Fish Community Dynamics Research Articles

Space–Time Variations of Fish Communities from the Belbek River

This paper presents the results of studying the species composition, diversity, and dynamics of fish communities from the middle reaches of the Belbek River, one of the small rivers of the Crimean Peninsula, which is actively used to eliminate the shortage of water supply in recent years. The river is currently exposed to significant anthropogenic impact. Trends of change in the structure of the fish community with the economic development of the river have been observed and the main forms of negative environmental impact have been identified. A deterioration in the structure of aquatic communities and a sharp decrease in their diversity have been recorded over a short time period. A decrease in the abundance and a deformation of the population structure of native fish species (Crimea shemaya Alburnus mentoides, Crimean barbel Barbus tauricus, and Salgir gudgeon Gobio krymensis) are also observed. The determining influence of hydraulic engineering and hydrotechnical measures on destructive processes in aquatic ecosystems is shown.

Fish community characterization of mid-shelf and shelf- edge mesophotic coral ecosystems in the expanded Flower Garden Banks National Marine Sanctuary

The mid to outer continental shelf in the northern Gulf of Mexico is composed of a patchy distribution of coral and rock reefs designated high priority for marine protection. To better understand the influence of deepwater habitat on fish community dynamics and conservation needs, we compared altiphotic-mesophotic transition (20– 40 m), upper mesophotic (40–60 m), and middle mesophotic (60–80 m) fish communities between mid-shelf (Sonnier Bank) and outer-shelf (McGrail Bank) banks from before their inclusion into the Flower Garden Banks National Marine Sanctuary. Surveys performed over two years with a remotely operated vehicle indicated that each bank and depth zone had distinct fish communities. Both banks were dominated by planktivores and piscivores, with an increase in depth specialists (e.g., deepwater anthiids and serranids) at the deeper zones surveyed, particularly in middle mesophotic depths at McGrail. An increased frequency of snappers, groupers, and amberjack was observed at Sonnier Bank, predominately in mesophotic depths, indicating the Sonnier Mesophotic Coral Ecosystem as either a hotspot or potential refuge for meso- and apex predators. This study fills a temporal gap in fish community dynamics of these two banks, serving to create a more continuous dataset available to assist in conservation assessments of the Flower Garden Banks National Marine Sanctuary.

Key factors determining water quality, fish community dynamics, and the ecological health in an Asian temperate lotic system

Identifying the key determinants and their impacts on water quality and fish community structure is imperative to environmental assessment and ecosystem conservation. The main objectives of this study were to assess the factors driving water quality, fish assemblages, and ecological health in a temperate lotic ecosystem using a long-term (12 years) dataset. The results indicate that sewage treatment plants significantly negatively impact river water quality and are primary pollution sources. Temporal fluctuations showed that the summer monsoon adversely influenced TN and EC due to dilution effects while increasing the COD and TSS in the water. Annual variations and the Mann–Kendall test demonstrated increasing trends in COD, TN:TP ratio, and CHL-a but a decreasing trend of TP with the impoundment. Artificial barriers (weirs) in the river created more favorable algal growth conditions due to increased water residence time. COD and TSS levels in the river were significantly affected by soil erosion. The environmental disturbance measures index indicated that all sites were severely degraded. Fish composition analysis suggested that exotic fish species such as, Lepomis macrochirus and Micropterus salmoides, dominated the community structure and negatively impacted native species after the installation of weirs. Our findings indicate that fish guilds and the IBI model are controlled by nutrient enrichment, organic matter, algal production, and land use/land cover. Water quality governs the biological health of the river. Overall, evaluation of the river's ecological health based on the multi-metric WPI and the IBI revealed that the river has a “poor” to “very poor” ecological status. The outcomes of this study may aid policymakers in managing and restoring the river.

Fish community structure and dynamics are insufficient to mediate coral resilience.

Coral reefs are being impacted by myriad stressors leading to drastic changes to their structure and function. Fishes play essential roles in driving ecosystem processes on coral reefs but the extent to which these processes are emergent at temporal or ecosystem scales or otherwise masked by other drivers (for example, climatic events and crown-of-thorns starfish outbreaks) is poorly understood. Using time series data on fish community composition and coral and macroalgae percentage cover between 2006 and 2017 from 57 sites around Mo'orea, Polynesia, we found that fish community diversity predicts temporal stability in fish biomass but did not translate to temporal stability of coral cover. Furthermore, we found limited evidence of directional influence of fish on coral dynamics at temporal and ecosystem scales and no evidence that fish mediate coral recovery rate from disturbance. Our findings suggest that coral reef fisheries management will benefit from maintaining fish diversity but that this level of management is unlikely to strongly mediate coral loss or recovery over time.

