It is a nearly universal truth that larger fish eggs have increased chances of survival. However, theory predicts that as temperatures rise, larger eggs may suffer the costs of oxygen limitation and lose their advantage altering the optimal life history trade-off between fecundity and offspring size. Whether these effects may happen under the predicted warming of climate change scenarios is still unknown. Here we show that wild Esox lucius females lay smaller eggs later in the season when temperatures are higher and that this was associated with a seasonal decline in female body size. Experimentally, we found that larger eggs had higher survival rates at lower temperatures, but this advantage diminished in rising temperatures and disappeared entirely in temperatures (~ 18 °C) that could arise under climate change. The results demonstrate that the optimal egg size is temperature dependent, and that future warming may favor smaller females with smaller, more numerous, eggs that may profoundly affect population dynamics and the evolutionary optima of life-history traits.

Skin microbiomes provide vital functions, yet knowledge about the drivers and processes structuring their species assemblages is limited-especially for non-model organisms. In this study, fish skin microbiome was assessed by high throughput sequencing of amplicon sequence variants from metabarcoding of V3-V4 regions in the 16S rRNA gene on fish hosts subjected to the following experimental manipulations: (i) translocation between fresh and brackish water habitats to investigate the role of environment; (ii) treatment with an antibacterial disinfectant to reboot the microbiome and investigate community assembly and priority effects; and (iii) maintained alone or in pairs to study the role of social environment and inter-host dispersal of microbes. The results revealed that fish skin microbiomes harbour a highly dynamic microbial composition that was distinct from bacterioplankton communities in the ambient water. Microbiome composition first diverged as an effect of translocation to either the brackish or freshwater habitat. When the freshwater individuals were translocated back to brackish water, their microbiome composition converged towards the fish microbiomes in the brackish habitat. In summary, external environmental conditions and individual-specific factors jointly determined the community composition dynamics, whereas inter-host dispersal had negligible effects. The dynamics of the microbiome composition was seemingly non-affected by reboot treatment, pointing towards high resilience to disturbance. The results emphasised the role of inter-individual variability for the unexplained variation found in many host-microbiome systems, although the mechanistic underpinnings remain to be identified.

Fish behavior often varies across a species' distribution range. Documenting how behaviors vary at fringes in comparison to core habitats is key to understanding the impact of environmental variation and the evolution of local adaptations. Here, we studied the behavior of Wels catfish (Silurus glanis) in Lake M & ouml;ckeln, Sweden, which represent a European northern fringe population. Adult individuals (101-195 cm, N = 55) were caught and externally marked with data storage tags (DSTs). Fifteen DSTs were recovered a year after tagging, of which 11 tags contained long-term high-resolution behavioral data on the use of vertical (depth) and thermal habitats. This showed that the catfish already became active in late winter (<5 degrees C) and displayed nocturnal activity primarily during summer and late autumn. The latter included a transition from the bottom to the surface layer at dusk, continuous and high activity close to the surface during the night, and then descent back to deeper water at dawn. During the daytime, the catfish were mainly inactive in the bottom layer. These behaviors contrast with what is documented in conspecifics from the core distribution area, perhaps reflecting adaptive strategies to cope with lower temperatures and shorter summers.

Variation in the composition of skin-associated microbiomes has been attributed to host species, geographical location and habitat, but the role of intraspecific phenotypic variation among host individuals remains elusive. We explored if and how host environment and different phenotypic traits were associated with microbiome composition. We conducted repeated sampling of dorsal and ventral skin microbiomes of carp individuals (Cyprinus carpio) before and after translocation from laboratory conditions to a semi-natural environment. Both alpha and beta diversity of skin-associated microbiomes increased substantially within and among individuals following translocation, particularly on dorsal body sites. The variation in microbiome composition among hosts was significantly associated with body site, sun-basking, habitat switch and growth, but not temperature gain while basking, sex, personality nor colour morph. We suggest that the overall increase in the alpha and beta diversity estimates among hosts were induced by individuals expressing greater variation in behaviours and thus exposure to potential colonizers in the pond environment compared with the laboratory. Our results exemplify how biological diversity at one level of organization (phenotypic variation among and within fish host individuals) together with the external environment impacts biological diversity at a higher hierarchical level of organization (richness and composition of fish-associated microbial communities).

