Lake Fish Research Articles

Consequences of changing water clarity on the fish and fisheries of the Laurentian Great Lakes

Human-driven environmental change underlies recent changes in water clarity in many of the world’s great lakes, yet our understanding of the consequences of these changes on the fish and fisheries they support remains incomplete. Herein, we offer a framework to organize current knowledge, guide future research, and help fisheries managers understand how water clarity can affect their valued populations. Emphasizing Laurentian Great Lakes findings where possible, we describe how changing water clarity can directly affect fish populations and communities by altering exposure to ultraviolet radiation, foraging success, predation risk, reproductive behavior, or territoriality. We also discuss how changing water clarity can affect fisheries harvest and assessment through effects on fisher behavior and sampling efficiency (i.e., catchability). Finally, we discuss whether changing water clarity can affect understudied aspects of fishery performance, including economic and community benefits. We conclude by identifying generalized predictions and discuss their implications for priority research questions for the Laurentian Great Lakes. Even though the motivation for this work was regional, the breadth of the review and generality of the framework are readily transferable to other freshwater and marine habitats.

Slimy sculpin depth shifts and habitat squeeze following the round goby invasion in the Laurentian Great Lakes

The collapse of Diporeia spp. and invasions of dreissenid mussels (zebra, Dreissena polymorpha; quagga, D. bugensis) and round goby (Neogobius melanostomus) have been associated with declines in abundance of native benthic fishes in the Great Lakes, including historically abundant slimy sculpin (Cottus cognatus). We hypothesized that as round goby colonized deeper habitat, slimy sculpin avoided habitat competition, predation, and aggression from round goby by shifting to deeper habitat. Accordingly, we predicted increased depth overlap of slimy sculpin with both round goby and deepwater sculpin (Myoxocephalus thompsonii) that resulted in habitat squeeze by both species. We used long-term bottom trawl data from Lakes Michigan, Huron, and Ontario to evaluate shifts in slimy sculpin depth and their depth overlap with round goby and deepwater sculpin. Lake Huron most supported our hypotheses as slimy sculpin shifted to deeper habitat coincident with the round goby invasion, and depth overlap between slimy sculpin and both species recently increased. Slimy sculpin depth trends in Lakes Michigan and Ontario suggest other ecological and environmental factors better predicted sculpin depth in these lakes.

Source apportionment of perfluoroalkyl substances in Great Lakes fish

Due to the complex sources and fate of perfluoroalkyl substance (PFAS), their source apportionment in the environment remains a challenge. A data set of 11 straight-chain PFAS in 139 samples of fish in the Great Lakes was analyzed using positive matrix factorization (PMF) to investigate their primary sources, whose spatial variations were examined against the surrounding environmental factors. PMF analysis produced five fingerprints. Factor 1 (72% of Σ11PFAS, dominated by PFOS) probably represented emissions from primary sources (such as consumer products) and secondary sources (precursors), and increased in average abundance from west to east across the Great Lakes. Factor 2 (13% of Σ11PFAS) and factor 3 (7% of Σ11PFAS), highly loaded with long-chain PFAS and PFNA, respectively, were thought to represent PVDF manufacture or processing in metal plating. They showed higher contributions in sparsely populated Lakes Superior and Huron. Factor 4 (5% of Σ11PFAS, highly loaded with PFOS and PFHxS) presented hot spots near current and former air force bases, suggesting it was related to aqueous film-forming foams (AFFFs). Factor 5 (4% of Σ11PFAS) contained primarily PFOS and PFOSA, which may imply metabolism of precursors (PFOSA) to PFOS in vivo. Unexpectedly, the spatial trends of the five sources all showed abnormally low values near the more urbanized Chicago and Milwaukee in Lake Michigan, which may be due to their unique wastewater and stormwater infrastructure or may arise from atmospheric transport of precursors. Our study indicated that PMF was an effective tool to identify sources of PFAS in fish despite absorption, distribution, metabolism, and excretion (ADME) processes which might alter fingerprints in fish relative to their surrounding environment.

