Comparisons between the roles played by estuaries in the life cycles of fishes in temperate Western Australia and Southern Africa

Synopsis The rivers of south-western Australia and south-eastern Africa lie at similar latitudes, open into the Indian Ocean and frequently have estuaries that are periodically closed off from the sea by sand bars at their mouths. The present study has compared the species, genera and families represented in the fish assemblages of estuaries in south-western Australia and temperate southern Africa, i.e. below 31°S, and the way in which these estuaries are used by fish. The Clupeidae, Mugilidae, Atherinidae and Gobiidae were important families in both regions. However, the Terapontidae and Tetraodontidae, and the tropical families Apogonidae and Gerreidae, were represented by large numbers of individuals only in the estuaries of south-western Australia. Although 45 out of a total of 112 families and 32 of 233 genera occurred in both south-western Australia and temperate southern Africa, only 15 of the 326 species were found in both regions. The contributions made by the number of marine species which regularly enter estuaries in large numbers (marine estuarine-opportunists) to the total number of species recorded in the estuaries of south-western Australia and temperate southern Africa were similar (13.4 and 12.2% respectively) and the same was also true of species capable of completing their life cycles in estuaries (8.8 and 8.2%). The number of fresh water and diadromous species recorded in both regions was small. By contrast, the species of marine stragglers contributed approximately 70% to the total number of species in both regions. The adaptations of marine estuarine-opportunists and estuarine spawners to life in estuaries, and particularly to the effects of the closure of estuary mouths, is discussed. Although only one marine species was restricted to estuaries at any particular interval of its life cycle in south-western Australia, the juveniles of a number of marine species were confined to estuaries in temperate southern Africa. It is suggested that this difference can be attributed to the presence of a greater area and quality of alternative nursery habitats in the inshore marine environments in south-western Australia than in southern Africa.     

The fish community of a newly restored southern California estuary: ecological perspective 3 years after restoration

Bays and estuaries are considered essential fish habitat, yet in many parts of the world, these areas have been degraded or destroyed. In southern California, habitat restoration has become a widely used approach for protecting coastal ecosystems; however, there is little information available on the success of these efforts. Monthly abundance surveys were employed to examine spatial and temporal trends in the fish assemblages 3 years after the restoration of the Bolsa Chica Full Tidal Basin (BCFTB). This was used as a short-term success assessment of the BCFTB restoration, as well as an important baseline against which future studies can determine the long-term trajectory of the restoration. Forty-four species of fish were caught inside the BCFTB, at an average density of 116.8 fish 100 m^?2 and an average biomass of 4.2 kg 100 m^?2. There was a seasonal pattern in fish abundances but no overall increase or decrease in abundances during the entire study period. Marine, estuarine and migrant fish species were found in the BCFTB, each showing different seasonal patterns in abundance, similar to nearby estuaries, with 14 species driving these patterns. Water temperature and season were the most influential factors on the species composition of the fish community in the BCFTB. Therefore, 3 years after restoration the BCFTB is providing habitat for coastal fish species where none existed previously, and shows a community structure similar to natural estuaries in southern California. The BCFTB restoration has been initially successful but needs to be monitored periodically to assess its long-term success.     

Factors influencing the behavior and duration of residence of adult Chinook salmon in a stratified estuary

The duration of residence, behavior, and thermal experience of adult Chinook salmon (Oncorhynchus tshawytscha) in a stratified lagoon-type estuary over a four-year period was determined using acoustic and radio biotelemetry transmitters and archival temperature tags. Results did not support the hypothesis that adult Chinook salmon would hold extensively and migrate slowly through the estuarine lagoon with return trips to the sea and use of the salt wedge for behavioral thermo-osmoregulation in response to high summer water temperatures. Passive tidal transport was not observed as predicted and entry from the sea to the estuarine lagoon occurred during all tidal phases suggesting that conserving energy was not a priority for fish during the estuarine phase of their migration. An unexpected finding was that the persistent pursuit pressure of pinnipeds, especially from California sea lions (Zalophus californianus), appeared to negate the suitability of estuarine lagoon as holding habitat. This finding led to the hypothesis that the risk of pinniped predation was primarily responsible for the brief residence (<24 h on average) and rapid migration (mean 1.2 km/h, 0.42 body lengths/s) of adult Chinook salmon in the estuarine lagoon, especially given its relatively small size (7 km). This hypothesis will be difficult to test without exclusion of pinnipeds. Thermal records of fish that retreated back to the sea after tagging suggested that use of the marine river plume could be important for osmotic adaption to allow rapid migration through estuaries, which often contain concentrations of predators anticipating the return of migrating salmon. Alternatively, use of the marine river plume could have occurred independent of predators. Fish retreating back to the sea should be considered a tagging effect as downstream movement commonly occurs when adult salmonids are released after tagging. There was no evidence of any other tagging effects or biased behavior by fish tagged in this study. Regardless of the reasons, the successful migration of fish that held for weeks in the sea after tagging indicates that there could be considerable flexibility in the river entry timing of some salmon stocks. Tagging adult salmonids in the sea as they approach their natal rivers is ideal but the associated logistical challenges and expense make tagging fish immediately upon entrance to river mouths the next best option when possible.     

