Oir River Research Articles

Intra- and interspecific densities shape life-history traits in a salmonid population.

Population dynamics can be regulated through intra- and interspecific density dependence. In species with close ecological requirements, interspecific competition for resources may add to intraspecific density, or even exceed its effect; it may impact single or multiple traits. However, the relative impact of intra- and interspecific densities on demographic parameters has been rarely empirically assessed. We analyzed 18years of capture-mark-recapture data from brown trout (Salmo trutta) coexisting with Atlantic salmon (Salmo salar) during the juvenile freshwater phase in the Oir River (France) to estimate the relative effects of intra- and interspecific density on trout early life. In trout, a species with optional migration, we estimated the migration probability of young-of-the-year trout out of their natal site, survival probability during the first winter, as well as body size, in relation to both intra- and interspecific density. Trout density correlated negatively with body size and with winter survival in resident trout but not with trout migration. Salmon density correlated positively with trout migration, but no impact was detected on trout body size or survival. Our study highlighted contrasting effects of intra- and interspecific density on trout early life, and the need to account for both factors when studying population dynamics in coexisting species. In particular, by affecting trout migration decision, salmon density may drive trout life history.

Factors determining survival of European eels in two unexploited sub‐populations

Summary Estimating accurate age‐specific survival probabilities and understanding the processes (density dependent or independent) that regulate this demographic parameter are fundamental to propose sustainable management options for the endangered European eel (Anguilla anguilla). In the present study, we used extensive mark‐recapture data sets (13 and 17 years) on eel >150 mm to estimate accurate natural survival probabilities of two eel sub‐populations, from the Oir and Frémur rivers, western France, and then we analysed survival probabilities in relation to density‐independent (temperature) and density‐dependent factors to identify those causing survival variability. The Frémur and Oir rivers are two small (<100 km2) river systems in close proximity (65 km apart). The Frémur River is a small river obstructed by dams with medium water quality, high eel recruitment and density (0.37 eel m−2), and a male dominant population; whereas, in the Oir River, low densities of eels were observed (0.04 eel m−2) and the sex ratio was skewed towards females. Furthermore, previous research suggested that the lotic habitats of the Frémur River have reached carrying capacity, whereas habitats in the Oir River are below habitat saturation. In the Frémur River, there were significant spatial and temporal variations in the survival probabilities. However, survival probabilities observed in the Oir River were stable over time and space. The results highlight that the differences in the characteristics of the two systems and the two sub‐populations prompt different responses to regulatory processes. The contrasting pressures applied on these two sub‐populations impact survival, which possibly lead to different life history strategies such as sex differentiation.

New insights regarding gonad development in European eel: evidence for a direct ovarian differentiation

In European eel, it has been proposed that the undifferentiated gonad would develop into either an intersexual stage (Syrski organ) or directly into an ovary. The Syrski organ could then develop into either an ovary or a testis. In the present study, glass eels were raised until they reached a minimum size of 29 cm for histological sex assessment. In addition, some undifferentiated individuals with size encompassing 15-28 cm were sampled in a female-biased population (Oir River). We also investigated aromatase gene expression, which is known to be involved in the process of fish sex differentiation. Gonad histology revealed that intersexual eels were characterized by a small number of oocytes within a predominant testis-like structure. Males were significantly smaller than intersexual eels, which suggests that all males do not necessarily pass through an intermediate intersexual stage. Aromatase transcript levels in intersexual eels gonads and testes were similar but significantly lower than in ovaries and were comparable between ovaries and undifferentiated gonads from the females-biased population. In addition, condition factor was lower in female than in intersexual individuals. Together, these results provide evidence that ovaries would not develop from the Syrski organ.

Assessment of the success of headwater restoration through the vegetation diversity analysis of four Oir river's tributaries (Normandy, France)

Safeguarding of riparian ecosystems is a field of major study in the comprehension and the maintenance of the health of rivers. The aim of our study is to analyze vegetation changes after passive restoration on two headwaters. Follow-ups of vegetation were carried out using 36 permanent plots of 15 m2 per brooks during five years. Our results suggest that after restoration, vegetation assemblages are comparable to those present on other headwaters considered in good ecological health. The effectiveness of the passive restoration method employed is discussed in terms of strong potential of resilience for comprehension of ecological functions and in term of sustainability for river managers.

