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.

Several wild bird and fish species across the Northern Hemisphere have been shown to episodically be thiamine deficient. This may lead to mass-mortality events, especially in offspring. To understand the mechanisms underlying thiamine deficiency we need a better understanding of the dynamics and somatic allocation of the vitamin. Here we focus on a common, ecologically and economically important species, that is, Atlantic cod (Gadus morhua), which has been suggested to be sensitive to thiamine deficiency. We sampled cod of varying sizes and maturity stages in a system where thiamine deficiency regularly occurs (i.e., Baltic Sea) and compare these with cod from the North Atlantic, where this deficiency has not been recorded. Results show that thiamine concentrations were tissue-specific. Concentrations in muscle and liver generally declined during growth and maturation, whereas concentrations in gonads increased. Of the total thiamine in a female's body, approximately 70% of the total pool was allocated to the gonads at the onset of reproduction, suggesting that micronutrients constitute a major investment when spawning. Free thiamine was the dominating vitamer in gonads and increased in proportion of total thiamine as gonads developed, whereas the muscle and liver's relative composition of vitamers was constant with thiamine diphosphate dominating. Transketolase activity and latency suggest that livers were saturated with thiamine and there was no evidence of ongoing thiamine deficiency. Likewise, thiamine concentrations were similar between areas with different histories of thiamine deficiency when accounting for differences in size and reproductive state, suggesting that thiamine statuses were comparable. We show that life cycle and tissue-specific dynamics in thiamine concentrations should be considered when assessing the thiamine status of a species. Furthermore, we discuss how specific life history traits related to spawning may put species at higher risk of thiamine deficiency.

Many coregonine species have declined drastically across the Northern Hemisphere, including populations of Coregonus maraena (whitefish) in the Baltic Sea, and the mechanisms leading to these declines are not well investigated. An abrupt population crash occurred in the 1990s, coinciding with heavy declines in salmonid recruitment, also known as thiamine deficiency syndrome. Thiamine, i.e., vitamin B1, is an essential micronutrient needed for a functional metabolism. Offspring with thiamine deficiency have a high mortality posing significant negative impact on populations. Here, we aim to determine if whitefish, like other salmonids in the Baltic Sea, is affected by thiamine deficiency. Anadromous whitefish were therefore sampled during spawning in rivers of Southeastern Sweden, and we compared tissue concentrations and thiamine-dependent enzyme latencies to published thresholds. Further, we tested whether the variation in thiamine concentrations among individuals could be explained by physiological and morphological traits. Results showed that latency of thiamine-dependent enzymes along with egg thiamine concentrations suggest no evident thiamine deficiency. Concentrations were generally higher in the liver compared to muscle tissues. While females had lower liver thiamine concentrations compared to males, the opposite was found for muscle tissues, suggesting sex-specific patterns of allocation of the vitamin. Concentrations in eggs were positively related to the condition of the females and, similar to muscle and liver tissues, tended to be negatively related to standardized gill raker length. The latter is often used as a proxy for characterizing the feeding niche of coregonines. As has been observed in a number of other organisms (e.g., fish and molluscs), there was a reduction in thiamine concentration with length. Hence, the populations studied here showed no evidence of exhibiting thiamine deficiency. The variation in thiamine concentrations could largely be attributed to intrinsic physiological traits as well as traits associated with coregonine feeding niche.

Human activities increase turbidity in aquatic environments worldwide, which often affects fish behaviour. However, predicting how species react to higher turbidity remains difficult, as responses vary depending on the species, their ecology and the ecosystem. It is thus important to improve our understanding of the responses of fishes living in ecosystems experiencing recent increases in turbidity, especially those with unique species compositions where biodiversity is most vulnerable. One such ecosystem is Lake Tanganyika in East Africa, which is home to a diverse fish community with a high degree of endemism. In this study, we conducted a laboratory experiment with the territorial cichlid, Neolamprologus pulcher, which is endemic to Lake Tanganyika, to investigate the effects of increased turbidity on territorial and exploratory behaviour. We found that moderate increases in turbidity led to reduced territory defence, decreased exploration and increased time spent in shelters. Given that these fish live in large colonies, feed on planktonic particles in the water column and defend their territory against conspecific and heterospecific intruders, these behavioural changes are likely to have substantial implications for their social structure and reproduction in their native environments. Our study raises important questions about whether these effects will persist in the long term as human activities are likely to continue to increase turbidity in the lake over the coming decades and whether the responses to turbidity affect the community composition of fishes in Lake Tanganyika.

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.

Climate change causes earlier and warmer springs in seasonal environments and a higher incidence of extreme weather events. In aquatic environments, this changes the thermal conditions during spawning, and the thermal performance of eggs and embryos may determine the consequences of climate change on recruitment. In iteroparous species with indeterminate growth, the eggs produced by a given female in successive years will increase in size as the female grows larger and likely be exposed to different temperatures during incubation due to annual variation in spring phenology. Still, we know little about whether differences in maternal size impact the temperature-dependent performance and viability of the offspring. Here we utilised a thermal gradient laboratory experiment on Baltic Sea perch (Perca fluviatilis) to investigate how maternal size influence the temperature dependent hatching success of the offspring. The results uncovered a positive relationship between maternal size and average hatching success, but the shape of the relationship (reaction norm) linking hatching success to incubation temperature was independent of maternal size. However, we did find an association between maternal size and the variance (S.D. and CV) in hatching success across temperatures, with larger females producing offspring with maintained performance (less sensitive) across temperature treatments, indicative of flatter reaction norms and broader thermal niches. This suggests that maintaining the size distribution of fish populations, for instance through regulations of size-selective fisheries, may be important to aid the long-term productivity and viability of fish populations and ultimately conserve the function and services of ecosystems.

Biological invasions are a major component of global change worldwide. But paradoxically, an invasive species might also have threatened populations within its native range. Designing efficient management policies is needed to prevent and mitigate range expansions of invasive alien species (IAS) in non-native areas, while protecting them within their native range. Characterizing genetic variation patterns for IAS populations and deciphering the links between their native and introduced populations is helpful to (i) assess the genetic state of both native and non-native populations, (ii) reveal potential invasion pathways, (iii) define IAS management strategies in invaded areas, and (iv) identify native populations requiring conservation measures. The European catfish (Silurus glanis) is the largest European predatory fish. Introduced since the seventies from Eastern to Western Europe, it has colonized many waterbodies. Yet, little is known about the genetic status of non-native populations and the invasion pathways used by the species. Besides, some native populations are threatened, requiring conservation actions. Here, we describe current patterns of genetic variability of native and non-native S. glanis populations across Europe. Using microsatellite markers, we first assessed genetic variation within and between native and non-native populations. Second, we performed genetic clustering analyses to determine the genetic structure of multiple catfish populations across Europe and highlight their potential links. We revealed that native populations are more genetically diverse than non-native populations, and highlight complex introduction pathways involving several independent sources of introduction, which likely explain the invasion success of this large predatory fish across western Europe.