Estuarine Fish Species Research Articles

Multi-habitat utilization and diverse life history of the endangered Sea Carp (Cyprinus acutidorsalis) in the Wanquan Estuary, Hainan island, China: Evidence from otolith microchemistry

The Sea Carp Cyprinus acutidorsalis (Wang) is an endemic species limited to a few estuaries in China, uniquely adapted to long-term residence in brackish water environments. This species is endangered due to anthropogenic disturbances, and its life history and habitat use remain poorly understood. This study is the first to investigate the life history and habitat utilization of Sea Carp in the Wanquan Estuary, Hainan Province, China, using otolith microchemistry. We analyzed the otolith samples from wild-captured Sea Carp and freshwater-reared juvenile fish. Significant differences in ratios of 7Li, 24Mg, and 88Sr between the two groups demonstrated that the otolith microchemistry can effectively reflect habitat conditions. We identified three distinct spawning areas, ranging from freshwater areas to nearshore habitats. Six life history contingents were identified, including four migratory and two resident types, with a preference for estuarine habitats. Our findings highlight the importance of multiple spawning grounds and diverse life histories in enhancing the adaptability and resilience of Sea Carp populations. We recommended that conservation efforts focus on maintaining habitat connectivity between estuarine and marine environments, protecting mangrove areas, and regulating fishing activities. This study provides valuable insights into the habitat utilization and life history of rare and endangered estuarine fish species, contributing to the development of effective management plans for their preservation worldwide.

Individual performance niches and responses to winter temperature change in three estuarine fishes from eastern Australia

Climate change may impact individual organisms in different ways, a consideration often overshadowed by predominant focus on population effects in studies. We examined three estuarine fish species to determine if individual fish performance, persisted across winter water temperatures. Fish performance at 16 °C (current Sydney winter estuarine water temperature) and 20 °C (predicted under climate change) with low and high food regimes was assessed using key physiological (growth, aerobic scope, burst speed) and behavioural parameters (foraging activity, boldness, shelter usage, predator escape response). We expected a strong positive relationship between performance at 16 °C and 20 °C for each parameter, and interactions with food level, however in general this was not found for any species. Relative performance was only maintained across temperatures for a few parameters, such as bite rate, boldness, and shelter response in one species (trumpeter Pelates sexlineatus), with aerobic scope in silver biddy Gerres subfasciatus, and boldness in fortescue Centropogon australis. Our results suggest that individuals’ fitness (directly via changes in growth, indirectly via behaviours) might be impacted by climate warming due to differences in relative performance among juvenile individuals across water temperatures. Changes in relative performance among individuals may initially compensate for a population-level response, thereby buffering the effects of climate change.

Acute toxicity testing of 6PPD-quinone on the estuarine-dependent sport fish, Sciaenops ocellatus.

Recently, large-scale fish kills in the Pacific Northwest were linked to tire wear particles (TWPs) left on roadways, with the lethality attributed to 6PPD-quinone. which has a median lethal concentration of <1 µg/L for selected salmonids. However, there remains a paucity of 6PPD-quinone toxicity values developed for estuarine fish species, which is particularly significant because estuaries receiving inflows from highly urbanized watersheds are especially vulnerable to TWP contamination. Therefore, the present study aimed to determine the toxicity of 6PPD-quinone to an economically and ecologically important estuarine-dependent fish-red drum (Sciaenops ocellatus). Here, we examined the relative sensitivities of three early life stages within red drum: embryonic, larval, and post-settlement for 24-72 hours, depending on the life stage. Exposure concentrations ranged from 10 μg/L to 500 μg/L. We also assessed the sub-lethal impacts of 6PPD-quinone exposure on development during embryonic and larval stages, including body and organ sizes. Our results indicate that red drum are not acutely sensitive to 6PPD-quinone at each early life stage tested. We also found that yolk-sac larvae did not exhibit sub-lethal morphological impacts in a dose-dependent manner, regardless of exposure during embryonic and larval stages. These data are the first to assess the impacts of 6PPD-quinone on estuarine-dependent non-model fishes.