Decadal changes in size, salinity, waterbirds, and fish in lakes of the Konya Closed Basin, Turkey, associated with climate change and increasing water abstraction for agriculture

ABSTRACT The Konya Closed Basin (KCB) in Turkey has a cold semiarid to warm Mediterranean climate and hosts the largest Turkish freshwater lake, Lake Beyşehir, and the iconic saline Lake Tuz. Using published as well as our own ground-truth and remote sensing data, we provide (1) a brief description of the paleoenvironmental changes in the KCB; followed by (2) a detailed description of the changes in land use, crop farming, groundwater and surface water levels, and climate; and (3) associated changes in lake water surface area and salinity as well as in waterbird and fish communities during the past 40 years. The KCB is intensively farmed, and the farming of mainly water intensive crops has increased substantially, especially since 2000. This, combined with climate warming, has led to a substantial rate of reduction of the groundwater level (up to 1 m/yr) and the surface area of the lakes and wetlands, followed by an increase in salinisation, and even complete loss of several wetlands. Three globally threatened waterbird species face extinction in the basin, and 18 of the 62 previous breeding species have already been lost. The KCB has 38 fish species, of which 74% are endemic and 61% are considered threatened or near threatened. Modelling projections using various climate and land use scenarios predict serious additional reductions of the water level in the future due to climate change, leading to deterioration (or complete loss) of lake ecosystems and the services they provide.

Variation of the heavy metals concentration in some wells and springs water between Haditha dam and the site of Al-Baghdadi dam

The study area is located between latitude 33° 51.9 -34° 19 and latitudes 42° 15.83 to 42° 40.3 in the northwest part of the Governorate of Anbar. The research focuses upon the analysis of Heavy Metal variation between the dam of Haditha and the dam of Al Baghdadi in (8) wells and (4) springs. Nine heavy metals, including Zn2+, Fe2+, Cd2+, Mn2+, Pb2+, B2+and As2+, were analyzed for the wells and springs samples in August 2019 and March 2020. Present study shows a spatial and temporal change in heavy metals, whereas all well samples did not exceed allowable limits suggested by WHO and IQS. A major source of High values of heavy metals in some water samples of springs may be due to the effects of Agriculture, Fertilizers, pesticides and fish lake. The results also show a variation in the depth and source of wells, reflected by different co-existing sources. In addition to some human activities, which raise the level of concentrations.

Validation of an imaging FTIR spectroscopic method for analyzing microplastics ingestion by Finnish lake fish (Perca fluviatilis and Coregonus albula)

Despite the ubiquitousness of microplastics, knowledge on the exposure of freshwater fish to microplastics is still limited. Moreover, no standard methods are available for analyzing microplastics, and the quality of methods used for the quantification of ingested microplastics in fish should be improved. In this study, we studied microplastic ingestion of common wild freshwater fish species, perch (Perca fluviatilis) and vendace (Coregonus albula). Further, our aim was to develop and validate imaging Fourier-transform infrared spectroscopic method for the quantification of ingested microplastics. For this purpose, enzymatically digested samples were measured with focal plane array (FPA) based infrared microscope. Data was analyzed with siMPle software, which provides counts, mass estimations, sizes, and materials for the measured particles. Method validation was conducted with ten procedural blanks and recovery tests, resulting in 75% and 77% recovery rates for pretreatment and infrared imaging, respectively. Pretreatment caused contamination principally by small <100 μm microplastics. The results showed that 17% of perch and 25% of vendace had ingested plastic. Most of the fish contained little or no plastics, while some individuals contained high numbers of small particles or alternatively few large particles. Perch from one sampling site out of five had ingested microplastics, but vendace from all sampling sites had ingested microplastics. The microplastics found from fish were mostly small: 81% had particle size between 20 and 100 μm, and most of them were polyethylene, polypropylene, and polyethylene terephthalate. In conclusion, the implemented method revealed low numbers of ingested microplastics on average but needs further development for routine monitoring of small microplastics.

