As he once described himself (Vøllestad 2024), we can imagine Asbjørn Vøllestad as a young boy growing up on a small farm on a remote island, spending his free time either reading or fishing with a simple bamboo rod. Years later, after completing his undergraduate studies in biology, he began exploring the fascinating dynamics of fish migration in his master's thesis.
In the 1980s, Asbjørn had his first contact with an international scientific journal and published his first paper—a remarkable achievement at a time when having an international focus in research was still uncommon in much of the scientific community. Since then, with nearly 200 published papers, Asbjørn has remained an active and respected member of the scientific community. Notably, he has had many very successful international collaborations, and he has served on numerous student committees, some internationally.
Asbjorn is famous for his dry wit, sensible approach to science and life, and wise counsel. Everything just seems calmer and clearer when he is in the room.
Now, as he approaches retirement and after 20 years of dedicated service as a senior editor of Ecology of Freshwater Fish (EFF, 2005–2025), Asbjørn steps down from the Editorial Board. Throughout all these years, Asbjørn has demonstrated a high level of professionalism, scientific rigour, and unwavering commitment to advancing and disseminating knowledge in the field of freshwater ecology. His editorial leadership has made a significant contribution to maintaining the journal's academic excellence and enhancing its international visibility and impact. His diligent coordination of the peer-review process, meticulous attention to content quality, and ability to foster constructive dialogue between authors and reviewers have been key to the journal's success and continued relevance in the scientific community. All those involved with the journal—fellow editors, reviewers, and Wiley staff—would like to acknowledge and express their sincere gratitude for his outstanding editorial work and unwavering commitment to the journal.
As he begins this new chapter, we hope that Asbjørn will continue to contribute to EFF as a valued author and reviewer, sharing his deep expertise and insight. We also hope he finds again the quiet pleasure of those childhood days—peaceful afternoons filled with reading and fishing.
Nuria Sanz wrote the original draft. Nuria Sanz and David Heins reviewed and edited the text.
The authors declare no conflicts of interest.
Most salmonid populations are declining across their entire habitat range, partly because of large-scale loss of crucial physical habitats. Alterations in river flow and temperature resulting from climate change are likely to further degrade habitat quality, particularly summer thermal conditions experienced by temperature-sensitive fish species. Understanding how summer thermal conditions control the spatial distribution of ectotherms is thus central to helping project the consequences of climate change and develop management solutions. This study uses snorkelling fish surveys collected over 10 years and airborne thermal infrared (TIR) mapping of surface temperature acquired in 2022 to assess the relationship between European grayling distribution and thermal habitats along a 9-km long reach of the Allondon River, Switzerland. Results show that all 3 grayling life stages (adults, sub-adults and juveniles) respond negatively to elevated summer temperature, with distribution patterns highlighting thermal structuring effects on fish populations. The presence of two cooler reaches appears critical to the survival of the Allondon's declining grayling population, while the warmest reach that separates these habitats potentially acts as a thermal barrier during critical summer conditions. These results were used to guide local stakeholders towards short-term and longer-term actions to be taken on the river, which include: concerted trans-national management to protect key upstream tributaries, tree planting to limit summer peak temperature and strategic protection of cold-water patches that may act as thermal refuges during critical periods.
Freshwater ecosystems are in the midst of a biodiversity crisis marked by non-random and often dramatic shifts in fish community composition. Yet, the patterns and underlying ecological drivers of these community dynamics within gravel pit lakes—widespread artificial lakes with significant socioeconomic and ecological value—remain understudied. This study examines patterns and drivers of beta-diversity of fish communities across 16 gravel pit lakes in southwestern France over 10 years. Our results unveiled strong shifts in fish community composition, with lake maturity (older and more productive) emerging as the main driver. Immature lakes experienced substantial species turnover over one decade, harbouring more unique fish communities, whereas mature lakes showed little change, contributing less to overall beta-diversity. Highly invaded lakes displayed higher levels of species nestedness than species turnover compared with uninvaded lakes. The contributions of native and non-native species to beta-diversity remained relatively constant on average over the study period, although with some notable exceptions. Non-native species such as common carp (Cyprinus carpio) and Wels catfish (Silurus glanis) had increasing contributions to beta-diversity, whereas native European perch (Perca fluviatilis) and roach (Rutilus rutilus) were increasingly more common (i.e., widespread) despite declining biomass when present. A number of abiotic and biotic factors likely contributed to the diversity patterns observed. Eutrophication is associated with increasing turbidity and hypoxia/anoxia, promoting the dominance of more tolerant, largely non-native, fish species. The higher proportion of species nestedness in invaded communities may stem from reduced invasibility or local extinctions over time. Finally, European perch, a pioneer species, exhibited increasing homogenisation of its biomass across lakes over the 10-year period, resulting in similar small populations distributed throughout the study area. These findings highlight temporal changes in fish community composition within gravel pit lakes, revealing the complex interplay between lake maturity and non-native species dynamics as major drivers.