Studies on the Spatial Distribution and Abundance of Selected Estuarine Fish Species in the Indus Delta Creek System Sindh, Pakistan

Studies on the Spatial Distribution and Abundance of Selected Estuarine Fish Species in the Indus Delta Creek System Sindh, Pakistan

Life cycles and challenges for fish species that breed in South African estuaries.

The literature currently recognizes four guilds of estuarine resident fish species, namely solely estuarine, estuarine and marine, estuarine and freshwater, and estuarine migrant. In this review the life cycles of actual representatives from these four guilds are assessed to determine whether the current definitions, which have never been formally tested, are appropriate to fish species resident in South African estuaries. Detailed information and diagrammatic life cycles are provided for the selected species covered by this review. A potential new estuarine resident guild category is also identified, namely, those taxa that are primarily estuarine but also have subpopulations recorded in both adjacent marine and freshwater habitats. The full range of reproductive characteristics employed by estuary resident species is examined, ranging from live bearers, pouch and nest brooders, to a suite of oviparous taxa that attach their ova to estuarine rocks, shells, and submerged vegetation, all of which assists with larval retention within the estuarine environment. The small size and early reproductive maturity of most estuarine resident species is highlighted, with reduced vulnerability to predation in shallow, sheltered, often turbid estuary waters offering considerable protection during spawning events when compared to the open ocean. In addition, these small fish would not have to move considerable distances at any stage of their life cycle, since egg, larval, juvenile, and adult stages all occur in the same place. The existence of contingent subpopulations within many estuarine resident species is noted, physico-chemical stresses on these species are highlighted, and the eurytopic nature of these small fish taxa emphasized.

Informing extinction risk: Summarizing population viability through a meta-analysis of multiple long-term monitoring programs for a declining estuarine fish species

Decisions about whether to designate a species for conservation and protection depend on the ability to summarize their population trajectories and their risk of extinction. Such decisions may rely on quantitative population viability analyses based on a time series of abundance index values that are derived from a monitoring program. In the case of the Longfin Smelt (Spirinchus thaleichthys), a decision to protect a distinct population segment of the species under the U.S. Endangered Species Act was informed by several indices of population abundance. In this paper, we combined individual population viability analyses into a single metric for extinction risk using a meta-analysis framework. Individual monitoring surveys for this species generally agreed that the trajectory of abundance was downward, but variation introduced uncertainty. Combining data from several surveys produced a better summary of the population growth rate. We also used the population growth rates in a simulation to estimate the probability that the abundance of Longfin Smelt dropped too low to recover. We found that this probability of quasi-extinction was substantial, exceeding 20 % over two decades. This study demonstrates a practical way that having multiple sources of information creates better information about the trajectory of a population. Individually, the surveys contribute information about specific life stages or ages to our understanding of the population. Combined into one metric and an associated graphical summary, this analysis succinctly communicates risk and creates a benchmark for evaluating future management decisions.

Assessing the application of the revised Remane Model to fish species in a fluvially dominated cool-temperate southern African coastal system

Estuaries are ecologically important areas which support a variety of aquatic species, particularly marine and estuarine fish species. This study represents a case study of the Orange River and Estuary (ORE) to understand patterns in fish assemblages and diversity trends that were compared to the revised Remane Model predictions in an estuary with poor marine fish species richness. A total of 30 species were recorded in the River continuum that comprised 14 freshwater, 10 marine and six estuarine species. Species diversity revealed seasonal variation in fish assemblages during the high-flow and low-flow seasons but not inter-annually. The results revealed that species diversity is lower in higher salinity areas when compared to low salinity areas. These patterns are consistent with the biogeographical trend of decreasing species richness along the South African coast from east to west, however, inconsistent with Remane predictions. The extremely low marine fish richness at its lower end and the extremely large freshwater influx at its upper end are the primary causes of the inconsistency. This may reflects the unsuitability of the Remane model for the Orange Estuary. In comparison to similar river-dominated South African estuaries, the ORE has a low marine species richness. When compared to more conventional South African estuaries, the ORE has a unique biotic environment with low fish species richness of estuarine-associated marine species adjacent to the Benguela upwelling zone and therefore the region does not offer suitable habitat for such species. As a result, the ORE is not a good candidate to test the Remane Model. The data does, however, confirm the left-hand part of the Remane model, which shows a decline in the fish species richness of freshwater fish species as salinity rises towards mesohaline and polyhaline levels.

