Changes In Fish Assemblage Structure Research Articles

Changes in Fish Assemblage Structure after Pen Culture Removal in Gehu Lake, China

The removal of the net enclosure has been used as a lake management strategy in various regions of China as ecological development is given more attention. Nevertheless, little is known about the substantive impact of this measure on fish communities in inland lakes. To this end, the fish community composition and structural features after the removal of the net enclosure in Gehu Lake were explored and evaluated in this study from 2021 to 2022 and compared to the investigation before the net enclosure removal from 2017 to 2018. Belonging to 7 orders, 10 families, and 46 species, a total of 17,151 fish were collected, with pelagic, sedentary, and omnivorous species dominating. In comparison, the number of species increased by 10 after removal, and fish alpha diversity increased. The Index of Relative Importance (IRI) revealed that the composition of dominating species remained constant, including Coilia nasus, Hypophthalmichthys molitrix, and Hypophthalmichthys nobilis; Parabramis pekinensis, Megalobrama amblycephal, and Culter mongolicus were upgraded from common species to general species. Culter alburnus, Hemiculter bleekeri, and Pseudobrama simoni were downgraded from general species to common species. Elopichthys bambusa had become a common species (IRI = 109.35), which was not discovered before removal. According to the hierarchical clustering (HC) and non-metric multidimensional sequencing (NMDS), the fish community of the northern reserve was highly aggregated. As the Abundance Biomass Comparison (ABC) curve and biodiversity index indicated, the fish community structure of the whole lake was in a state of moderate anthropogenic disturbance with reduced stability, while that of the northern reserve was in a state of light anthropogenic disturbance with greater stability. The number of fish species increased in this survey compared to the period before removal, species and dominant species composition altered dramatically, and total lake stability declined. This study demonstrates that the fish diversity in Gehu Lake increased after the removal of the net enclosure. Meanwhile, the stability of the fish community structure was decreased temporarily. Lake restoration is a long-term process, and the underlying impact of the removal of the net enclosure still requires continuous monitoring and further studies.

Long-Term Changes in Fish Assemblage Structure in the Korea Strait from 1986 to 2010 in Relation with Climate Change

Climate change is expected to impact on marine-capture fisheries around the world. The Korea Strait showes the highest fisheries production among the adjacent seas of Korea, but studies on changes in the fish community related with climate change are scarce. We evaluated the spatio-temporal changes in fish assemblages, and their relationships with oceanic conditions in the Korea Strait (KS, 126°–129°5′E, 33°5′–35°N) to provide a scientific basis for developing fisheries policies to adapt to climate change and global warming. We used the inflow indices of the Korea Strait Bottom Cold Water and the Tsushima Warm Current, water temperature and salinity, and commercial fisheries catch data from 1986 to 2010. To detect temporal shifts in the oceanic conditions, we applied a sequential t test of regime shift. To evaluate spatio-temporal changes of the fish assemblage structure by year, month, and fishing block, we graphically summarized the species compositions of the commercial fisheries catch data by correspondence analysis (CA). We conducted cross-correlation and canonical CA to evaluate the relationship between the oceanic conditions and fish assemblages. Sardine and filefish dominated in spring and were mainly distributed far away from the coast. Anchovy were mostly caught in summer near the southeast coast of Korea, and their annual catch steadily increased. Squid and hairtail dominated in autumn–winter, and were mainly distributed from the northern part off Jeju Island to the southeast water of Korea. Chub mackerel were relatively constant in annual and seasonal catch. CA detected a shift in the fish assemblage between 1990 and 1991, which we speculated was triggered by the 1988–1989 regime shift in the North Pacific. Surface temperatures at 0–20 m depths, especially with a time lag of 1 year, were significantly correlated with the 1990–1991 shift in fish assemblage structure. We concluded that (1) the KS is an intermediate area between the waters of Ieodo and the East Sea with respect to the timing of shift in fish assemblage structure, (2) the regional shifts were characterized by the replacement of dominant fish species from sardine and filefish to chub mackerel and squid. We hope that further multidisciplinary studies between regional oceanographers and fisheries scientists will contribute to the development of fisheries policies through better understanding of the interactions between oceanographic processes and fishes at the regional scale in adaptation to climate change.