Species and Functional Dynamics of the Demersal Fish Community and Responses to Disturbances in the Pearl River Estuary

Fishery resources are threatened by environmental changes and anthropogenic pressures, particularly in coastal ecosystems. It is crucial to understand the changes of fish communities and their responses to environmental changes and human disturbances to formulate rational fisheries and ecosystem-based management. The Pearl River Estuary (PRE) is a typical sub-tropic coastal ecosystem located in the center of the Guangdong-Hong Kong-Macao Greater Bay Area in the northern South China Sea. The demersal fish in the PRE is traditionally targeted as commercial fishing and severely impacted by overexploitation and hypoxia in the last few decades. In this study, we analyze the fish survey data during the period of 2020~2021 using multivariate statistics to investigate the impacts of human disturbances on the species and functional dynamics of the demersal fish community in the PRE. The results reveal that dissolved oxygen and temperature have significant correlations with the functional traits of the demersal fish community. The impacts of hypoxia on the demersal fish vary with species and locations. We found that the mean functional redundancy of the demersal fish community in the PRE was high across three surveys, but the functional diversity was low in this region. The abundance and richness of the demersal fish community increased during the summer fishing moratorium in the South China Sea in 2021, but the functional diversity did not increase significantly. We conclude that the high functional redundancy in the PRE might not be sufficient to buffer against environmental disturbances because of its low functional diversity. Our study highlights the complicated interactions between the demersal fish community and disturbances in the PRE. Understanding the traits structure and functional diversity of the fish community can help elucidate the factors determining the dynamic responses of the fish community to disturbances.

Influence of internal seiche dynamics on vertical movement of fish

Abstract Internal seiches are common in stratified lakes, with significant effects on stratification patterns, hydrodynamics and vertical nutrient transport. In particular, seiches can change the vertical distribution of the thermocline and the cold hypolimnetic and warm epilimnetic water masses by several metres on a timescale of a few hours, leading to rapid and strong changes in temperature profiles and oxygen availability, with profound effects on mobile and sessile organisms. This could affect fish communities directly, through physiological stress and elevated mortality, and indirectly, through prey distribution. The aim of this study was to analyse the effects of internal seiche dynamics on lacustrine fish behaviour, and to characterise fish reaction patterns, with the main focus on vertical movement of fish in the vicinity of a shifting thermocline, and avoidance of cold hypolimnetic water. The analysis was based on acoustic telemetry data from Lake Milada, a post‐mining lake in the Czech Republic, with a total of 61 tracked individuals of four species: northern pike (Esox lucius), wels catfish (Silurus glanis), tench (Tinca tinca) and rudd (Scardinius erythropthalmus). The effects of seiche dynamics on the four species studied were weak but significant during the day, while at night they affected only rudd. Upward seiches elicited stronger responses in fish than downward seiches, and the impacts occurred only during the strongest seiche events. Thermocline shifting during seiche events may induce a transient reduction in habitat for seiche‐reacting species, and thus affect predation and other inter‐ and intra‐specific interactions, as well as fish community dynamics.

Size spectrum model reveals importance of considering species interactions in a freshwater fisheries management context

AbstractInland fisheries have a significant cultural and economic value around the globe, providing dietary protein, income, and recreation. Consequently, methods for monitoring and managing these important fisheries are continually being refined. In marine systems, multispecies size spectrum models have been increasingly used to explore management scenarios of important fish stocks within an ecosystem‐based fisheries management framework; however, these models have not been applied as extensively in freshwater systems. In this study, we developed a multispecies size spectrum model for the fish community of Lake Nipissing, a large, productive lake in Ontario, Canada. To the best of our knowledge, this is the first fully calibrated multispecies size spectrum model for an inland fishery. Using this model, we explored the impacts of potential fishing regimes and management scenarios on fish community dynamics while taking species interactions into account. Specifically, we examined how changes in fishing mortality affect (1) species biomass, (2) community size structure, and (3) stock recovery times. We found that community dynamics following changes in fishing mortality were driven by complex interactions among species, including competition and predation. The greatest changes in biomass and community size structure were observed following changes in fishing mortality of top predators, with community size structure most strongly influenced by changes in the mortality of the largest species. Counter to predictions based on generation time, the smallest species in our model exhibited the longest time to recovery due to strong competition and predation. Our results demonstrate the importance of considering species interactions in the management of inland fisheries and highlight the potential of size spectrum model use in freshwater systems.

