Estuarine Fish Species Research Articles

Genetic basis for rapidly evolved tolerance in the wild: adaptation to toxic pollutants by an estuarine fish species.

Atlantic killifish (Fundulus heteroclitus) residing in some urban and industrialized estuaries of the US eastern seaboard demonstrate recently evolved and extreme tolerance to toxic aryl hydrocarbon pollutants, characterized as dioxin-like compounds (DLCs). Here, we provide an unusually comprehensive accounting (69%) through quantitative trait locus (QTL) analysis of the genetic basis for DLC tolerance in killifish inhabiting an urban estuary contaminated with PCB congeners, the most toxic of which are DLCs. Consistent with mechanistic knowledge of DLC toxicity in fish and other vertebrates, the aryl hydrocarbon receptor (ahr2) region accounts for 17% of trait variation; however, QTL on independent linkage groups and their interactions have even greater explanatory power (44%). QTL interpreted within the context of recently available Fundulus genomic resources and shared synteny among fish species suggest adaptation via interacting components of a complex stress response network. Some QTL were also enriched in other killifish populations characterized as DLC-tolerant and residing in distant urban estuaries contaminated with unique mixtures of pollutants. Together, our results suggest that DLC tolerance in killifish represents an emerging example of parallel contemporary evolution that has been driven by intense human-mediated selection on natural populations.

Modification of nektonic fish distribution by piers and pile fields in an urban estuary

Large urban piers degrade habitat value for several estuarine benthic fish species by shading, but their effects on mobile nektonic species is less well understood due to sampling challenges. Dual Frequency Identification Sonar (DIDSON) allowed equal access to sampling in the water column of structured shaded and unshaded vs. open environments in both dark and light conditions by methods similar to video but without light. Sampling (n=228, 5-min transects) occurred under and around four large municipal piers of varying dimensions in the Hudson River estuary during day and night from summer and fall in 2007–2009. The distribution of small (5–25cm in length) and large (25–850cm) fishes were analyzed separately in recognition of functional guild differences. Small fishes occupied open water, shaded under-pier, and un-decked relict piling habitats, but were significantly more abundant during the day in open unshaded water than under adjacent piers or in piling habitats. Small fish occurred under 3 of 4 piers of varying size and configuration at 10–20% of the median abundances of adjacent open water. However, while schools were rare under piers they could be very large, so that abundance greatly exceeded mean open water abundance variance so as to preclude confidence in differences among piers. The differences among habitats were not significant at night, and the difference among piers was also not significant at night. School membership for small fish appeared to mitigate adverse effects of shading and may influence scaling of their response to shading and could therefore influence pier design. Large (>25cm) predatory fish were uncommon but responded similarly to habitat effects as did small fish. Habitats did not segregate fish by guild as small forage fish co-occurred in 65.8% of samples with large piscivores. Studies that provide species-specific and mechanistic interpretation of dynamic habitat use as well as further quantification of scaling effects could improve our understanding of how fishes respond to piers and other structures on urban shorelines.

Long-term changes in species composition of demersal fish and epibenthic species in the Jade area (German Wadden Sea/Southern North Sea) since 1972

Within this long-term study, the short- and long-term variability of demersal fish and epibenthic species in relation to temperature and climate-driven environmental changes in the inshore tidal bay system of the Jade area was investigated. Semiquantitative sampling took place once per spring and summer period from 1972 to 2014 by using a 2 m beam trawl at one station in the Jade area (German Wadden Sea/southern North Sea). Min/max autocorrelation analysis (MAFA) and Mann-Kendall analysis revealed significant increasing trends in total abundance and species number. Homogeneity analysis revealed shifts for abundance in spring and summer in the late 1980s and for species number in the late 1980s in spring and early 2000s in summer. Abundances of the estuarine crustacean species Carcinus maenas and Liocarcinus holsatus and of the estuarine fish species Pomatoschistus spp. showed significant increasing abundances since the late 1980s. The marine juvenile species Pleuronectes platessa and Limanda limanda showed significant decreasing abundances, while abundances of Solea solea showed significant increasing abundances since the early 2000s. Abundances of L.holsatus and C. maenas showed mass occurrences since the early 2000s. Spearman correlation analysis revealed significant correlations of temperature and abundance data of some characteristic species. Statistical downscaling analysis revealed significant correlations between observations and climate indicators such as the North Sea Environmental (NSE) Index for spring. Thus, it appears that climate effects influenced the long-term variability of species number and abundance of epibenthic and demersal fish species in the Jade area, resulting in community shifts in the late 1980s and early 2000s.

