Search

Climate change and other anthropogenic stressors have led to long-term changes in the thermal structure, including surface temperatures, deepwater temperatures, and vertical thermal gradients, in many lakes around the world. Though many studies highlight warming of surface water temperatures in lakes worldwide, less is known about long-term trends in full vertical thermal structure and deepwater temperatures, which have been changing less consistently in both direction and magnitude. Here, we present a globally-expansive data set of summertime in-situ vertical temperature profiles from 153 lakes, with one time series beginning as early as 1894. We also compiled lake geographic, morphometric, and water quality variables that can influence vertical thermal structure through a variety of potential mechanisms in these lakes. These long-term time series of vertical temperature profiles and corresponding lake characteristics serve as valuable data to help understand changes and drivers of lake thermal structure in a time of rapid global and ecological change.

Globally, lake surface water temperatures have warmed rapidly relative to air temperatures, but changes in deepwater temperatures and vertical thermal structure are still largely unknown. We have compiled the most comprehensive data set to date of long-term (1970-2009) summertime vertical temperature profiles in lakes across the world to examine trends and drivers of whole-lake vertical thermal structure. We found significant increases in surface water temperatures across lakes at an average rate of + 0.37 °C decade-1, comparable to changes reported previously for other lakes, and similarly consistent trends of increasing water column stability (+ 0.08 kg m-3 decade-1). In contrast, however, deepwater temperature trends showed little change on average (+ 0.06 °C decade-1), but had high variability across lakes, with trends in individual lakes ranging from - 0.68 °C decade-1 to + 0.65 °C decade-1. The variability in deepwater temperature trends was not explained by trends in either surface water temperatures or thermal stability within lakes, and only 8.4% was explained by lake thermal region or local lake characteristics in a random forest analysis. These findings suggest that external drivers beyond our tested lake characteristics are important in explaining long-term trends in thermal structure, such as local to regional climate patterns or additional external anthropogenic influences.

Certain spawning areas of the critically endangered Lake Saimaa landlocked Atlantic salmon (Salmo salar m. sebago) have been recently restored by excavator- and helicopter-scattered gravel, but the success of applied methodologies has remained open. Here, we monitored the spawning-related movements and redds occurrence of transported semi-wild and hatchery-reared salmon in the restored River Ala-Koitajoki, eastern Finland, using radiotelemetry and snorkeling. Nearly all fish accepted the newly restored spawning habitats in the river that maintains a low flow rate (only 4 [m.sup.3] * [s.sup.-1]). The movements of females released up to 2 weeks before spawning time were restricted mainly to the closest rapids from the release site, and most of the movements were directed downstream. Semi-wild and hatchery background fish exhibited similar mobility, which was more variable among males. A majority of redds were found in restored areas, and both helicopter- and excavator-scattered gravels were accepted as spawning substrates. Our results indicate that natural reproduction of the studied landlocked salmon population can be successfully returned to its original spawning river by restoration of habitats and transfer of spawners. Resume : Certains lieux de frai du saumon atlantique de lac (Salmo salar m. sebago) du lac Saimaa, qui est en voie de disparition, ont ete restaures a l'aide de gravier repandu par excavatrice et par helicoptere, mais le succes des methodes employees n'est pas encore etabli. Nous avons surveille les deplacements associes au frai et la presence de nids de frai de saumons semi-sauvages et eleves en ecloserie transportes dans la riviere Ala-Koitajoki, une riviere restauree de l'est de la Finlande, en utilisant la radiotelemetrie et la plongee avec tuba. Presque tous les poissons ont accepte les habitats de frai nouvellement restaures dans cette riviere qui maintient un faible debit (seulement 4 [m.sup.3] * [s.sup.-1]). Les deplacements des femelles relachees jusqu'a 2 semaines avant le moment du frai se sont principalement limites aux rapides les plus proches du site de lacher, et la plupart des deplacements etaient vers l'aval. Les poissons semi-sauvages et issus d'ecloseries presentaient une mobilite semblable, qui etait plus variable parmi les males. La majorite des nids de frai trouves etaient dans des zones restaurees, et tant les graviers repandus par helicoptere que par excavatrice etaient acceptes comme substrats de frai. Nos resultats indiquent que la restauration d'habitats et le transfert de geniteurs peuvent permettre le retour de la reproduction naturelle dans la riviere de frai d'origine pour la population de saumons de lac etudiee. [Traduit par la Redaction], Introduction Restorations of the spawning and nursery habitats are widely applied to enhance ecological state of regulated and degraded rivers (Pennell et al. 2007; Marttila et al. 2019; Pulg et [...]

