Estuarine-dependent Fish Research Articles

Early growth and environmental conditions control partial migration of an estuarine-dependent fish

Partial migration is a widespread phenomenon in animals, whereby multiple groups follow different migration behaviors (i.e. contingents) within a single population. Fishes exhibit particularly high diversity in their early dispersal behaviors; however, whether these represent conditional partial migration behaviors remains unclear. We combined otolith microstructure and chemistry to assess the influence of early life conditions and environmental drivers on juvenile-stage partial migration of anadromous striped bass Morone saxatilis in the Hudson River (USA) in 2 consecutive years with contrasting hydrologic conditions. Time series clustering on otolith strontium (Sr) and barium (Ba) profiles revealed 4 dominant early migration contingents in both years: freshwater residents, oligohaline migrants, small mesohaline migrants, and large mesohaline migrants. In both years, juvenile partial migration appeared to be a conditional strategy linked to a growth-mediated threshold. The propensity to migrate early was related to slower larval growth, whereas freshwater residency and delayed migration were associated with faster larval growth. Differences in hatch dates may have indirectly affected migration contingents by exposing larvae to varying environmental conditions. In the dry year, dispersal timing to mesohaline habitats coincided with high freshwater flow and tidal currents, but not so in the higher-flow year. Recruitment to coastal nurseries outside the Hudson River occurred primarily during the juvenile phase. Early migration contingents could carry over to population dynamics, whereby diverse estuarine nursery habitats contribute differently to recruitment.

The Mediterranean Killifish Aphanius fasciatus (Valenciennes, 1821) (Teleostei: Cyprinodontidae) as a Sentinel Species for Protection of the Quality of Transitional Water Environments: Literature, Insights, and Perspectives

Transitional waters are fragile ecosystems with high ecological values, representing the breeding and resting sites for rare and threatened species. They warrant particular attention in regards to protection, as they experience numerous anthropogenic threats. The present review aims to analyze the recent literature on Aphanius fasciatus, currently considered one of the most strictly estuarine-dependent fish species, thus affected by the degradation of lagoon habitats, and to discuss its suitability as a sentinel species for protection of the quality of transitional water environments. The analysis and discussion highlight the potential applicability of the molecular, cellular, and physiological responses of this species as diagnostic tools for detecting the subtle effects induced by environmental pollution on the biota in transitional water environments. Moreover, the suitability of the responses of this species is suggested in the wider framework of the One Health perspective, which considers human and animal health and the environmental state to be highly interconnected, sharing common aspects. To date, omics technologies show great potential in reacquiring novel knowledge on the responses of the organisms to environmental changes and to the alterations of the environmental health status. Therefore, considering the relevant potential of this organism as a sentinel species, many efforts are required in the near future to improve the quantity and quality of the omics tools that refer to A. fasciatus.

Is Chelidonichthys lucerna (Linnaeus, 1758) a Marine Estuarine-Dependent Fish? Insights from Saccular Otolith Microchemistry

Tub gurnard, Chelidonichthys lucerna (Linnaeus, 1758), is a Mediterranean-Atlantic benthic species usually captured as by-catch by Portuguese traditional fisheries and considered the most important commercial fish species of the family Triglidae. However, to date, little is known about its habitat residency and whether the species can be considered a marine estuarine-dependent fish. Otolith microchemistry has proven effective in providing information about fish movement patterns throughout different water salinities. In this study, core-to-edge transects of Sr:Ca in the fish saccular otoliths of 35 juveniles of C. lucerna captured in March 2007 by a scientific survey along the Portuguese coast were used to assess the species movement between brackish and marine waters. Data suggest that most individuals (97%) have an estuarine-dependent profile, with 63% showing a clear presence in marine waters during the early life history periods. Evidence of an estuarine residence throughout the fish life cycle was found in only 3% of individuals. Ba:Ca profiles did not reflect an inverse relationship with Sr:Ca salinity results but corroborated findings from other authors about the influence of upwelling processes and freshwater runoffs on Ba incorporation into the otoliths of coastal fish. Furthermore, the data also showed that C. lucerna can occupy and migrate among habitats with diverse salinity degrees, showing high environmental plasticity and adaptation.

Olfactory cueing responses in the early stages of a marine estuarine-dependent fish species in South Africa

Many marine fish species depend on estuaries as nurseries for their vulnerable life stages. Consequently, successful recruitment into estuaries by the larvae and juveniles of marine estuarine-dependant fish such as Rhabdosargus holubi (Sparidae) is vital to completing their lifecycles and thus maintaining populations. This species is known to be attracted to the water of riverine and estuarine origin, potentially via olfaction of chemical cues. However, the specific cues and cueing mechanisms involved in these recruitment responses have yet to be fully elucidated for many species utilising estuaries as nursery areas. In this study, choice chamber flume experiments were used to test the chemical attraction of fish to water from different sources to understand further the mechanisms and cues involved. Rhabdosargus holubi juveniles were attracted to the water of riverine and estuarine origin, as the larvae were previously shown to do. These fish were also attracted to exudates from seagrass that occur naturally in their preferred estuarine habitats. The results of the study highlight both the importance of conserving vegetated habitats and managing adequate freshwater flow into estuaries, as well as the need for more research on fish cueing into nursery areas.