Comparing the utility of fishery‐independent and fishery‐dependent methods in assessing the relative abundance of estuarine fish species in partial protection areas

AbstractIn partial protection management areas such as Recreational Fishing Havens (RFHs) where commercial fishing is prohibited there are fewer sampling methods available for quantifying the relative abundance of species. Consequently, a framework is needed in such areas to decide whether sampling should be based on: (1) fishery‐independent techniques; (2) fishery‐dependent methods that rely on recreational fishery harvests; or (3) a combination of the two. This study compared fishery‐independent data derived from baited remote underwater video stations (BRUVS) and independent fishing surveys (IFS) with fishery‐dependent data based on recreational angler surveys (AS) to identify the fish species that were best enumerated by each method in two Australian RFHs. Each method captured/detected disparate species and each consistently showed correlations with the relative abundance of particular suites of species. The species that were correlated with and best enumerated by BRUVS were Microcanthus strigatus (Cuvier) and Pelates sexlineatus (Qouy & Gaimard); those correlated with IFS were Gerres subfasciatus Cuvier, Girella tricuspidata (Qouy & Gaimard), Mugil cephalus L. and Pomatomus saltatrix (L.); and those correlated with AS were Platycephalus fuscus Cuvier and Acanthopagrus australis (Owen). By identifying the species that were best quantified by each method, this study facilitates accurate monitoring of key species within RFHs.

Salinity stress response in estuarine fishes from the Murray Estuary and Coorong, South Australia.

Estuaries are unstable ecosystems and can be changed by the environmental and anthropogenic impact. The Murray Estuary and Coorong were degraded by drought and low freshwater input in the last decade and therefore transformed into the largest hyper-saline lagoon in Australia. This study evaluates the physiological stress of two estuarine fish species (small-mouthed hardyhead Atherinosoma microstoma and Tamar goby Afurcagobius tamarensis) to the induced salinity change in captivity. The test fishes were collected from the Coorong and transported to the laboratory in the water from the Coorong. Each fish species was exposed to different levels of salinity, and a number of enzymes were assessed to measure the stress response of fish to salinity change. The activity of reactive oxygen species was significantly increased with the salinity change in both fish species compared with the fish in the control. Significant salinity effect on superoxide dismutase activity was observed on Tamar goby but not on small-mouthed hardyhead. Conversely, the impact of salinity on catalase activity was detected on small-mouthed hardyhead but not on Tamar goby. The study reveals that the induction of physical stress by salinity changes occurred in both Tamar goby and small-mouthed hardyhead despite the varying response of antioxidant enzymes between fish species. The study provides an insight into the understanding of physiological adaptation in estuarine fish to salinity change. The results could improve our knowledge on stress response and resilience of estuarine fish to hypo- and hyper-salinity stress.

Mercury bioaccumulation in an estuarine predator: Biotic factors, abiotic factors, and assessments of fish health

Estuarine wetlands are major contributors to mercury (Hg) transformation into its more toxic form, methylmercury (MeHg). Although these complex habitats are important, estuarine Hg bioaccumulation is not well understood. The longnose gar Lepisosteus osseus (L. 1758), an estuarine predator in the eastern United States, was selected to examine Hg processes due to its abundance, estuarine residence, and top predator status. This study examined variability in Hg concentrations within longnose gar muscle tissue spatially and temporally, the influence of biological factors, potential maternal transfer, and potential negative health effects on these fish. Smaller, immature fish had the highest Hg concentrations and were predominantly located in low salinity waters. Sex and diet were also important factors and Hg levels peaked in the spring. Although maternal transfer occurred in small amounts, the potential negative health effects to young gar remain unknown. Fish health as measured by fecundity and growth rate appeared to be relatively unaffected by Hg at concentrations in the present study (less than 1.3 ppm wet weight). The analysis of biotic and abiotic factors relative to tissue Hg concentrations in a single estuarine fish species provided valuable insight in Hg bioaccumulation, biomagnification, and elimination. Insights such as these can improve public health policy and environmental management decisions related to Hg pollution.

