Effects of species invasion on population dynamics,vital rates and life histories of the native species

Invasions occurring in natural environments provide the opportunity to study how vital rates change and life histories evolve in the presence of a competing species. In this work, we estimate differences in reproductive traits, individual growth trajectories, survival, life histories and population dynamics between a native species living in allopatry and in sympatry with an invasive species of the same taxonomic Family. We used as a model system marble trout Salmo marmoratus (native species) and rainbow trout Oncorhynchus mykiss (non-native) living in the Idrijca River (Slovenia). An impassable waterfall separates the stream into two sectors only a few 100 meters apart: a downstream sector in which marble trout live in sympatry with rainbow trout and an upstream sector in which marble trout live in allopatry. We used an overarching modelling approach that uses tag-recapture and genetic data (>2,500 unique marble and rainbow trout were sampled and genotyped) to reconstruct pedigrees, test for synchrony of population dynamics and model survival and growth, while accounting for individual heterogeneity. The population dynamics of the two marble trout populations and of rainbow trout were synchronous. We found higher prevalence of younger parents, higher mortality and lower population density in marble trout living in sympatry with rainbow trout than in marble trout living in allopatry. There were no differences in the average individual growth trajectories between the two marble trout populations. Faster life histories of marble trout living in sympatry with rainbow trout are consistent with predictions of life history theory.     

Trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) mitochondrial DNA polymorphism in the centre of the marble trout distribution area

The marble trout, a lineage of the Salmo trutta complex, is endemic to the Southern Alpine region. Although it is endangered throughout its entire distribution range, population genetic data were lacking for the central area, including the upper Etsch/Adige River system (South Tyrol, Northern Italy). A total of 672 Salmo trutta specimens, comprising phenotypic marble trout and phenotypic brown trout, from 20 sampling sites throughout South Tyrol were analysed by sequencing the complete mitochondrial DNA control region. Thirteen distinct haplotypes were identified, which clustered within three major genetic lineages: the Marmoratus (MA), the Atlantic (AT) and the Danubian (DA) lineage. 41.7% of the investigated individuals carried haplotypes of the MA lineage, 47.9% of the AT lineage and 10.4% of the DA lineage. It is noticeable that AT haplotypes were present at all sampling sites and no “pure” marble trout population with exclusively MA haplotypes was found. This points to a considerable impact of stocking with allochthonous brown trout, given that there is no evidence for natural colonisation by individuals of the AT lineage. However, our data indicate, for at least four localities, a limited gene flow between the native marble trout and hatchery-reared strains. Future conservation and rehabilitation measures will thus have to concentrate on the identification of remnant pure marble trout individuals from such mixed populations. Handling editor: C. Sturmbauer     

Non‐additive effects of density‐dependent and density‐independent factors on brown trout vital rates

Interactions between density‐dependent and density‐independent processes can lead to variation in both growth and survival rates. Detecting such effects, however, will often require sampling on an individual level and at the appropriate spatial and temporal scale. This study documents substantial variation in survival and growth of stream‐dwelling brown trout Salmo trutta from a small Norwegian stream. The data is based on seasonal capture–recaptures of individually marked trout on fixed stations during eight years. The fish were small‐sized, rarely reaching sizes larger than 20 cm and ages older than seven years. Density varied between 0.2–0.8 fish m^?2. Variation in survival and recapture probabilities was analysed using program MARK. Apparent survival (the probability of being alive and present within the study area) generally decreased with increasing trout density and increasing drought level (measured as lowest observed water flow) during both winter and summer. Further, there was a significant interaction effect between density and water flow, indicating that density‐dependent effects on survival predominated when environmental conditions were benign (no drought), while density‐independent processes were most important under harsh environmental conditions (drought). Observed length‐at‐age during autumn indicated a more or less linear growth trajectory throughout life, and no effect of density, water flow or temperature was found. However, using the individual‐based capture–recapture data to estimated specific growth rate, significant positive effects of water flow and temperature and a negative effect of density were identified. Thus, the capture–recapture data suggest a strong potential for population regulation at the rather low densities found in this stream, and regulation may occur both through effects on survival and growth.     

