Climate change impact on high latitude freshwater fish populations

Climate change is one of the greatest threats to animal wildlife in high latitude freshwater ecosystems. Climate warming is rapidly increasing water temperatures in these areas, affecting biological processes of ectotherms such as growth, maturation and reproduction, which in turn trigger population responses. The magnitude of the effects of climate warming will vary depending on the thermal niche and phenotype of species. Climate change will continue to redistribute species, and fish species from warmer temperature guilds will invade and possibly take over areas where cold water fish currently dominate. Hence, it is important to establish the performance of cold vs warmer water species in a warming Arctic. The aim of this thesis is therefore to provide novel insights and predictions on population level implications of climate change for both cold- and cool water fish at high latitudes. The primary focus is on climate effects mediated by direct and indirect individual-level responses to increasing water temperatures, addressed using long-term empirical investigations and modelling in retrospective and prospective studies. In addition, the thesis addresses interactions between climate change and size-selective harvesting, a main pressure on high latitude fish populations, by modelling their cumulative effects to evaluate risks and reveal potential synergistic threats. The thesis documents how both cold- and cool water fish at their northern range edge have increased their somatic growth rates during the last three decades of warming. However, the cool-water adapted vendace and perch displayed a higher increase in juvenile somatic growth with warming compared to cold-water Arctic charr and whitefish, stressing how the thermal niche modulates the magnitude of warming effects. The individual based models developed for this thesis predict a further increase in somatic growth towards year 2100 under warming scenarios (RCP-4.5, -8.5), with cool water fish displaying a greater increase in somatic growth rate than cold water fish. The documented and projected climate driven increase in somatic growth rate mediates changes in survival rates and life history, including a likely increase in juvenile survival, and earlier maturation, the latter being contingent on species’ maturation reaction norm. The demographic implications of these individual effects were investigated via modelling and long-term empirical studies. The population level response to climate warming, mediated by individual effects, was evident in the cool water adapted perch, which experienced a substantial increase in density and importance relative to the cold-water adapted whitefish, which is dominant in the investigated lakes. The population response of this cool water fish was mediated by an increase in juvenile growth rate which resulted in larger size at age and earlier maturation, but also a likely increase in survival through the first critical winter. The modelled populations displayed higher biomass and yield as size at age increased with warming, but this effect was larger in the cool water specie than in the cold water species. In sum, cool water fish will benefit more from climate warming than cold water fish at high latitudes, and where they coexist, cool water fish may become the dominant player in the fish community. The climate driven increase in size at age affects the age-specific exposure to size-selective harvesting, increasing the risk of younger individuals being caught by gillnets. The population level effect of earlier gillnet exposure is an increased age truncation, as illustrated by individual based model outcomes. Also, larger size at age increased the proportion of immature individuals being caught, with the magnitude of the effect being contingent on growth trajectories, their temperature dependence, and orientation of the maturation reaction norm. The increased juvenile mortality and more pronounced age truncation reduce recruitment, increasing the vulnerability of exploited populations to environmental stressors. Fish species with large size, slow growth, and late maturation like Arctic charr were more vulnerable to warming and harvesting than species with a faster life history, like vendace. In conclusion, the stronger positive effects of warming on the performance of cool-water adapted species relative to cold-water salmonids, and the greater vulnerability of the latter when exposed to size-selective harvesting, warn of incipient reorganizations of Arctic fish communities, and invite climate adaptation in the management of high latitude populations

Twig selection on mountain birch Betula pubescens by winter-feeding willow grouse Lagopus lagopus in a subarctic forest