The abundance of pike, a keystone top-predator, have declined dramatically in the Baltic Sea since the 1990s likely owing to recruitment failure. It has been proposed that wetland restoration can aid the recovery of the pike stock by increasing the number of recruits produced by anadromous populations. Yet, no previous studies have addressed whether wetland restorations are associated with higher abundances of adult pike in the coastal habitat. To address this, we performed standardised rod-and-reel survey fishing in paired bays with and without wetlands across three coastal areas and 3 years. To estimate dispersal and the contribution of wetland pike to the coastal stock, we tagged captured pike with passive integrated responders (PIT) and employed PIT reader stations in wetland inlets. The results showed that pike abundances were on average 90% higher in bays with an adjacent wetland although the effect varied among areas. Moreover, PIT-data uncovered that wetland pike constituted a high proportion of the pike found in adjacent coastal habitats and that some wetland fish dispersed up to 10 km. These results support that wetland restoration is a valuable tool to aid the coastal pike stock and ultimately restore the function and services of the coastal ecosystem.

Predator-prey interactions can modify population dynamics and influence ecosystem functioning. An interesting, but less understood, aspect of such species interactions is predator-prey role reversal where a mesopredator can reduce the recruitment success, and ultimately the abundance, of their own predator. Such role reversal is ongoing in the Baltic Sea where increasing abundances of mesopredatory sticklebacks (Gasterosteus aculeatus) have resulted in intense predation on the recruits of their predator the northern pike (Esox lucius). Still, the mechanistic underpinning of this reversal remains poorly understood. Here we investigate the role of chemical communication in this predator-prey role reversal with the aim to understand whether and how sticklebacks use olfactory cues to assess whether pike constitutes a predator or prey. By performing a laboratory preference experiment, we show that chemical communication is indeed used by sticklebacks for assessing the predation risk emanated by adult pike but that it requires additional alarm cues (Schreckstoff) informing that sticklebacks are predated upon. Adult pike kairomones or alarm cues alone did not result in any adaptive response by sticklebacks nor did olfactory cues of pike as prey (pike larvae kairomones and dietary cues of zooplankton). This knowledge contributes to the understanding of predator-prey interactions as well as the dynamics of the shifting coastal fish community in the Baltic Sea. The results also suggest that addition of Schreckstoff to confined pike (and perch) spawning and recruitment habitats may deter sticklebacks from entering which should be further explored by management.

Fish skin-associated microbial communities are highly variable among populations and species and can impact host fitness. Still, the sources of variation in microbiome composition, and particularly how they vary among and within host individuals, have rarely been investigated. To tackle this issue, we explored patterns of variation in fish skin microbiomes across different spatial scales. We conducted replicate sampling of dorsal and ventral body sites of perch (Perca fluviatilis) from two populations and characterized the variation of fish skin-associated microbial communities with 16S rRNA gene metabarcoding. Results showed a high similarity of microbiome samples taken from the left and right side of the same fish individuals, suggesting that fish skin microbiomes can be reliably assessed and characterized even using a single sample from a specific body site. The microbiome composition of fish skin differed markedly from the bacterioplankton communities in the surrounding water and was highly variable among individuals. No ASV was present in all samples, and the most prevalent phyla, Actinobacteria, Bacteroidetes, and Proteobacteria, varied in relative abundance among fish hosts. Microbiome composition was both individual- and population specific, with most of the variation explained by individual host. At the individual level, we found no diversification in microbiome composition between dorsal and ventral body sites, but the degree of intra-individual heterogeneity varied among individuals. To identify how genetic and phenotypic characteristics of fish hosts impact the rate and nature of intra-individual temporal dynamics of the skin microbiome, and thereby contribute to the host-specific patterns documented here, remains an important task for future research.

Environmental conditions that vary in space and time influence the distribution, abundance, diversity and evolution of individuals, populations, species and communities. This thesis explores how environmental variation affects diversity at different levels of biological organization, and across a wide range of spatiotemporal scales, by studying fish and their associated microbiomes. The specific aims were to investigate i) effects of coarse- and fine-scale environmental variation for the performance of fish populations and individuals, and ii) ecological drivers impacting the structure and dynamics of microbial communities associated with fish hosts.