Analytical study of the current status of the fisheries of Lake Burullus in Kafr El-Sheikh Governorate

The research aims to identify the time stages of the quantity and value of fish production from Lake Burullus fisheries, and to determine the relationship between the quantity of fish production from Lake Burullus as a dependent variable and the following economic variables studied (the value of fish production in million pounds, average wholesale price in thousand pounds per ton, the number of fishing boats in the boat, and the number of Fishermen by fishermen, the amount of wastewater in cubic meters, and finally to identify the most important problems affecting fish production from Lake Burullus in the research area from the viewpoint of the researched fishermen.This research relied on achieving its objectives on descriptive and quantitative economic analysis, where a simple linear regression analysis was used. To estimate the equations of the general time trend of the economic variables associated with the study, the surplus production model of Schaefer was used (1957 Schaefer). Per year, from fishing effort amounted to about 6.32 thousand boats, and the actual production exceeded the permissible production quantity (MSY) during the last years of this period. To study, and that the actual fishing effort exceeds the optimal fishing effort in that lake, which illustrates the extent to which Lake Burullus suffers from the increase in the phenomenon of illegal fishing practices, and then it is expected that the fish stocks of the lake will decrease, which will result in the decline of the lake's fish production in the following years

Microplastic pollution characteristic in surface water and freshwater fish of Gehu Lake, China.

Much more attention has been poured into microplastic pollution in freshwater systems recently. In the present study, the pollution of microplastics (MPs) in surface water and freshwater fish (crucian carp, etc.) were investigated from Gehu Lake, which is the second largest lake in southern Jiangsu after Taihu Lake. The result manifested that the average abundance of MPs was respectively 6.33±2.67 n/L for surface water and 10.7 items per individual for freshwater fish. The distribution of MPs in Gehu Lake varied from place to place, with the highest abundance of MPs was observed in the two estuaries of the eastern part of the lake. It was speculated that topographical factors and human factors were the main factors affecting the abundance and distribution of MPs. Transparent fibers were the main type of MPs in water samples, accounting for 69.70% of all detected particles. Meanwhile, most of the MPs ingested by freshwater fish were fibers, and the main colors were transparent and blue. In addition, the dominant size of the MPs was between 0.1 to 0.5 mm in water and fish samples. Moreover, PES, man-made fiber, and PP were the dominant polymer types in the surface water and fish samples. The results of this investigation can provide basic data for the research and management of MPs in freshwater systems.

Congruency between two traditional and eDNA-based sampling methods in characterising taxonomic and trait-based structure of fish communities and community-environment relationships in lentic environment

Recent developments in environmental DNA (eDNA) metabarcoding suggest that eDNA-based representation of ecological communities can be a promising tool in both fundamental ecological research and environmental assessment. However, it is less known, how eDNA performs in characterising ecological communities and community-environment relationships at the regional scale compared with traditional sampling methods. Here, we used electrofishing (EF), gillnetting (GN) and eDNA-based surveys to compare their congruency in characterising the taxonomic and trait-based structure of (oxbow) lake fish communities and their structuring mechanisms. eDNA proved to be more effective in detecting taxa in the total samples and by traits than EF and GN. Principal coordinate analysis and multiple factor analysis showed a moderate separation of communities according to sampling methods for the taxon and the trait-based structures, respectively, but eDNA samples were always located in intermediate position in the ordination plots. Procrustes analyses indicated significant among-method congruency in community structure. However, in general, eDNA-based community patterns always showed higher correlation with either the EF or the GN-based community patterns, than the two traditional methods to each other. Variance partitioning in redundancy analyses indicated large differences among the sampling methods in the importance of environmental and spatial variables in shaping metacommunity structure. These results thus suggest that the sampling method can largely influence the identified mechanisms which govern fish metacommunity organisation. Our results suggest, that eDNA metabarcoding can be the best universal method for understanding the taxonomic and trait-based organisation of lake fish metacommunities.