In a subarctic forest at Kvaløya, northern Norway, willow grouse Lagopus lagopus fed at snow level by clipping bits of twigs from mountain birch Betula pubescens during winter. Birch has two types of twigs ending in a terminal bud: long twigs with a smooth bark, and short twigs with rings of thicker bark. The grouse selected ringed twigs above smooth twigs despite a surplus of smooth twigs in the forest. Ringed twigs had more bark cm−1 of twig length and a higher relative bark/wood ratio than smooth twigs. Smooth twigs had growth nodes that increased in diameter inwards from the tip. Because of the non-linear relation between the area and the circumference of a circle, the bark/wood ratio decreased for each node. Although being able to clip much thicker twigs, 90% of smooth twigs clipped by grouse were ≤ 2 mm in diameter. It is concluded that willow grouse fed optimally on birch in winter by selecting twigs to minimize fibrous wood intake

Climate change impact on high latitude freshwater fish populations

Climate change is one of the greatest threats to animal wildlife in high latitude freshwater ecosystems. Climate warming is rapidly increasing water temperatures in these areas, affecting biological processes of ectotherms such as growth, maturation and reproduction, which in turn trigger population responses. The magnitude of the effects of climate warming will vary depending on the thermal niche and phenotype of species. Climate change will continue to redistribute species, and fish species from warmer temperature guilds will invade and possibly take over areas where cold water fish currently dominate. Hence, it is important to establish the performance of cold vs warmer water species in a warming Arctic. The aim of this thesis is therefore to provide novel insights and predictions on population level implications of climate change for both cold- and cool water fish at high latitudes. The primary focus is on climate effects mediated by direct and indirect individual-level responses to increasing water temperatures, addressed using long-term empirical investigations and modelling in retrospective and prospective studies. In addition, the thesis addresses interactions between climate change and size-selective harvesting, a main pressure on high latitude fish populations, by modelling their cumulative effects to evaluate risks and reveal potential synergistic threats. The thesis documents how both cold- and cool water fish at their northern range edge have increased their somatic growth rates during the last three decades of warming. However, the cool-water adapted vendace and perch displayed a higher increase in juvenile somatic growth with warming compared to cold-water Arctic charr and whitefish, stressing how the thermal niche modulates the magnitude of warming effects. The individual based models developed for this thesis predict a further increase in somatic growth towards year 2100 under warming scenarios (RCP-4.5, -8.5), with cool water fish displaying a greater increase in somatic growth rate than cold water fish. The documented and projected climate driven increase in somatic growth rate mediates changes in survival rates and life history, including a likely increase in juvenile survival, and earlier maturation, the latter being contingent on species’ maturation reaction norm. The demographic implications of these individual effects were investigated via modelling and long-term empirical studies. The population level response to climate warming, mediated by individual effects, was evident in the cool water adapted perch, which experienced a substantial increase in density and importance relative to the cold-water adapted whitefish, which is dominant in the investigated lakes. The population response of this cool water fish was mediated by an increase in juvenile growth rate which resulted in larger size at age and earlier maturation, but also a likely increase in survival through the first critical winter. The modelled populations displayed higher biomass and yield as size at age increased with warming, but this effect was larger in the cool water specie than in the cold water species. In sum, cool water fish will benefit more from climate warming than cold water fish at high latitudes, and where they coexist, cool water fish may become the dominant player in the fish community. The climate driven increase in size at age affects the age-specific exposure to size-selective harvesting, increasing the risk of younger individuals being caught by gillnets. The population level effect of earlier gillnet exposure is an increased age truncation, as illustrated by individual based model outcomes. Also, larger size at age increased the proportion of immature individuals being caught, with the magnitude of the effect being contingent on growth trajectories, their temperature dependence, and orientation of the maturation reaction norm. The increased juvenile mortality and more pronounced age truncation reduce recruitment, increasing the vulnerability of exploited populations to environmental stressors. Fish species with large size, slow growth, and late maturation like Arctic charr were more vulnerable to warming and harvesting than species with a faster life history, like vendace. In conclusion, the stronger positive effects of warming on the performance of cool-water adapted species relative to cold-water salmonids, and the greater vulnerability of the latter when exposed to size-selective harvesting, warn of incipient reorganizations of Arctic fish communities, and invite climate adaptation in the management of high latitude populations

