Cryptic persistence and loss of local endemism in Lake Constance charr subject to anthropogenic disturbance.

In the welcome circumstance that species believed extinct are rediscovered, it is often the case that biological knowledge acquired before the presumed extinction is limited. Efforts to address these knowledge gaps, in particular to assess the taxonomic integrity and conservation status of such species, can be hampered by a lack of genetic data and scarcity of samples in museum collections. Here, we present a proof-of-concept case study based on a multidisciplinary data evaluation approach to tackle such problems. The approach was developed after the rediscovery, 40 years after its presumed extinction, of the enigmatic Lake Constance deep-water charr Salvelinus profundus. Targeted surveys led to the capture of further species and additional sympatric normal charr, Salvelinus cf. umbla. Since the lake had been subject to massive stocking in the past, an evaluation of the genetic integrity of both extant forms was called for in order to assess possible introgression. A two-step genomic approach was developed based on restriction site associated DNA (RAD). Diagnostic population genomic (single nucleotide polymorphism [SNP]) data were harvested from contemporary samples and used for RNA bait design to perform target capture in DNA libraries of archival scale material, enabling a comparison between extant and historic samples. Furthermore, life history traits and morphological data for both extant forms were gathered and compared with historical data from the past 60-120 years. While extant deep-water charr matched historical deep-water specimens in body shape, gill raker count, and growth rates, significant differences were discovered between historical and extant normal charr. These resulted were supported by genomic analyses of contemporary samples, revealing the two extant forms to be highly divergent. The results of population assignment tests suggest that the endemic deep-water charr persisted in Lake Constance during the eutrophic phase, but not one of the historical genomic samples could be assigned to the extant normal charr taxon. Stocking with non-endemic charr seems to be the most likely reason for these changes. This proof-of-concept study presents a multidisciplinary data evaluation approach that simultaneously tests population genomic integrity and addresses some of the conservation issues arising from rediscovery of a species characterized by limited data availability.

Stickleback mass occurrence driven by spatially uneven parasite pressure? Insights into infection dynamics, host mortality, and epizootic variability.

Since 2012, a massive invasion of the three-spined stickleback (Gasterosteus aculeatus) has taken place into the pelagic area of Lake Constance. This species, which had previously been restricted to the littoral zone, is now the dominant pelagic fish and the previously dominant whitefish (Coregonus wartmanni) has suffered severe reductions in growth and recruitment. In this study, in total, 2871 sticklebacks were collected via monthly sessions over a 4-year period in pelagic and benthic areas of Lake Constance and examined for signs of infection with Schistocephalus solidus, a parasite known to be potentially fatal. The infection risk to sticklebacks increases throughout the course of the year and is size- and sex-dependent. Habitat has only a marginal impact. All parasite-induced harm is imparted after stickleback spawning and parental care is over. The results did not support the hypothesis that the invasion of the pelagic area might be driven by parasite-avoiding behaviour. Furthermore, the impact of the parasite is likely to be limited to post-reproductive adults, thereby ensuring stable reproduction of the hosts despite high rates of transmission and mortality. In consequence, stickleback stock development is independent of S. solidus infection, leading to secure coexistence of host and parasite even at extraordinary high host levels.

Rapid and Efficient Method for the Detection of Microplastic in the Gastrointestinal Tract of Fishes.

The rising evidence of microplastic pollution impacts on aquatic organisms in both marine and freshwater ecosystems highlights a pressing need for adequate and comparable detection methods. Available tissue digestion protocols are time-consuming (>10 h) and/or require several procedural steps, during which materials can be lost and contaminants introduced. This novel approach comprises an accelerated digestion step using sodium hydroxide and nitric acid in combination to digest all organic material within 1 h plus an additional separation step using sodium iodide which can be used to reduce mineral residues in samples where necessary. This method yielded a microplastic recovery rate of ≥95%, and all tested polymer types were recovered with only minor changes in weight, size, and color with the exception of polyamide. The method was also shown to be effective on field samples from two benthic freshwater fish species, revealing a microplastic burden comparable to that indicated in the literature. As a consequence, the present method saves time, minimizes the loss of material and the risk of contamination, and facilitates the identification of plastic particles and fibers, thus providing an efficient method to detect and quantify microplastics in the gastrointestinal tract of fishes.