Ephemeral habitat supports high fish α‐diversity and β‐diversity during drought in a subtropical semiarid wetland

ABSTRACTGlobally, the number of rivers with intermittent flow is increasing due to climate change and water abstraction for human consumption. Currently, our understanding of how hydrology in subtropical rivers with intermittent flow affects the structure and dynamics of aquatic communities is poorly understood. Here, we investigated how fish α‐ and β‐diversity patterns in intermittent channels in the lower reaches of the Okavango Delta respond to seasonal flooding and drought. Under low‐flow conditions, ephemeral habitat had higher α‐diversity, and this was influenced by a combination of fish aggregation and apparent transient legacy effects as habitat patches became smaller and more isolated. Investigation of changes in fish assemblage structure across different hydrological periods and habitat types showed significant species turnover when water levels fell, suggesting a strong influence from species sorting. During high‐water periods, species assemblages were homogenized both at local and regional scales, suggesting a greater influence of mass effects. Our findings support hydrology as a major factor regulating diversity patterns in intermittent rivers of a major wetland in a semiarid region of subtropical Africa. We infer from these findings that maintenance of a relatively natural flow regime will be necessary for conserving aquatic ecosystem structure and function in this system.

Effects of Hurricane Harvey on the Trophic Status of Juvenile Sport Fishes (Cynoscion nebulosus, Sciaenops ocellatus) in an Estuarine Nursery

Understanding the implications of environmental disturbance is of growing importance as climate change and extreme weather events increasingly threaten estuaries and other coastal ecosystems. Among species at risk in estuarine habitats, sport fishes are of particular interest to conservation and management agencies, because of their ecological and economic value. Here, we evaluate the effects of Hurricane Harvey on the abundance, diet, and body condition of age-0 red drum (Sciaenops ocellatus) and spotted seatrout (Cynoscion nebulosus) in Powderhorn Lake, a nearshore estuary in Texas, USA. Catch data from seines indicated that the abundances of age-0 red drum and spotted seatrout were not affected by the hurricane in the study system, but stomach contents and morphometric data show that diet and body condition changed after the event. Both species exhibited declines in body condition based on weight-length ratios, and diets included a greater proportion of invertebrates (i.e., shrimp, crab) post-disturbance, which may have been attributed to changes in fish assemblage structure within the estuary. Trophic niche overlap between age-0 red drum and spotted seatrout also decreased by up to 50% after the event, potentially serving as a mechanism to reduce competition in response to decreased prey availability. Thus, while more easily measured indicators of resistance/resilience (i.e., abundance) did not indicate effects of Hurricane Harvey on sport fishes in Powderhorn Lake, body condition and diet suggest impacts were observed among age-0 fish. Declines in body condition and greater reliance on crustacean prey could have implications for survival, fitness, and harvestability, particularly if more energy is allocated to maintenance during recovery, rather than growth.

Mass coral bleaching causes biotic homogenization of reef fish assemblages.

Global climate change is altering community composition across many ecosystems due to nonrandom species turnover, typically characterized by the loss of specialist species and increasing similarity of biological communities across spatial scales. As anthropogenic disturbances continue to alter species composition globally, there is a growing need to identify how species responses influence the establishment of distinct assemblages, such that management actions may be appropriately assigned. Here, we use trait-based analyses to compare temporal changes in five complementary indices of reef fish assemblage structure among six taxonomically distinct coral reef habitats exposed to a system-wide thermal stress event. Our results revealed increased taxonomic and functional similarity of previously distinct reef fish assemblages following mass coral bleaching, with changes characterized by subtle, but significant, shifts toward predominance of small-bodied, algal-farming habitat generalists. Furthermore, while the taxonomic or functional richness of fish assemblages did not change across all habitats, an increase in functional originality indicated an overall loss of functional redundancy. We also found that prebleaching coral composition better predicted changes in fish assemblage structure than the magnitude of coral loss. These results emphasize how measures of alpha diversity can mask important changes in the structure and functioning of ecosystems as assemblages reorganize. Our findings also highlight the role of coral species composition in structuring communities and influencing the diversity of responses of reef fishes to disturbance. As new coral species configurations emerge, their desirability will hinge upon the composition of associated species and their capacity to maintain key ecological processes in spite of ongoing disturbances.

