Is coexistence mediated by microhabitat segregation? An in-depth exploration of a fish invasion

Vendace Coregonus albula has invaded the subarctic Pasvik hydrosystem, northern Norway and Russia, after being translocated from its native Finnish range into Lake Inari (Finland), upstream of Pasvik. The development of the invader and the effects on two native lake communities in the upstream and downstream part of the hydrosystem, respectively, have been monitored since 1991. In the upstream lake location, the invader relegated the native whitefish Coregonus lavaretus from the pelagic habitat within a few years. An expected similar development in the downstream location was, however, not observed. Data are presented on the diet and microhabitat use (vertical pelagic zone) for three types of replicated situation: (1) the invader v. the native species, (2) upstream v . downstream lakes and (3) early and late invasion stages, to explore the outcome of the competitive interactions. The use of different microhabitats available within the deeper pelagic zone in the downstream location was observed to have delayed the negative effects on native whitefish, possibly enabling a persistent coexistence between the two species in the pelagic zone of this lake.     

Seasonal habitat use of three predatory fishes in a freshwater ecosystem

Hydrobiologia - To understand the spatiotemporal overlap in the habitat use of sympatric predators, we studied longitudinal activity and reservoir section and depth use of pike (Esox lucius),...     

Invasion genetics of vendace (Coregonus albula (L.)) in the Inari‐Pasvik watercourse: revealing the origin and expansion pattern of a rapid colonization event

Species invasions can have wide‐ranging biological and socio‐economic effects and are generally unwanted by legislation. Identification of the source population as well as the ecology and genetics of both the invader population and the receiving community is of crucial importance. The rapid invasion of a small coregonid fish vendace (Coregonus albula) in a major northern European subarctic watercourse has resulted in a labile ecological situation in the receiving community. The ecological impact of the invasion has been thoroughly documented, but the genetics of the invasion remains to be explored. We analyzed the genetic diversity and divergence patterns among the two possible source populations from southern Finnish Lapland and three colonists populations within the Inari‐Pasvik watercourse using ten microsatellite loci in order to (i) identify the most likely source of the invasion, (ii) reveal the dispersal pattern and genetic structure of the secondary expansion, and (iii) to investigate whether the initial introduction and the secondary expansion were associated with founder effects. We revealed that repeated translocation of vendace from Lake Sinettäjärvi into a tributary lake of L. Inari in 1964–1966 is the most plausible source for the invasion. Both the initial introduction and the secondary expansion were found not to be associated with significant founder effects. The secondary expansion followed a stepping stone pattern and the source and colonist populations of this expansion have undergone rapid genetic divergence within a period of 15–35 years (ca. 8–17 generations). The rapid divergence may be contributed to lack of gene flow among the source and colonist populations due to the extensive hydroelectric damming in the watercourse. Multiple introductions and substantial genetic variation in combination with the boom‐and‐bust population development of the species thus likely counteracted the founder effects as well as fueled the rapid establishment and expansion of this species within the Inari‐Pasvik watercourse.     

Contrasting Population and Life History Responses of a Young Morph-Pair of European Whitefish to the Invasion of a Specialised Coregonid Competitor,Vendace

Invasions of non-native species represent a global problem of great scientific interest. Here we study in detail the response in population and life history characteristics of closely related native species, with divergent habitat preferences, that are impacted by an invading species over a sufficient time period to allow a new stable state to become established. A time series of 20 years starting at the first occurrence of the invader (vendace Coregonus albula (L.)) allows exploration of the long term population and life history response of two ecologically, morphologically, and genetically different native sympatric morphs (DR- and SR-) of congeneric whitefish C. lavaretus (L.). The whitefish morphs are taxonomically equally related to the invading vendace, but only the planktivorous DR-whitefish share its pelagic niche. We would expect that the ecological differences between the whitefish morphs may be used as a predictor of competitive effects. Vendace exhibited an initial boom-and-bust development, and has continued to fluctuate in density. The responses of the pelagic DR-whitefish were: i) an immediate habitat shift, ii) a subsequent population decline caused by increased annual mortality, and iii) a new stable state at a lower density and apparently relaxed competition. The ecologically more distant benthivorous SR-whitefish also showed significant, but a much more limited response during this process, indicating damped indirect interactions through the food-web. This long-term case-study found that in two native eco-species equally related to the invader, only one of the eco-species was highly affected. Direct competition for resources is obviously important for species interactions, whereas the taxonomic relatedness per se seems to offer little predictive power for invasion effects.     

