Escaping malnutrition by shifting habitats: A driver of three-spined stickleback invasion in Lake Constance.

Fatty acids, and especially long-chain polyunsaturated fatty acids, are biologically important components in the metabolism of vertebrates, including fish. Essential fatty acids (EFA) are those that in a given animal cannot be synthesized or modified from precursors and must therefore be acquired via the diet. Because EFAs are often unevenly distributed in nature, this requirement may drive species to make behavioral or ecological adaptations to avoid malnutrition. This is especially true for fish like the three-spined stickleback (Gasterosteus aculeatus L.) of Upper Lake Constance (ULC), whose recent marine ancestors evolved with access to EFA-rich prey, but which found themselves in an EFA-deficient habitat. An unexpected and unprecedented ecological shift in the ULC stickleback population from the littoral to pelagic zones in 2012 might be linked to EFA availability, triggering ecological release and enabling them to build a hyperabundant population while displacing the former keystone species, the pelagic whitefish Coregonus wartmanni. To test this hypothesis, sticklebacks from the littoral and pelagic zones of ULC were sampled seasonally in two consecutive years, and their stomach contents and fatty acid profiles were analysed. Pelagic sticklebacks were found to possess significantly higher values of an important EFA, docosahexaenoic acid (DHA), especially during autumn. Evaluation of the DHA supply suggests that sticklebacks feeding in the littoral zone during autumn could not meet their DHA requirement, whereas DHA availability in the pelagic zone was surplus to demand. During autumn, pelagic sticklebacks consumed large amounts of DHA-rich prey, that is, copepods, whereas littoral sticklebacks relied mainly mostly on cladocerans, which provide much lower quantities of DHA. Access to pelagic zooplankton in 2012 was possibly facilitated by low densities of previously dominant zooplanktivorous whitefish. The present study offers a convincing physiological explanation for the observed expansion of invasive sticklebacks from the littoral to the pelagic zones of Lake Constance, contributing to a phase shift with severe consequences for fisheries.

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.

Mitigating human impacts including climate change on proliferative kidney disease in salmonids of running waters.

Over the last two decades, an increasing number of reports have identified a decline in salmonid populations, possibly linked to infection with the parasite Tetracapsuloides bryosalmonae and the corresponding disease, that is, proliferative kidney disease (PKD). The life cycle of this myxozoan parasite includes sessile bryozoan species as invertebrate host, which facilitates the distribution of the parasite in running waters. As the disease outcome is temperature dependent, the impact of the disease on salmonid populations is increasing with global warming due to climate change. The goal of this review is to provide a detailed overview of measures to mitigate the effects of PKD on salmonid populations. It first summarizes the parasite life cycle, temperature-driven disease dynamics and new immunological and molecular research into disease resistance and, based on this, discusses management possibilities. Sophisticated management actions focusing on local adaptation of salmonid populations, restoration of the riverine ecosystem and keeping water temperatures cool are necessary to reduce the negative effects of PKD. Such actions include temporary stocking with PKD-resistant salmonids, as this may assist in conserving current populations that fail to reproduce.

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.

Size matters? Species- and size-specific fish predation on recently established invasive quagga mussels Dreissena rostriformis bugensis Andrusov 1897 in a large, deep oligotrophic lake.

Since its first appearance in Lake Constance in 2016 the invasive quagga mussel Dreissena rostriformis bugensis has come to dominate the mussel community and now occurs in hyperabundant densities over the whole lake bottom. A lake-wide field study was conducted between 2019 and 2020 to obtain a systematic insight into the importance of this novel source of potential prey for the native fish community. In total 664 fish of 20 different species were caught and their digestive tracts were analysed. Meanwhile, quagga mussels were sampled to assess their calorific energy value by size and season. Regressions of septum length on maximum valve length were used to evaluate size-specific mussel consumption by the three dominant quagga-eating fish species. The study shows that nearly all benthivorous fish are able to forage efficiently on quaggas. However, in the case of one keystone species with very high commercial interest, the benthic whitefish Coregonus spp., quagga consumption is more limited as only individuals larger than 35 cm consumed quaggas in relevant amounts. The energy content of quagga mussels is positively size dependent with seasonal effect and elevated values during summer for medium- and large-sized mussels. Even at its peak, the calorific value of quaggas by weight is much lower than that of endemic aquatic invertebrates, while single mass is high. Future implications for fish stocks, food web integrity and possible fishery management options are discussed.