Several Finnish populations of salmonids have been maintained exclusively by stocking hatchery-reared fish for several generations, and it is crucial to know whether domestication has affected fitness-related traits and to assess how the developmental environment influences fish phenotypes. Here, we focused on Lake Saimaa landlocked salmon (Salmo salar m. sebago) and studied the role of the early rearing environment in trait formation by comparing juveniles (fingerlings) from three backgrounds: (i) semi-wild (stocked as alevins), (ii) standard hatchery, and (iii) enriched hatchery conditions We compared juvenile morphology, swimming performance, and capability of feeding on natural prey in early winter. We found no differences between standard and enriched hatchery conditions. Semi-wild fish consumed more prey items, but wet mass of stomach contents did not differ among backgrounds. Swimming endurance was comparable among backgrounds. Semi-wild fish had slenderer body shape and longer fins than hatchery-reared fish. The lack of performance differences between hatchery juveniles and their semi-wild conspecifics is possibly due to reduced phenotypic plasticity caused by extremely low levels of genetic diversity in this population. Comme plusieurs populations finlandaises de salmonides ont ete maintenues exclusivement grace a l'ensemencement de poissons eleves en ecloserie pendant plusieurs generations, il importe de savoir si la domestication a eu une incidence sur des caracteres associes a l'aptitude et d'evaluer l'influence du milieu de developpement sur les phenotypes des poissons. Nous nous penchons sur les saumons d'eaux interieures du lac Saimaa (Salmo salar m. sebago) et etudions le role du milieu de l'elevage precoce dans la formation de caracteres en comparant de jeunes saumons (alevins d'un an) de trois origines distinctes, a savoir (i) des individus semi-sauvages (alevins vesicules au moment de l'ensemencement), (ii) eleves en ecloserie dans des conditions normales et (iii) eleves en ecloserie dans des conditions enrichies. Nous comparons la morphologie des jeunes saumons, leur performance de nage et leur capacite a se nourrir de proies naturelles au debut de l'hiver. Nous ne relevons aucune difference entre les individus issus d'ecloseries dans des conditions normales et enrichies. Les poissons semi-sauvages consomment un plus grand nombre de proies, mais la masse humide des contenus stomacaux ne varie pas selon l'origine des poissons. Les poissons semi-sauvages ont des corps plus elances et des nageoires plus longues que les poissons eleves en ecloserie. L'absence de differences sur le plan de la performance entre les jeunes poissons issus d'ecloseries et leurs congeneres semi-sauvages est possiblement due a une plasticite phenotypique reduite causee par des degres extremement faibles de diversite genetique dans cette population. [Traduit par la Redaction], Introduction Many natural salmonid populations are threatened by various human activities, such as overharvesting and construction of hydropower dams. Dams often severely block the movements of migratory fishes and thus [...]

Endangered wild fish populations are commonly supported by hatchery propagation. However, hatchery‐reared fish experience very different selective pressures compared to their wild counterparts, potentially causing genotype‐by‐environment interactions (G × E) in essential fitness traits. We experimentally studied early selection in a critically endangered landlocked Atlantic salmon population, first from fertilization to the swim‐up stage in a common hatchery setting, and thereafter until the age of 5 months in two contrasting rearing environments. Swim‐up progeny were moved either to standard indoor hatchery tanks involving conventional husbandry or to seminatural outdoor channels providing only natural food. After the first summer, sampled survivors were assigned to their families by genotyping. Early survival until the swim‐up stage was mostly determined by maternal effects, but also involved significant variation due to sires and full‐sib families (potential genetic effects). High on‐growing survival in hatchery tanks (88.7%) maintained a more even distribution among families (relative share 1.5%–4.2%) than the seminatural environment (0.0%–5.4%). This heterogeneity was mostly maternal, whereas no independent paternal effect occurred. Heritability estimates were high for body size traits in both environments (0.62–0.69). Genetic correlations between the environments were significantly positive for body size traits (0.67–0.69), and high body condition in hatchery was also genetically linked to rapid growth in the seminatural environment (0.54). Additive and phenotypic growth variation increased in the seminatural environment, but scaling effects probably played a less significant role for G × E, compared to re‐ranking of genotypes. Our results suggest that not only maternal effects, but also genetic effects, direct selection according to the environmental conditions experienced. Consistently high genetic variation in growth implies that, despite its low overall genetic diversity and long history in captive rearing (>50 years), this landlocked Atlantic salmon population still possesses adaptive potential for response to change from hatchery rearing back to more natural conditions. [ABSTRACT FROM AUTHOR]

Management of inland recreational fisheries would benefit from stock abundance and size structure data. Feasibly standardised angling methods such as ice fishing could produce representative catch‐per‐unit‐effort (CPUE) information on the abundance of different‐sized fish in small lakes. Here, we first used standard Nordic multimesh gillnets to obtain number‐per‐unit‐effort (NPUE), biomass‐per‐unit‐effort (BPUE) and size structure data on Eurasian perch (Perca fluviatilis) stocks in 11 small boreal lakes in summer. Second, the same lakes were ice‐fished by voluntary anglers using a pre‐defined, loosely standardised protocol to obtain angling‐based NPUE, BPUE, and length frequency distributions. Effects of environmental variables such as water oxygen concentration and light penetration on angling catch rates were controlled statistically. Neither perch Nordic gillnet NPUE nor BPUE corresponded to ice‐fishing CPUEs. However, the length distribution of the catch did not differ between methods. Our results imply that traditional ice fishing applying natural baits is relatively unselective for fish size and could produce valid length‐based indicators for management purposes while angling CPUE was poorly related to Nordic gillnet CPUE. [ABSTRACT FROM AUTHOR]