Length-Weight Relationship and the Condition Factor of some Important Estuarine Fish Species from Kakum Estuary of Ghana

This study assessed the length-weight relationships and the condition factors of nine commercial and abundant fish species namely Liza dumerilli, Mugil curema, Liza falcipinnis, Mugil bananensis, Lutjanus goreensis, Lutjanus fulgens, Sardinella aurita, Caranx hippos, and Eucinostomus melanopterus from the Kakum River Estuary. This study was conducted from November 2016 to April 2017 to provide primary information about the biometrics, the length-weight model parameters, and the state of well-being of fish species inhabiting the Kakum Estuary. Sampling was done monthly using a mini boat of 9 m long with a depth of 1.5 m for both diurnal (06:00, 10:00, 14:00, 18:00, 06:00 h) and nocturnal (22:00, 02:00 h) periods with a medium-mesh cast net of 20 mm stretched. Sampling was done within thirty (30) minutes on all sampling times. The total length of the species from the estuary ranged from 2.2 cm in Liza dumerilli to 29.8 cm in Mugil curema while the total weight ranged from 0.2 g to 251 g in Mugil curema. The slope b values of the species varied from 2.3787 for Liza dumerilli to 3.1009 for Eucinostomus melanopterus. Three species from the Kakum Estuary exhibited negative allometric growth and 6 species were growing isometrically. The mean condition factors of the fish species ranged from 0.82 ± 0.36 (SD) in Sardinella aurita to 1.82 ± 0.23 (SD) in Lutjanus goreensis. The results of the present study provide essential input parameters for further assessment of the species standing stock biomass.

Development of modelling and mapping methods to predict spatial distributions and abundance of estuarine and coastal fish species life‐stages in Florida

AbstractSuitability modelling and mapping methods were developed to predict spatial distributions and population abundance of fish and macroinvertebrate species in Florida estuaries and coastal zones. Habitats were mapped in Pensacola Bay, Tampa Bay, and Charlotte Harbor using data from fisheries‐independent monitoring. Starting in 1997, suitability functions from habitat suitability index models were linked to habitat grids to create seasonal maps for early‐juvenile, juvenile, and adult life‐stages. After 2003, habitat suitability models (HSM) were linked to seasonal habitat grids to create seasonal maps validated by reciprocal transfer of suitability functions between estuaries. A quantitative method was used with five factor models and models with fewer environmental factors for four species in 2005 and for 11 species by life‐stages in 2009. In 2006, suitability functions transferred from Charlotte Harbor were linked with habitat grids for Rookery Bay and Fakahatchee Bay and HSM maps produced for three time periods to test transferability to estuaries lacking long‐term monitoring. Analyses in 2012 demonstrated that HSM maps for Tampa Bay derived independently from earlier suitability functions were almost identical to HSM maps created using recent suitability functions. Salinity was the most significant environmental variable for predicting abundance in models for species life‐stages in the Peace River and Charlotte Harbor, although nutrient concentrations from upriver may have influenced species’ abundances associated with low (&lt;5 psu) to moderate (5–10 psu) salinities. Population abundance estimates for Charlotte Harbor were derived from seasonal HSM maps created in 2019 and 2021. The 2021 paper compared changes in seasonal population numbers between Baseline and Minimum Flows associated with projected water withdrawals from the Peace River. Electronic logbooks were developed in 2001 and 2004 to collect data on shrimp fishing vessels and HSM maps for the West Florida Shelf created for 16 months in 2004–2005. Methodologies evolved as enabling technology became available.