Global warming is projected to lead to increased freshwater growth potential and changes in pace of life in Atlantic salmon Salmo salar

Global warming has been implicated in widespread demographic changes in Atlantic salmon Salmo salar populations, but projections of life-history responses to future climate change are lacking. Here, we first exploit multiple decades of climate and biological data from the Burrishoole catchment in the west of Ireland to model statistical relationships between atmospheric variables, water temperature, and freshwater growth of juvenile Atlantic salmon. We then use this information to project potential changes in juvenile growth and life-history scheduling under three shared socioeconomic pathway and representative concentration pathway scenarios from 1961 to 2100, based on an ensemble of five climate models. Historical water temperatures were well predicted with a recurrent neural network, using observation-based atmospheric forcing data. Length-at-age was in turn also well predicted by cumulative growing degree days calculated from these water temperatures. Most juveniles in the Burrishoole population migrated to sea as 2-year-old smolts, but our future projections indicate that the system should start producing a greater proportion of 1-year-old smolts, as increasingly more juveniles cross a size-based threshold in their first summer for smoltification the following spring. Those failing to cross the size-based threshold will instead become 2-year-old smolts, but at a larger length relative to 2-year-old smolts observed currently, owing to greater overall freshwater growth opportunity. These changes in age- and size-at-seaward migration could have cascading effects on age- and size-at-maturity and reproductive output. Consequently, the seemingly small changes that our results demonstrate have the potential to cause significant shifts in population dynamics over the full life cycle. This workflow is highly applicable across the range of the Atlantic salmon, as well as to other anadromous species, as it uses openly accessible climate data and a length-at-age model with minimal input requirements, fostering improved general understanding of phenotypic and demographic responses to climate change and management implications.     

Resource partitioning between pelagic coregonids in a subarctic watercourse following a biological invasion

If a pelagic ecosystem is invaded by an efficient planktivorous fish, competition theory predicts that habitat and/or diet segregation should occur if the invader and native planktivores are to co-exist. We compared the diet and pelagic habitat use between invading vendace (Coregonus albula) and two native morphs of whitefish (Coregonus lavaretus) in three subarctic lakes located within the same watercourse. No clear vertical habitat segregation was found as vendace prevailed throughout the entire depth range. Zooplankton generally dominated the diet of all three coregonids. No obvious resource partitioning was found in two of the lakes, while in the third lake the diets of whitefish and vendace differed significantly from each other. Here, the predominant pelagic whitefish morph fed extensively on water mites whereas vendace mainly consumed surface insects and to some extent small fish, apparently as a result of highly depleted zooplankton resources. The results suggest that various levels of resource depletion following the vendace invasion caused the observed differences between the lakes, indicating that different levels of resource partitioning were associated with dissimilar competition intensities. At present, coexistence of these congeneric species seems to be possible, although the vendace invasion has apparently resulted in a challenging and unstable situation for the planktivorous whitefish.     

The role of gill raker number variability in adaptive radiation of coregonid fish

Gill raker divergence is a general pattern in adaptive radiations of postglacial fish, but few studies have addressed the adaptive significance of this morphological trait in foraging and eco-evolutionary interactions among predator and prey. Here, a set of subarctic lakes along a diversifying gradient of coregonids was used as the natural setting to explore correlations between gill raker numbers and planktivory as well as the impact of coregonid radiation on zooplankton communities. Results from 19 populations covering most of the total gill raker number gradient of the genus Coregonus, confirm that the number of gill rakers has a central role in determining the foraging ability towards zooplankton prey. Both at the individual and population levels, gill raker number was correlated with pelagic niche use and the size of utilized zooplankton prey. Furthermore, the average body size and the abundance and diversity of the zooplankton community decreased with the increasing diversity of coregonids. We argue that zooplankton feeding leads to an eco-evolutionary feedback loop that may further shape the gill raker morphology since natural selection intensifies under resource competition for depleted prey communities. Eco-evolutionary interactions may thus have a central role creating and maintaining the divergence of coregonid morphs in postglacial lakes.     

A planktivorous specialist turns rapacious: piscivory in invading vendace Coregonus albula

Following invasion and establishment in a subarctic watercourse, small-sized vendace Coregonus albula were surprisingly found to include fish prey in their diet. The zooplankton resource was severely depleted and only made a minor dietary contribution.     

Three-dimensional migratory behaviour of European silver eels (Anguilla anguilla) approaching a hydropower plant

The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.