Climatic seasonality, hydrological variability, and geomorphology shape fish assemblage structure in a subtropical floodplain

AbstractWe applied a multiscale approach to understand the patterns of longitudinal, lateral, and temporal variability in fish assemblage structure of a large floodplain river and explored their relationships with climatic seasonality, hydrological variability, and floodplain geomorphology. We studied the fish assemblage structure of 35 connected lakes along a 600-km section of the Parana River over 5 y. Longitudinal changes in fish assemblage structure were associated with a latitudinal climatic gradient, whereas temporal changes were associated with climatic seasonality and hydrological variability. Longitudinal and seasonal changes in assemblage structure were probably based on different thermal preferences of species that migrate seasonally along the main river axis. Extraordinary floods in summer promoted downstream dispersal of several species, which increased similarity between downstream and upstream regions. Thermal seasonality of this subtropical system was a strong factor driving temporal chang...

Temporal fish community responses to two cascade run‐of‐river dams in the Madeira River, Amazon basin

AbstractWe examined responses from shore fish assemblages and bottom channel fish assemblages before and after 2 cascade run‐of‐river dams in the Madeira River, in the Amazon. Those dams were installed in a river corridor where historical factors, represented by the presence of the Teotônio and Jirau falls, were the best predictors of the fish fauna assemblage composition. Jirau and Santo Antônio were the first dams constructed with Kaplan horizontal bulb turbines in the Amazon. These bulb turbines have the ability to operate by forming small reservoirs known as run‐of‐river, which should minimize impacts on fish assemblages. Overall, there were major short‐term impacts in shore fish assemblages by increasing species richness, abundance, and biomass in experimental catches. Shore fish assemblages dissimilarities before and after the dams' closure were also noticed and were related to an increase in water temperature and dissolved oxygen. However, the historical factor represented by Teotônio Fall is still the best predictor of fish assemblage dissimilarities, combined with effects of dams' closure. Bottom channel fish assemblage dissimilarities can be explained only by dams closure and are related especially to changes in fish species abundance inside the reservoirs. Our study revealed an increase in native opportunistic fish species and changes in fish assemblage structure at local scale. Kaplan horizontal bulb turbines employed in run‐of‐river dams seem to be less deleterious than vertical axis turbines typically used in accumulation reservoirs, and should be preferred in the face of burgeoning new hydroelectricity development plans for rivers across the Amazon basin.

Changes in trophic ecology of fish assemblages after no take Marine Protected Area designation in the southwestern coast of Portugal

Changes in fish assemblage structure caused by human activities, such as fishing, can alter trophic relations in fish assemblages. In this context, Marine Protected Areas (MPA) are efficient tools for habitat recovery and ideal environments for evaluating changes on the trophic structure resulting from human activities. The present work targeted fish assemblages from two no-take MPAs from the northern half of South Alentejo and Costa Vicentina Marine Park, established in 2011. Previous works reported positive effects on local fish assemblages after no-take MPA designation, and it is therefore important to further study its impact on local fish assemblages, especially concerning trophic interactions. Local fish assemblages were sampled (summer 2011, winter 2012, summer 2013 and winter 2013) using trammel nets. Diets were characterized and digestive tract contents of the 10 most abundant fish species were compared between the no take MPAs (treatment) and adjacent areas (controls), and changes evaluated as a function of time since protection. Results revealed significant differences between the diets of fish from protected and non protected areas, with crabs being the preferential prey in both protected and control areas but being more ingested outside the no-take areas. However, these differences were evident since the beginning of the study. Fish assemblages from the northern area presented significantly larger niche breadth and significantly increasing with time. This way, the main effects of no-take MPA implementation were directly visible on the niche breadth but did not directly impact the diet composition of the sampled fish assemblages, contributing however to reinforce the already naturally existent differences. This work provides important information regarding the effect of changes in the fish assemblage caused by MPA designation on the trophic ecology of fish.