Quantification of alizarin red S uptake in coregonid eggs after mass-marking.

An easy method to measure the uptake rate of the persistent dye alizarin red S (ARS) during marking of whitefish eggs was established and used to measure the ARS content in three different whitefish species during and at the end of the marking procedure. Those values show that only 6-10% of the ARS in the marking solution will be absorbed by the eggs (0.0061-0.0119 mg per egg). Additional analyses 6, 15 and 36 months after marking showed ARS levels below the response level (<6.9 µg kg-1 ).

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.

Thermotactic behaviour in lacustrine and riverine forms of Salmo trutta and its relevance to an emerging parasitic disease (PKD) in the wake of climate change.

The thermotactic response of brown trout (Salmo trutta) was examined with the goal to investigate potential effects of the emerging temperature-dependent fatal trout disease PKD (proliferative kidney disease). First the differences in cold-water preferences of two forms of brown trout, lacustrine (migratory) and riverine, were determined. Second, it was studied whether this preference was changed in fish infected with PKD. The experiment involved a one-week habituation period at 14 °C in a two-chamber runway followed by a week of 3 °C temperature difference between the two runways. The fish could freely move between lanes via an opening at the end where food was provided. The temperature manipulation was repeated twice, and there were 3 trials per experimental group. All fish developed a clear spatial preference in the test. Lacustrine trout demonstrated a preference for warmer water, while riverine trout preferred cooler water. This may increase the risk to PKD in the lacustrine form. Most strikingly, riverine trout experimentally exposed to Tetracapsuloides bryosalmonae, the parasite that causes PKD, demonstrated stronger cold-seeking behaviour than control fish. Cold seeking behaviour suggests the occurrence of a disease-induced behavioural chill response, which may play an important role in disease recovery. This demonstrates the significance of protecting river connectivity and cold-water sanctuaries as management strategies for preserving salmonid populations in a warming climate.

Unravelling the origins of boldness behaviour: a common garden experiment with cavefish (Barbatula barbatula).

Many animals show an aversion to bright, open spaces, with significant variability seen across species, populations and individuals within populations. Although there is much interest in the underlying causes of this behaviour, few studies have been able to systematically isolate the role of heritable and environmental effects. Here, we addressed this gap using a common garden experiment with cavefish. Specifically, we bred and cross-bred cave loaches (Barbatula barbatula), Europe's only known cavefish, in the laboratory, raised the offspring in complete darkness or normal light conditions, and studied their light avoidance behaviour. Cavefish spent much more time in a light area and ventured further out, while surface fish spent considerable time in risk-assessment behaviour between the light and dark areas. Hybrids behaved most similarly to cavefish. Light treatment and eye quality and lens size only had a modest effect. Our results suggest light avoidance behaviour of cavefish has a heritable basis and is fundamentally linked to increased boldness rather than reduced vision, which is likely adaptive given the complete lack of macropredators in the cave environment. Our study provides novel experimental insights into the behavioural divergence of cavefish and contributes to our broader understanding of the evolution of boldness and behavioural adaptation.

Obtaining accurate population estimates with reduced workload and lower fish mortality in multi-mesh gillnet sampling of a large pre-alpine lake.