Assessing interspecific variation in life-history traits of three sympatric tropical mullets using age, growth and otolith allometry

Life history theory predicts that organisms optimize their life-history traits according to evolutionary context and environmental constraints. Sympatric estuarine-dependent fish are intriguing subjects for studying life-history variation, as their traits are molded by their complex habitat use and co-occurrence with ecologically similar species. In this study, we investigated life-history traits of three tropical mullet species (Mugil curema, M. liza and M. rubrioculus) using age, growth, and otolith allometry. Our results indicate that otolith allometric patterns are useful predictors of specific life history traits: the faster-growing species M. rubrioculus (K = 0.31) was characterized by earlier stanza changing points (SCPs), with 0.8 years of life. In contrast, the slower-growing species, M. liza (K = 0.21) was characterized by later changing points, 1.3 years of life. M. curema is intermediate (K = 0.25), with 0.95 years of life. The first SCP may register juvenile movements between estuarine and coastal habitats before the first year of life, and the second SCPs appear to reflect the migration to the sea and age at maturation. More generally, our study species exhibit fine-scale temporal differences in life cycle events that may be related to growth rates and body size that can be predicted by otolith allometry, as well as a gradient of interspecific divergences in life-history traits. These are likely to be driven by differences in their migratory behavior and habitat use that permit coexistence.

Trophic adaptability shapes isotopic niche of the resident fish Aphanius fasciatus across lagoon habitats

Coastal lagoons are characterized by high habitat heterogeneity where natural habitats coexist with artificial ones, historically set up to support human activities. Increasing anthropogenic pressure may lead to progressive degradation of the most vulnerable lagoonal habitats and the associated biological communities. One of the strictly estuarine-dependent fish species that may be affected by the degradation of lagoon habitats is the South European toothcarp Aphanius fasciatus, archetype of Mediterranean lagoon residents.Carbon and nitrogen stable isotopes were used to disentangle the influence of habitat types (natural vs artificial) and fish community (multi-trophic context) on the trophic niche features of A. fasciatus. Fish communities and organic matter sources were sampled in two Mediterranean lagoons located at the northernmost (Lagoon of Venice) and at the mid-latitude (Stagnone di Marsala, Trapani) of A. fasciatus distribution.Results showed evidence of high trophic adaptability of A. fasciatus, whose omnivorous feeding habit resulted in seasonal changes in isotopic niche width and niche partitioning with co-occurring species, constituting an advantage in habitats characterized by high seasonal variation of resources availability. Furthermore, while macrophytes drove the main trophic pathways leading to A. fasciatus in natural habitats, artificial habitats were mainly based on sedimentary organic matter routes. These outcomes broaden our knowledge on how natural and artificial habitats trophically support A. fasciatus populations and results are discussed in light of the ecological implications for environmental management of coastal lagoons.

Daytime intra-estuary movements and harmful algal bloom avoidance by Mugil cephalus (family Mugilidae)

The estuarine movement patterns of Mugil cephalus (Mugilidae) in response to Harmful algal blooms (HABs) was investigated in the eutrophied Sundays Estuary on the warm temperate coast of South Africa. The movements of six tagged M. cephalus (31–35 cm FL) were manually tracked using a Vemco VR100 acoustic receiver over 27 days during the typical summer bloom period. Fish were released above, within and below the mesohaline zone where Heterosigma akashiwo bloom maxima are known to occur. Physico-chemical variables and chlorophyll-a biomass were collected at each fish location and at four bloom monitoring sites, which locations were based on surface water salinity of 18, 10, 8 and 5. Mugil cephalus mostly occupied the upper mesohaline to lower oligohaline zones moving with the tidal ebb and flow. Phytoplankton biomass peaked at sites with a salinity of 10 and 8, with tagged fish only briefly (<0.63% of total time free roaming) venturing into bloom maxima areas. This is the first account of small scale intra-estuary movements of a mugilid worldwide and also the first to suggest avoidance behaviour of estuarine fishes in response to HABs. More work is needed to further understand how anthropogenic threats may impact the movements of important estuarine-dependent fish species in order to prevent the possible loss of ecosystem services induced by eutrophication.