Translocation of stream-dwelling salmonids in headwaters: insights from a 15-year reintroduction experience

Translocation programs are a common strategy to increase the number of viable populations of threatened freshwater fishes. Yet, only in a minority of cases the success or failure of translocations has been assessed through a quantitative analysis of demographic traits, compensatory responses, life-histories and population dynamics of the threatened species. A paradigmatic case a translocation program combining both management- and research-oriented activities is represented by the Marble Trout Conservation Program, which started in 1993 in the upper reaches of the Soca, Idirjca and Baca river basins (Slovenia) for the conservation of stream-dwelling marble trout Salmo marmoratus. In order to enhance the viability of the species, two new populations were created in 1996 by stocking 500 marble trout aged 1+ in previously fishless streams (Gorska and Zakojska) within the core habitat of the species. The new populations have been systematically monitored for 15 years by individually tagging and sampling marble trout. Our analyses show that deterministic extinction of marble trout populations are unlikely and that high-magnitude environmental stochasticity (i.e., severe floods) is the only main cause of local population extinction, despite the high resilience to flood-induced massive mortalities exhibited by marble trout through compensatory mechanisms (e.g., relaxation of density-dependent body growth and survival at low densities). Fishless headwaters, probably characterized by a history of recurrent severe floods, should not be considered as candidate sites for the creation of new populations. Fewer individuals than originally reintroduced (i.e., 500 fish aged 1+ in each stream) might be sufficient to establish viable populations, since compensatory mechanisms are likely to regulate population size around stream carrying capacity in a few years. Besides enhancing the species viability, translocation programs can provide an excellent framework for the estimation of ecological traits (e.g., life-histories, demography, population dynamics etc.), identify potential vulnerabilities and thus guide well-formed management actions for the threatened species.     

The selection of the ‘wild’: A combined molecular approach for the identification of pure indigenous fish from hybridised populations

The identification of pure indigenous fish from hybridised populations represents a key issue in fisheries management and conservation biology. In the present study an approach for selection of purebred marble trout (Salmo trutta marmoratus C.) individuals out of admixed populations was set up and assessed. In a first step, baseline data sets of pure marble trout and pure brown trout specimens based on twelve microsatellite loci were used to simulate five consecutive generations of admixture. The baseline and the resulting simulation data sets were then combined with data of a ‘real’ hybridised marble trout population to perform a single individual assignment test as implemented in STRUCTURE. By this procedure the assignment approach was calibrated and it was possible to compare admixture coefficients obtained for individuals from different populations. The ranking of individual admixture coefficients on a plot and comparison with simulated data revealed that the test population was composed of pure marble trout individuals, first generation hybrids between marble trout and brown trout, and hybrid backcross specimens between both groups. However, by defining a critical q-value of 0.1 and additionally integrating individual sequence data of the mtDNA control region, it was possible to indicate individuals, which could be selected for the establishment of a pure marble trout strain.     

Museum samples could help to reconstruct the original distribution of Salmo trutta complex in Italy

Partial D‐loop sequences of museum specimens of brown trout and marble trout (Salmo trutta species complex) collected from Mediterranean rivers in the late 19th century were analysed to help to describe the native distribution of these species. All the individuals studied carried native haplotypes, the geographic distribution of which is consistent with published data. These results indicate that museum specimens from the 19th century could represent an opportunity to get a picture of the original genetic diversity distribution of this species complex.     