Suitability of compensatory saltmarsh habitat for feeding and diet of multiple estuarine fish species

AbstractSaltmarsh provides essential fish feeding and nursery habitat but has globally declined by 50%. We used a statistical block design to compare fish feeding activity within human‐engineered or “re‐aligned” saltmarsh to established saltmarsh. Linear and multivariate modelling highlighted that Thinlip Mullet (Chelon ramada) and European Bass (Dicentrarchus labrax) feeding rates were 16% and 31% lower within re‐aligned than established saltmarshes, whereas Gobies (Pomatoschistus spp.) fed at the same rate as in both habitats. Analysis of European bass and Goby gut contents highlighted that important detritivorous prey species were up to 85.6% lower in re‐aligned sites. Lower vegetation density may have negatively affected the feeding ecologies of fishes within re‐aligned sites. However, due to the ecological value and potential for further improvement or habitat development, continued assessment of the beneficial effects of re‐aligned sites for fisheries and net gain perspectives is needed.

The role of carbonic anhydrase-mediated tissue oxygen extraction in a marine teleost acclimated to hypoxia.

With the growing prevalence of hypoxia (O2 levels ≤2 mg l-1) in aquatic and marine ecosystems, there is increasing interest in the adaptive mechanisms fish may employ to better their performance in stressful environments. Here, we investigated the contribution of a proposed strategy for enhancing tissue O2 extraction - plasma-accessible carbonic anhydrase (CA-IV) - under hypoxia in a species of estuarine fish (red drum, Sciaenops ocellatus) that thrives in fluctuating habitats. We predicted that hypoxia-acclimated fish would increase the prevalence of CA-IV in aerobically demanding tissues to confer more efficient tissue O2 extraction. Furthermore, we predicted the phenotypic changes to tissue O2 extraction that occur with hypoxia acclimation may improve respiratory and swim performance under 100% O2 conditions (i.e. normoxia) when compared with performance in fish that have not been acclimated to hypoxia. Interestingly, there were no significant differences in relative CA-IV mRNA expression, protein abundance or enzyme activity between the two treatments, suggesting CA-IV function is maintained under hypoxia. Likewise, respiratory performance of hypoxia-acclimated fish was similar to that of control fish when tested in normoxia. Critical swim speed (Ucrit) was significantly higher in hypoxia-acclimated fish but translated to marginal ecological benefits with an increase of ∼0.3 body lengths per second. Instead, hypoxia-acclimated fish may have relied more heavily on anaerobic metabolism during their swim trials, utilizing burst swimming 1.5 times longer than control fish. While the maintenance of CA-IV may still be an important contributor for hypoxia tolerance, our evidence suggests hypoxia-acclimated red drum are using other mechanisms to cope in an O2-depleted environment.

Neuroactive pharmaceuticals in estuaries: Occurrence and tissue-specific bioaccumulation in multiple fish species

Contamination of surface waters by pharmaceuticals is an emerging problem globally. This is because the increased access and use of pharmaceuticals by a growing world population lead to environmental contamination, threatening non-target species in their natural environment. Of particular concern are neuroactive pharmaceuticals, which are known to bioaccumulate in fish and impact a variety of individual processes such as fish reproduction or behaviour, which can have ecological impacts and compromise fish populations. In this work, we investigate the occurrence and bioaccumulation of 33 neuroactive pharmaceuticals in brain, muscle and liver tissues of multiple fish species collected in four different estuaries (Douro, Tejo, Sado and Mira). In total, 28 neuroactive pharmaceuticals were detected in water and 13 in fish tissues, with individual pharmaceuticals reaching maximum concentrations of 1590 ng/L and 207 ng/g ww, respectively. The neuroactive pharmaceuticals with the highest levels and highest frequency of detection in the water samples were psychostimulants, antidepressants, opioids and anxiolytics, whereas in fish tissues, antiepileptics, psychostimulants, anxiolytics and antidepressants showed highest concentrations. Bioaccumulation was ubiquitous, occurring in all seven estuarine and marine fish species. Notably, neuroactive compounds were detected in every water and fish brain samples, and in 95% of fish liver and muscle tissues. Despite variations in pharmaceutical occurrence among estuaries, bioaccumulation patterns were consistent among estuarine systems, with generally higher bioaccumulation in fish brain followed by liver and muscle. Moreover, no link between bioaccumulation and compounds’ lipophilicity, species habitat use patterns or trophic levels was observed. Overall, this work highlights the occurrence of a highly diverse suite of neuroactive pharmaceuticals and their pervasiveness in waters and fish from estuarine systems with contrasting hydromorphology and urban development and emphasizes the urgent need for toxicity assessment of these compounds in natural ecosystems, linked to internalized body concentration in non-target species.