The conservation status of Niugini black bass: a world‐renowned sport fish with an uncertain future

AbstractThe Niugini black bass, Lutjanus goldiei Bloch, is an estuarine and freshwater fish species endemic to New Guinea and the surrounding islands. It is the focus of a growing sport fishing industry that has the potential to provide long‐standing benefits to local people. Plantation agriculture, mining and logging are expanding in many catchments where L. goldiei is found, creating the potential for these industries to impact on L. goldiei and the environments it relies on. Understanding of the current status of the species, including its biology, ecology and distribution, is essential for its sustainable management. However, very little is known about the species. Here, the published literature, unpublished data and interviews with anglers and fisheries officers were used to draw together existing knowledge, assess the current conservation status, make a preliminary analysis of threats and identify key areas for research that will support the sustainable development of the L. goldiei sport fishing industry while fostering positive conservation outcomes.

Body size mediated effects of multiple abiotic stressors on the growth and social behaviour of an estuarine fish, Australian Bass (Macquaria novemaculeata)

Anthropogenic climate change is expected to result in dramatic shifts in the abiotic conditions within estuaries, including an elevation of temperature and salinity levels. Even so, few studies have addressed the impacts of multiple abiotic stressors on the behaviour and life history of key estuarine species, such as those of biological and commercial importance. Here we used a cross-factored experimental design to tease apart the effects of temperature and salinity on intraspecific aggression and growth rates of a native estuarine fish species, the Australian bass (Macquaria novemaculeata). Juvenile bass were exposed to one of four treatment conditions: 1) baseline temperature and salinity, 2) elevated temperature, 3) elevated salinity, and 4) elevated temperature and salinity. Elevated salinity increased rates of aggression, and elevated temperature decreased rates of growth, although the effects of both factors were mediated by the body size of individual bass. These results therefore highlight complex and variable effects of abiotic stressors and body size, emphasising the importance of considering individual-level attributes when evaluating the impacts of climate change on estuarine fishes.

Can bycatch in a flatfish gillnet fishery be reduced with rectangular mesh?

Gillnets fished in North Carolina, USA, estuaries have high rates of bycatch relative to the target catch of southern flounder, Paralichthys lethostigma Jordan & Gilbert. This study tested whether rectangular-mesh gillnets would maintain catch rates of southern flounder and reduce fish bycatch relative to conventional diamond-mesh gillnets in two North Carolina estuaries. In the Neuse River estuary, catch rates of legal southern flounder were not different between the two mesh shapes, but the bycatch of red drum, Sciaenops ocellatus (Linnaeus) and other fish species was reduced with rectangular-mesh net. In the Newport River estuary, southern flounder and red drum catches were reduced in rectangular-mesh net, but the decrease was greater for red drum. Catches of sublegal southern flounder were reduced in the rectangular-mesh net in both estuaries. Reduced catch rates of sublegal southern flounder and bycatch species suggest rectangular mesh may help manage stocks of estuarine fish species in areas where gillnets are used to target flatfishes.

Evaluating the Hypoxia Response of Ruffe and Flounder Gills by a Combined Proteome and Transcriptome Approach.

Hypoxia has gained ecological importance during the last decades, and it is the most dramatically increasing environmental factor in coastal areas and estuaries. The gills of fish are the prime target of hypoxia and other stresses. Here we have studied the impact of the exposure to hypoxia (1.5 mg O2/l for 48 h) on the protein expression of the gills of two estuarine fish species, the ruffe (Gymnocephalus cernua) and the European flounder (Platichthys flesus). First, we obtained the transcriptomes of mixed tissues (gills, heart and brain) from both species by Illumina next-generation sequencing. Then, the gill proteomes were investigated using two-dimensional gel electrophoresis and mass spectrometry. Quantification of the normalized proteome maps resulted in a total of 148 spots in the ruffe, of which 28 (18.8%) were significantly regulated (> 1.5-fold). In the flounder, 121 spots were found, of which 27 (22.3%) proteins were significantly regulated. The transcriptomes were used for the identification of these proteins, which was successful for 15 proteins of the ruffe and 14 of the flounder. The ruffe transcriptome dataset comprised 87,169,850 reads, resulting in an assembly of 72,108 contigs (N50 = 1,828 bp). 20,860 contigs (26.93%) had blastx hits with E < 1e-5 in the human sequences in the RefSeq database, representing 14,771 unique accession numbers. The flounder transcriptome with 78,943,030 reads assembled into 49,241 contigs (N50 = 2,106 bp). 20,127 contigs (40.87%) had a hit with human proteins, corresponding to 14,455 unique accession numbers. The regulation of selected genes was confirmed by quantitative real-time RT-PCR. Most of the regulated proteins that were identified by this approach function in the energy metabolism, while others are involved in the immune response, cell signalling and the cytoskeleton.