Field observations on the lepidophagous teleost Terapon jarbua (Forsk»l)

Synopsis Foraging behaviour of the lepidophagous Indo-Pacific teleost Terapon jarbua in the Bulolo and Mtumbane estuaries of southern Africa is described. These observations suggest that scale removal from large fish and the complete ingestion of fish fry represents a modified form of predation. Prey reaction to T. jarbua shoals is also discussed.     

Mesophotic depths as refuge areas for fishery-targeted species on coral reefs

Coral reefs are subjected to unprecedented levels of disturbance with population growth and climate change combining to reduce standing coral cover and stocks of reef fishes. Most of the damage is concentrated in shallow waters (<30 m deep) where humans can comfortably operate and where physical disturbances are most disruptive to marine organisms. Yet coral reefs can extend to depths exceeding 100 m, potentially offering refuge from the threats facing shallower reefs. We deployed baited remote underwater stereo-video systems (stereo-BRUVs) at depths of 10–90 m around the southern Mariana Islands to investigate whether fish species targeted by fishing in the shallows may be accruing benefits from being at depth. We show that biomass, abundance and species richness of fishery-targeted species increased from shallow reef areas to a depth of 60 m, whereas at greater depths, a lack of live coral habitat corresponded to lower numbers of fish. The majority of targeted species were found to have distributions that ranged from shallow depths (10 m) to depths of at least 70 m, emphasising that habitat, not depth, is the limiting factor in their vertical distribution. While the gradient of abundance and biomass versus depth was steepest for predatory species, the first species usually targeted by fishing, we also found that fishery-targeted herbivores prevailed in similar biomass and species richness to 60 m. Compared to shallow marine protected areas, there was clearly greater biomass of fishery-targeted species accrued in mesophotic depths. Particularly some species typically harvested by depth-limited fishing methods (e.g., spearfishing), such as the endangered humphead wrasse Cheilinus undulatus, were found in greater abundance on deeper reefs. We conclude that mesophotic depths provide essential fish habitat and refuge for fishery-targeted species, representing crucial zones for fishery management and research into the resilience of disturbed coral reef ecosystems.     

Causes and consequences of human induced impacts on a ubiquitous estuary-dependent marine fish species

Decreasing populations of common estuary-associated marine fishes are being documented globally and red flags associated with such declines are often ignored due to the high relative abundance of these species when compared to more rare and threatened taxa. The Cape stumpnose Rhabdosargus holubi (Steindachner 1881) is an abundant and widespread southern African sparid that is dependent on estuaries as a primary nursery area. Historical and current information on the species is comprehensive and the accumulated evidence strongly suggests that this ubiquitous fish is under pressure, mainly due to degradation of estuaries and associated catchments, increasing fishing pressures from recreational and subsistence anglers, habitat loss relating to reduced submerged macrophyte areas in many systems, industrial and agricultural pollution, infrastructural developments in and around estuaries, and climate change. In particular, the temporary loss of Lake St Lucia, which accounts for approximately 50% of the estuarine surface area in South Africa, has drastically reduced the nursery area availability for R. holubi on the subcontinent. Overall we present strong evidence to support the hypothesis that present-day stocks of R. holubi are much smaller than those in the pristine state and that urgent management measures need to be considered and implemented to prevent current depleted populations of the species from declining further.     

