to provide context for its use to minimize escapees.We found that the abundance of escaped farmed Atlantic salmon in rivers is correlated to aquaculture intensity. Furthermore, the abundance of wild Atlantic salmon, mean yearly discharge, and the interaction between fjord placement and wild salmon abundance were important predictors of escapee abundance in rivers.The model was 40% accurate when predicting the abundance of farmed escaped salmon in rivers. However, the accuracy improved to 75% when using risk categories derived from modelled intrusion rates that induced long‐term genetic changes to the wild population (low < 4%, medium 4%–10%, and high > 10% escaped farmed salmon).Synthesis and applications. This study links aquaculture production, at relevant spatiotemporal scales (75 km from rivers), to the abundance of escaped farmed Atlantic salmon in rivers, and provides governmental agencies with a tool to help regulate domesticated salmon production based on the carrying capacity of the system to buffer against introgression between conspecifics. Furthermore, understanding this relationship will be beneficial when establishing new aquaculture sites in pristine ecosystems where they would overlap with wild Atlantic salmon. Finally, future mitigation efforts should continue to focus on new technologies (e.g. triploid females) that can eliminate the risk of introgression without limiting aquaculture production. [ABSTRACT FROM AUTHOR]

The habitat use of adult Atlantic salmon was evaluated in a fjord complex in western Norway. A representative sample of 9 wild and 29 hatchery origin Atlantic salmon were tagged with acoustic tags and tracked with 28 stationary receivers from June 2012 to river entry. Using a movement-based kernel density estimation, we identified near shore habitat use in relation to a National Salmon Fjord (NSF) and adjoining aquaculture sites. Furthermore, we compared progression rates and vertical distributions among habitat units. Mean residence time for salmon with acoustic tags from marking to river entry was 30 days (range 3–83 days). Moreover, recaptures of individuals with T-bar anchor tags supported the long residency of Atlantic salmon within the fjord (0–63 days). The main habitat use occurred outside the NSF (68%), with 62% of the individuals overlapping with one or more aquaculture sites. Median progression rates ranged from 15 km d−1up to 42 km d−1among the different sections of the fjord with a mean vertical distribution <2 m from the surface. Whereas direct implications from the overlap with aquaculture are unclear, the results illustrate the challenge encountered when evaluating the effectiveness of marine protected areas in close proximity to aquaculture sites. [ABSTRACT FROM AUTHOR]

The restoration of 4 partial stream barriers was evaluated in watersheds of Terra Nova National Park, Newfoundland, Canada from 2009 to 2011. Brook trout (n = 462) were tagged and tracked moving through our study sites using passive‐integrated transponder telemetry and the restoration actions were assessed using 3 different measures: passage success rates; the range of passable flows; and the availability of passable flows. We considered the observed results within a before‐after‐control‐impact (BACI) design that included reference reaches and pre‐restoration observations. The conclusions of BACI analyses were also contrasted with those that would have been obtained from commonly used before‐after (B‐A) or control‐impact (C‐I) study designs. While the restoration actions changed hydrological conditions in a way that should facilitate fish passage, our biological measures indicated that success was variable across culverts and within culverts depending on the measure evaluated. Furthermore, the natural temporal and spatial variability of fish movements often resulted in different conclusions between the more robust BACI design and the more commonly used B‐A and C‐I designs. Our results demonstrate that restoration of partial barriers may not always yield dramatic improvements. Furthermore, without suitable controls, the chances of drawing false conclusions regarding restorations in temporally and spatially dynamic systems are substantial. [ABSTRACT FROM AUTHOR]

The effects of sea lice on the marine survival of wild salmonids are widely debated. In Norway this debate has reached a crescendo as the Norwegian government has recently ratified a management system where the growth in the salmonid aquaculture industry will be conditional on regional estimated impact of salmon lice on wild fish. Sea lice have thus become the most prominent obstacle to the stated political aim of quintupling aquaculture production in Norway by 2050. Scientific documentation that salmon lice impact the marine survival of salmon is robust. However, it is also evident that marine survival of salmon is strongly impacted by other factors, and that the effect of salmon lice is most likely an integral part of these other mortality factors. In this paper, our goal is to discuss and give advice on how managers and policy makers should handle this complexity, and to identify the greatest challenges in using scientific results to construct robust management rules. Inadequate extrapolation from the scale of known effects to the scale of management implementation may initially give a false impression of scientific certainty, but will eventually fuel upsetting disagreements among stakeholders as they gradually uncover the shaky foundation of the implemented policy. Thus, using a single model and parameter to determine management advice is not warranted, as no single data point reflects the natural complexity of nature. Furthermore, robust management rules should be based on unambiguous definitions of key concepts. Finally, despite the scientific consensus that salmon lice are a risk to salmon, studies on wild populations in situ that accurately quantify the impact of salmon lice are still urgently needed. We give advice on how this can be accomplished. [ABSTRACT FROM AUTHOR]

Anthropogenic barriers to fish passage, such as culverts and dams, are major factors impeding the persistence and recovery of aquatic species. Considerable work has focused on mitigating these impacts; however, activities associated with measuring and restoring connectivity of aquatic ecosystems often face challenges in determining the passability of barriers by aquatic species. Hydrological modeling software that incorporates biological aspects of a focal species is often used as a relatively inexpensive method for assessing barrier passability for restoration decisions. However, the biological relevance of these approaches remains to be rigorously tested. We assessed passage rates of PIT-tagged Brook TroutSalvelinus fontinalisthrough four road culverts and adjacent reference sites (unaltered areas of the streams) on the island of Newfoundland to determine whether upstream passage through road culverts was more restrictive than unaltered reference areas of the stream. Next, we examined the usefulness of barrier passability predictions derived from FishXing software by comparing them with in situ movement data for this species. Brook Trout passage for three of the four reference sites had a significantly higher range of passable stream flows compared with that for culverts, indicating the presence of velocity barriers in culverts. However, FishXing predictions of suitable fish passage discharges were conservative, and tagged fish successfully navigated partial barriers that were at least 2–3 times the upper limits of stream flow predicted to allow successful passage. The results of our study show a clear need for an improved understanding of fish movement through these structures so that barrier assessment techniques can be refined. The implications of not doing so may lead to restoration actions that result in limited biological benefit. Received February 6, 2013; accepted July 2, 2013 [ABSTRACT FROM AUTHOR]