Ecological linkages in a Caribbean estuary bay

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 533:29-46 (2015) - DOI: https://doi.org/10.3354/meps11342 Ecological linkages in a Caribbean estuary bay H. Andrade1,2,*, J. Santos1, M. J. Ixquiac3 1Norwegian College of Fishery Science, University of Tromsø, 9037 Tromsø, Norway 2Akvaplan-niva AS, Framsenteret, 9296 Tromsø, Norway 3Centro de Estudios del Mar y Acuicultura, Universidad de San Carlos de Guatemala, Guatemala 01012, Guatemala *Corresponding author: hector.andrade@akvaplan.niva.no ABSTRACT: Central America and the western Caribbean regions form a center of freshwater and marine biodiversity that is increasingly becoming the focus of ecological and evolutionary studies. We conducted an integrated ecological study of Amatique Bay, Guatemala, a major estuary lagoon connected to the low-lying Lake Izabal and to the Mesoamerican Reef System, and provide novel information for the management and conservation of similar systems across the Caribbean. Precipitation and wind regimes constitute important environmental drivers of ecosystem functioning, which partially compensate for the weak tidal-forcing that is characteristic of the Caribbean Sea. Seasonal peaks in temperature and precipitation were strongly correlated to the reproduction of marine, catadromous and estuarine fish species, suggesting that the ensuing increase in primary production provides larval fish with an abundant food source. Increased abundances of transient marine species were observed during the dry season, which may be explained by passive transport, feeding migration, or both, considering that prey may be more abundant inshore and that environmental conditions are dominated by higher salinity and stronger onshore winds during this period. Despite being a stopover site for many long-range migrating shorebird species, the Bay serves primarily as a resting place since it lacks extensive tides and tidal flats, and thus provides limited access to invertebrate prey. Furthermore, this sheltered environment, featuring abundant freshwater, seasonally high water clarity, and low tidal amplitude, is likely to provide good habitat for abundant seagrasses and manatees. The Lake Izabal-Amatique Bay complex demonstrates a wide range of teleconnections and connectivity among terrestrial, freshwater, and marine oceanic and reef ecosystems. Understanding the evolution and ecology underlying this highly connected system is required for the management of the multi-trophic, small-scale fisheries it sustains. KEY WORDS: Fisheries · Migratory shorebirds · Manatee · Life history · Environmental drivers · Tropical conservation · Evolution · Central America Full text in pdf format Supplementary material PreviousNextCite this article as: Andrade H, Santos J, Ixquiac MJ (2015) Ecological linkages in a Caribbean estuary bay. Mar Ecol Prog Ser 533:29-46. https://doi.org/10.3354/meps11342 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 533. Online publication date: August 06, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Long-term patterns in estuarine fish growth across two climatically divergent regions.

Long-term ecological datasets are vital for investigating how species respond to changes in their environment, yet there is a critical lack of such datasets from aquatic systems. We developed otolith growth 'chronologies' to reconstruct the growth history of a temperate estuarine fish species, black bream (Acanthopagrus butcheri). Chronologies represented two regions in south-east Australia: South Australia, characterised by a relatively warm, dry climate, and Tasmania, characterised by a relatively cool, wet climate. Using a mixed modelling approach, we related inter-annual growth variation to air temperature, rainfall, freshwater inflow (South Australia only), and El Niño-Southern Oscillation events. Otolith chronologies provided a continuous record of growth over a 13- and 21-year period for fish from South Australia and Tasmania, respectively. Even though fish from Tasmania were sourced across multiple estuaries, they showed higher levels of growth synchronicity across years, and greater year-to-year growth variation, than fish from South Australia, which were sourced from a single, large estuary. Growth in Tasmanian fish declined markedly over the time period studied and was negatively correlated to temperature. In contrast, growth in South Australian fish was positively correlated to both temperature and rainfall. The stark contrast between the two regions suggests that Tasmanian black bream populations are more responsive to regional scale environmental variation and may be more vulnerable to global warming. This study highlights the importance of examining species response to climate change at the intra-specific level and further validates the emerging use of growth chronologies for generating long-term ecological data in aquatic systems.