The EU Water Framework Directive requires monitoring of the ecological status of lakes, with fish as a relevant class of biotic quality indicator, but monitoring fish populations in large lakes is demanding. This study evaluated use in Lake Constance of a novel multi-mesh gillnet modified to reduce catch numbers. In direct comparison with conventional European Committee for Standardization (CEN) nets we achieved 48% reduction in fish mortality with 38% less labour for tasks directly influenced by fish catch numbers, while maintaining comparable species composition and catch per unit effort. Comparison of mesh sizes indicated no significant reduction in species detection in area-reduced panels of the small mesh sizes, while total observed species richness was greater when using the modified nets. Differences in benthic species communities among depth strata were common, while those of pelagic zones were more homogeneous and did not differ significantly with depth. Catches of different net types from the same depth stratum did not exhibit significant differences. The dominance structure of the most common species, relevant to lake assessment, was similar in catches of both net types, suggesting overall superiority of the modified nets in Lake Constance. Sampling conducted according to standard European CEN protocol, while deploying 60% fewer nets, yielded sufficiently precise abundance estimates for monitoring shallow areas of the benthic zone. A 50% difference in the abundance of dominant species was detected among sampling events with a certainty of 95%. The sample did not provide comparable accuracy in deep benthic strata or the pelagic zone, but was adequate to record complete inventories of species present. Based on this trial data, a new stratified sampling design is proposed for monitoring large lake fish communities for ecological assessment. Depth-dependent fish communities were used to calculate the required number of nets, which resulted in a 69% reduction for the entire lake compared to the CEN calculation method. Using the modified nets increases the feasibility of performing WFD surveys, by reducing effort and cost, while the simultaneous halving of fish mortality minimises the negative impact of fish surveys.

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.

Improving estimates of trophic shift (Δδ(trophic)) for diet reconstruction studies using enzyme activities.

For diet reconstruction studies using stable isotopes, accurate estimates of trophic shift (Δδtrophic) are necessary to get reliable results. Several factors have been identified which affect the trophic shift. The goal of the present experiment was to test whether measurements of the activities of enzymes could improve the accuracy of estimation of trophic shift in fish. Forty-eight Nile tilapia (Oreochromis niloticus) were fed under controlled conditions with two diets differing in their protein content (21 and 41%) each at four different levels (4, 8, 12 and 16gkg(-0.8)d(-1)). At the end of the feeding experiment, proximate composition, whole body δ(13)C and δ(15)N as well as the activities of enzymes involved in anabolism and catabolism were measured. Step-wise regression specified contributing variables for Δδ(15)N (malic enzyme, aspartate aminotransferase and protein content) and Δδ(13)Clipid-free material (aspartate aminotransferase and protein content). Explained variation by using the significant main effects was about 70% for Δδ(15)N and Δδ(13)Clipid-free material, respectively. The results of the present study indicate that enzyme activities are suitable indicators to improve estimates of trophic shift.

Systemic effects of BMP2 treatment of fractures on non-injured skeletal sites during spaceflight.

Unloading associated with spaceflight results in bone loss and increased fracture risk. Bone morphogenetic protein 2 (BMP2) is known to enhance bone formation, in part, through molecular pathways associated with mechanical loading; however, the effects of BMP2 during spaceflight remain unclear. Here, we investigated the systemic effects of BMP2 on mice sustaining a femoral fracture followed by housing in spaceflight (International Space Station or ISS) or on Earth. We hypothesized that in spaceflight, the systemic effects of BMP2 on weight-bearing bones would be blunted compared to that observed on Earth. Nine-week-old male mice were divided into four groups: 1) Saline+Earth; 2) BMP+Earth; 3) Saline+ISS; and 4) BMP+ISS (n = 10 mice/group, but only n = 5 mice/group were reserved for micro-computed tomography analyses). All mice underwent femoral defect surgery and were followed for approximately 4 weeks. We found a significant reduction in trabecular separation within the lumbar vertebrae after administering BMP2 at the fracture site of mice housed on Earth. In contrast, BMP2 treatment led to a significant increase in trabecular separation concomitant with a reduction in trabecular number within spaceflown tibiae. Although these and other lines of evidence support our hypothesis, the small sample size associated with rodent spaceflight studies limits interpretations. That said, it appears that a locally applied single dose of BMP2 at the femoral fracture site can have a systemic impact on distant bones, affecting bone quantity in several skeletal sites. Moreover, our results suggest that BMP2 treatment works through a pathway involving mechanical loading in which the best outcomes during its treatment on Earth occurred in the weight-bearing bones and in spaceflight occurred in bones subjected to higher muscle contraction.