The Neotropical catfish family Trichomycteridae contains parasitic fishes that include scale and mucus feeders (subfamily Stegophilinae) and blood feeders (i.e., ‘candirus’, subfamily Vandelliinae). The influence of parasitic catfishes on food web dynamics is unknown. Most ecological information on the feeding ecology of freshwater parasite catfishes has been based on visual observation of diets via stomach contents analysis, a method that does not provide information about the origin of ingested blood or mucus (interspecific interactions) or vertical trophic position of these parasites. We used stable isotope analysis of carbon (d13C) and nitrogen (d15N) to explore the role of parasitic catfishes in a river food web in Guyana, South America. We obtained tissue samples from over 100 fish species, including three genera of trichomycterid parasitic catfishes. Parasitic catfishes were estimated to have the highest trophic positions in the food web, which, given their small size (~5 cm total length), contributed to a weak relationship between trophic position and fish body size for the fish community. Ochmacanthus, a mucus-feeder, had higher d15N and estimated trophic position than the two blood feeders, Paracanthopoma and Vandellia. Results from stable isotope analysis indicate that parasitic catfishes should be considered among the top consumers in this hyperdiverse fish community.
The black basses of the southeastern United States are an excellent study group for investigating the relative roles of habitat and biotic interactions for determining species co-occurrence. This group is comprised of generalist species such as Largemouth Bass Micropterus nigricans, as well as specialist species such as those within the M. coosae complex. Bartram's Bass M. sp. cf. coosae is a provisional species of particular concern endemic to the upper Savannah River basin of Georgia, South Carolina and North Carolina, USA. Bartram's Bass and Largemouth Bass may occur in the same rivers, but do not necessarily co-occur in the same micro- or mesohabitats. Recent surveys in numerous Savannah River tributaries where Bartram's Bass was previously known to have existed have either detected only Largemouth Bass or no black basses at all. The apparent loss of endemic Bartram's Bass and frequent replacement with cosmopolitan Largemouth Bass is concerning from a conservation perspective because it could indicate either (a) ongoing biotic homogenisation via habitat shifts, (b) habitat-mediated asymmetric interspecific interactions resulting incompetitive exclusion, or (c) some combination of these processes. In this study, we assessed differences in landscape-level factors and proximity to reservoirs on the occurrence and co-occurrence of these species using a multispecies occupancy model developed for two or more potentially interacting species. This approach can be used to untangle the relative roles that biotic interactions or habitat covariates have on species distributions. The most compelling model evidence suggested that Bartram's Bass and Largemouth Bass were associated with land cover effects on instream habitat and provided less evidence that co-occurrence was driven by interspecific interactions. Human land use change and impoundment of free-flowing rivers have created new opportunities for Largemouth Bass to access upstream lotic habitats and are associated with the loss of lotic habitats and restricted the distribution of Bartram's Bass.
Sea-run brown trout use lake habitat extensively when it is available both before and after spawning. However, the ways in which brown trout actually use lakes remain poorly resolved. In Norway, some of these lakes are used as reservoirs for the effluent of high-head power stations that tunnel mountain water through turbines, discharging the water into the lakes that support migratory species like trout. In this study, we calculated the positions of tagged trout in Lake Vassbygdivatnet, western Norway from July–November 2021 using acoustic telemetry and YAPS software in the R programming environment. We fitted kernel density estimators to the positioning data to estimate core areas where the trout were found within the lake and identified hotspots close to shore near locations in the lake where cascades discharged from the surrounding mountains. However, a resource selection function type analysis identified no affinity of the trout for the area immediately adjacent to the high-head power station outlet. Finally, a hidden Markov model determined that the nearshore lake margin was mostly used by trout for more passive states (low directionality, short steps), whereas the middle of the lake was more frequently used for active states (high directionality, long steps). The findings demonstrate for the first time that mountain cascades discharging into lakes provide habitat that attracts trout and that there is no clear attraction towards the power station outlet during the pre-spawn period. Risk of entrainment in the power station tunnel therefore does not seem to be a major risk factor for migrating trout. However, fisheries managers may wish to use these findings when regulating fishing activities in lakes like Vassbygdivatnet where trout stage in preparation for spawning.