Ecomorphology and resource use by dominant species of tropical estuarine juvenile fishes

Eleven ecomorphological attributes and diet of seventeen juvenile fish species were examined to test the hypothesis that morphological patterns determine resource uses in estuarine habitats. Species were separated according to the apparatus to food capture and habitat use (benthic or pelagic) in three different groups: (1) a group with depressed fish body, strong caudal peduncle and enlarged pectoral fins; (2) a second group laterally flattened with a wide protruding mouth, and (3) a third group strongly flattened with small pectorals fins. The following six trophic groups were organized based on prey categories: Zooplanktivores, Benthivores, Omnivores, Detritivores, Macrocarnivores and Insectivores. Significant results (PERMANOVA) between ecomorphological indices and habitat and between ecomorphological indices and trophic groups were found. These data indicate that similarity of ecomorphological forms, which minimize the influence of environment and partitioning of food, would help facilitate the co-existence of these fish when they are abundant in this tropical estuary.

Rescaling stable isotope data for standardized evaluations of food webs and species niches

Many human and natural events can impact aquatic populations and communities, leav- ing strong imprints as altered food web dynamics. Stable C and N isotopes in fish can record these altered trophic dynamics in an integrated way, and a new simple methodology is presented to extract measures of food web change from isotope measure- ments of fish species. Measured C and N isotope data are re-scaled as modified Z-scores to equalize the effects of average food inputs, then graphed in (x,y) space to obtain food web shapes. Pairs of food webs are compared for shifts in Euclidean distance of isotope values for individual species and pairs of spe- cies, and these shifts are used to assess overall con- stancy of species niches and fish food webs. Appli - cation of the methodology shows that (1) on average, estuarine fish species are relatively fixed in their trophic niches, with only 14 to 24% variability in the niches; (2) relatively undisturbed natural fish com- munities often show substantially different food web structure even when the same species of fish are present; and (3) factors such as species invasions and dredging result in marked differences between food webs. Isotope evaluations of possible food web shifts should be valuable in many conservation and resto- ration contexts.

Evaluation of catch-and-release angling practices for the fat snook Centropomus parallelus in a Brazilian estuary

Recreational fisheries are developing for many iconic marine and estuarine fish species and are generating important social and economic returns in Brazil. Fat snook is the primary target species in southeastern Brazil and it supports a vibrant fisheries sector, including recreational fishing tournaments. To evaluate the impacts of recreational angling practices, we examined external hooking injuries, secondary physiological stress responses (i.e., blood glucose and lactate concentrations; N = 24), reflex impairment (N = 39), and overnight survival (N = 39) of fat snook angled with and without air exposure. We also examined physiological stress and reflex impairment of fat snook captured in a fishing tournament (N = 43) immediately post weigh-in and 1 h post weigh-in. Values were compared to baseline reflexes (N = 10) and physiology (N = 8) collected from minimally stressed fat snook. Fish captured by anglers infrequently had hooking injury but all angling and tournament treatments resulted in significantly elevated blood glucose and lactate, indicating a considerable physiological response to angling. Angling with air exposure and tournament weigh-in (in a dry container) resulted in the highest blood lactate values. The body flex reflex was occasionally impaired after angling or tournament practices, but this was consistent with baseline reflex actions, and the body flex reflex was therefore not a valuable indicator of fisheries stress or mortality in fat snook. Although angling and tournament practices caused a marked physiological stress response, low mortality of snook indicated that the species is resilient to brief bouts of angling at moderate water temperatures. However, high lactate values associated with air exposure suggest that handling practices be refined, particularly during tournament weigh-in. We recommend that anglers reduce air exposure intervals when handling snook prior to release and that recreational tournaments adopt wet weigh-in procedures to maximize the utility of catch-and-release in this fishery.

Contrasting signals of genetic diversity and historical demography between two recently diverged marine and estuarine fish species

Estuaries, at the confluence of marine and freshwater systems, are mostly of geolog- ically recent origin and as such make excellent models for understanding recent speciation events. Using molecular approaches, we compared genetic diversity and demographic histories in 2 closely related southern African klipfish species, the marine Clinus superciliosus and the estuar- ine C. spatulatus. Strong genetic differentiation was identified using both mtDNA control region and nDNA S7 sequencing, despite some haplotype sharing. Coalescent-based modelling suggests that species divergence occurred during the Late Pleistocene or, more likely, during the Early Holocene, when present-day estuaries formed. Analyses of population demography suggest that C. superciliosus has undergone historical population expansion, whereas C. spatulatus is charac- terized by a population decline, potentially driven by repeated cycles of population crashes linked to the opening and closing of estuarine systems. This is also reflected in values of genetic diversity, which are almost an order of magnitude lower in the estuarine than in the marine species. Given the unique evolutionary history of C. spatulatus, a species that is restricted to only 2 South African estuaries, we highlight the need for a better understanding of the processes that have shaped the evolution of estuarine populations. The identification of unique genetic lineages in estuaries can help to better guide conservation and management efforts for some of South Africa's most fragile habitats.