Genetic differentiation and phenotypic plasticity drive troglomorphic character development in European cavefish.

The Aach cave loach (Barbatula barbatula), a recently discovered member of the Nemacheilidae family, offers a unique opportunity to understand the mechanisms underlying evolutionary change. In a common garden experiment, we reared groups of laboratory-bred cave, surface, and hybrid loach under different light conditions. Troglomorphic characters varied significantly among the fish, influenced to a different extent by parental origin and light conditions. Cavefish progeny consistently exhibited smaller eyes, lighter pigmentation, longer barbels, and larger olfactory epithelia than surface fish, while hybrids displayed intermediate characteristics. Surface and hybrid fish raised in complete darkness resembled the cavefish phenotype, while cavefish raised under a natural photoperiod approached the surface form. Characters associated with eye degeneration were found to be primarily heritable. Conversely, traits related to chemo- and mechano-reception were enhanced in the surface and hybrid groups reared in complete darkness, suggesting phenotypic plasticity. Our findings offer valuable insights into the interplay between genetic differentiation and phenotypic plasticity to troglomorphic adaption. This contributes to the broader understanding of the early stages of adaptation, where phenotypic plasticity, drift, and selection shape phenotypes. Relatively recently established cavefish, such as the Aach cave loach, are promising candidates for comparative research investigating evolutionary mechanisms.

Inland freshwater aquaculture in a warming world.

Climate change potentially threatens the sustainable production of highly valued cold-water fish species in flow-through systems, such as salmonids. By analysing the relationship of water temperature to hydrological characteristics, air temperature, solar exposure, and precipitation, this study predicted temperature dynamics of five temperate cold-water aquaculture facilities under four projected climate change scenarios. Air temperature was found to be directly associated with facility site water temperature, and based on rational assumptions, two of the five facilities were predicted to face critical warming by mid-century. Extreme precipitation events induced acute short-term increases in water temperature of up to 5 °C. Significantly lower warming, roughly equal to the projected climate change-induced increase, was seen with artificial shading lowering temperature by 1 °C. Complementary niche modelling revealed that 37-77 % of current cold-water facilities will likely incur suboptimal climate conditions by the end of the century. Shading of raceways, more efficient water use, and disease management are proposed as key actions to preserve cold-water aquaculture.

A systematic study of the microplastic burden in freshwater fishes of south-western Germany - Are we searching at the right scale?

In a comprehensive study of microplastic contamination in southern Germany, 1167 individual fish of 22 different species were sampled from 11 rivers and 6 lakes across the state. The microplastic burden of investigated fish was analyzed on the basis of habitat type, location, and a number of abiotic and biotic factors. A particle size distribution analysis of the detected microplastics was carried out. The results showed a relatively low plastic prevalence of 18.8%, with significant differences between rivers (20.6%) and lakes (16.5%). The number of ingested plastic particles ranged between 1 and 4 particles per fish. The majority of abiotic and biotic factors seem to play little or no role in the ingestion of microplastics, suggesting that in most cases uptake is passive or accidental. It is notable that piscivorous fish appeared significantly less burdened, suggesting a low transfer rate and no accumulation in the food web. However, size distribution analysis identified a power law growth fit in particle numbers at the smallest end of the distribution. This carries a worrying implication, that >95% of particles are likely to be smaller than 40 µm and thereby beyond the detection range of this and most other microplastic surveys conducted so far. When the frequency development of small particles is taken into account, the likely microplastic prevalence in the present study increases to 100%, with an average intensity of around 23 predominantly small particles per fish. A striking 70% of those particles would be smaller than 5 µm and therefore eligible for translocation into tissues, with critical implications for fish health and consumer exposure. This raises a question as to whether current estimates of microplastic burden in fishes generally might be overlooking a majority of potential contamination within the critical smaller particle size classes.