The new tool.

We reviewed 199 published data sets (21 species) to assess the relative frequency of various density‐dependent processes in salmonids. We examined studies for the presence or absence of density‐dependence: in growth, mortality, fecundity and recruitment. Based on data from all studies pooled 71% showed density‐dependence in growth, 23% found density‐dependence in mortality, 2% detected density‐dependence in fecundity and 4% displayed density‐dependence in recruitment. Most studies at the population level (e.g. changes in abundance or means of growth, mortality or fecundity) demonstrated density‐dependence; however, these studies were not common, likely because they require long time series of data (at least 3–4 times mean generation time for the species). Our review indicated there was no evidence that the probability of finding density‐dependent effects differed among the 21 species examined, but the number of studies per species varied substantially (5 [minimum for analysis] – 20+); hence, this finding should be viewed as tentative. Most salmonid species showed evidence of density‐dependence in growth; however, few studies examined density‐dependent effects on fecundity or recruitment, and further study is needed on these phenomena. There was no evidence that density‐dependent effects were restricted to a particular habitat type or geographic region.

https://onlinelibrary.wiley.com/doi/epdf/10.1111/eff.12523

Density‐dependent effects on salmonid populations: A review

Recently, Minto et al. (2008), based on a fishery data set including marine, estuarine, and freshwater fishes, described higher variability in the survival rates of juveniles at low rather than at ...

Recruitment and survival rate variability in fish populations: density-dependent regulation or further evidence of environmental determinants?

Emigration is a fundamental process affecting species’ local, regional, and large-scale dynamics. The paradigmatic view in ecology is that emigration is density independent (DIE) or positiv...

Frequency of Occurrence and Population-Dynamic Consequences of Different Forms of Density-Dependent Emigration.

Alteration of natural river flow regimes is a major threat to freshwater biodiversity. Restoration of natural flow regimes requires an understanding of linked hydrological and ecological processes. In this study, we investigated how annual seasonal flow characteristics and physical habitat attributes could interact to explain yearly changes in brown trout population densities (for 0+ fish, corresponding to the young of the year, and for the >0+ age class) in 112 sites widespread in France. Using an information theoretic approach and general linear modeling, we identified the physical habitat attributes and seasonal hydrological variables which explained 0+ proportion, total, >0+ and 0+ brown trout density dynamics. A decrease in total and 0+ brown trout densities were mainly linked to high water levels in both high and low flows during the emergence period and to a lesser extent, to physical habitat attributes reflecting the river size. 0+ proportions were only linked to the level of low flows, and to a lesser extent to high flows, during the emergence period. Our study demonstrated that seasonality of flow is a main driver of brown trout population dynamics, especially magnitude of high and low flows for the 0+ age class, and should be considered in the definition of environmental flows. This article is protected by copyright. All rights reserved.

Hydrological drivers of brown trout population dynamics in France

Les facteurs environnementaux peuvent causer des fluctuations synchrones de densites entre populations. Une meilleure comprehension des processus expliquant la synchronie est fondamentale pour predire des pertes de resilience des metapopulations sujettes a des changements environnementaux. Nous etudions la synchronie des chroniques de densites de trois classes d’âge de la truite brune (Salmo trutta) (0+, 1+ et adultes) entre 36 troncons de cours d’eau. Nous utilisons des tests de Mantel pour discriminer les effets relatifs de la proximite geographique, de la synchronie de variables environnementales clefs (debit, temperature de l’eau, conditions hydrauliques et mobilite du substrat) et de la dispersion densite-dependante. La synchronie environnementale expliquait fortement la synchronie de la truite jusqu’a des distances de 75 km. Cet effet etait du en partie a l’influence negative sur les 0+ des hauts debits pendant l’emergence et une influence de la mobilite du substrat pendant la periode de ponte. La dispersion entre troncon influencait faiblement nos resultats. Les densites de juveniles et d’adultes etaient fortement structurees par des processus de survie, mais n’etaient pas influencees par la synchronie des conditions environnementales. Les resultats suggerent que l’environnement peut avoir des effets generaux sur la dynamique de population qui peuvent influencer la resilience des metapopulations

/pdf/influence-of-discharge-hydraulics-water-temperature-and-1k4vtunwtt.pdf

Influence of discharge, hydraulics, water temperature, and dispersal on density synchrony in brown trout populations (Salmo trutta)

Successful management and protection of wild animal populations relies on good understanding of their life cycles. Because population dynamics depends on intricate interactions of biological and ecological processes at various scales, new approaches are needed that account for the variability of demographic processes and associated parameters in a hierarchy of spatial scales. A hierarchical Bayesian model for the resident brown trout (Salmo trutta) life cycle was built to assess the relative influence of local and general determinants of mortality. The model was fitted to an extensive data set collected in 40 river reaches, combining abundance and environmental data (hydraulics, water temperature). Density-dependent mortality of juveniles increased at low water temperatures and decreased with shelter availability. High water temperature increased density-dependent mortality in adults. The model could help to predict monthly juvenile and adult mortality under scenarios of global warming and changes in shel...