Influence of warming temperatures on coregonine embryogenesis within and among species

The greatest known global response of lakes to climate change has been an increase in water temperatures. The responses of many lake fishes to warming water temperatures are projected to be inadequate to counter the speed and magnitude of climate change. We experimentally evaluated the responses of embryos from a group of cold, stenothermic fishes (Salmonidae Coregoninae) to increased incubation temperatures. Study groups included cisco (Coregonus artedi) from lakes Superior and Ontario (USA), and vendace (C. albula) and European whitefish (C. lavaretus) from Lake Southern Konnevesi (Finland). Embryos from artificial crossings were incubated at water temperatures of 2.0, 4.5, 7.0, and 9.0°C, and their responses were quantified for developmental and morphological traits. Embryo survival, incubation period, and length-at-hatch were inversely related to incubation temperature whereas yolk-sac volume increased with incubation temperature within study groups. However, varying magnitudes of responses among study groups suggested differential levels of developmental plasticity to climate change. Differential levels of parental effects indicate genetic diversity may enable all study groups to adapt to cope with some degree of changing environmental conditions. Our results suggest that the coregonines sampled within and among systems may have a wide range of embryo responses to warming incubation conditions.

Microplastic contamination in Great Lakes fish.

Freshwater ecosystems, generally adjacent to human population and more contaminated relative to adjacent marine ecosystems, are vulnerable to microplastic contamination. We sampled 7 species of fish from Lake Ontario and Lake Superior and assessed their gastrointestinal (GI) tracts to quantify ingested microplastics and other anthropogenic particles. A subset of the microparticles were chemically analyzed to confirm polymer types and anthropogenic origins. We documented the highest concentration of microplastics and other anthropogenic microparticles ever reported in bony fish. We found 12,442 anthropogenic microparticles across 212 fish (8 species) from nearshore Lake Ontario, 943 across 50 fish (1 species) from Humber River, and 3094 across 119 fish (7 species) from Lake Superior. Fish from Lake Ontario had the greatest mean abundance of anthropogenic microparticles in their GI tracts (59 particles/fish [SD 104]), with up to 915 microparticles in a single fish. Fish from Lake Superior contained a mean [SD] of 26 [74] particles/fish, and fish from Humber River contained 19 [14] particles/fish. Most particles were microfibers. Overall, ≥90% of particles were anthropogenic, of which 35-59% were microplastics. Polyethylene (24%), polyethylene terephthalate (20%), and polypropylene (18%) were the most common microplastics. Ingestion of anthropogenic particles was significantly different among species within Lake Ontario (p < 0.05), and the abundance of anthropogenic particles increased as fish length increased in Lake Ontario (ρ = 0.62). Although we cannot extrapolate the concentration of microplastics in the water and sediments of these fish, the relatively high abundance of microplastics in the GI tracts of fish suggests environmental exposure may be above threshold concentrations for risk.

The Organoleptic Properties of Food Products from Tempe Lake Fish

The potential freshwater fish of Lake Tempe has long been consumed by the local community. However, its use is minimal. This article describes the organoleptic properties of fish cakes and fish floss as the development of various forms of food made from fish of Tempe Lake. The types of fish used as essential ingredients are Channa Striata and Monopterus Albus. The organoleptic test involved 70 panelists from the Department of Agricultural Technology, Universitas Negeri Makassar. Panelists assessed the color, taste, smell, and texture of the two food products. This result of the study describes the results of the organoleptic test with a comparison figure and t-test. Organoleptic test results showed that the panelists preferred the fish floss from Channa Striata for color, smell, and taste compared to fish floss from Monopterus Albus. Panelists' assessment of the fish cake from Channa Striata was better on smell and texture, while Monopterus Albus was preferred on the color and taste test. The T-test results showed that the panelists' assessments for fish cakes from Channa Striata and Monopterus Albus were similar. However, the organoleptic characteristics of fish cake are different between two kinds of lake fish.

