Catch and release (C&R) is widely promoted to reduce harvest mortality and support sustainable recreational fisheries. However, understanding its ecological, social, and management implications remains limited by the lack of large-scale, long-term data sets revealing whether C&R practices differ among species and change over time. We analyzed over 1 million caught fish (2011–2024) across 39 species and 1,286 Swedish inland fisheries, reported via the online license platform iFiske AB. The prevalence of C&R increased from ∼53% to 71% during 2011–2024, though trajectories and magnitudes differed among species. Size-specific analyses showed that longer individuals were released less frequently than shorter conspecifics for four of the six main target species (Northern Pike Esox lucius, European Perch Perca fluviatilis, Zander Sander lucioperca, European Grayling Thymallus thymallus, Brown Trout Salmo trutta, and Arctic Char Salvelinus alpinus). Overall, our findings indicate that recreational fisheries are shifting towards lower exploitation rates. Nevertheless, the lower release rates of large individuals, critical for recruitment and ecosystem functioning, highlight an important demographic and management concern. These insights are essential for assessing the sustainability of recreational fishing and for guiding management strategies that balance harvest opportunities with long-term population viability, while emphasising angler education to ensure responsible release practices.

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.

The spatial synchrony framework suggests that asynchrony among subpopulations in different branches of a river network should stabilize the metapopulation. However, how barriers affect this framework remains poorly understood. This is a significant knowledge gap given that population synchrony arises from dispersal and environmental similarity, both of which are influenced by barriers. We empirically evaluated how barriers impact fish population synchrony and, subsequently, the associations between synchrony and metapopulation persistence, productivity, stability and trajectory within fragments. We found that barriers demographically decouple populations by decreasing synchrony in brown trout (Salmo trutta) and Eurasian minnow (Phoxinus phoxinus), but not in northern pike (Esox lucius), suggesting species-specific responses to fragmentation. Additionally, asynchrony had a stabilizing portfolio effect on metapopulation stability at the fragment level that was statistically significant for S. trutta. Higher fragment synchrony also made S. trutta and P. phoxinus populations less stable. The impact of barriers on riverine fish population synchrony emphasizes the need to include barriers in future studies on the causes and consequences of synchrony in rivers. That asynchrony stabilizes populations in some riverine fishes suggests that conservation priorities should lie in restoring or retaining larger fragment sizes and higher branching complexity with intact connectivity.

The alloherpesvirus esocid herpesvirus 1 (EsHV-1) causes epidermal hyperplasia on the skin and fins of northern pike (Esox lucius). For the first time, we present a near-complete genome sequence of EsHV-1, directly obtained from a pike skin sample. The 223,553 bp sequence of the genome has a GC-content of 56.47% and is organised into a long, unique segment (148,159 bp) and a short, unique segment (45,925 bp). The short segment is flanked by inverted repeat sequences (IRSs) of 14,733/6 bp, with the IRS length difference attributed to a codon deletion. The genome is predicted to contain 144 open reading frames, including eight duplicated within the IRSs. The leftmost third of the genome contains genes of unknown function, but many of which exhibit extensive inter-gene homology, suggesting gene duplication. Six paralogous groups were identified, each containing two to thirteen gene members. Homologues of all twelve alloherpesvirus core genes are present. The ATPase subunit of the terminase and the DNA polymerase is composed of three and two exons, respectively. However, an alternate splicing pattern is found, for which, speculatively, a role is suggested in the terminase assembly at the capsid portal.

Vertebrate brain function is particularly sensitive to the effects of hypoxia, with even brief periods of oxygen deprivation causing significant brain damage and impaired cognitive abilities. This study is the first to investigate the cognitive consequences of hypoxia in fish, specifically induced by exhaustive exercise and air exposure, conditions commonly encountered during catch-and-release (C&R) practices in recreational fishing. Angling exerts substantial pressure on inland fish populations, underscoring the need for sustainable practices like C&R. While C&R survival rates are generally high, understanding its sublethal impacts is crucial for evaluating the practice’s ethical and ecological sustainability. We examined the effects of these stressors on the cognitive function of 238 rainbow trout, using the free movement pattern Y-maze method to assess working memory through navigational search patterns during free exploration sessions. Our results showed that air exposure led to short-term (3–4 h post-treatment), but transient impairments in working memory, with no long-term cognitive deficits observed at one week and one month post-treatment. These findings emphasize the high tolerance of fish to hypoxia and support the sustainability of C&R as a tool in fisheries management.

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.

Size- and species-selective harvest inevitably alters the composition of targeted populations and communities. This can potentially harm fish stocks, ecosystem functionality, and related services, as evidenced in numerous commercial fisheries. The high popularity of rod-and-reel recreational fishing, practiced by hundreds of millions globally, raises concerns about similar deteriorating effects. Despite its prevalence, the species and size selectivity of recreational fisheries remain largely unquantified due to a lack of combined catch data and fisheries-independent surveys. This study addresses this gap by using standardised monitoring data and over 60,000 digital angling catch reports from 62 distinct fisheries. The findings demonstrate a pronounced selectivity in recreational fisheries, targeting top predators and large individuals. Catch-and-release practices reduced the overall harvest by 60% but did not substantially alter this selectivity. The strong species- and size-specific selectivity mirror patterns observed in other fisheries, emphasising the importance of managing the potential adverse effects of recreational fisheries selective mortality and overfishing.