Mutually promoting or constraining? Disentangling the superimposed effect of velocity and illuminance on fish motion in low‐velocity flows with a novel metric

Abstract Global climate change, species invasion, and human activities such as the construction of hydraulic facilities are contributing to the worldwide degradation of fish stocks. As a pivotal environmental factor that affects fish migration and population distribution, the low‐velocity flow has not drawn enough attention despite its extensive presence in various ecoregions. To understand the behavioural profiles of fish in low‐velocity flow, we examined the motility and swimming performance of two representative cyprinids, grass carp (GC, Ctenopharyngodon idella) and silver carp (SC, Hypophthalmichthys molitrix), under the combined effect of different flow velocities and illuminance ranges. The two species were exposed to two flow velocities (0.15 and 0.25 m/s) and three illuminance ranges (100.0–0.9 lx, 300.0–2.5 lx, and 700.0–4.9 lx). A novel behavioural metric, composed of light preference, swimming speed, rheotaxis index (RI), swimming stability (SS), and the probability of moving forwards/backwards over a certain distance (PMf‐0.1 and PMb‐0.1), was developed to fully characterise the motion strategy of fish. By analysing the individual distribution in light gradient, we confirmed the scototaxis of GC and found the oscillating light preference in SC that changed from 84.41–10.18 to 6.00–2.08 lx and disappeared with increasing illuminance. The RI and SS increased at the fast flow velocity (0.25 m/s) for both species, and SC was more forward‐prone and more stable than GC. Despite the fluctuating light preference, the higher illuminances heightened the RI and SS in SC, while in GC, higher illuminances inhibited the SS and had no impact on the RI. The results of PMf‐0.1 and PMb‐0.1 indicated that a higher flow velocity could facilitate the temporary upstream swimming in both species, while illuminance could only promote the same effect in SC. Finally, our study proposed possible manoeuvres in fine‐scale spatio‐temporal scenarios such as attraction flow and non‐traditional passages and provided valuable insights into fish community dynamics, laying the groundwork for alleviating the threat from invasive species and for future fish population recovery.

Similar fish species composition despite larger environmental heterogeneity during severe hypoxia in a coastal ecosystem.

Environmental heterogeneity is one of the most influential factors that create compositional variation among local communities. Greater compositional variation is expected when an environmental gradient encompasses the most severe conditions where species sorting is more likely to operate. However, evidence for stronger species sorting at severer environment has typically been obtained for less mobile organisms and tests are scarce for those with higher dispersal ability that allows individuals to sensitively respond to environmental stress. Here, with the dynamics of fish communities in a Japanese bay revealed by environmental DNA metabarcoding analyses as a model case, we tested the hypothesis that larger environmental heterogeneity caused by severe seasonal hypoxia (lower concentration of oxygen in bottom waters in summer) leads to larger variation of species composition among communities. During summer, fish species richness was lower in the bottom layer, suggesting the severity of the hypoxic bottom water. In contrast to the prediction, we found that although the environmental parameters of bottom and surface water was clearly distinct in summer, fish species composition was more similar between the two layers. Our null model analysis suggested that the higher compositional similarity during hypoxia season was not a result of the sampling effect reflecting differences in the alpha or gamma diversity. Furthermore, a shift in the species occurrence from bottom to surface layers was observed during hypoxia season, which was consistent across species, suggesting that the severe condition in the bottom adversely affected fish species irrespective of their identity. These results suggest that larger environmental heterogeneity does not necessarily lead to higher compositional variation once the environmental gradient encompasses extremely severe conditions. This is most likely because individual organisms actively avoided the severity quasi‐neutrally, which induced mass effect‐like dispersal and lead to the mixing of species composition across habitats. By showing counter evidence against the prevailing view, we provide novel insights into how species sorting by environment acts in heterogeneous and severe conditions.