All day-long: Sticklebacks effectively forage on whitefish eggs during all light conditions.

The three-spined stickleback Gasterosteus aculeatus invaded Lake Contance in the 1940s and expanded in large numbers from an exclusively shoreline habitat into the pelagic zone in 2012. Stickleback abundance is very high in the pelagic zone in winter near the spawning time of pelagic whitefish Coregonus wartmanni, and it is hypothesized that this is triggered by the opportunity to consume whitefish eggs. Field sampling has qualitatively confirmed predation of whitefish eggs by stickleback, but quantification has proven difficult due to stormy conditions that limit sampling. One fundamental unknown is if freshwater stickleback, known as visual feeders, can successfully find and eat whitefish eggs during twilight and night when whitefish spawn. It is also unknown how long eggs can be identified in stomachs following ingestion, which could limit efforts to quantify egg predation through stomach content analysis. To answer these questions, 144 individuals were given the opportunity to feed on whitefish roe under daylight, twilight, and darkness in controlled conditions. The results showed that stickleback can ingest as many as 100 whitefish eggs under any light conditions, and some individuals even consumed maximum numbers in complete darkness. Furthermore, eggs could be unambiguously identified in the stomach 24 hours after consumption. Whitefish eggs have 28% more energy content than the main diet of sticklebacks (zooplankton) based on bomb-calorimetric measurements, underlining the potential benefits of consuming eggs. Based on experimental results and estimates of stickleback abundance and total egg production, stickleback could potentially consume substantial proportions of the total eggs produced even if relatively few sticklebacks consume eggs. Given the evidence that stickleback can feed on eggs during nighttime spawning and may thereby hamper recruitment, future studies aimed at quantifying actual egg predation and resulting effects on the whitefish population are urgently needed.

The hunter and the hunted-A 3D analysis of predator-prey interactions between three-spined sticklebacks (Gasterosteus aculeatus) and larvae of different prey fishes.

Predator-prey interactions play a key life history role, as animals cope with changing predation risk and opportunities to hunt prey. It has recently been shown that the hunting success of sticklebacks (Gasterosteus aculeatus) targeting fish larvae is dependent on both the size of the prey and the prior exposure of its species to stickleback predation. The purpose of the current study was to identify the behavioural predator-prey interactions explaining the success or failure of sticklebacks hunting larvae of three potential prey species [roach (Rutilus rutilus), perch (Perca fluviatilis) and whitefish (Coregonus wartmannii)] in a 3D environment. Trials were carried out for each prey species at four different size classes in a standardised laboratory setup and were recorded using a slow motion, stereo camera setup. 75 predator-prey interactions including both failed and successful hunts were subject to the analysis. 3D track analysis indicated that sticklebacks applied different strategies. Prey with less complex predator escape responses, i.e. whitefish larvae, were hunted using a direct but stealthy approach ending in a lunge, while the behaviourally more complex roach and perch larvae were hunted with a faster approach. A multivariate logistic regression identified that slow average speed and acceleration of the prey in the initial stages of the hunt increased the probability of stickleback success. Furthermore, predators adjusted their swimming direction more often when hunting larger whitefish compared to smaller whitefish. The results suggest that appropriate and adequately timed avoidance behaviours, which vary between prey species and ontogenetic stages, significantly increase the chances of outmanoeuvring and escaping stickleback predation. Small whitefish larvae can reach similar levels of swimming performance compared to older conspecifics, but display ineffective anti-predator behaviours, resulting in higher hunting success for sticklebacks. Thus, the development of appropriate anti-predator behaviours depending on size appears to be the crucial factor to escaping predation.