Length–weight relationships of three fish species from the Yangtze River Estuary

The present study provides the length–weight relationships (LWRs) for common estuarine fish species collected from the Yangtze River Estuary by bottom trawl from 2017 to 2021. The net port width of bottom trawl net was 6 m, and the mesh size was 20 mm. This study provides new information on LWRs for three species in FishBase. Also, this study updates the information on the maximum length for three species.

Chronic toxicity of tire crumb rubber particles to mummichog (Fundulus heteroclitus) in episodic exposures

Microrubber (MR) encompasses all tire-related particles in the micro-scale and has recently drawn increased attention as a subclass of the broader group of microplastics. While tire particles entered the environment since the introduction of rubber tires for vehicles, the concern regarding tire wear particles (TWP) as an environmental contaminant is relatively new. Recent studies have examined physical and chemical toxicity of MR particles and leachates to a variety of organisms. However, there is a lack of information on the long-term effects of tire particle exposure under environmentally realistic conditions. The current study examined the chronic toxicity of crumb rubber (CR) particles to the estuarine fish species, mummichog (Fundulus heteroclitus) under episodic exposures at environmentally relevant concentrations. Immunohistochemistry (IHC) of fish gill, intestine, and liver was performed to assess CYP1A induction in these organs. Bile fluorescence was measured as an indicator of exposure to polycyclic aromatic hydrocarbons (PAHs) from CR. DNA damage was measured through the formation of 8-hydroxy-2′-deoxyguanosine (8-OHdG) together with other oxidative stress measures as lipid peroxidation (TBARS assay), free glutathione (GSH), and oxidized glutathione (GSSG) concentrations. Upregulation of CYP1A in gill, intestine, and liver was observed especially in gill filaments and general vasculature. Increased bile fluorescence demonstrated exposure to aromatic compounds, especially pyrene-like PAHs. Data for DNA damage indicated greater plasma 8-OHdG concentrations as a result of increased DNA repair. There was a decrease in malondialdehyde (MDA) production and an increase in total GSH at higher concentrations of CR. It appeared that under long-term repeated dosing, antioxidant systems in mummichog were upregulated to deal with exogenous stressors released by the CR particles. Combined, these data demonstrate that fish exposed to tire crumb rubber particles illicit significant biomarker responses under environmentally relevant CR concentrations, but induced antioxidant and detoxification pathways may prevent mortality and serious physiological effects in F. heteroclitus when exposed to environmentally relevant concentrations of CR.

Estuarine Fish Feeding Changes as Indicator to Mangrove Restoration Success in Seasonal Karstic Wetlands

The loss of mangrove areas due to anthropogenic activities has triggered efforts to recover or restore these ecosystems, their functions, and associated diversity. Such functions include nursery areas and energy flow through trophic relationships for a large variety of inhabitant species (e.g., fish). The food webs and trophic dynamics of estuarine fish provide important information on the food resources in mangrove ecosystems and their response to restoration processes. Nonetheless, few studies focus on using fish feeding characteristics as an approximation to assess the conservation or recovery status of these aquatic ecosystems. Thus, the aim of the present study was to compare the trophic dynamics of an ichthyic community in a mangrove ecosystem related to karstic wetlands in the Mexican–Caribbean using freshwater, estuarine, and marine fish species as bioindicators of the restoration process in mangroves. Stomach contents were analyzed for eight species of fish inhabiting specific mangrove zones (1—conserved zone, and 2—restored zone; a zone exposed to ecological restoration processes due to impacts of anthropic activities) related to karstic wetlands in the Mexican–Caribbean. Four feeding characteristics were considered: trophic guild, trophic level, feeding strategy, and prey abundance. Results showed differences and changes in the use of food sources at the trophic level mainly for Floridichthys polyommus, changing from a secondary consumer in the conserved zones to a primary consumer in the restored zones. This suggests that the feeding characteristics of the inhabiting fish are related to the mangrove’s conservation/restoration status and the trophic dynamics in the community. The results of this study are relevant as a tool for mangrove restoration plans regarding the analysis of fish and their food prey, in order to perform an easy and rapid assessment to determine the conservation/restoration status of these aquatic ecosystems from a functional perspective.