Habitat partitioning in juvenile fishes associated with three vegetation types in selected warm temperate estuaries,South Africa

Three common vegetation types were studied to assess habitat partitioning in juvenile fishes in select warm temperate estuaries of South Africa. Vegetated habitat types are known as productive and important areas for predator avoidance and feeding and are often preferred by juvenile fishes. Habitat partitioning is not well understood, with previous studies mostly focusing on seagrass (Zostera capensis). This study aimed to assess three common vegetated fish nursery areas in estuaries to aid conservation planning. Fishes were collected by means of double-winged, six-hooped fyke nets, 1 mm mesh size secured in placed on the nocturnal flood tide at each inundated vegetation type (Phragmites australis, Zostera capensis, Spartina maritima) and at an adjacent unvegetated site for three consecutive months in 2014 and 2015 during the summer recruitment period for juvenile fishes. Higher catches were frequently recorded in vegetated areas for solely estuarine and marine estuarine dependent species. In general, the previously unstudied reed, P. australis showed the highest species richness and abundance of juvenile fishes overall, followed by, Z. capensis and the intertidal salt marsh species, S. maritima. Results from this study supports international trends on the value of vegetated areas as refugia for young fishes in estuaries.     

Opportunistic use of estuarine habitat by juvenile bull trout, <Emphasis Type="Italic">Salvelinus confluentus</Emphasis>, from the Elwha River before,during, and after dam removal

Estuaries are used by anadromous fishes, either as the definitive marine habitat or as transition habitat as they move to fully marine waters, and extent of estuary use may vary with habitat conditions and fish attributes. Bull trout (Salvelinus confluentus) are commonly fluvial or adfluvial, though anadromous populations also exist. However, little is known about estuary use, especially by juveniles of this threatened species. We sampled the estuaries of the Elwha River, where a spawning population exists, and the nearby Salt Creek, where none exists, to reveal seasonal timing of estuarine use by juvenile bull trout, size of those using the estuary, and possible use of the non-natal estuary. We captured juvenile bull trout (all ≥100 mm FL, most <300 mm) in the Elwha River estuary in all months except August, but primarily December through May. None was captured in Salt Creek’s estuary despite comparable sampling effort. We also evaluated how dam removal on the Elwha River influenced bull trout estuarine occupancy by sampling before, during, and after dam removal, because this process enlarged the estuary but also increased turbidity and sediment transport in the lower river. Catches were low before dam removal, increased during and immediately after removal, and returned to low levels in recent years, suggesting that juveniles temporarily sought refuge from conditions associated with dam removal. Our findings indicate juvenile bull trout occupy estuarine habitat opportunistically; this information may aid conservation efforts as anadromous populations occur elsewhere in rivers with estuaries altered by human development.     

Fine-scale movement and assimilation of carbon in saltmarsh and mangrove habitat by resident animals

Despite theories of large-scale movement and assimilation of carbon in estuaries, recent evidence suggests that in some estuaries much more limited exchange occurs. We measured the fine-scale movement and assimilation of carbon by resident macroinvertebrates between adjacent saltmarsh and mangrove habitats in an Australian estuary using δ¹³C analysis of animals at different distances into adjacent patches of habitat. δ¹³C values of crabs (Parasesarma erythrodactyla –15.7 ± 0.1‰, Australoplax tridentata –14.7 ± 0.1‰) and slugs (Onchidina australis –16.2 ± 0.3‰) in saltmarsh closely matched that of the salt couch grass Sporobolus virginicus (–15.5 ± 0.1‰). In mangroves, δ¹³C values of crabs (P. erythrodactyla –22.0 ± 0.2‰, A. tridentata –19.2 ± 0.3‰) and slugs (–19.7 ± 0.3‰) were enriched relative to those of mangroves (–27.9 ± 0.2‰) but were more similar to those of microphytobenthos (–23.7 ± 0.3‰). The δ¹³C values of animals across the saltmarsh-mangrove interface fitted a sigmoidal curve, with a transition zone of rapidly changing values at the saltmarsh-mangrove boundary. The width of this transition indicated that the movement and assimilation of carbon is limited to between 5 and 7 m. The δ¹³C values of crabs and slugs, especially those in saltmarsh habitat, clearly indicate that the movement and assimilation of carbon between adjacent saltmarsh and mangrove habitat is restricted to just a few metres, although some contribution from unmeasured sources elsewhere in the estuary is possible. Such evidence demonstrating the extent of carbon movement and assimilation by animals in estuarine habitats is useful in determining the spatial arrangement of habitats needed in marine protected areas to capture food web processes.     