Changes in fish assemblage structure after implementation of Marine Protected Areas in the south western coast of Portugal

Marine Protected Areas (MPAs) are increasingly being recommended as management tools for biodiversity conservation and fisheries. With the purpose of protecting the region's biodiversity and prevent the over exploitation of marine resources, in February 2011 the no-take MPAs of Ilha do Pessegueiro and Cabo Sardão were implemented within the “Parque Natural do Sudoeste Alentejano e Costa Vicentina “(PNSACV) Marine Park, south western coast of Portugal. As such, commercial and recreational fishing became prohibited in these areas. In order to evaluate the effects of these no-take MPAs, the structure of their fish assemblages and of adjacent control areas without fishing restrictions were studied between 2011 (immediately after implementation) and 2013 (two years after implementation). A total of 4 sampling campaigns were conducted (summer 2011, winter 2012, summer 2013 and winter 2013) using trammel nets and bottom trawl. Ichthyofaunal assemblages from the no-take MPAs (treatment) were compared with adjacent areas (controls) and changes evaluated as a function of time since protection. Results revealed significant increase in fish abundance after the implementation of the no-take MPAs. Furthermore, significant differences in the structure of fish assemblages (abundance and fish size) between protected and neighbouring areas were rapidly observed upon the implementation of the no-take MPAs. In addition, specimens of larger size occurred more frequently within Ilha do Pessegueiro no-take MPA in the last year of the study. Overall, despite the young age of these no-take MPAs, changes on the structure of their fish assemblages were already evident after only two years of protection, indicating that management measures such as MPA designation may play an important role to promote fisheries sustainable exploitation as well as to protect species with conservation interest.

Seasonal changes in fish assemblage structure at a shallow seamount in the Gulf of California.

Seamounts have generally been identified as locations that can promote elevated productivity, biomass and predator biodiversity. These properties attract seamount-associated fisheries where elevated harvests can be obtained relative to surrounding areas. There exists large variation in the geological and oceanographic environment among the thousands of locations that fall within the broad definition of seamount. Global seamount surveys have revealed that not all seamounts are hotspots of biodiversity, and there remains a strong need to understand the mechanisms that underlie variation in species richness observed. We examined the process of fish species assembly at El Bajo Espiritu Santo (EBES) seamount in the Gulf of California over a five-year study period. To effectively quantify the relative abundance of fast-moving and schooling fishes in a ‘blue water’ habitat, we developed a simplified underwater visual census (UVC) methodology and analysis framework suitable for this setting and applicable to future studies in similar environments. We found correlations between seasonally changing community structure and variability in oceanographic conditions. Individual species responses to thermal habitat at EBES revealed three distinct assemblages, a ‘fall assemblage’ tracking warmer overall temperature, a ‘spring assemblage’ correlated with cooler temperature, and a ‘year-round assemblage’ with no significant response to temperature. Species richness was greatest in spring, when cool and warm water masses stratified the water column and a greater number of species from all three assemblages co-occurred. We discuss our findings in the context of potential mechanisms that could account for predator biodiversity at shallow seamounts.

The Review of Ecological and Genetic Research of Ponto-Caspian Gobies (Pisces, Gobiidae) in Europe

Abstract Invasive Ponto-Caspian gobies (monkey goby Neogobius fluviatilis, round goby Neogobius melanostomus and bighead goby Ponticola kessleri) have recently caused dramatic changes in fish assemblage structure throughout European river systems. This review provides summary of recent research on their dietary habits, age and growth, phylogenetic lineages and gene diversity. The principal food of all three species is invertebrates, and more rarely fish, which depends on the type of habitat, part of the year, as well as the morphological characteristics of species. According to the von Bertalanffy growth model, size at age is specific for the region, but due to its disadvantages it is necessary to test other growth models. Phylogenetic analysis of monkey goby and round goby indicates separation between the Black Sea and the Caspian Sea haplotypes. The greatest genetic diversity is found among populations of the Black Sea, and the lowest among European invaders. The lack of molecular research on bighead goby requires further studies.