Functional models for growth and food consumption of Atlantic salmon parr, Salmo salar, from a Norwegian river

1. The chief objectives were to analyse and model experimental data for maximum growth and food consumption of Atlantic salmon parr (Salmo salar) collected from a cold glacier fed river in western Norway. The growth and feeding models were also applied to groups of Atlantic salmon growing and feeding at rates below the maximum. The growth models were validated by comparing their predictions with observed growth in the river supplying the experimental fish.
2. Two different models were fitted, one originally developed for British salmon and the other based on a model for bacterial growth. Both gave estimates for optimum temperature for growth at 18–19 °C, somewhat higher than for Atlantic salmon from Britain. Higher optimal temperature for growth in salmon from a cold Norwegian river than from British rivers does not concur with predictions from the thermal adaptation hypothesis.
3. Model parameter estimates differed among growth groups in that the lower critical temperature for growth increased from fast to slow growing individuals. In contrast to findings for brown trout (Salmo trutta), the optimum temperature for growth did not decrease with decreasing levels of food consumption.
4. A new and simple model showed that food consumption (expressed in energy terms) peaked at 19.5–19.8 °C, which is similar to the optimal temperature for growth. Feeding began at a temperature 1.5 °C below the lower temperature for growth and ended about 1 °C above the maximum temperature for growth. Model parameter estimates for consumption differed among growth groups in a manner similar to the growth models. Maximum consumption was lower for Atlantic salmon than for brown trout, except at temperatures above 18 °C.
5. By combining the growth and food consumption models, growth efficiency was estimated and reached a maximum at about 14 °C for fast growing individuals, increasing to nearly 17 °C for slow growing ones, although it was lower overall for the latter group. Efficiency also declined with increasing fish size. Growth efficiency was generally higher for Atlantic salmon than for brown trout, particularly at high temperature.     

Trout Salmo spp. complex in Serbia and adjacent regions of the western Balkans: reconstruction of evolutionary history from external morphology

The multivariate phenetic approach to the classification of Salmo spp. samples from Serbia and adjacent regions of western Balkans for 22 continuous external morphological characters suggests the occurrence of the following distinct stocks: West Danubian (Crno Osoje Stream and upper Zeta River) Salmo taleri , marble trout Salmo marmoratus (Trebuščica River), hatchery-reared Atlantic Salmo trutta , Mlava River drainage (Mlava and Krupaja rivers and Buk Stream) trout Salmo cf. trutta , Velika Morava River system (Godljevača, Bela and Resava rivers) trout S. cf. trutta , Ohrid Lake belvica Salmo ohridana and Aegean coastal drainage Salmo macedonicus (Božica River). In contrast to the phenetic similarity, the phylogenetic reconstruction places the Lake Ohrid belvica as part of an unresolved polytomy with other trout groups. Salmo cf. trutta in the Mlava River appears to form the basal group for the trout species in the region. The position of marble trout implies its independent and more recent origin from the West Danubian trout stock.     

Lack of molecular genetic divergence between sea-ranched and wild sea trout (Salmo trutta)

The supportive breeding programme for sea trout (Salmo trutta) in the River Dalälven, Sweden, is based on a sea‐ranched hatchery stock of local origin that has been kept ‘closed’ to the immigration of wild genes since the late 1960s (about seven generations). In spite of an apparent potential for substantial uni directional gene flow from sea‐ranched to wild (naturally produced) trout, phenotypic differences with a presumed genetic basis have previously been observed between the two ‘stocks’. Likewise, two previous studies of allozyme and mitochondrial DNA variation based on a single year of sampling have indicated genetic differentiation. In the present study we used microsatellite and allozyme data collected over four consecutive years, and tested for the existence of overall genetic stock divergence while accounting for temporal heterogeneity. Statistical analyses of allele frequency variation (F‐statistics) and multilocus genotypes (assignment tests) revealed that wild and sea‐ranched trout were significantly different in three of four years, whereas no overall genetic divergence could be found when temporal heterogeneity among years within stocks was accounted for. On the basis of estimates of effective population size in the two stocks, and of F_ST between them, we also assessed the level of gene flow from sea‐ranched to wild trout to be ≈ 80% per generation (with a lower confidence limit of ≈ 20%). The results suggest that the reproductive success of hatchery and naturally produced trout may be quite similar in the wild, and that the genetic characteristics of the wild stock are largely determined by introgressed genes from sea‐ranched fish.     