Daily and seasonal dynamics of littoral zone fish communities in the lowland Włocławek Reservoir (central Poland), with a special emphasis on alien invasive gobies

The aim of the study was to examine the daily and seasonal dynamics of changes in fish communities in two environmentally different littoral zones of the Włocławek Reservoir (central Poland), i.e., the floodplain (limnic Dobiegniewo site) and lotic area (Murzynowo site). During seasonal catches using trawl nets, in the daily cycle (at 00.00, 06.00, 12.00, and 18.00 hours), the presence of 18 fish species representing 4 families (Cyprinidae, Percidae, Gasterosteidae, Gobiidae) was confirmed. In both localities, the domination of native taxa, such as roach Rutilus rutilus (L.) and Eurasian perch Perca fluviatilis L., in Dobiegniewo (26.6 and 26.6%, respectively) and Murzynowo (13.6 and 22.7%, respectively) of all caught specimens was noted. The quantitative share of three alien invasive gobies, racer goby Neogobius gymnotrachelus (Kessler, 1857), monkey goby Neogobius fluviatilis (Pallas, 1814), and western tubenose goby Proterorhinus semilunaris (Heckel, 1837), varied in the range between 2.7 and 35.3% in samples throughout the season. The results indicate the significant role of gobies in the structure of the sublittoral fish communities of Włocławek Reservoir and the necessity of expansion process studies, as well as the observation of the interactions with native fish species in invaded areas.

Phenotypic responses in fish behaviour narrow as climate ramps up

Natural selection alters the distribution of phenotypes as animals adjust their behaviour and physiology to environmental change. We have little understanding of the magnitude and direction of environmental filtering of phenotypes, and therefore how species might adapt to future climate, as trait selection under future conditions is challenging to study. Here, we test whether climate stressors drive shifts in the frequency distribution of behavioural and physiological phenotypic traits (17 fish species) at natural analogues of climate change (CO2 vents and warming hotspots) and controlled laboratory analogues (mesocosms and aquaria). We discovered that fish from natural populations (4 out of 6 species) narrowed their phenotypic distribution towards behaviourally bolder individuals as oceans acidify, representing loss of shyer phenotypes. In contrast, ocean warming drove both a loss (2/11 species) and gain (2/11 species) of bolder phenotypes in natural and laboratory conditions. The phenotypic variance within populations was reduced at CO2 vents and warming hotspots compared to control conditions, but this pattern was absent from laboratory systems. Fishes that experienced bolder behaviour generally showed increased densities in the wild. Yet, phenotypic alterations did not affect body condition, as all 17 species generally maintained their physiological homeostasis (measured across 5 different traits). Boldness is a highly heritable trait that is related to both loss (increased mortality risk) and gain (increased growth, reproduction) of fitness. Hence, climate conditions that mediate the relative occurrence of shy and bold phenotypes may reshape the strength of species interactions and consequently alter fish population and community dynamics in a future ocean.

Environmental DNA Metabarcoding: A Novel Method for Biodiversity Monitoring of Marine Fish Communities.

Environmental DNA (eDNA) is genetic material that has been shed from macroorganisms. It has received increased attention as an indirect marker for biodiversity monitoring. This article reviews the current status of eDNA metabarcoding (simultaneous detection of multiple species) as a noninvasive and cost-effective approach for monitoring marine fish communities and discusses the prospects for this growing field. eDNA metabarcoding coamplifies short fragments of fish eDNA across a wide variety of taxa and, coupled with high-throughput sequencing technologies, allows massively parallel sequencing to be performed simultaneously for dozens to hundreds of samples. It can predict species richness in a given area, detect habitat segregation and biogeographic patterns from small to large spatial scales, and monitor the spatiotemporal dynamics of fish communities. In addition, it can detect an anthropogenic impact on fish communities through evaluation of their functional diversity. Recognizing the strengths and limitations of eDNA metabarcoding will help ensure that continuous biodiversity monitoring at multiple sites will be useful for ecosystem conservation and sustainable use of fishery resources, possibly contributing to achieving the targets of the United Nations' Sustainable Development Goal 14 for 2030.