Size-selective predation by three estuarine zooplanktivorous fish species

Context Zooplanktivorous fish are a key link between abundant zooplankton and higher trophic levels but the foraging behaviour of zooplanktivorous fish is not fully understood. Selective feeding behaviours have been observed, with many species of planktivorous fish targeting certain species and sizes of zooplankton for prey. However, why certain size classes of zooplankton are preferred remains unclear. Aim This study investigated prey selection by three zooplanktivorous fish species through the lens of optimal foraging theory. Methods We assessed the size structure of zooplankton in the environment and compared this with the size distribution of zooplankton in gut contents from three zooplanktivorous fish. Key results The targeted prey size of Atypichthys strigatus and Scorpis lineolata aligns with the prey size classes in the environment that contain the highest overall biomass. Trachurus novaezelandiae showed little evidence of targeting these size classes. Conclusions These prey sizes therefore represent the most efficient prey to target because the return on foraging effort is greatest. By contrast, T. novaezelandiae showed only an underselection of large and small prey. Implications By incorporating this information on this key trophic link between zooplankton and fish, ecosystem models could better resolve the size dependant predation, particularly in size-based models.

Effects of forest cover on richness of threatened fish species in Japan.

Estuaries––one of the most vulnerable ecosystems globally––face anthropogenic threats, including biodiversity loss and the collapse of sustainable fisheries. Determining the factors contributing to the maintenance of estuarine biodiversity, especially that of fish, is vital for promoting estuarine conservation and sustainability. We used environmental DNA metabarcoding analysis to determine fish species composition in 22 estuaries around Japan and measured watershed‐scale land‐use factors (e.g., population size, urban area percentage, and forest area percentage). We sought to test the hypothesis that the richness of the most vulnerable estuarine fish species (i.e., registered by the Japanese Ministry of the Environment in the national species red‐list) is determined by watershed‐scale land‐use factors. The richness of such species was greater, where forest cover was highest; thus, forest cover contributes to their conservation. The proportion of agriculture cover was associated with low species richness of red‐listed fishes (redundancy analysis, adjusted R 2 = 43.9% of total variance, df = 5, F = 5.3843, p = 0.0001). The number of red‐listed species increased from 3 to 11 along a watershed land‐use gradient ranging from a high proportion of agriculture cover to a large proportion of forest cover. Furthermore, the results showed that throughout Japan all the examined watersheds that were covered by >74.8% forest had more than the average (6.7 species per site) richness of red‐listed fish species. This result can be attributed to the already high average forest cover in Japan of 67.2%. Our results demonstrate how the land use of watersheds can affect the coastal sea environment and its biodiversity and suggest that proper forest management in conjunction with land‐use management may be of prime importance for threatened fish species and coastal ecosystems in general.

Ontogenetic stability in the trophic niche of a common Gulf of Mexico fish, Ariopsis felis

Ariopsis felis (Hardhead Catfish) is an abundant yet understudied estuarine fish species in the US Atlantic and Gulf of Mexico. Very little is known about the species’ trophic role and place in coastal food webs. This study was designed to provide baseline information on A. felis stable isotope values and to examine for any ontogenetic changes. n = 126 A. felis were sampled in the summer of 2018 in Terrebonne Bay, LA, USA, and muscle tissue was analyzed for stable carbon (δ13C) and nitrogen (δ15N) values. Overall, we found little evidence for any ontogenetic trophic niche shifts, expansions, or contractions; mature males and females have similar isotopic values and little to no differences were detected between immature and mature fish. Ultimately, A. felis occupy a trophic niche similar to that of other common estuarine fishes; however, the lack of any ontogenetic change in trophic position suggests that A. felis may be occupying a more stable and long-term place in coastal food webs.