Are nest sites a limiting resource for speleophilic nesting fishes in sand dominated temporarily open/closed estuaries?

Southern African temporarily open/closed estuaries are often characterized by sand dominated mouth regions, a feature in contrast to many permanently open estuaries of the region which regularly have substantial rocky areas. Here we explore how habitat characteristics may reduce nesting opportunities and potentially explain population trends for a common estuarine fish species, the speckled sand goby Psammogobius knysnaensis, in many temporarily open/closed estuaries in South Africa. The present study is the first to document that P. knysnaensis is a speleophilic nester, utilizing the underside of rocks and other hard substrata for egg attachment. Furthermore, our results suggest that ideal nesting habitat for the speckled sand goby is potentially a limiting resource in sand-dominated temporarily open/closed estuaries. We postulate that this is a factor contributing to the relatively low numbers of adult P. knysnaensis found in sandy temporarily open/closed estuaries of the region, although the role of marine connectivity, food availability, predation pressure and other environmental factors in this regard still remain largely unassessed.     

Incorporation of strontium into otoliths of an estuarine fish

Patterns of Sr/Ca variability in fish otoliths have been widely applied as tracers of movement between freshwater and marine habitats, with the assumption that low salinity habitats correspond to lower otolith levels of Sr/Ca. On the other hand, fluvial estuaries can contain steep gradients in Sr/Ca, and in some estuaries, freshwater values of Sr/Ca can exceed marine values, which are relatively constant across marine habitats. Therefore, to interpret Sr/Ca variability in otoliths of fish that move through estuaries, information is needed about both the incorporation of strontium into otoliths and the nature of the gradient of Sr/Ca in the water. We conducted four experiments to evaluate the incorporation of strontium into fish otoliths under estuarine conditions, using white perch (Morone americana) as a model estuarine fish. One laboratory and the two field experiments tested the relationship between Sr/Ca in the otolith and that in the water. A fourth experiment investigated the effect of salinity, independently of the water chemistry (Sr was manipulated while maintaining a constant salinity and Ca level). All four experiments supported a direct relationship between Sr/Ca in the otolith and the water, across a range of estuarine salinities. Results also indicated that the incorporation of strontium into otoliths of estuarine fishes should be constant across broad gradients of Sr/Ca in estuarine waters. While the experiments supported past applications of tracing estuarine and diadromous movements with otolith Sr/Ca chronologies, we emphasize the need to understand the underlying nature of Sr/Ca gradients in estuaries, which may limit or confound reconstructions of estuarine habitat use.     

Fish communities in temporarily open/closed estuaries from the warm- and cool-temperate regions of South Africa: A review

The majority of estuaries along the coastline of southern Africa are termed temporarily open/closed estuaries (TOCEs) and are closed off from the sea for varying periods by a sandbar which forms at the mouth. It is therefore important to understand the processes occurring within TOCEs and their importance to fishes in order to make sound management recommendations. Estuaries along the coast of South Africa and their associated fish assemblages are biogeographically distinct and occur in either a subtropical, warm-temperate or cool-temperate zone. There are 125 TOCEs found within the cool-temperate and warm-temperate zones. Most fish species found in TOCEs are the juveniles of marine taxa that breed at sea. Permanently open estuaries generally have a higher diversity of species than TOCEs, but TOCEs still provide important nursery areas for many marine species and numerically often have a higher proportion of estuarine resident species. Important taxa in terms of abundance and biomass in warm-temperate TOCEs include the sparids Rhabdosargus holubi and Lithognathus lithognathus, several mugilid species, estuarine residents (particularly Gilchristella aestuaria and Atherina breviceps) and the freshwater cichlid Oreochromis mossambicus. The diversity of fishes in cool-temperate TOCEs is low when compared with warm-temperate systems and Liza richardsonii tends to dominate catches by number and mass in most systems. Several species recorded in TOCEs show clear longitudinal distribution trends. For example Atherina breviceps is generally more abundant in the lower reaches of estuaries. Mouth state, particularly the frequency, timing and duration of mouth opening plays a key role in determining species richness, composition, diversity and abundance in TOCEs. Mouth state is directly linked to freshwater input. Reduced river inflow leads to prolonged mouth closure and shorter open phases, which inhibits immigration and emigration of marine fish species between estuaries and the sea. Understanding of the effects of various processes occurring within these systems, particularly variation in freshwater input, on the biota of these important systems facilitates the development of informed management recommendations.     

