Evidence of unidirectional gene flow in a fragmented population of Salmo trutta L.

Selection, genetic drift, and gene flow affect genetic variation within populations and genetic differences among populations. Both drift and selection tend to decrease variation within populations and increase differences among populations, whereas gene flow increases variation within populations but leads to populations being related. In brown trout (Salmo trutta L.), the most important factor in population fragmentation is disrupted river-segment connectivity. The main goal of the study was to use genetic analysis to estimate the level of gene flow among resident and migratory brown trout in potential hybridization areas located downstream of impassable barriers in one river basin in the southern Baltic Sea region. First, spawning redds were counted in the upper river basin downstream of impassable barriers. Next, samples were collected from juveniles in spawning areas located downstream of barriers and from adults downstream and upstream of barriers. Subsequently, genetic analysis was performed using a panel of 13 microsatellite loci and the Salmo trutta 5 K SNP microarray. The genetic differentiation estimated between the resident form sampled upstream of the barriers and the anadromous specimens downstream of the barriers was high and significant. Analysis revealed that gene flow occurred between the two forms in the hybridization zone investigated and that isolated resident specimens shared spawning grounds with sea trout downstream of the barriers. The brown trout population from the river system investigated was slightly, internally diversified in the area accessible to migration. Simultaneously, the isolated part of the population was very different from that in the rest of the basin. The spawning areas of the anadromous form located downstream of the barriers were in a hybridization zone and gene flow was confirmed to be unidirectional. Although they constituted a small percentage, the genotypes typical upstream of the barriers were admixed downstream of them. The lack of genotypes noted upstream of the barriers among adult anadromous individuals might indicate that migrants of upstream origin and hybrids preferred residency.

Increased brain growth in escaped rainbow trout

Recent examples of rapid brain size plasticity in response to novel laboratory environments suggest that fish brain size is a flexible trait, allowing growth or shrinkage of brain tissue based on short term needs. Nevertheless, it remains to be seen if plasticity of fish brain size is relevant to natural environmental conditions. Here, using rainbow trout escaped from a farming operation as a natural experiment, we demonstrate that adult fish brain size can change rapidly in response to life in a natural lake environment. Specifically, escaped trout had on average 15% heavier brains relative to body size than captive trout after living for about 7 months in the lake. Because relative brain size of most escaped trout fell above the range of variation seen within the captive trout population, we conclude that increased brain size was achieved by plasticity after escape. Brain morphology analysis showed that the most anterior regions (olfactory bulbs and rest of telencephalon) contributed most to the increase in overall brain size in escaped trout. Relative size of the heart ventricle, another organ which can be subject to plastic changes under variable environmental conditions in fish, did not differ between escaped and captive trout. Massive and selective brain growth under the changed environmental conditions associated with escape from holding pens highlighted the plastic potential of fish brain size and suggests that a shift to increased complexity of life in the wild setting of a lake imposed greatly increased cognitive requirements on escaped trout.

The Wild Rivers Program: Development of a “Wild Rivers” Conservation Label to be used by River Management

