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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.

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

Before and after relicensing of Pont-Haut hydropower station on the Roizonne (in the French Alps), fish monitoring surveys were conducted over a fifteen-year period, complemented by measurement of discharge and characterization of habitat. A model of the dynamics of the trout population (MODYPOP) was calibrated for the period to analyze the fluctuations in the different age classes upstream and downstream of the power plant. Several hydrological scenarios were developed over a twenty-year period on the basis of random picking from the available daily natural discharge time series. For the bypassed reach, four management modes were reconstituted, with MIF(i) constant throughout the year, (ii) varying in accordance with the discharge upstream of the water intake, and (iii) varying in accordance with the season (high in winter and low in summer, or the opposite). The corresponding habitat time series were reconstituted and compared. The response of the population to these different scenarios and dam operation options was tested with the aid of MODYPOP imulations, combining two hypotheses of population regulation as a function of the habitat: i) density-dependent mortality, which tends to push the population toward a size compatible with the local carrying capacity in a hydrologically typical year under low-flow conditions, and ii) adjustment of the adult biomass to the local carrying capacity using a one-month time step. The results of the simulations of population dynamics are compared with the results obtained on the upstream reach under natural flow conditions. This comparison is helpful to a discussion of the natural flow regime paradigm adopted in global instream flow methodologies that are based essentially on hydrology.

https://hal.archives-ouvertes.fr/hal-00468741/document

Environmental flow regimes based on natural flow regime: myth or reality for trout population dynamics?

Le retour au premier plan de la notion de sante des ecosystemes met en evidence la preoccupation de la societe a mettre en œuvre des mesures pour la protection des milieux anthropises. Les criteres d’evaluation de la sante des peuplements piscicoles mis en œuvre sont rarement debattus au sein de la communaute scientifique. L’exercice propose ici devait permettre d’avoir ce debat sur la truite commune (Salmo trutta ), espece phare des cours d’eau situes en tete de bassin, qui representent la majorite du reseau hydrographique francais. La demarche visait a recueillir la conception qu’avait un nombre restreint d’experts, sur le fonctionnement de ces populations et sur les criteres d’evaluation de leur fonctionnement. Les parametres cle regulant chaque stade de developpement ont ete identifies et hierarchises. Un consensus s’est degage sur le fait que, dans l’etat actuel des connaissances, le diagnostic peut etre etabli sur la base de l’analyse des parametres abiotiques essentiels pour la biologie et, plus difficilement, sur l’analyse des parametres biotiques. Pour tous les stades de developpement, les parametres identifies sont lies a l’habitat (qualite du substrat, conditions hydrauliques, temperature et qualite de l’eau), l’hydrologie et la connectivite. Des connaissances demandent d’etre acquises pour definir des criteres biologiques. Cela implique de renforcer la surveillance biologique a long terme et les actions de recherche pour caracteriser la variabilite des parametres biologiques et identifier les echelles spatio-temporelles pertinentes et les processus fonctionnels.

https://www.hydroecologie.org/articles/hydro/pdf/2014/01/hydro140001.pdf

Véronique Gouraud

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

Environmental flow regimes based on natural flow regime: myth or reality for trout population dynamics?

Sur quelles connaissances se baser pour évaluer l’état de santé des populations de truite commune (Salmo trutta)?