δ15N of estuarine fishes as a quantitative indicator of urbanization

Nitrogen stable isotope values (δ15N) have commonly been used as a qualitative indicator of catchment urbanization in estuaries, but no quantitative relationship has to date been established between δ15N and degree of urbanization. We sampled five species of common estuarine fish (Mugil cephalus, Acanthopagrus australis, Sillago ciliata, Gerres subfasciatus and Herklotsichthys castelnaui) of different trophic levels from six estuaries in Southeast Queensland, Australia, to test if a quantitative relationship exists between fish δ15N and urbanization in the catchment. Degree of urbanization (urban %) in the catchment adjacent to the tidal estuary was measured using polygons constructed in Google Earth images, and verified using data from the detailed Queensland Land Use Mapping Project (QLUMP). Significant linear relationships exist between fish δ15N and urban % in most cases irrespective of the section of the estuary (upper, middle and lower) where the fish were caught, with no difference in slope and elevation between lines established for different sections. Inclusion of additional fish δ15N data from other Australian estuaries in a combined multiple regression model adding trophic level as a predictor generated a significant relationship predicting increase in fish δ15N values with increasing urban % and trophic level. There is therefore a generic quantitative relationship between estuarine fish δ15N values and degree of urbanization applicable to different fish species, geographic location or feeding habit. While the generality of this relationship needs to be tested further in other geographic locations, our findings potentially improve the applicability of fish δ15N values as a quantitative indicator of urban influence in estuaries.

Differing importance of salinity stratification and freshwater flow for the recruitment of apex species of estuarine fish

Table S1. Results of AIC-based model selection investigating the role of flow and salinity during the spawning season in driving short-term (2004-onwards) black bream recruitment variability. k= number of model parameters; MDF= mean daily flow; P10= flow exceeded 10% of the time; lowersal, midsal and uppersal= stratification in lower, middle and upper sections of the estuary respectively. Each site’s best model is highlighted in bold. Note: upper salinity data was not available for Eumeralla and no salinity data was available for Snowy

Molecular analyses confirm genetically distinct populations of two indigenous estuarine fish species in an isolated coastal lake: implications for the management of introduced ichthyofauna

Groenvlei is a coastal, near-freshwater lake in the Garden Route region of South Africa, which became isolated from the ocean about 4,000 years ago due to sea level regression. It contains only two native fish species—Atherina breviceps and Gilchristella aestuaria—and several non-native species, including the illegally introduced common carp (Cyprinus carpio). Options for controlling this highly invasive species in Groenvlei are presently being considered and as such, it is pertinent that the conservation status of native species be assessed. Mitochondrial DNA analyses support significant divergence of both native fish species inhabiting Groenvlei from nearby populations. Additionally, genetic structuring of A. breviceps populations inhabiting other sampling localities was also detected. In Groenvlei, populations of both species displayed slightly lower haplotype diversity and much lower nucleotide diversity than those inhabiting nearby estuaries. Given the level of genetic divergence, native fish populations in Groenvlei should be recognised as a distinct management unit. It is acknowledged that invasive species, such as carp, pose a serious threat to the Groenvlei ecosystem. However, the divergence of the Groenvlei populations—as evidenced by this study—implies that any large-scale actions aimed at removing invasive ichthyofauna should not impact the native species because the unique evolutionary signals of both populations could be destroyed. Our results highlight the importance of incorporating a genetic approach to understanding the evolutionary history of southern African estuarine species and decision-making processes supporting conservation of biodiversity in the region.

The effects of salinity on swimming performance of two estuarine fishes, Fundulus heteroclitus and Fundulus majalis.

Prolonged and high-speed swimming performance measurements were used to explore the swimming abilities of two species of estuarine fishes, the mummichog Fundulus heteroclitus and the striped killifish Fundulus majalis, under different salinities. Critical swimming performance was significantly higher for F. majalis in high salinity than in low salinity, but no difference was observed in brief constant acceleration swimming trials in this species; however, the swimming performance of F. heteroclitus was not significantly affected by salinity changes, indicating that this species is well adapted to regular estuarine salinity oscillations. Fundulus majalis displayed higher swimming speeds than F. heteroclitus in both high and low salinities, and while this cannot be explained by their respective salinity preferences, the specific habitat preferences of F. majalis for sandy subtidal habitats and F. heteroclitus for vegetated marshes could explain the better swimming performance of F. majalis.