In recent years (post 2010), the recruitment of populations of cisco (Coregonus artedi) in the Great Lakes and surrounding inland lakes has generally increased. Conversely, recruitment of lake whitefish (C. clupeaformis) populations is declining, in some cases at alarming rates. Differential survival of the two species is likely set during early life stages and may be partially explained by differences in avoidance behaviours, particularly in the presence of more recently introduced nearshore predators, like round goby (Neogobius melanostomus) and rainbow smelt (Osmerus mordax). We evaluated the predation success of three sizes of round goby (small, 59–59 mm total length (TL)), medium, 82–92 mm TL and large, 111–132 mm TL) and one size of rainbow smelt (130–144 mm TL) consuming lake whitefish and cisco at various early life stages—larval, early juvenile and juvenile. Controlled predation trials were conducted and recorded for 30 min. to examine foraging behaviours of potential predators and avoidance patterns of the two coregonine species at the three early life stages. Round goby capture success and attacks per pursuit were primarily related to coregonine life stage (i.e., size), but medium and large round goby had higher success with captures and/or predation efficiency on early juvenile and juvenile lake whitefish than cisco. The number of attacks per pursuit by rainbow smelt was higher on juvenile lake whitefish than cisco, which did not lead to different capture rates. Juvenile cisco exhibited stronger anti-predator response (i.e., tighter group cohesion) than juvenile lake whitefish, which could contribute to lower predation success and/or lower efficiency by introduced predators. Our results suggest that round goby (> 65 mm TL) may have an impact on survival of early juvenile and juvenile lake whitefish, and rainbow smelt may have a larger impact on juvenile lake whitefish than cisco during times of spatial overlap in late spring. Predation by these two invasive fish species in the Great Lakes has the potential to at least partially explain recruitment differences between lake whitefish and cisco.
Understanding the ecology and behaviour of freshwater fish species is essential for effective management and conservation. Within the genus Barbus, small-sized mountain species remain understudied despite conservation concerns. In this study, we investigated the movement patterns, habitat use and flow preferences of the endangered species Barbus caninus in a Mediterranean mountain stream in northern Italy using passive integrated transponder telemetry. Over a 15-month period, 164 tagged fish were tracked between 2 and 30 times. Most individuals exhibited small home ranges, with a median linear range of 33.7 m, although a few fish moved over hundreds of metres. Fish size did not influence movement patterns. Movements were seasonally variable, with a significantly larger linear range observed during spring compared to other seasons. The barbel showed a weak preference for fast-flowing mesohabitats, and this preference was pronounced during winter. Despite this, B. caninus predominantly utilised low-velocity confined zones within hydromorphological units, potentially indicating energetically efficient microhabitat use. These findings shed light on the habitat requirements and movement patterns of an endangered species that inhabits small mountain streams and is subject to numerous anthropogenic threats.
Successful strategies of predator deterrence or avoidance are necessary for juvenile organisms to survive to adulthood. Individuals should balance the benefits and risks of any strategy they choose. One strategy used by many fish species is shoaling by forming a loose grouping of conspecifics. This may deter predators through dilution or confusion, though it also increases competition between shoal mates. Another strategy is hiding to avoid predation, though hiding spots are only effective to avoid visual predators and may also be used by predators. Juvenile convict cichlids (Amatitlania nigrofasciata) were given a choice between a shoal of juvenile conspecifics and a potential shelter (plants/rocks), in the presence and absence of a predator (Parachromis dovii). We predicted that the fish would prefer the shoal when the predator was present and would prefer the potential shelter when the predator was absent because while shoaling is a predator deterrent it increases competition for resources. We found that the juvenile convict cichlids preferred to spend more time with a shoal than the potential shelter and did not prefer the potential shelter alone, and this preference did not change with predator presence. This suggests that not only did they prefer the shoal, but that they may have been avoiding the potential shelters, indicating a flexible strategy.
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