Fish size spectra are affected by nutrient concentration and relative abundance of non-native species across streams of the NE Iberian Peninsula

Aquatic ecosystems are strongly body-size structured with a decline of numerical abundance with increasing body size (hereafter, the size spectrum). Marine and lake fish studies have reported consistent variations of size spectra in relation to environmental conditions and biotic composition, but little is known about stream fishes. Accordingly, in this study we test several hypotheses about the effects of local water conditions, biotic introductions and cumulative pressures (measured as the IMPRESS index) on the fish size-spectrum slope (that is, the linear rate of decline of fish abundance as body size increase in a log-log scale) and the size-spectrum intercept (commonly used as proxy for carrying capacity) among 118 local fish assemblages in streams of the NE Iberian Peninsula. To our knowledge, this is the first time that an extensive river fish dataset is used in a dendritic network to cover systematic changes of size-spectrum parameters. We find that the slope and intercept of the fish size spectrum are negatively correlated with nutrient concentration (mainly total phosphorus), with a greater relative abundance of small fishes but a decline of overall carrying capacity. Moreover, fish assemblages with greater relative abundance of non-native species have flatter size-spectrum slopes. In contrast, the IMPRESS index and climate-related variables are poor predictors of the shape of the fish size spectra. This study contributes to better understanding of the main factors structuring fish assemblages in lotic environments of the Iberian Peninsula. We encourage more research on this line to further explore the use of fish size structure to evaluate the ecological health of riverine ecosystems.

Effects of Artificial Reefs on Phytoplankton Community Structure in Baiyangdian Lake, China

The habitat and feeding environment of freshwater fish in freshwater lakes have been destroyed, with the problem of miniaturization and simplification of catches being serious. An artificial reef is an effective technical measure to protect and proliferate offshore fishery resources, but little research has been conducted on its application in freshwater lakes. A small artificial reef for freshwater lakes was designed according to the water depth of the lake and the habits of benthic fish. The artificial reef is composed of biomass modules, each of which is 900 × 120 mm. The community structure of phytoplankton around the artificial reef and its adjacent waters was studied. The results showed that 77 species from seven phyla were identified, with a high number of species from the Chlorophyceae. In terms of density composition, the density of cyanobacteria decreased month by month, while the phylum Chlorophyta and Cryptophyta increased first and then decreased. As for biomass composition, Chlorophyta and Cryptophyta increased first and then decreased. RDA analysis showed that water temperature, dissolved oxygen, and total phosphorus were the main influencing factors. To sum up, the artificial reef can improve the algae phase in the surrounding water column, inhibit the growth and reproduction of cyanobacteria to a certain extent, and have a significant enrichment and promotion effect on diatoms. Artificial fish reef affects the phytoplankton community structure of the surrounding water bodies mainly through the absorption of phosphorus nutrients. Artificial reefs can be popularized and applied in freshwater lakes to provide foraging and shelter for benthic fish in the lake.

Impacts of volcanic eruptions and early recovery in freshwater environments and organisms.

Volcanic eruptions modify environments physically and chemically with serious consequences for the biota. In this review, we analysed 80 papers reporting the effects of volcanic eruptions in freshwater environments and on freshwater organisms. An increase in water turbidity is the most common reported physical effect while increases in concentrations of inorganic elements, many representing nutrients for primary producers, are the most common chemical effects. Bacterial growth is usually stimulated, while autotrophs can be either positively or negatively affected depending on the type of impact. A persistent effect reported in the biota is changes to the assemblage, which could generate further changes in terms of ecosystem functions. This analysis also identifies some information gaps, particularly involving the effects of eruptions on heterotrophic biofilms in streams and on invertebrates and fish in lakes. Most studies were carried out soon after the volcanic eruption, so it is difficult to assess the recovery of the ecosystems. Eruptions present unique opportunities for scientific discovery, although such studies are often hindered by a lack of pre-eruption data, which would allow for a more comprehensive assessment of the effects.