Communal spawning of brook and river lampreys (Lampetra planeri and L. fluviatilis) is common in the Oir River (France)

Lasne E, Sabatié M‐R, Evanno G. Communal spawning of brook and river lampreys (Lampetra planeri and L. fluviatilis) is common in the Oir River (France). Ecology of Freshwater Fish 2010: 19: 323–325. © 2010 John Wiley & Sons A/SAbstract – The taxonomic status of river Lampreys (Lampetra fluviatilis) and brook lampreys (L. planeri) has been repeatedly questioned due to the high morphological similarity of these species. Here, we report a high frequency (54%) of communal spawning involving both species in the Lampetra redds of a French coastal river. These observations suggest that reproductive isolation between the two species may not be as strong as previously supposed.

Linking migratory patterns and diet to reproductive traits in female brown trout (Salmo trutta L.) by means of stable isotope analysis on ova

Abstract – Many populations of vertebrates display alternative reproductive strategies involving delays in maturation processes and migrations to more productive habitats. Notably for females, migratory patterns and diet could influence reproductive traits such as ova size and number. To test this hypothesis, we targeted anadromous and freshwater‐resident female brown trout of various sizes during their return to spawn in tributaries, and carried out stable isotope analyses (δ15N and δ13C) on ova to identify their feeding areas and diet. The study was performed in the Oir River (France), a small coastal system where trout migrations to sea and within the river basin have been monitored for the last 20 years. General trends were observed in reproductive traits, such as the positive correlation between female fork length, fecundity and ova size. Fecundity was size‐dependent whatever the migratory origin of females, but major divergences were seen for ova size and isotopic ratios. Freshwater residents displayed a constant increase in ova size and a 15N‐enrichment in ova, suggesting an investment in larger ova and a shift to piscivory as the fish become larger and older. Conversely, anadromous females had smaller ova compared with freshwater‐resident females of similar body size and achieved higher fecundity as they grew bigger. Our results suggest the existence of a continuum of reproductive traits for freshwater‐resident females whereas anadromous females clearly show a break with this continuum. This major dissimilarity could be explained by the difference in growing environments in terms of food availability and quality.

Hierarchical Bayesian modelling with habitat and time covariates for estimating riverine fish population size by successive removal method

We present a hierarchical Bayesian modelling (HBM) framework for estimating riverine fish population size from successive removal data via electrofishing. It is applied to the estimation of the population of Atlantic salmon (Salmo salar) juveniles in the Oir River (France). The data set consists of 10 sampling sites sampled by one or two removals over a period of 20 years (1986–2005). We develop and contrast four models to assess the effect of temporal variations and habitat type on the density of fish and the probability of capture. The Bayes factor and the deviance information criterion are used to compare these models. The most credible and parsimonious model is the one that accounts for the effects of the years and the habitat type on the density of fish. It is used to extrapolate the population size in the entire river reach. This paper illustrates that HBM successfully accommodates large but sparse data sets containing poorly informative data for some units. Its conditional structure enables it to borrow strength from data-rich to data-poor units, thus improving the estimations. Predictions of the population size of the entire river reach can be derived, while accounting for all sources of uncertainty.

Genetic differentiation between anadromous and freshwater resident brown trout (Salmo truttaL.): insights obtained from stable isotope analysis

Abstract – A genetic survey was carried out on recently emerging fry to investigate interbreeding between anadromous and freshwater resident brown trout in La Roche Brook, a major spawning tributary of the Oir River system (Normandy, France). Emerging fry were sampled at three different sites in spring 2002 and 2003. Stable isotope analysis (SIA) allowed the classification of each individual as the progeny of resident (N = 76) or anadromous (N = 58) female trout. Microsatellite DNA analysis showed that genetic diversity between fry samples can be explained by microgeographical (various sampling sites) and temporal (2 years of sampling) variations rather than maternal origin (anadromous vs. freshwater resident). We conclude that a high level of gene flow exists between the two morphs when anadromous adults have access to the spawning grounds of residents. We highlight the relevance of using SIA in conjunction with genetic analysis to study interactions between the two morphs.