Detection and apparent survival of PIT‐tagged stream fish in winter

Environmental fluctuations exert strong control on behavior, survival, and fitness of stream biota. Technical improvements increasingly allow for tracking the response of large numbers of individuals to environmental fluctuations, for instance, by remote detection of animals equipped with PIT (passive integrated transponder) tags. PIT tags were implanted into 393 juvenile and adult brown trout Salmo trutta L. and European sculpin Cottus gobio L. in a boreal stream subjected to considerable ice formation. With weekly trackings over 6 months, we quantified apparent survival and detection probability in relation to biological, environmental, and methodological factors. Individuals with a higher physical condition in autumn showed a higher apparent survival; this pattern was consistent across all species and age classes. Detection probability decreased with increasing thickness of the surface ice layer; this effect was most pronounced for juvenile trout and benthic‐living sculpin, both tagged with smaller‐sized tags. Detection probability was reduced in structurally complex habitats. Our study demonstrates that apparent survival and particularly detection probability may show pronounced spatiotemporal variation. In order to compare results from different sampling occasions and sites, a good knowledge of the study site and of the regulating factors is crucial.     

Temporal changes in allele frequencies in a small marble trout Salmo marmoratus population threatened by extreme flood events

The effect of extreme floods on the genetic composition of marble trout Salmo marmoratus living in Lipovscek, a tributary of the Soca River in Slovenia, which has been affected by multiple destructive flood events for centuries was investigated. By monitoring genetic variability during the period 2004–2011, apparent signatures of genetic erosion including a decline in observed and expected heterozygosities and allelic richness were observed. Contemporary effective population size was estimated between 11 and 55 individuals, which is congruent with census data. The data suggest asymmetric gene flow between the two sections of the river. The existence of substantial downstream migration (15–19%) was confirmed by paternity analysis. A small (1–3%) upstream migration was also suggested, which was confirmed by tagging data. Overall, low genetic diversity has not prevented the survival of the Lipovscek population, which might be a common feature of salmonid freshwater populations.     

Effective population size and temporal genetic change in stream resident brown trout (Salmo trutta, L.)

Temporal genetic data may be used forestimating effective population size (N _e) and for addressing the `temporal stability' of population structure, two issues of central importance for conservation and management. In this paper we assess the amount of spatio-temporal genetic variation at 17 di-allelic allozyme loci and estimate current N _e in two populations of stream resident brown trout (Salmo trutta) using data collected over 20 years. The amount ofpopulation divergence was found to bereasonably stable over the studied time period.There was significant temporal heterogeneitywithin both populations, however, and N _e was estimated as 19 and 48 for the twopopulations. Empirical estimates of theprobability of detecting statisticallysignificant allele frequency differencesbetween samples from the same populationseparated by different numbers of years wereobtained. This probability was found to befairly small when comparing samples collectedonly a few years apart, even for theseparticular populations that exhibit quiterestricted effective sizes. We discuss someimplications of the present results for browntrout population genetics and conservation, andfor the analysis of temporal genetic change inpopulations with overlapping generations ingeneral.     

The role of density-dependent individual growth in the persistence of freshwater salmonid populations

Theoretical and empirical models of populations dynamics have paid little attention to the implications of density-dependent individual growth on the persistence and regulation of small freshwater salmonid populations. We have therefore designed a study aimed at testing our hypothesis that density-dependent individual growth is a process that enhances population recovery and reduces extinction risk in salmonid populations in a variable environment subject to disturbance events. This hypothesis was tested in two newly introduced marble trout (Salmo marmoratus) populations living in Slovenian streams (Zakojska and Gorska) subject to severe autumn floods. We developed a discrete-time stochastic individual-based model of population dynamics for each population with demographic parameters and compensatory responses tightly calibrated on data from individually tagged marble trout. The occurrence of severe flood events causing population collapses was explicitly accounted for in the model. We used the model in a population viability analysis setting to estimate the quasi-extinction risk and demographic indexes of the two marble trout populations when individual growth was density-dependent. We ran a set of simulations in which the effect of floods on population abundance was explicitly accounted for and another set of simulations in which flood events were not included in the model. These simulation results were compared with those of scenarios in which individual growth was modelled with density-independent Von Bertalanffy growth curves. Our results show how density-dependent individual growth may confer remarkable resilience to marble trout populations in case of major flood events. The resilience to flood events shown by the simulation results can be explained by the increase in size-dependent fecundity as a consequence of the drop in population size after a severe flood, which allows the population to quickly recover to the pre-event conditions. Our results suggest that density-dependent individual growth plays a potentially powerful role in the persistence of freshwater salmonids living in streams subject to recurrent yet unpredictable flood events. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Addressing Temporal Variability in Bird Calling with Design and Estimation: A Northern Bobwhite Example