Внутри- и межпопуляционная изменчивость цист и взрослых стадий артемии (Branchiopoda: Anostraca) в сибирских популяциях (морфометрия)

Размеры цист артемии являются важным показателем ценности их как кормового ресурса и в некоторой степени позволяют идентифицировать популяции. В статье проанализированы показатели цист артемии партеногенетических популяций (диаметр, толщина хориона, наличие пятен на оболочке), отобранных в гипергалинных озёрах Западной Сибири в разные годы, и морфометрические показатели рачков, выращенных из цист при одинаковой солёности. Установлена значительная внутри- и межпопуляционная изменчивость рассмотренных показателей. Абсолютные значения диаметра цист находились в пределах 210–330 мкм, средние значения по пробам — 243,5–282,9 мкм, средние по популяциям — 257,8–279,6 мкм; абсолютные значения диаметра декапсулированных цист — в пределах 196–294 мкм, средние значения по пробам — 236,5–262,6 мкм, средние по популяциям — 239,9–253,2 мкм; абсолютные значения толщины хориона цист — 3,3–16,9 мкм, средние значения по популяциям — 6,6–12,4 мкм. В основных промысловых озёрах, на которые приходится около 70 % от всего вылова цист артемии в России, цисты имели близкие среднепопуляционные размеры (262–268 мкм). Дано заключение об отсутствии внутрипопуляционной закреплённости таких признаков, как диаметр цист и толщина хориона, то есть они не могут служить надёжными показателями, идентифицирующими сибирские популяции. Установлена статистически значимая связь (r = −0,5) между солёностью материнского водоёма и диаметром эмбрионов артемии. Пятнистость цист, не превышающая 5 % почти во всех их пробах, у цист озера Кучукское составила 24 %. Анализ морфометрических показателей рачков, выращенных из цист, показал, что средняя длина рачков (9,27–11,63 мм), ширина абдомена (0,53–0,69 мм) и расстояние между глазами (1,36–1,52 мм) тесно коррелировали с солёностью материнского водоёма (значения r составили −0,76; −0,62; −0,67 соответственно). Кластерный анализ совокупности морфометрических признаков рачков указывает на объединение популяций по признаку солёности.

Response of fish assemblages to restoration of rapids habitat in a Great Lakes connecting channel

Rapids habitats are critical spawning and nursery grounds for multiple Laurentian Great Lakes fishes of ecological importance such as lake sturgeon, walleye, and salmonids. However, river modifications have destroyed important rapids habitat in connecting channels by modifying flow profiles and removing large quantities of cobble and gravel that are preferred spawning substrates of several fish species. The conversion of rapids habitat to slow moving waters has altered fish assemblages and decreased the spawning success of lithophilic species. The St. Marys River is a Great Lakes connecting channel in which the majority of rapids habitat has been lost. However, rapids habitat was restored at the Little Rapids in 2016 to recover important spawning habitat in this river. During the restoration, flow and substrate were recovered to rapids habitat. We sampled the fish community (pre- and post-restoration), focusing on age-0 fishes in order to characterize the response of the fish assemblage to the restoration, particularly for species of importance (e.g. lake whitefish, walleye, Atlantic salmon). Following restoration, we observed a 40% increase in age-0 fish catch per unit effort, increased presence of rare species, and a shift in assemblage structure of age-0 fishes (higher relative abundance of Salmonidae, Cottidae, and Gasterosteidae). We also observed a “transition” period in 2017, in which the assemblage was markedly different from the pre- and post-restoration assemblages and was dominated by Catostomidae. Responses from target species were mixed, with increased Atlantic salmon abundance, first documented presence of walleye and no presence of lake sturgeon or Coregoninae.