Adaptive Resolution Imaging Sonar (ARIS) as a tool for marine fish identification

Assessment and monitoring of marine biodiversity, including fish populations, is essential for evidence-based conservation management of coastal marine resources. The effectiveness of monitoring techniques for stock assessment varies with sea conditions. In dynamic marine environments with high turbidity, such as those found in estuaries, mangroves, coastal straits, fjords, and bays, traditional assessment methods include the use of destructive techniques such as trawling. Hydroacoustic sampling techniques overcome such restrictions, equipment such as echosounders have commonly been used for biodiversity assessments including fish community structure, biomass, behaviour, and dynamics studies. However, hydroacoustic methods have been shown to be less reliable for species identification.The high frequency Adaptive Resolution Imaging Sonar (ARIS) is widely used for underwater object detection and imaging. Our study investigated the suitability of ARIS 3000 for the species identification of North-East Atlantic marine species using experimental aquarium studies, field surveys and multi investigator assessments. Aquaria results showed that 82 % of species were detected by observers, of which five were identified correctly identified consistently. The remaining four species were identified correctly <67 % of the time. During field surveys, a 150 % higher confidence in identification was given to more morphologically distinct groups such as elasmobranchs.Whilst our results highlight the suitability of the ARIS for accurate and repeatable identification of some of the model species used in this study, we have also shown that factors such as size and morphological traits limit the accuracy of identification for all species. We suggest that monitoring techniques combine the use of ARIS sonars alongside other sampling tools for assessing motile faunal communities.

Evaluating the impacts of fishing and migratory species in a temperate bay of China using the ecosystem model OSMOSE-JZB

Small-scale fisheries (SSFs) play a vital role in the sustainability of local economies. Migratory species moving into and out of an ecosystem may influence the dynamics of local fish communities and SSFs. We used the end-to-end model, OSMOSE-JZB (Object-oriented Simulator of Marine ecOSystEms), to evaluate the impacts of fishing and a migratory shrimp (Trachypenaeus curvirostris) on the ecosystem of Jiaozhou Bay, China. Increased fishing intensity (i.e., annual fishing effort) resulted in the decline of four ecological indicators, including the total biomass of the community, mean trophic level of the community, inverse fishing pressure, and large fish index. Compared to managing fish stocks under uniform fishing mortality over the fishing season, landings and community biomass were higher when a “race to fish” (i.e., large catches in a short period) occurred. The results suggested that managing fishing seasonality (i.e., temporal allocation of fishing effort) could mitigate the negative impact of fishing intensity. Two resident high-trophic-level fishes were sensitive to changes in fishing intensity and fishing seasonality. The changes in trophic interactions had larger impacts on species at low trophic levels than fishing. Pearson's correlation analysis showed that T. curvirostris biomass was negatively correlated with the biomass of resident species and positively correlated with the biomass of other migratory species. We also found that fishing changed the impact of varying T. curvirostris migratory biomass on other species. Resident species were more sensitive to changes in fishing and T. curvirostris migratory biomass than other migratory species. We argue that SSFs management can benefit from the development of temporal fishing strategies and consideration of trophic interactions stemming from migratory species dynamics.

Effective Management of Closed Hypereutrophic Estuaries Requires Catchment-Scale Interventions

Increased nutrient loading associated with rapid population growth is the leading cause of deteriorating water quality in urbanized estuaries globally. Small estuaries are particularly sensitive to changes when connection with the marine environment is restricted, or lost, because of high water retention. The temporarily closed Hartenbos Estuary (South Africa) is an example of how such pressures can culminate in a severely degraded ecosystem. Wastewater treatment work (WWTW) discharges introduce substantial volumes of freshwater (8,000 m3 d–1) and nutrient loads (38 kg DIN d–1 and 22 kg DIP d–1) into this estuary. This constant inflow has necessitated frequent artificial breaching (inducing alternating states) of the estuary mouth to prevent flooding of low-lying developments and, occasionally, to mitigate against extreme events such as fish kills and sewage spills. This study investigated the efficacy of artificial mouth breaching practices in eliciting responses in selected abiotic and biotic parameters. Microalgal (phytoplankton and benthic diatoms), benthic macrofauna and fish community dynamics were assessed in response to mouth state and water quality conditions using a seasonal monitoring programme. The hypereutrophic nature of the Hartenbos Estuary was highlighted by persistent high-biomass phytoplankton accumulations (&amp;gt;100 μg Chl-a l–1), extreme dissolved oxygen conditions (0.4–20.5 mg O2 l–1) and the predominance of harmful algal bloom (HAB) events comprising Nannochloropsis sp. and Heterosigma akashiwo. Artificial breaching of the mouth facilitated limited tidal exchange and occurred approximately bimonthly once water levels exceeded 1.9 m above mean sea level (MSL). Current pressures and management interventions have culminated in an ecosystem void of natural fluctuations and instead characterised by low diversity and shifts between undesirable states. This is highlighted by the near year-round dominance of only a few opportunistic species/groups tolerant of adverse conditions (e.g., Nannochloropsis sp., Halamphora coffeiformis, oligochaetes, estuarine round herring Gilchristella aestuaria, and southern mullet Chelon richardsonii). Therefore, catchment-scale interventions such as the diversion of WWTW discharges and restoration of hydrodynamic variability are management priorities for improving the health and biodiversity of small, closed microtidal systems such as the Hartenbos Estuary.