Imprecise or biased density estimates can lead to inadequate conservation action, overexploitation of game species, or lost recreational opportunities. Common approaches to estimating density of avian populations often either ignore the probability that an individual is present within the sampling area but is not available to be sampled (e.g., not vocalizing), or do not consider covariates that could influence availability. Additionally, management decisions made at the management unit scale are often informed by inadequate monitoring practices, such as limited sampling intensity. In such cases, management agencies calculate density by applying correction factors (e.g., detection probabilities estimated using empirical data from a different study system) to count data, rather than estimating a detection function directly using statistical models. We conducted a simulation study using northern bobwhite (Colinus virginianus; bobwhite) as a model species to quantify the consequences of mis-specifying avian point count models on bias and precision of density estimates. We compared bias and precision of estimates from a fully specified distance-sampling model that estimates availability and detection to 4 different mis-specified approaches, including 2 approaches to calculating density using correction factors. Using correction factors to calculate density produced estimates with low bias but relatively lower precision compared to the fully specified model (CV of density estimates at 35 sites over 5 years: fully specified = 10%, correction factors = 25% and 30%). Although the mean precision and bias of the fully specified model improved with more data (70 sites over 5 years, CV = 9%; 35 sites over 10 years, CV = 9%), precision of correction factors did not (70 sites over 5 years, CV = 22% and 27%; 35 sites over 10 years, CV = 24% and 29%). The fully specified model captured the underlying temporal variation in detection and availability. Increasing sampling duration from 5 to 10 years improved modeled estimates of growth rate, even for mis-specified models, but not derived growth rates using pre-determined detection functions. We demonstrated that conducting point counts 3 times/year at a feasible number of sites can produce relatively unbiased estimates of bobwhite density. Pre-determined detection functions can be fortuitously unbiased for certain years, but they are not a reliable method for determining density or identifying trends in density over time. © 2020 The Wildlife Society.     

Microsatellite DNA data point to extensive but incomplete admixture in a marble and brown trout hybridisation zone

Marble trout, endemic to the Adriatic drainage basin, is severely threatened by hybridisation with non-native brown trout. In the present study, we analysed 12 microsatellite DNA loci to assess genetic population structure and differentiation between sympatric phenotypic marble and brown trout at nine sampling sites in the upper Etsch/Adige River system. F _ST and AMOVA analyses revealed significant genetic differentiation between marble and brown trout samples. Thus, admixture between brown and marble trout appears to be incomplete. However, factorial correspondence analysis depicted marble trout, Atlantic brown trout and intermediate genotypes. Bayesian-based individual assignment tests identified indigenous marble trout at five sampling sites. In four other samples no ‘pure’ marble trout were detected. Bidirectional, first-generation hybridisation, involving both sexes of both parental species was observed. In locations where ‘pure’ marble trout still exist, post-F1 hybridisation appears to be directed towards brown trout. This has likely slowed the rate of hybridisation between the two trout species and the decline of relic marble trout populations. Based on these results, restoration management actions are proposed, such as the abandonment of brown trout stocking activities, sharper angling policies, establishment of indigenous marble trout breeding strains and the elaboration of a conservation priority list.     