Possible impacts of non-native plant, pathogen, invertebrate and fish taxa on the indigenous ichthyofauna in South African estuaries: a preliminary review

We review the possible impacts of non-native biota on the indigenous fishes of South African estuaries, including macrophytes, algae, pathogens, invertebrates, and fishes. Freshwater macrophytes are one of the primary non-native groups in the oligohaline reaches of some predominantly open estuaries, lake and river mouth type estuaries, as well as the entire area of certain low salinity, temporarily closed estuaries. Anoxia and hypoxia in the water column below Salvinia molesta and Pontederia crassipes floating mats have caused fish kills in certain temporarily closed estuaries. Mass mortalities of fish in estuaries have arisen from harmful algal blooms (HABs) and a catchment-derived pathogenic water mould, Aphanomyces invadans. Non-native invertebrate species in local estuaries are derived from freshwater, estuarine and marine sources. The freshwater gastropod Tarebia granifera has invaded many subtropical estuaries and may be negatively impacting their food webs, with estuarine zoobenthivorous fishes not appearing to consume this mollusc. The marine polychaete Ficopomatus enigmaticus has invaded many South African estuaries and, in some of them, changed the zoobenthic food web by encrusting on hard surfaces and filtering particulate matter from the water column. This species also does not appear to be eaten by zoobenthivorous fishes within these systems. No non-native marine or estuarine fish species have been recorded in South African estuaries but non-native freshwater fish species now occur in 25% of estuaries in the region. Degraded estuaries in particular are more vulnerable to colonisation by non-native and translocated fish species than unimpacted systems. During the 1990s, a fish survey of 191 estuaries revealed that only 0.04% of the catch comprised non-native or translocated species but this percentage has increased in many estuaries in recent decades. Non-native and translocated freshwater fish species have successfully colonised the oligohaline and mesohaline reaches of many systems and there may be an impact due to predation on the eggs and larvae of resident estuarine taxa and the recruitment success of catadromous and some estuarine-associated fish species. However, most non-native fishes have a limited tolerance for the salinity regimes found in the lower and middle reaches of many South African estuaries, with an even larger threat to the indigenous estuarine ichthyofauna coming from non-native plant, invertebrate and pathogen invaders. Based on this review, and other similar global studies, there is a developing paradigm that non-native invasions by fishes and other organisms into South African and global estuaries are driven primarily from freshwater taxa and not estuarine or marine species.

How do natural changes in flow magnitude affect fish abundance and diversity in temperate regions? A systematic review protocol

Abstract Natural flow regimes play important roles in maintaining the ecological integrity and diversity of aquatic ecosystems. Wildlife has adapted over time to the natural dynamics of their environment, including changes in flow regimes. Changes in flow, including changes in magnitude, frequency, duration, timing and rate of change, may affect the physical characteristics of aquatic habitats, access to habitats, food availability, population dynamics and community composition. Given the importance of natural flow regimes for fish, it is necessary to understand the extent to which natural flow regimes alter fish abundance and diversity. Here we present a protocol for a systematic review that will estimate how fish abundance and diversity are affected by natural variation (resulting from climatic variability and broad‐scale drivers such as climate‐induced change) in flow. This systematic review will use evidence published before 2016 that was identified in a recent systematic mapping exercise on the broader topic of flow regime change impacts (both natural and anthropogenic) on direct outcomes of freshwater or estuarine fish productivity. An updated English language search will be performed using six bibliographic databases, Google Scholar and networking tools to include commercially published and grey literature that has been published after 2016. Eligibility screening will be conducted at two stages: title and abstract, and full‐text. We will include all studies that evaluate the effect of natural changes in flow magnitude on fish abundance (broadly defined to also capture density and biomass metrics) and species diversity (broadly defined to also capture species richness and composition metrics). Any freshwater or estuarine fish species in temperate regions will be considered. Included eligible studies will be assessed for study validity. We will extract information on study characteristics, intervention/comparator details, measured outcomes and effect modifiers. A narrative synthesis will describe the quantity and characteristics of the available evidence, and where sufficient numbers of similar studies are available, a meta‐analysis will be conducted to estimate an overall mean and variance of effect.