Differentiation of two swim bladdered fish species using next generation wideband hydroacoustics.

Monitoring fish populations in large, deep water bodies by conventional capture methodologies requires intensive fishing effort and often causes mass mortality of fish. Thus, it can be difficult to collect sufficient data using capture methods for understanding fine scale community dynamics associated with issues such as climate change or species invasion. Hydroacoustic monitoring is an alternative, less invasive technology that can collect higher resolution data over large temporal and spatial scales. Monitoring multiple species with hydroacoustics, however, usually requires conventional sampling to provide species level information. The ability to identify the species identity of similar-sized individuals using only hydroacoustic data would greatly expand monitoring capabilities and further reduce the need for conventional sampling. In this study, wideband hydroacoustic technology was used in a mesocosm experiment to differentiate between free swimming, similar-sized individuals of two swim-bladdered species: whitefish (Coregonus wartmanni) and stickleback (Gasterosteus aculeatus). Individual targets were identified in echograms and variation in wideband acoustic responses among individuals, across different orientations, and between species was quantified and visually examined. Random forest classification was then used to classify individual targets of known species identity, and had an accuracy of 73.4% for the testing dataset. The results show that species can be identified with reasonable accuracy using wideband hydroacoustics. It is expected that further mesocosm and field studies will help determine capabilities and limitations for classifying additional species and monitoring fish communities. Hydroacoustic species differentiation may offer novel possibilities for fisheries managers and scientists, marking the next crucial step in non-invasive fish monitoring.

Morphological and molecular data show no evidence of the proposed replacement of endemic Pomphorhynchus tereticollis by invasive P. laevis in salmonids in southern Germany.

Changes in parasite communities might result in new host-parasite dynamics and may threaten local fish populations. This phenomenon has been suggested for acanthocephalan parasites in the river Rhine and Danube where the species Pomphorhynchus tereticollis is becoming replaced by the Ponto-Caspian P. laevis. Developing knowledge on morphologic, genetic and behavioural differences between such species is important to follow such changes. However, disagreements on the current phylogeny of these two acanthocephalan species are producing conflicts that is affecting their correct identification. This study is offering a clearer morphological and genetic distinction between these two species. As P. tereticollis is found in rhithral tributaries of the Rhine, it was questioned whether the local salmonid populations were hosts for this species and whether P. laevis was expanding into the Rhine watershed as well. In order to test for this, brown trout, Salmo trutta, and grayling, Thymallus thymallus from South-Western Germany watersheds have been samples and screened for the occurrence of acanthocephalan parasites. For the first time, both species were confirmed to be hosts for P. tereticollis in continental Europe. P. tereticollis was found to be common, whereas P. leavis was found only at a single location in the Danube. This pattern suggest either that the expansion of P. laevis through salmonid hosts into rhithral rivers has not yet occurred, or that not yet ascertained biotic or abiotic features of rhithral rivers hinder P. laevis to spread into these areas.

Effects of climate and land-use changes on fish catches across lakes at a global scale.

Globally, our knowledge on lake fisheries is still limited despite their importance to food security and livelihoods. Here we show that fish catches can respond either positively or negatively to climate and land-use changes, by analyzing time-series data (1970-2014) for 31 lakes across five continents. We find that effects of a climate or land-use driver (e.g., air temperature) on lake environment could be relatively consistent in directions, but consequential changes in a lake-environmental factor (e.g., water temperature) could result in either increases or decreases in fish catch in a given lake. A subsequent correlation analysis indicates that reductions in fish catch was less likely to occur in response to potential climate and land-use changes if a lake is located in a region with greater access to clean water. This finding suggests that adequate investments for water-quality protection and water-use efficiency can provide additional benefits to lake fisheries and food security.

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.