Chitobiase activity as an indicator of aquatic ecosystem health

The sampling of benthic arthropods is one of several common approaches to monitor and assess ecosystem health in both lotic and lentic systems. This type of environmental effects monitoring has a number of drawbacks that support the development of new methodologies to improve our ability to detect and mitigate effects in the field. We propose an enzymatic technique to assess the status of arthropod communities and ecosystem health rapidly and efficiently. The method is based on measuring the activity of free chitobiase, an arthropod moulting enzyme, in natural waters. The rationale behind using chitobiase activity and how it could be applied to general field monitoring and assessment situations are discussed. An example of results obtained from field level monitoring using this assay is presented from the Oir River catchment and one of its tributaries, the La Roche, in Lower Normandy, France. Chitobiase activity was detected and quantified with low variability in the Oir and La Roche with the enzyme's activity statistically related (p < 0.0001, r2 = 0.77) to standard water quality physico-chemical parameters using multiple linear regression. A decrease in chitobiase activity was noted downstream of a sewage treatment plant releasing effluent into the Oir. This is the first time chitobiase activity has been measured in a freshwater ecosystem as an indicator of ecosystem health. However, extensive validation is required before chitobiase activity can be used in an environmental monitoring context.

A continuum of life history tactics in a brown trout (Salmo trutta) population

Life history tactics of the brown trout (Salmo trutta) population of the Oir River (Normandy, France) were studied using passive integrated transponder (PIT) tagging data of five consecutive cohorts (5900 individuals) monitored between 1995 and 2002. Results demonstrate that (i) life history traits vary among cohorts, chiefly caused by environmental variability, (ii) juvenile growth, particularly second-year growth, plays an important role in the determination of the growing environment and trout exhibit variable migratory behaviour (from remaining in the natal brook to migrating in the sea) related to their juvenile growth rate, and (iii) the description of life history tactics (including juvenile growth, fine-scale migratory behaviour, and reproduction) can be clarified. Tactics are expressed along a continuum in time (age to reproduce) and space (distance of migration). Flexible life history tactics varying with juvenile growth is consistent with previous studies, but the use of empiric data on growth and migration from PIT tagging allows refining the description of life history tactics, taking into account their continuous distribution in time and space.

Lack of genetic differentiation between anadromous and resident sympatric brown trout (Salmo trutta) in a Normandy population

The level of genetic differentiation between anadromous and non-andromous trout (Salmo trutta L.) coex- isting in a Norman river (Oir river) was examined at 15 microsatellites markers. Despite a large amount of variation, no genetic differentiation was found between forms. Four domesticated stocks were also genotyped with the same loci. Highly significant differences were found with natural samples allowing to exclude a recent stocking effect on the Oir population. We conclude that the resident and anadromous trout form a single naturally reproducing unit in this river system. However, this conclusion could reflect particular ecological or demographic conditions in the Oir basin and needs to be validated in other hydrographical and ecological contexts.

Role of habitat variability in trout population dynamics: Application of a dynamic population model to three French rivers

The Instream Flow Incremental Methodology (IFIM) was developed to determine flows that must be maintained downstream of hydropower plants to preserve aquatic populations. It is based on the hypothesis that the characteristics of the habitat in periods of low flow have a crucial impact on the dynamics of these populations. Other parameters that may also affect populations include: water quality, temperature, flood episodes, strategies for stocking, fishing, etc. Dynamic models of fish populations are now being developed in an attempt to integrate all these factors. The model presented here was applied on 3 populations (on Oir River, Neste d'Oueil River, and Roizonne River). The study has highlighted the fundamental role of the temporal variability of environmental parameters (particularly temperature, discharge and Weighted Surface Area) in structuring trout populations. Particularly, the flow-related habitat role simulated in the model – through a phenomenon of compensation during the first months of life and through displacement resulting in mortality, and mortality among fry when discharge is high – was illustrated by these examples.