Assessment of coastal anthropo-ecological system dynamics in response to a tsunami catastrophe of an unprecedented magnitude encountered in Japan

On 11 March 2011, a catastrophic earthquake and subsequent tsunami hit the Pacific coast of northern Japan, devastating many of the towns, villages and coastal ecosystems located along the shoreline. To assess the impacts of the disaster, we investigated temporal dynamics of fish and epibenthic megafaunal community structure in relation to changes in a range of physical, biological and anthropogenic variables between 2007 and 2018 in Onagawa Bay. Commercially important fish such as greenlings, Japanese anchovy, flatfishes, rockfishes were consistently abundant in both larval and adult fish assemblages. While abundance, species richness, and Shannon index H′ for adult fish and epibenthic megafaunal assemblages increased significantly soon after the disaster to peak values towards the end of the study period, the same metrics did not change accordingly for larval fish assemblages. Temporal dynamics of larval fish community clearly demonstrated significant seasonal variation along with changes in large-scale environmental conditions such as temperature and nutrients. However, anthropogenic components such as decline in human population, reduction in fishing pressure and the recovery of aquaculture operations significantly explained the observed post-disaster change in adult fish and epibenthic megafaunal communities. The pelagic and benthic components of Onagawa Bay appeared to have responded to the 2011 disaster very differently, and this study suggests the post-disaster recovery and dynamics of the coastal ecosystems may be regulated by how human societies respond to the impacts of a tsunami catastrophe through their influences on benthic habitat of ecosystems.

Understanding shifts in estuarine fish communities following disturbances using an ensemble modeling framework

Ecological disturbances may result in mortality events that alter biotic communities and ecosystems. In many coastal zones disturbances are increasing, including algal blooms and fish kills. These two disturbances are often related, with blooms releasing toxins or depleting oxygen, ultimately killing fish. Depending on the intensity, duration, and geographic extent of an algal bloom, the fish community can take days to years to recover from disturbances. To explore the relationship among environmental disturbances, sport fish, and forage fish communities, this study examines a non-toxic brown algal bloom (Aureoumbra lagunensis) occurring from December 2015 through March 2016. Using an ensemble modelling framework combining generalized linear models (GLM), Bayesian modelling, and Bayesian structural equation modeling (SEM), this complementary framework helped elucidate complex relationships among environmental variables and the fish community following a disturbance. The algal bloom crashed over a three-day period in March 2016 and resulted in a fish kill when dissolved oxygen concentrations dropped below hypoxic levels (DO < 2 mg/L). The bloom and subsequent fish kill led to shifts in both forage and sport fish communities, and their relationships, when compared to non-disturbed years. Both sport fish and forage fish abundances decreased following the bloom, but the response of the forage fish community was more rapid. When looking at direct correlations between individual sport fish and forage fish community metrics during the bloom, a large amount of variation in sport fish abundance was explained by forage fish abundance (R2 = 0.34). Also, the variation in forage fish abundance was explained well by pH (R2 = 0.72). Forage fish community dynamics were more closely related to water quality metrics than sport fish communities during non-disturbed periods. However, during this algal bloom, sport fish community dynamics were more closely associated with water quality metrics than forage fish community dynamics. Furthermore, sport fish community dynamics were strongly related to bloom dynamics during the three months prior to the fish kill. In the three months following the kill, the forage and sport fish communities were less strongly linked than in non-disturbed years. These large shifts in community dynamics and relationships following a disturbance suggest both forage and sport fish communities, food webs, and trophic dynamics may be at increasing risk of crossing ecological thresholds as algal blooms become more common in coastal ecosystems.