/pdf/understanding-inter-reach-variation-in-brown-trout-salmo-45c1e12ixl.pdf

Understanding inter-reach variation in brown trout (Salmo trutta) mortality rates using a hierarchical Bayesian state-space model

The key role of hydrological variability in structuring brown trout populations is well established. However, the influence of additional drivers is more difficult to identify. The implementation of long-term monitoring and the development of reliable tools can help to reveal fine local drivers structuring fish populations in contrasted flow regimes. This study used data series for nine reaches monitored for nine to nineteen years in four French salmonid streams. Study reaches were within five bypassed sections influenced by instream flow. A deterministic trout population dynamics model was applied on each reach, with calibration and validation procedures. Results revealed that biological drivers structured all reaches similarly. In addition, seven other drivers were identified. Among these additional drivers, hydrology mainly explained temporal fluctuations in trout density, regardless of reach. Three drivers independent of hydrology were also revealed: poor water quality, limited spawning area and the effect of power plant operations (overtopping, flushing or plant shutdown). All drivers influenced the whole bypassed section and were never limited to the scale of the reach (sampling area). Further analyses of each driver are now needed, to regionalise and quantify their respective impact precisely. Thus, assessment of trout population status would be simplified , enabling implementation of efficient management rules.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/eff.12295

Main potential drivers of trout population dynamics in bypassed stream sections

Investigating the variables driving synchrony in populations and the spatial scales at which they operate is worthwhile for both a basic understanding of population dynamics and for management. Various environmental classifications have been developed to group sites according to predefined similarities in abiotic variables (with spatial and/or temporal components) and to explain population dynamics in running waters. These classifications may be useful to managers and decision makers, as studies have shown relationships between time-dependent population variability and environmental classifications on different spatial scales. We investigated whether synchrony among brown trout (Salmo trutta) populations could be explained by three regional (hydro-ecoregions, hydrological units, and natural flow regime classification) and one local environmental classification and their associated variables. We first analysed whether densities of two size classes (0+ and >0+ fish) varied over a large spatial scale (i.e. 112 sites spread throughout France sampled for >11 years). We then determined whether the degree of synchrony matched the spatial patterns defined by the environmental classifications. Finally, we investigated whether variables such as river distance between sites, hydrology, and temperature were associated with patterns of synchrony. Overall, relationships observed between synchronies in environmental variables and trout population densities were weak. Synchrony was best explained by the classification based on hydrological units, defined as discrete physical units that provided boundaries with real biological significance for river assemblages (12% for 0+ density and 9% for >0+ density and total densities). However, when considering environmental variables alone, synchrony in trout populations was mainly associated (from 11% to 23%) with seasonal flow variables such as high flows during reproductive and emergence periods. Temperature and river distance did not explain synchrony between trout populations. High flows during emergence appear to be a key driver of brown trout population dynamics. Floods during the reproductive period could explain synchrony among brown trout populations, although the effect was weaker than that of high flows during the emergence period. We concluded that existing physical, morphological, and biogeographical classifications at regional spatial scales were not sufficiently integrative to account for environmental drivers (such as flow variables) that were most associated with the spatiotemporal patterns of synchrony in trout populations. Flow explained synchrony better than classifications did. Further investigations should focus on the spatial patterns of flow variations during the emergence and reproductive periods to derive geographical areas that best capture synchrony patterns in trout dynamics. This approach could be extended to other freshwater fish species for which climatic drivers are important, and may help to explain the distributional changes of exotic species, the propensity of populations to fluctuate synchronously and the scale of synchrony patterns under different scenarios of climate change, or the threat of extinction of native species if the Moran effect is high. © 2019 John Wiley and Sons Ltd.

Victor Bret

Papers

Influence of discharge, hydraulics, water temperature, and dispersal on density synchrony in brown trout populations (Salmo trutta)

Understanding inter-reach variation in brown trout (Salmo trutta) mortality rates using a hierarchical Bayesian state-space model

Main potential drivers of trout population dynamics in bypassed stream sections

Similarity in seasonal flow regimes, not regional environmental classifications explain synchrony in brown trout population dynamics in France