Recolonization Process and Fish Assemblage Dynamics in the Guadiamar River (SW Spain) After the Aznalcóllar Mine Toxic Spill

The Guadiamar River (SW Iberian Peninsula) received a major toxic spill (6 hm3) from a tailing pond in 1998 that defaunated 67 km of the main stem. Following early mud removal works, the fish assemblage was annually monitored at four affected sampling sites and one located in the upstream non-affected reach of the Guadiamar River as reference. Fish abundance and assemblage structure were analysed. Principal response curve was applied to assess the recovery trends and to identify the most influential species. A non-metric multidimensional scaling ordination and permutational multivariate analysis of variance were applied to evaluate changes in fish assemblage structure between sites and years. Overall, the affected reaches harboured fish within 2 years of the spill. Colonists arrived mainly from the upstream and downstream non-affected Guadiamar River reaches and, to a lesser extent, from three lateral tributaries. It is likely that the proximity, connectivity and environmental conditions of non-affected fish sources greatly influenced the recolonization process in each site. The structure of the fish community in the affected sites was initially similar to that in the unaffected reference stretch but changed dramatically with time, and each site followed its own trajectory. Currently, long-term threats such as mining leachates, urban sewage, agricultural pollution and exotic fish species expansion have probably exceeded the initial spill effect. This highlights the large effect of anthropogenic factors on freshwater ecosystem resilience, and the need to significantly reduce both pollution and exotic species if the affected reach of the Guadiamar River is to recover fully. Copyright © 2015 John Wiley & Sons, Ltd.

Trophic downgrading results in complex ecosystem dynamics in experimental tropical floodplain food webs

Species or functional guild loss from upper trophic positions, i.e., trophic downgrading, will likely have important consequences for ecosystem functioning due to cascading direct and indirect effects. Using 1000 l mesocosms, we examined how sequential loss of species occupying upper trophic positions influenced ecosystem functioning of experimental floodplain lagoon food webs. Treatments were developed based on fish assemblage data from long-term field surveys of tropical floodplain lagoons, and response variables represented multiple components of ecosystem functioning. Sequential loss of species occupying upper trophic positions significantly influenced multiple ecosystem responses including changes in fish assemblage structure, nutrient concentrations, and zooplankton density. Although loss of species from specific functional roles is expected to facilitate predictive understanding of ecosystem consequences, we observed complex and dynamic responses to trophic downgrading that did not follow expectations of strong predicted top-down effects. The highly connected food web structure in our system and relative balance between top-down and bottom-up processes likely suppressed cascading effects. Consequences of biodiversity loss in highly connected multitrophic ecosystems may be difficult to predict as ecosystem responses will likely deviate from simplified food chain dynamics or from patterns that emerged from single trophic level studies.

FISH SPECIES COMPOSITION IN SEAGRASS BEDS OF TANJUNG MERAH (NORTH SULAWESI), INDONESIA

Seagrass beds are found along the shore of Tanjung Merah (North Sulawesi, Indonesia). This ecosystem covers approximately 20 hectars, and are well known to support local fisheries. Thalassiahemprichii was the dominant seagrass species at the study site. This study in the given area was aimed to monitor any changes in fish assemblage structure, and examine the function of seagrass beds for marine fish. Fishes were sampled using the small beam trawl in March 2003, April 2004 and September 2005. A total of 137 species belonging to 34 families were identified. Twenty four species were found abundantly in terms of individual numbers, however Apogon margaritophorus and A. hartzfeldii were the most dominant. These two species were permanent resident, indicated by a catch of large number in different stages of life cycle. We also noticed that many adults of A. margaritophorus kept fertilized eggs in their mouths. This evidence also confirmed that seagrass beds function as both spawning and nursery ground for marine fishes. During three years’ monitoring, there was a slightly seasonal variety in fish assemblage structures.