Influence of hydrologic attributes on brown trout recruitment in low-latitude range margins

Factors controlling brown trout Salmo trutta recruitment in Mediterranean areas are largely unknown, despite the relevance this may have for fisheries management. The effect of hydrological variability on survival of young brown trout was studied during seven consecutive years in five resident populations from the southern range of the species distribution. Recruit density at the end of summer varied markedly among year-classes and rivers during the study period. Previous work showed that egg density the previous fall did not account for more than 50% of the observed variation in recruitment density. Thus, we expected that climatic patterns, as determinants of discharge and water temperature, would play a role in the control of young trout abundance. We tested this by analyzing the effects of flow variation and predictability on young trout survival during the spawning to emergence and the summer drought periods. Both hatching and emergence times and length of hatching and emergence periods were similar between years within each river but varied considerably among populations, due to differences in water temperature. Interannual variation in flow attributes during spawning to emergence and summer drought affected juvenile survival in all populations, once the effect of endogenous factors was removed. Survival rate was significantly related to the timing, magnitude and duration of extreme water conditions, and to the rate of change in discharge during hatching and emergence times in most rivers. The magnitude and duration of low flows during summer drought appeared to be a critical factor for survival of young trout. Our findings suggest that density-independent factors, i.e., hydrological variability, play a central role in the population dynamics of brown trout in populations from low-latitude range margins. Reported effects of hydrologic attributes on trout survival are likely to be increasingly important if, as predicted, climate change leads to greater extremes and variability of flow regimes.     

Limestone Treatment of Whetstone Brook, Massachusetts. II. Changes in the Brown Trout (Salmo trutta)and Brook Trout (Salvelinus fontinalis)Fishery

Density, age structure, and growth rates of wild brook trout (Salvelinus fontinalis)and brown trout (Salmo trutta)in Whetstone Brook in northcentral Massachusetts were monitored for 4 years before and 3 years during limestone treatment to mitigate acidic conditions. The population density of brook trout increased significantly during treatment. Liming did not have any significant effects on the growth rates of brook trout or brown trout. Actual survival rates of brook trout and brown trout were not calculated due to the low density of both species, but more older individuals of both species were captured during the treatment period. Fulton condition factors (an index of fish condition) increased significantly for both brook trout and brown trout during treatment. Seven-day in situ bioassays of brown trout and rainbow trout demonstrated that liming improved the chemical environment for fish in Whetstone Brook. During a pretreatment bioassay in 1987, 100% rainbow trout mortality was observed at both the control and treatment stations in Whetstone Brook. Brown trout mortality was 67% in the control station and 70% in the treatment station. The pH during the 1987 bioassay averaged 4.90 in the control station and 4.99 in the treated station. During a bioassay conducted in 1990 after treatment began, rainbow trout mortality was 100% in the control station and 0% in the treatment station. Brown trout mortality was 17% in the control station and 0% in the treatment station. The pH during the 1990 bioassay averaged 5.23 in the control station and 6.60 in the treatment station. Analysis of total aluminum in the gills of fish from the 1990 bioassay revealed higher levels in fish from the control station than in those from the treatment station.     

A length-based hierarchical model of brown trout (Salmo trutta fario) growth and production

We present a hierarchical Bayesian model (HBM) to estimate the growth parameters, production, and production over biomass ratio (P/B) of resident brown trout (Salmo trutta fario) populations. The data which are required to run the model are removal sampling and air temperature data which are conveniently gathered by freshwater biologists. The model is the combination of eight submodels: abundance, weight, biomass, growth, growth rate, time of emergence, water temperature, and production. Abundance is modeled as a mixture of Gaussian cohorts; cohorts centers and standard deviations are related by a von Bertalanffy growth function; time of emergence and growth rate are functions of water temperature; water temperature is predicted from air temperature; biomass, production, and P/B are subsequently computed. We illustrate the capabilities of the model by investigating the growth and production of a brown trout population (Neste d'Oueil, Pyrénées, France) by using data collected in the field from 2005 to 2010.