Visual transect counting of large‐bodied fish using artificial light at night in a shallow littoral zone (<1 m water depth) is introduced as a complementary survey method for fishes such as common bream Abramis brama and northern pike Esox lucius that are under‐represented in standard gillnet surveys. The results suggest that transect counting at night and applying necessary corrections for environmental variables could provide a simple and repeatable method to assess the presence and abundance of large‐bodied fishes in lakes with satisfactory water clarity. [ABSTRACT FROM AUTHOR]

Historically, Finnish rivers supported vital populations of migratory salmonids. Presently, these species are more or less endangered due to extensive damming and hydropower production. In this article, we study the main legal and scientific drivers for re-evaluating some of the existing hydropower operations in Finland. We argue that there is a need for re-evaluation on the basis of legal obligations stemming largely from EU law and new scientific knowledge. Theoretically, our setting opens up a classical adaptive governance problem in how to address laws and past decisions that are based on outdated assumptions about the functioning of social-ecological systems. [ABSTRACT FROM AUTHOR]

The European whitefish, Coregonus lavaretus (L.), is a highly polymorphic species consisting of several distinct ecotypes or forms. In the Koitajoki watercourse, south-eastern Finland, there exists a pair of indigenous sympatric whitefish ecotypes with distinct gill raker numbers. In the watercourse, supplemental stocking was initiated in the 1960s to compensate recruitment failure of whitefish caused by water level regulation for hydroelectric purposes. We investigated the genetic differentiation and possible hybridization between the two forms using 21 microsatellite loci and by assessing the temporal changes in the distribution of gill raker numbers. The distribution of gill raker number shifted from bimodality to trimodality in less than 15 years during the 1980s and 1990s, implying putative hybridization of these forms. Significant genetic differentiation was found between the two forms (FST = 0.034). In addition, Bayesian inference of population structure as well as a discriminate analysis of principal components (DAPC) identified two genetic clusters largely coincident with these forms. A high degree of genetic admixture was found in phenotypically intermediate individuals, which also differed slightly from the indigenous forms as indicated by the DAPC scatterplot. Taken together, the results suggest that human actions such as stockings and water level regulation are the most likely factors contributing to the reduction of reproductive isolation between the sympatric whitefish forms. [ABSTRACT FROM AUTHOR]

Sexual dimorphism is common across the animal kingdom, but the contribution of environmental factors shaping differences between the sexes remains controversial. In ectotherms, life-history traits are known to correlate with latitude, but sex-specific responses are not well understood. We analyzed life-history trait variation between the sexes of European perch ( Perca fluviatilis L.), a common freshwater fish displaying larger female size, by employing a wide latitudinal gradient. We expected to find sex-dependent latitudinal variation in life-history variables: length at age, length increment, and size at maturity, with females showing consistently higher values than males at all latitudes. We further anticipated that this gender difference would progressively decrease with the increasingly harsh environmental conditions toward higher latitude. We hypothesized that growth and length increment would decrease and size/age at maturity would increase at higher latitudes. Our results confirmed female-biased sexual size dimorphism at all latitudes and the magnitude of sexual dimorphism diminished with increase in latitude. Growth of both sexes decreased with increase in latitude, and the female latitudinal clines were steeper than those of males. Hence, we challenge two predominant ecological rules (Rensch's and Bergmann's rules) that describe common large-scale patterns of body size variation. Our data demonstrate that these two rules are not universally applicable in ectotherms or female-biased species. Our study highlights the importance of sex-specific differences in life-history traits along a latitudinal gradient, with evident implications for a wide range of studies from individual to ecosystems level. [ABSTRACT FROM AUTHOR]

Pikeperch populations typically differ substantially in life-history traits. Here, we evaluated the potential of variable size limits to ensure ecologically and evolutionarily sustainable fishing of six important Finnish inland pikeperch stocks using a carefully parameterized age-, size-, and maturity-structured evolutionary population model. Individual growth rates of the Oulujärvi and Vesijärvi stocks were the fastest and least resource-limited. The Höytiäinen, Vanajavesi and Pääjärvi stocks were strongly regulated by density-dependent food availability. Minimum size limit ensuring the highest yields was clearly higher for the fast-growing stocks than for the strongly food-limited stocks. Implementation of a maximum size limit 200 mm above the minimum size limit (MSL) would allow the stocks to tolerate stronger fishing mortality rate, but the 200 mm slot appeared too wide to significantly prevent evolution towards earlier maturation. To ensure maximal stability of yield and minimal evolutionary effect, fishing mortality rates should be restricted to relatively low values. [ABSTRACT FROM AUTHOR]