Downstream fish assemblage response to river impoundment varies with degree of hydrologic alteration

River impoundments can fundamentally restructure downstream fish assemblages by altering flow regimes. However, the degree of alteration and associated ecological change may depend on pre-existing hydrologic regimes. We used long-term datasets to compare downstream hydrological and fish assemblage responses to impoundment in two catchments classified as having intermittent and perennial-flashy natural hydrologic regimes. We observed significant shifts in fish assemblage structure at both sites after stream impoundment. The historically intermittent stream shifted to a stable perennial flow regime. Changes in fish assemblage structure covaried with changes in five different components of the flow regime; most species that increased in abundance require fluvial habitats and likely benefited from increased flows during historically low flow seasons. Shifts in fish assemblage structure were also observed in the perennial stream, despite minimal flow alteration after impoundment; however, most species shifts were associated with lentic environments, and were more likely related to proximity of reservoirs in the drainage system rather than changes in stream flow. Findings from this study confirm that downstream fish assemblage response to river impoundment can be associated with high levels of hydrologic alteration, but other factors including expansion of lentic species into lotic environments also influence shifts in assemblage structure.

Historical Changes in Fish Assemblage Structure in Midwestern Nonwadeable Rivers

Historical change in fish assemblage structure was evaluated in the mainstems of the Des Moines, Iowa, Cedar, Wapsipinicon, and Maquoketa rivers, in Iowa. Fish occurrence data were compared in each river between historical and recent time periods to characterize temporal changes among 126 species distributions and assess spatiotemporal patterns in faunal similarity. A resampling procedure was used to estimate species occurrences in rivers during each assessment period and changes in species occurrence were summarized. Spatiotemporal shifts in species composition were analyzed at the river and river section scale using cluster analysis, pairwise Jaccard's dissimilarities, and analysis of multivariate beta dispersion. The majority of species exhibited either increases or declines in distribution in all rivers with the exception of several “unknown” or inconclusive trends exhibited by species in the Maquoketa River. Cluster analysis identified temporal patterns of similarity among fish assemblages in the Des Moines, Cedar, and Iowa rivers within the historical and recent assessment period indicating a significant change in species composition. Prominent declines of backwater species with phytophilic spawning strategies contributed to assemblage changes occurring across river systems.

Historical Changes in Nebraska's Lotic Fish Assemblages: Implications of Anthropogenic Alterations

Abstract The plains of midwestern North America have undergone significant anthropogenic alterations following European settlement with consequent effects to lotic fish assemblage structure. We examined trends in fish assemblage structure and function in Nebraska's lotic systems using site-specific, presence-absence data from historical (1939–1940) and contemporary surveys (2003–2005; n = 183). Shifts in fish assemblage structure were characterized by declines of specialist species (e.g., western silvery minnow Hybognathus argyritis) and increases in nonnative, sport, and generalist species (e.g., common carp Cyprinus carpio). Our research illustrates differences between historical and contemporary surveys for both taxonomic and functional metrics. Changes in fish assemblage structure were correlated with a contemporary measure of anthropogenic alteration (Human Threat Index; HTI) and were most pronounced for large-scale threats (i.e., watershed HTI, overall HTI). The HTI is a composite index of cumulat...

Long‐Term Changes in Fish Assemblage Structure in the Yellow River Estuary Ecosystem, China