For the first aim, I studied effects of environmental variation both within and between local habitats, by comparing populations of spawning migrating pike and monitor sun-basking behaviour of carp individuals. Results revealed that natal spawning site fidelity can promote evolution of local adaptations and population differentiation on relatively fine spatial scales in relation to the species dispersal capacity. I also demonstrated that fish can actively thermo-regulate and attain body temperatures in excess of the surrounding water by sun-basking, and that this translates into faster growth. Homing and sun-basking behaviour thus are important drivers of phenotypic diversity among and within populations and can also - as it turned out - influence the microbial communities associated with fish skin.

For the second aim, I used a mixture of observational and experimental approaches to characterize and identify sources of variation in microbial communities associated with fish skin of perch, roach and carp. An important finding was that fish skin microbiomes are highly dynamic biodiversity hotspots. Results further suggested that variation in the assembly, composition, spatial structure, and temporal shifts of these microbiomes are influenced by stochastic events in combination with ecological filtering imposed by environment and host phenotype, most notably behaviour. 

A key conclusion that emerges from this thesis is that diversity at one level of biological organisation seems to support and increase diversity at a higher hierarchical level of organisation. My thesis thus adds to the knowledge, and contribute new understanding and insight into, how environmental heterogeneity and the complex interplay between different species and hierarchical levels generate and maintain biodiversity.

In terrestrial environments, cold-blooded animals can attain higher bodytemperatures by sun basking, and thereby potentially benefit from broaderniches, improved performance and higher fitness. The higher heat capacityand thermal conductivity of water compared with air have been universallyassumed to render heat gain from sun basking impossible for aquaticectotherms, such that their opportunities to behaviourally regulate body temperatureare largely limited to choosing warmer or colder habitats. Here wechallenge this paradigm. Using physical modelswe first showthat submergedobjects exposed to natural sunlight attain temperatures in excess of ambientwater. We next demonstrate that free-ranging carp (Cyprinus carpio) canincrease their body temperature during aquatic sun basking close to thesurface. The temperature excess gained by basking was larger in dark thanin pale individuals, increased with behavioural boldness, and was associatedwith faster growth. Overall, our results establish aquatic sun basking as a novelecologically significant mechanism for thermoregulation in fish. The discoveryof this previously overlooked process has practical implications for aquaculture,offers alternative explanations for behavioural and phenotypicadaptations, will spur future research in fish ecology, and calls for modificationsof models concerning climate change impacts on biodiversity inmarine and freshwater environments.

Spatial sorting is a process that can contribute to microevolutionary change by assemblingphenotypes through space, owing to nonrandom dispersal. Here we first buildupon and develop the “neutral” version of the spatial sorting hypothesis by arguingthat in systems that are not characterized by repeated range expansions, the evolutionaryeffects of variation in dispersal capacity and assortative mating might not beindependent of but interact with natural selection. In addition to generating assortativemating, variation in dispersal capacity together with spatial and temporal variationin quality of spawning area is likely to influence both reproductive success and survivalof spawning migrating individuals, and this will contribute to the evolution of dispersal-enhancingtraits. Next, we use a comparative approach to examine whether differencesin spawning migration distance among 18 species of freshwater Anguilla eelshave evolved in tandem with two dispersal-favoringtraits. In our analyses, we use informationon spawning migration distance, body length, and vertebral number thatwas obtained from the literature, and a published whole mitochondrial DNA-basedphylogeny. Results from comparative analysis of independent contrasts showed thatmacroevolutionary shifts in body length throughout the phylogeny have been associatedwith concomitant shifts in spawning migration. Shifts in migration distance werenot associated with shifts in number of vertebrae. These findings are consistent withthe hypothesis that spatial sorting has contributed to the evolution of more elongatedbodies in species with longer spawning migration distances, or resulted in evolution oflonger migration distances in species with larger body size. This novel demonstrationis important in that it expands the list of ecological settings and hierarchical levels ofbiological organization for which the spatial sorting hypothesis seems to have predictivepower.