Connectivity between estuaries and marine environment: Integrating metrics to assess estuarine nursery function

Natural or anthropogenic induced variations in estuaries and the dynamics of marine fish populations potentially promote differences in connectivity between estuaries and marine areas, i.e. in their importance as nursery grounds. Within this context, an integrated assessment of the differential nursery function of the main estuaries along the Portuguese coast for commercial fish species common sole Solea solea, Senegalese sole Solea senegalensis, flounder Platichthys flesus and sea bass Dicentrarchus labrax was performed through several indicators based on available data. Contribution of individual estuaries to marine subpopulations was measured with potential metrics (juvenile density, habitat quantity, juvenile number and habitat quality within estuaries) and effective metrics (estuarine source of young adults in marine environment measured via otolith elemental fingerprints). The relationship between the two types of metrics was also assessed. Estuaries identified as important nursery and/or effective juvenile habitat (EJH) differed with species and no single estuary was best for all, highlighting species-specific regulation of nursery function. Multiple species assessment of nursery and EJH function differed among estuaries. Management and conservation of estuaries should focus on sites with higher contributions to adult subpopulations of multiple species. The importance of defining precise scientific and management objectives was emphasized by the different rankings of estuaries obtained with nursery or EJH criteria. Potential and effective contribution of estuaries were not significantly correlated, but in a quantitative analysis juvenile densities and number of juveniles seem related with effective contribution in some species. An agreement between potential and effective contributions of estuaries is concurrent with the acknowledged minor role of juvenile stage processes in regulation of recruitment to adult subpopulations.     

Turning passive detection systems into field experiments: an application using wetland fishes and enclosures to track fine-scale movement and habitat choice

Understanding habitat selection and movement remains a key question in behavioral ecology. Yet, obtaining a sufficiently high spatiotemporal resolution of the movement paths of organisms remains a major challenge, despite recent technological advances. Observing fine-scale movement and habitat choice decisions in the field can prove to be difficult and expensive, particularly in expansive habitats such as wetlands. We describe the application of passive integrated transponder (PIT) systems to field enclosures for tracking detailed fish behaviors in an experimental setting. PIT systems have been applied to habitats with clear passageways, at fixed locations or in controlled laboratory and mesocosm settings, but their use in unconfined habitats and field-based experimental setups remains limited. In an Everglades enclosure, we continuously tracked the movement and habitat use of PIT-tagged centrarchids across three habitats of varying depth and complexity using multiple flatbed antennas for 14 days. Fish used all three habitats, with marked species-specific diel movement patterns across habitats, and short-lived movements that would be likely missed by other tracking techniques. Findings suggest that the application of PIT systems to field enclosures can be an insightful approach for gaining continuous, undisturbed and detailed movement data in unconfined habitats, and for experimentally manipulating both internal and external drivers of these behaviors.     

Size-related mortality due to gnathiid isopod micropredation correlates with settlement size in coral reef fishes

The transition between the planktonic and the benthic habitat is a critical period for the larvae of many demersal marine organisms. Understanding the potential constraints on the timing of this habitat transition, called settlement, is important to understanding their biology. Size-specific mortality can set the limits on lifestyle and help explain ontogenetic habitat shifts. We examined whether size-based mortality risks after settlement may include micropredation by ectoparasites by testing whether survival of settlement-stage fish varies with fish size when exposed to a reef-associated micropredator. Fish (14 species) were exposed to one blood-sucking gnathiid isopod overnight, with appropriate controls; gnathiid feeding success and survival, and fish mortality were recorded relative to fish size. After adjusting for fish relatedness, we found the relationship between fish mortality and size differed with gnathiid exposure: for gnathiid-exposed fish, the mean mortality of the smallest fish was much higher (57%) than unexposed controls (10%), and decreased to ~0% for fish >12 mm standard length (SL); mortality was almost nil in controls. Thus, a predicted optimal size to switch habitat and reduce mortality risk from micropredation should be >12 mm SL. We then asked what species might be at greater risk and if the steep increase in survival at ~12 mm SL might coincide with settlement at larger sizes among fishes. Across 102 other species (32 families), 61% settled at ≥12 mm SL. After adjusting for relatedness, mean fish settlement size was 15.0 mm and this was not significantly different from 12 mm. Thus, settlement size clusters around the minimum fish size threshold our gnathiid experiment predicted would be large enough to survive a gnathiid encounter. These results suggest micropredators may contribute to size-selective mortality during settlement processes and are consistent with the hypothesis that the pelagic phase provides fish an escape from certain micropredators.     