Land cover alteration shifts ecological assembly processes in floodplain lakes: Consequences for fish community dynamics

Habitat degradation is expected to alter community structure and consequently, ecosystem functions including the maintenance of biodiversity. Understanding the underlying abiotic and biotic assembly mechanisms controlling temporal and spatial community structure and patterns is a central issue in biodiversity conservation. In this study, using monthly time series of fish abundance data collected over a three-year period, we compared the temporal community dynamics in natural habitats and poplar plantations in one of the largest river-lake floodplain ecosystems in China, the Dongting Lake. We found a prevailing strong positive species covariance, i.e. species abundance changes in the same way, in all communities that was significantly negatively impacted by higher water nutrient levels. In contrast to species covariance, community stability, which was measured by the average of aggregated abundance divided by temporal standard deviation, was significantly higher in poplar plantations than in natural habitats. The positive species covariance, which was consistent for both wet and dry years and among habitat types, had significantly negative effects on community stability. Furthermore, our results demonstrated that the ecological stochasticity (i.e. community assembly processes generating diversity patterns that are indistinguishable from random chance) was significantly higher in natural sites than in poplar plantations, suggesting that deterministic processes might control the community composition (richness and abundance) at the modified habitat through reducing species synchrony and positive species covariance observed in the natural habitats, leading to significantly lower temporal β-diversity. When combined, our results suggest that habitat modification created environmental conditions for the development of stable fish community in the highly dynamic floodplains, leading to niche-based community with lower temporal β-diversity.

Spatial Variations in Fish Abundance in the Mekong Delta

For the first time, the density of fish population has been quantitatively assessed in the Mekong River delta in the territory of the Socialist Republic of Vietnam. The studies were carried out in the main courses of two main branches, Hau and Tien rivers, into which the Mekong River splits in its lower course, and in the mouth branches of the delta. When comparing the fish abundance at 28 stations of the delta, statistically significant differences in the average specific abundance were found between the main river course and a mixohaline zone of the estuarine ecotone where values of the fish abundance were much lower. The analysis of the relationship between the fish abundance dynamics and different natural and anthropogenic environmental factors has demonstrated a weak significant correlation with salinity, distance from the mouth, the type of fish community and the amount of anthropogenic debris on the bottom. Large fluctuations in the fish abundance parameters associated with aggregated distribution of such species as Polynemusmelanochir, Ariusmaculatus and some representatives of the families Akysidae, Cynoglossidae, and Ariidae are observed in the delta. The results may be used as a baseline for subsequent monitoring of the abundance dynamics of bottom-dwelling fish communities in the Mekong Delta.

Вплив евтрофікації на багаторічну динаміку угруповань молоді риб

The article reveals the patterns of the temporal dynamics of juvenile fish communities under conditions of eutrophication of water bodies. The study was conducted in the reservoirs of the nature reserve “Dniprovsko-Orilsky”. Ichthyological sampling was carried out according to standard methods used in the study of the quantitative and qualitative composition of fish communities. During the study period, representatives of 38 species of juvenile fish were established in the surveys. Numbers of fish communities showed temporal patterns and differed significantly between habitat types. At presence of the general temporary trends of abundance, each of types of biotopes is characterized by the features of temporary dynamics. The highest level of abundance was characteristic of the channel part of the Dnieper River. Lake ecosystems of the Dnieper River floodplain were characterized by a much lower level of fish abundance. Among lake ecosystems, the highest number of juvenile fish communities was established for the reservoirs of the Taromsky ledge. The highest biomass was established for the channel part of the Dnieper River. The biomass of juvenile fish communities was significantly lower in the lacustrine water bodies. Species diversity of juvenile fish communities statistically reliably depends on habitat type and year as a categorical variable. The number of species increases with the growth of community abundance up to a certain level, after which there is an exit to a plateau: the growth of abundance is not accompanied by a significant increase in the number of species in the community. Biotopes can be divided into three groups according to the level of diversity. It has been substantiated that the establishment of protected areas and the maintenance of a fish conservation regime within them is an important factor in stabilizing the temporal dynamics of abundance, diversity and sustainability of fish populations over a much larger area than the territory of the reserve itself. Young fish communities are a sensitive indicator of the temporal dynamics of the ecological state of water bodies and respond both to the negative impact of anthropogenic activities and to positive measures of protection of biological diversity in protected areas.