Assessment of heavy metal accumulation in freshwater fish of Dongting Lake, China: Effects of feeding habits, habitat preferences and body size

We measured the concentrations of Cr, Fe, Ni, Cu, Zn, Cd, Pb and Hg, and the stable isotope ratios (δ13C and δ15N) in 87 fish samples within 12 economic fish species collected from the Dongting Lake, the second largest freshwater lake in China. With few exceptions in concentration of Cr, most of fish species showed lower concentrations of the 8 metals than legislation thresholds. Piscivorous fishes had significantly higher values of δ15N (possessing higher trophic level) and metal concentrations than planktivorous and herbivorous fishes. Moreover, demersal fishes showed higher concentrations of Cu, Zn, Cd and Pb than pelagic and benthopelagic fishes. We found positive correlations between concentrations of Fe, Pb and Hg and δ15N ratio, confirming the biomagnification of the three metals through trophic transfer. In contrast, fishes showed clear growth dilution effect for Ni, Zn, Cu, Pb and Hg, indicated by the negative correlations between their concentrations and fish weight or length. Multiple regression analysis demonstrated that growth dilution and biomagnification effects simultaneously governed the metal concentrations in fish muscle, and the two effects' importance varied among different metals. The human health risk assessment indicated that all 8 metals gave target hazard quotient (THQ) values < 1.0, i.e., the estimated daily intake (EDI) of metals did not exceed the oral reference dose (RfD), indicating a safe consumption of these fish species for consumer. Our study provides comprehensive approaches to better understand the determining processes and potential risk of heavy metals in freshwater lake fishes.

Size Spectra of Pelagic Fish Populations in a Deep Lake—Methodological Comparison between Hydroacoustics and Midwater Trawling

Net sampling by trawling and hydroacoustics was used to methodologically compare size spectra (SS) of the pelagic fish community in a deep lake across 12 years of sampling. Hydroacoustic SS were generated based on either single-echo detections (SEDs) or tracked-echo groups (TEGs) from 20 cross-lake transects. Trawl SS were obtained by a midwater trawl in four pelagic depth layers. All SS were derived from maximum likelihood estimations of exponent b of a continuous fish body mass distribution. The arithmetic mean exponent b was similar for all methods, and there were no significant differences of b among the three methods across years. However, visual inspection indicated that the SS differed considerably between trawling and hydroacoustics in some of the years, primarily when high densities of 0+ coregonid fishes were strongly spatially aggregated and hence caught by the trawl. Accordingly, there was no correlation between SS generated by trawling and hydroacoustics. In contrast, SS generated by SEDs and TEGs were significantly correlated, indicating reliability and reproducibility of obtaining SS by hydroacoustics. The SS estimated by TEGs revealed a positive trend of exponent b over the years since 2005, potentially reflecting the recent eutrophication of Lake Stechlin, which may lead to higher fish growth rates. We conclude that hydroacoustics may help to generate more precise SS of the pelagic fish community in our study lake than midwater trawling. However, the truthfulness of SS estimates cannot be evaluated because of the inherent difficulty in determining the true densities and sizes of fishes in lakes.

Food Webs and Fish Size Patterns in Insular Lakes Partially Support Climate-Related Features in Continental Lakes

Disentangling the effects of climate change on nature is one of the main challenges facing ecologists nowadays. Warmer climates forces strong effects on lake biota for fish, leading to a reduction in size, changes in diet, more frequent reproduction, and stronger cascading effects. Space-for-time substitution studies (SFTS) are often used to unravel climate effects on lakes biota; however, results from continental lakes are potentially confounded by biogeographical and evolutionary differences, also leading to an overall higher fish species richness in warm lakes. Such differences may not be found in lakes on remote islands, where natural fish free lakes have been subjected to stocking only during the past few hundred years. We studied 20 species-poor lakes located in two remote island groups with contrasting climates, but similar seasonality: the Faroe Islands (cold; 6.5 ± 2.8 °C annual average (SD) and the Azores Islands (warm; 17.3 ± 2.9 °C)). As for mainland lakes, mean body size of fish in the warmer lakes were smaller overall, and phytoplankton per unit of phosphorus higher. The δ13C carbon range for basal organisms, and for the whole food web, appeared wider in colder lakes. In contrast to previous works in continental fresh waters, Layman metrics of the fish food web were similar between the two climatic regions. Our results from insular systems provide further evidence that ambient temperatures, at least partially, drive the changes in fish size structure and the cascading effects found along latitude gradients in lakes.