Life history strategies of a trimorphic population of Arctic charr (Salvelinus alpinus (L.)) in Skogsfjordvatn, northern-Norway

Polymorphisms are widespread throughout many different taxa of vertebrates. Discrete polymorphisms or morphs usually differentiate in morphology, ecology and life history, most likely driven by adaptations to different habitats and resources. For sympatric morphs to be able to maximize fitness in different niches and habitats, they may develop differences in several life history traits. Arctic charr (Salvelinus alpinus (L.)) is a good model species for verifying and understanding ecological driven polymorphism and speciation. Here, I address the life history traits of three sympatric living Arctic charr morphs in Skogsfjordvatn in northern Norway. The charr morphs differ in diet and habitat utilization; including one morph that feeds on benthic invertebrates and zooplankton in the littoral-pelagic zone (referred to as the LO-morph), a second that feeds on chironomids and small mussels in the profundal zone (PB-morph), and a third that also inhabits the profundal zone, but has a piscivorous diet (PP-morph). The littoral and the two profundal spawning morphs are differentiated in time and place of spawning and all morphs are genetically differentiated, indicating complete reproductive isolation. These morphs also had contrasting differences in numerous life history traits

The trade-off between fecundity and egg size in a polymorphic population of Arctic charr (Salvelinus alpinus (L.)) in Skogsfjordvatn, subarctic Norway

Reproductive traits differ between intralacustrine Arctic charr morphs. Here, we ex - amine three sympatric lacustrine Arctic charr morphs with respect to fecundity, egg size and spawning time/site to assess reproductive investments and trade- offs, and possible fitness consequences. The littoral omnivore morph (LO- morph) utilizes the upper water for feeding and reproduction and spawn early in October. The large pro - fundal piscivore morph (PP- morph) and the small profundal benthivore morph (PB- morph) utilize the profundal habitat for feeding and reproduction and spawn in December and November, respectively. Females from all morphs were sampled for fecundity and egg- size analysis. There were large differences between the morphs. The PB- morph had the lowest fecundity (mean = 45, SD = 13) and smallest egg size (mean = 3.2 mm, SD = 0.32 mm). In contrast, the PP- morph had the highest fecundity (mean = 859.5, SD = 462) and the largest egg size (mean = 4.5 mm, SD = 0.46 mm), whereas the LO- morph had intermediate fecundity (mean = 580, SD = 225) and egg size (mean = 4.3, SD = 0.24 mm). Fecundity increased with increasing body size within each morph. This was not the case for egg size, which was independent of body sizes within morph. Different adaptations to feeding and habitat utilization have apparently led to a difference in the trade- off between fecundity and egg size among the three different morphs

Environmentally modulated repeat evolution of polymorphic Arctic charr life history traits

Sympatric Arctic charr, Salvelinus alpinus (L. 1758), morphs have flexible but repeated life history strategies tested across five Norwegian lakes. In several Scandinavian polymorphic Arctic charr populations differentiated by their diet and habitat use, a large littoral omnivorous (LO) morph commonly cooccurs with a smaller profundal spawning (PB/PZ) morph. A third, large piscivorous (PP) morph is also known to occur within a portion of Arctic charr populations in the profundal habitat along with the PB/PZ individuals. Life history traits, such as age at maturity, growth, and diet are known to differ among coexisting morphs. Notably, the PP morph was the longest morph with the oldest age at maturity while the PB/PZ morph showed the shortest lengths overall and youngest age with LO morph being intermediate in both traits. Growth parameters differed across all the morphs. When examining growth within morph groups, the LO morph was found to have different growth across all lakes, while similar reproductive investments and different energy acquisition patterns were seen within the PB/PZ and PP morphs. These results suggest repeat evolution in several life history strategies of reproductively isolated Arctic charr sympatric morphs, notably for the first time in the PP morph, while also highlighting the importance of the local environment in modulating life history traits

Climate warming accelerates somatic growth of an Arctic fish species in high-latitude lakes