AbstractThe Yellow River estuary ecosystem is an important spawning ground for many species found in the Bohai Sea and Yellow Sea and contributes substantially to the fishery resource structure and biological reproduction in the northern China Sea. Based on long‐term ecosystem surveys in the Yellow River estuary during the main spawning period (May) of most fishery species from 1959 to 2011, the responses of the ecosystem, including regime shifts in species composition, biomass, diversity, and other related factors, were analyzed in this study. Since the 1980s, the dominant large‐size species of high economic value (e.g., Largehead Hairtail Trichiurus lepturus) have been replaced by short‐lived, low‐trophic‐level, planktivorous pelagic species (e.g., Scaly Hairfin Anchovy Setipinna taty and Japanese Anchovy Engraulis japonicus). Currently, traditional commercially targeted fishes, such as the Largehead Hairtail, Red Seabream Pagrus major, and Pacific Herring Clupea pallasii, are locally extinct. There has been a rapid shift of dominant species from highly valued, high‐trophic‐level, large‐sized demersal species with complicated age structures to low‐value, low‐trophic‐level, small‐sized pelagic species with simple age structures; this shift has resulted in major changes to the ecological cycle and restoration of fishery resources. The fish catch declined from 421.66 kg/h in 1959 to 0.25 kg/h in 2008 and then increased to 3.62 kg/h in 2011. Diversity and evenness indices showed a continuously increasing trend during 1959–2011. The Yellow River estuary may be significantly compromised by overfishing, climate change, dam construction, and pollution, resulting in the decline of traditional fishing industries and reduced biodiversity in this ecosystem.Received August 30, 2012; accepted January 11, 2013

Surf zone fish abundance and diversity at two sandy beaches separated by long rocky jetties

In this work, we evaluated the influence of long rocky jetties (~5 km) on fish abundance and diversity between sheltered and exposed marine sandy beaches. We also described and compared the fish community structure and investigated the relationships between environmental variables and fish assemblages. Fish were collected monthly with a beach seine net from May 2001 to May 2002 at the Cassino and Mar Grosso beaches. Twenty-nine taxa were caught and the fish assemblage showed similar composition between beaches (Sj = 62.1%; %Min = 52.3%), with 18 species in common. Most of the fish were juveniles, mainly young-of-the-year with sizes ≤60 mm total length. Eight species (Trachinotus marginatus, Mugil liza, Brevoortia pectinata, Menticirrhus littoralis, Menticirrhus americanus, Odontesthes argentinensis and Oncopterus darwinii) were the most abundant, accounting for 95.6% of the total catch. At both beaches, only T. marginatus, M. liza and M. littoralis were frequent and abundant, but with some differences in their relative abundance. The canonical correspondence analysis results showed that temperature had the highest correlation with fish abundance. Seasonal changes in fish assemblage structure were evident, with a greater species diversity and abundance in the spring and summer. The overall results indicate that the presence of jetties had no effect on fish assemblages of adjacent sandy beaches. Moreover, differences between beaches were related to some species abundance and not to differences in the number and composition of species between beaches.

Impacts of climate warming on the long-term dynamics of key fish species in 24 European lakes

Fish play a key role in the trophic dynamics of lakes. With climate warming, complex changes in fish assemblage structure may be expected owing to direct effects of temperature and indirect effects operating through eutrophication, water level changes, stratification and salinisation. We reviewed published and new long-term (10–100 years) fish data series from 24 European lakes (area: 0.04–5,648 km2; mean depth: 1–177 m; a north–south gradient from Sweden to Spain). Along with an annual temperature increase of about 0.15–0.3°C per decade profound changes have occurred in either fish assemblage composition, body size and/or age structure during recent decades and a shift towards higher dominance of eurythermal species. These shifts have occurred despite a reduction in nutrient loading in many of the lakes that should have benefited the larger-sized individuals and the fish species typically inhabiting cold-water, low-nutrient lakes. The cold-stenothermic Arctic charr has been particularly affected and its abundance has decreased in the majority of the lakes where its presence was recorded. The harvest of cool-stenothermal brown trout has decreased substantially in two southern lakes. Vendace, whitefish and smelt show a different response depending on lake depth and latitude. Perch has apparently been stimulated in the north, with stronger year classes in warm years, but its abundance has declined in the southern Lake Maggiore, Italy. Where introduced, roach seems to take advantage of the higher temperature after years of low population densities. Eurythermal species such as common bream, pike–perch and/or shad are apparently on the increase in several of the lakes. The response of fish to the warming has been surprisingly strong and fast in recent decades, making them ideal sentinels for detecting and documenting climate-induced modifications of freshwater ecosystems.