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Heat waves are threatening fish around the world, leading sometimes to mass mortality events. One crucial function of fish failing in high temperatures is oxygen delivery capacity, i.e. cardiovascular function. For anadromous salmonids, increased temperature could be especially detrimental during upstream migration since they need efficiently working oxygen delivery system in order to cross the river rapids to reach upstream areas. The migration also occurs during summer and early autumn exposing salmonids to peak water temperatures, and in shallow rivers there is little availability for thermal refuges as compared to thermally stratified coastal and lake habitats. In order to shed light on the mechanisms underpinning the capacity of migrating fish to face high environmental temperatures, we applied a physiological and molecular approach measuring cardiovascular capacities of migrating and resident Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta) in Northern Norway. The maximum cardiovascular capacity of migrating fish was significantly lower compared to the resident conspecifics. The onset of cardiac impairment started only 2°C higher than river temperature, meaning that even a small increase in water temperature may already compromise cardiac function. The migrating fish were also under significant cellular stress, expressing increased level of cardiac heat shock proteins. We consider these findings highly valuable when addressing climate change effect on migrating fish and encourage taking action in riverine habitat conservation policies. The significant differences in upper thermal tolerance of resident and migrating fish could also lead changes in population dynamics, which should be taken into account in future conservation plans.

Climate change and pollution are some of the greatest anthropogenic threats to wild animals. Transgenerational plasticity—when parental exposure to environmental stress leads to changes in offspring phenotype—has been highlighted as a potential mechanism to respond to various environmental and anthropogenic changes across taxa. Transgenerational effects may be mediated via multiple mechanisms, such as transfer of maternal hormones to eggs/foetus. However, sources of variation in hormone transfer are poorly understood in fish, and thus the first step is to characterise whether environmental challenges alter transfer of maternal hormones to eggs. To this end, we explored the population variation and environmental variation (in response to temperature and endocrine disrupting copper) in maternal thyroid hormone (TH), transfer to offspring in a common fish model species, the three-spined stickleback (Gasterosteus aculeatus) using multiple approaches: (i) We compared ovarian TH levels among six populations across a wide geographical range in the Baltic Sea, including two populations at high water temperature areas (discharge water areas of nuclear power plants) and we experimentally exposed fish to (ii) environmentally relevant heat stress and (iii) copper for 7 days. We found that populations did not differ in intraovarian TH levels, and short-term heat stress did not influence intraovarian TH levels. However, copper exposure increased both T4 and T3 levels in ovaries. The next step would be to evaluate if such alterations would lead to changes in offspring phenotype.

Heat waves constitute a challenge for aquatic ectotherms. However, the thermal tolerance of animals and their individual phenotypic plasticity to respond to heat waves may be influenced by thermal history. We tested these hypotheses by comparing the upper thermal tolerance and the individual capacities of three‐spined sticklebacks from populations with different thermal histories to respond to heat waves. Two populations originated from thermally polluted nuclear power plant (NPP) habitats, while four locations represented geographically adjacent control areas. To disentangle the genetic adaptation from the phenotypic plastic response, we measured the individual upper thermal tolerance and the responses at molecular level in common‐garden conditions before and after a laboratory‐mimicked heat wave. We found that the sticklebacks exhibit considerable phenotypic plasticity in thermal tolerance since the heat wave increased fish upper thermal tolerance significantly. The individual plasticity to respond to the heat wave was also negatively correlated with initial thermal tolerance. On the other hand, neither the thermal tolerance nor the plastic responses differed between NPP and control sites despite detection of significant but low genome‐wide divergence in 10 out of 15 pairwise comparisons. Our results suggest that five decades of NPP activity with warmer water have not resulted in a detectable evolutionary change in either the upper thermal tolerance or its plasticity in three‐spined sticklebacks potentially rendering them sensitive to frequent heat waves.

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