High-latitude aquatic ecosystems are responding to rapid climate warming. A longer ice-free season with higher water temperatures may accelerate somatic growth in lake ectotherms, leading to widespread ecological implications. In fish, rising temperatures are expected to boost rates of food intake and conversion, and predictions based on empirical relationships between temperature and growth suggest a substantial increase in fish growth rates during the last decades. Fish abundance negatively affects growth by limiting food availability. This field study addresses the effects of climate warming on growth of a subarctic population of Arctic charr (Salvelinus alpinus (L.) over nearly 40 years. Juvenile growth of 680 individuals of Arctic charr, was reconstructed by sclerochronological analysis using sagittal otoliths sampled annually from the early 1980s to 2016. Statistical modelling revealed a positive effect of water temperature, and a negative effect of abundance on somatic growth in juvenile individuals. Temperature dependence in growth was significant for average and fast-growing individuals across all investigated age classes. These findings suggest that, as temperatures rise, somatic growth of Arctic charr will increase in high latitude lakes. Climate warming will thus influence cold water fish life history and size-structured interactions, with important consequences for their populations and ecosystems

The association between parasite infection and growth rates in Arctic charr: do fast growing fish have more parasites?

Trophically transmitted parasites are known to impair fish growth in experimental studies, but this is not well documented in natural populations. For Arctic charr [Salvelinus alpinus (L.)], individual growth is positively correlated with food consumption. However, increased food consumption will increase the exposure to trophically transmitted parasites. Using a correlative approach, we explore the association between parasite abundance and the individual growth of Arctic charr from five lakes within the same watercourse. The studied parasite species differ in their life cycles and cost to the host. We predicted a positive association between parasite abundance and fish growth for parasites of low pathogenicity reflecting high consumption rates, and a negative association at higher parasite abundances for more costly parasites. We found no direct negative associations between parasite abundance and fish growth. The relationship between parasite abundance and growth was linearly positive for the low costly Crepidostomum sp. and concave for the more costly Eubothrium salvelini. In natural fish populations, the negative effects of parasites on fish growth might be outweighed by the energy assimilated from feeding on the intermediate host. However, experimental studies with varying food consumption regimes are needed to determine the mechanisms underlying our observations

How vulnerable is the European seafood production to climate warming?

The main challenge for the European seafood industry is to ensure sustainable production volume while adapting to climate warming. Marine fisheries mainly target 41 species which account for 80% of the seafood production in Europe. The remaining 20% comes from marine and freshwater aquaculture, which harvest mainly 5 and 11 species, respectively. European seafood production volume (2004–2014) recorded by FAO was combined with indices of temperature sensitivity and biological sensitivity (BS) based on the life histories of the main exploited species. We found that the marine sectors are more vulnerable to global warming than the freshwater sector. The vulnerability to warming of a country’s production is defined by the temperature sensitivity and the BS of the main exploited seafood species, weighted by their production volume. Production vulnerability in the marine sector increases with latitude due to the temperature sensitivity of the harvested species and their high production volume. No such gradient is found in the freshwater sector because most of the production is based on two species with opposite temperature sensitivity. To ensure a sustainable European seafood production, national climate strategies and action plans should include both fisheries and aquaculture and be integrated at a regional level

Data on European seafood biomass production by country, sectors, and species in 2004–2014 and on ecological characteristics of the main species produced

In this data article, we present the 2004–2014 average European seafood production volume by production sector, country, and species. The production data originates from the Food and Agriculture Organisation of the United Nations (FAO) and covers three production sectors: Marine fisheries, marine aquaculture, and freshwater production. We present the main ecological characteristics of each species produced or harvested. These species characteristics were retrieved from various published sources and include biological sensitivity to harvesting and temperature ranges for the most important species. These indices were weighted by each species production volume in order to produce maps of European country’s color-coded by their overall temperature range, maximum temperature, and biological sensitivity within each production sector