A wild river is a living river, which is at baseline, well-preserved, and which runs freely and is home to a rich biodiversity in its high quality waters and on its banks. In Europe there are very few rivers which could be considered “wild”, which function at a high ecological level, since wild rivers, in the true sense of the term, no longer exist. Based on the fact that these rivers remain threatened, and that the existing tools (technical, regulatory, and financial) are insufficient and not adapted to ensure their preservation over the long term, the Wild Rivers project was founded in 2007, through a meeting of environmental defenders, scientists, fishermen, managers of land and river natural resources, and elected officials, all of whom were anxious to save the last of the French rivers which were still preserved, with a human impact that would be compatible with the conservation of the ecosystem. In 2014 the “Wild Rivers Site” label was created in France, as a conservation tool for rivers, both voluntary and non-regulatory, which allows the support necessary to enable the territorial players to preserve their rivers in harmony with the activity in the surrounding valleys. It also identifies and highlights these unique watercourses. The Valserine in the Ain region was the first river to obtain the Wild Rivers Site label. Today 28 rivers in France are labelled “Wild Rivers Sites” and the 22 management structures of these rivers are members of the Wild Rivers Site Network. To obtain the label, a river must fulfill two sets of criteria 1. The criteria grid: The watercourse must obtain a mark over 70/100. The grid is composed of 47 criteria evaluating the quality of the area, of which 12 are eliminatory, 8 are unrated, and 9 are under a bonus/penalty scheme 2. The program of actions taken by local players: The local managers must put in place a system of governance built around actions to be taken over a period of years, shared among them, and ambitious, going beyond the regulatory objectives of the European Directive Framework. It allows for the restoration of penalty points and the establishment of innovative conservation activities. The Wild Rivers Sites are also an open air laboratory for the development and use of innovative methods in order to provide new information on aquatic environments, and to improve their management and conservation. Numerous steps have already been taken within the network, such as the Ecosystem Services Study (Costa and Hernandez 2019); on the study of the genetic makeup of the brown trout population. Recently, the use of genetic study using environmental DNA to complete biodiversity inventories has also been deployed to study benthic diatoms (DNA of Diatoms Project 2020-2022). This project seeks to use DNA metabarcoding to respond to a number of objectives: i) inventory of the species of diatoms and their community structure in these watercourses which are generally seldom studied; ii) complete ecological status studies; iii) develop new genetic metrics and taxonomies adapted to the conservation of wild river watercourses. It is in this spirit that the Wild Rivers program was developed, and has received numerous positive responses on the behalf of watercourse management in France. Thanks to this impetus, work has been conducted to extend this conservation label to water sources in other countries (Switzerland, Ireland, Spain), with the future plan of building a European network dedicated to the conservation of Wild Rivers.

The genetic composition of wild recruits in a recovering lake trout population in Lake Michigan

Strain performance evaluations are vital for developing successful fishery management and restoration strategies. Here, we utilized genotypes from 36 microsatellites to investigate hatchery strain contribution to collections of naturally produced lake trout sampled across Lake Michigan. Strain composition varied by area, with recoveries of Seneca Lake strain exceeding expectations based on stocking records in northern Lake Michigan but performing similarly to other strains in southern Lake Michigan. Interstrain hybrids were present at moderate frequencies similar to expectations based on simulations suggesting that strains are interbreeding randomly. We hypothesize that the superior performance of the Seneca Lake strain in northern Lake Michigan is partially due to adaptive advantages that facilitate increased survival in areas with high mortality from sea lamprey predation such as northern Lake Michigan. However, when this selective pressures is lessened, the Seneca Lake strain performs similarly to other strains. Our study demonstrates that strain performance can vary across small spatial scales and illustrates the importance of conducting thorough strain evaluations to inform management and conservation.

Impacts of reduced Lepidurus arcticus availability on brown trout life history traits in a mountain reservoir

Lepidurus arcticus (the Arctic tadpole shrimp) is a vulnerable keystone species in Arctic and alpine water bodies where its occurrence and population size may influence the viability and life history traits of resident salmonids. Using data from a Norwegian mountain hydropower reservoir, Aursjoen, we illustrate how reduced availability of L. arcticus as prey resulted in the reduced condition, growth and delayed maturation of resident brown trout (Salmo trutta). We further link changes in the relative abundance of L. arcticus as prey to changing reservoir conditions, e.g. water level changes in the spring period, thereby establishing an indirect link between reservoir operation regimes and brown trout population traits. While no evidence for decreased brown trout survival was found, the results indicate that alternative brown trout prey resources, i.e. the small chydorid cladoceran Eurycercus lamellatus, do not appear to have successfully offset the caloric loss from reduced consumption of large-sized L. arcticus. Although the fundamental explanation for the observed L. arcticus collapse remains largely unknown, the present findings provide strong evidence that this vulnerable crustacean species can affect the abundance, viability and life history traits of valued resident salmonid populations in oligotrophic alpine lakes and reservoirs exposed to climate- and hydropower-driven changes in water levels and temperature.