The impact of bird herbivory on macrophytes and the resilience of the clear-water state in shallow lakes: a model study

Land-use change associated with human development can alter aquatic habitat and imperil aquatic species. Fish are challenged when urban streams are altered, for example for stormwater conveyance, but little is known about how such activities influence the space use of individual fish. Electronic tagging and experimental displacement of fish can be used to explore site fidelity and homing behaviour of fish and can therefore be useful for testing hypotheses about space use and habitat selection. In this study, we used experimental displacement to determine how longnose dace (LND, Rhinichthys cataractae) utilize reaches within a watershed that have varying degrees of degradation. LND were tagged using passive integrated transponders (PIT tags), transported upstream, and released either into the natural stream reach, impaired stormwater drain reach, or at their confluence. Fixed PIT antennas were used to monitor movement of the PIT-tagged fish among the three reaches for a period of 3 weeks. LND exhibited dramatic and rapid selection against the stormwater drain. No LND moved into the drain and 97% of fish transported to the drain left within 24 h. LND were actively avoiding the stormwater drain, emphasizing the need for enhancement work to improve the biological connectivity of the system.     

First record of <Emphasis Type="Italic">Spartina alterniflora</Emphasis> in southern Africa indicates adaptive potential of this saline grass

Spartina alterniflora was recorded in 2004 in the Great Brak Estuary, a system along the southern coast of South Africa that closes to the sea. This is alarming as this is a species with a known history as an aggressive invasive plant which has now been found 8000 km from its nearest known location and furthermore it is spreading under atypical conditions of submergence. This first recorded population in Africa indicates the adaptive potential of this invasive grass which survives inundation and non-tidal conditions for months at a time. Spartina alterniflora spread from 2566 m² in 2006 to a maximum area covered of 10,221 m² in 2011. There was an increase in silt, sediment organic matter and a significant reduction in sediment redox potential at sites invaded by S. alterniflora. When the estuary closes to the sea the water level rises and S. alterniflora is flooded, limiting opportunities for mechanical and chemical control. Application of a glyphosate-based herbicide in 2012 showed that chemical control was more effective in reducing the stands than mechanical removal. The additional use of imazapyr in 2014 significantly reduced stem density and the proportion of live stems. Spread of this invasive plant to the intertidal marshes in adjacent estuaries is a potential biodiversity threat although, fortunately, this population does not seem to produce viable seed. There is also the concern that hybridization with the resident S. maritima may occur. Important research and management questions remain i.e. how quickly will the natural marsh re-establish following eradication and how can we prevent movement of the grass to other estuaries?     

FISH CONSERVATION IN SOUTH AFRICAN ESTUARIES: PRESSURES,PROBLEMS AND PROSPECTS

Summary

A review of the conservation status of fish in the estuarine environment around the South African coastline reveals that some species face serious problems associated either with habitat destruction, and its associated biological, physical and chemical components or exploitation. The 65 species considered fall into three categories; truly estuarine species, which are dependent on estuaries for their entire life; marine species dependent on estuaries during the juvenile phase of their life cycle; and marine species whose juveniles occur mainly in estuaries but are also found at sea. Included in the first two categories are 14 species of fish which are on the South African Red Data list. These comprise one species which is endangered, five which are vulnerable and eight which are rare. All groups are considered in relation to factors in estuaries which are affecting their populations. A conservation strategy is suggested for certain estuarine types or for specific estuaries which could ensure the survival of the entire range of estuarine faunas.