Reduction of reproductive parameters of Pogonias courbina (Perciformes, Sciaenidae) in southern Brazil

In southern Brazil, the Southern Black Drum Pogonias courbina is an estuarine-dependent fish, endangered due to intense fishing in the past decades. This study calculated the reproductive parameters, age (A50) and length of first maturation (L50), batch fecundity, total fecundity and extension of the reproductive period of the P. courbina, in order to test the hypothesis of reduction, caused by heavy exploitation. The value for A50, calculated for both sexes, was 2.13 years. The L50 values for males and females were 42.5 and 46.4 cm respectively, while batch and total fecundity were 63,300 and 2,532,000 oocytes, in a reproductive period that extends from late October to March. Except for the extension of the reproductive period, it was observed that there was a reduction of all the reproductive parameters when compared with specimens, caught before overfishing. The data indicate that management strategies aimed at ecological and fishery sustainability are urgent and necessary.

Assimilation of Allochthonous Matter by Estuarine Consumers During the 2015 El Niño Event

The El Nino phenomenon refers to a warming of the tropical Pacific basin whose meteorological effects influence the dynamics of aquatic ecosystems around the world. Prior studies have shown that strong El Nino events are highly correlated with high rainfall episodes and high freshwater discharge into subtropical estuaries, with subsequent changes in species composition, abundance, and diversity of their biota. In this work, we evaluated the hypothesis that riverine allochthonous matter associated with the strong 2015 El Nino event is assimilated by decapod crustaceans and fishes of a southwestern Atlantic estuary. We analyzed carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of primary food sources and consumers in the estuary and of riverine allochthonous matter. Our findings revealed that higher water surplus and lower salinity associated with the 2015 El Nino coincided with an increase in the number of freshwater fish species and a decrease in the number of marine- and estuarine-dependent fishes inside the estuary. In addition, most estuarine consumers had lower average δ13C values during the wet period associated with the 2015 El Nino. This seemed to reflect the assimilation of 13C-depleted riverine matter, which according to Bayesian isotope mixing models ranged from 11% (adult resident decapod crustaceans) to 60% (adult resident fishes) during the wet season. Further studies are needed to evaluate the role of El Nino events on structuring food web organization in estuaries under the influence of this climatic phenomenon, which may become more frequent and intense in a global warming scenario.

The effects of hypersalinity on the growth and skeletal anomalies of juvenile Cape stumpnose, &lt;em&gt;Rhabdosargus holubi&lt;/em&gt; (Sparidae)

Estuarine organisms are exposed to hypersaline conditions for prolonged periods during drought conditions and under severely restricted river flow resulting from freshwater abstraction and impoundments. Consequently, marine estuarine-dependent fish such as Rhabdosargus holubi may be subjected to extreme conditions, such as hypersalinity prevailing for long periods ( &gt; 2 months). Hypersalinity may impact the energetic demands of fish due to osmoregulation leading to compromised growth. This study assessed the impact of high salinity on the growth and skeletal development of R. holubi juveniles. Skeletons of juveniles grown at different salinities in the wild and in aquaria were analysed for anomalies. The impact of hypersaline conditions on juvenile R. holubi growth was also determined in aquaria. Aquarium experiments indicated that hypersalinity of 50 did not significantly impact growth rates over two months. Overall, anomalies were rare and vertebral-related anomalies specifically did not differ significantly between salinities. However, fin rays were significantly impacted in fish growing at higher salinities in the wild. It was concluded that the strong osmoregulatory ability of R. holubi offers protection against hypersalinity affecting internal structures, but external structures may remain vulnerable. As such, from a locomotory standpoint, R. holubi may be vulnerable to long periods of exposure to hypersaline conditions.

Fishes associated with living stromatolite communities in peritidal pools: predators, recruits and ecological traps

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 580:153-167 (2017) - DOI: https://doi.org/10.3354/meps12300 Fishes associated with living stromatolite communities in peritidal pools: predators, recruits and ecological traps Gavin M. Rishworth1,*, Nadine A. Strydom2, Renzo Perissinotto1 1DST/NRF Research Chair: Shallow Water Ecosystems, Nelson Mandela University, Port Elizabeth 6031, South Africa 2Department of Zoology, Nelson Mandela University, Port Elizabeth 6031, South Africa *Corresponding author: gavin.rishworth@gmail.com ABSTRACT: Lithified microbial mats (microbialites) dominated shallow oceans during the Precambrian but are rare in modern environments. Factors restricting the prevalence of modern microbialites have been linked predominantly with reduced marine calcium carbonate levels and metazoan disruption. Some living microbialites only manage to persist together with an abundant metazoan community because of, among other factors, the regulatory pressure of fish predators on the grazing macrofauna. In peritidal pools formed by layered microbialites (termed ‘stromatolites’) along the South African coastline, fishes occur regularly, yet the dynamics of their interactions with co-occurring metazoans or other habitat features are unknown. We therefore aimed to link multivariate fish abundance to correlates of potential food resources and physico-chemical variability, using a generalised linear modelling approach. The fish community comprised both a resident species (the gobiid Coryogalops sordidus) and a number of transient fishes, most notably Myxus capensis (Mugilidae) and Rhabdosargus holubi (Sparidae) that dominated the fish biomass in the pools yet displayed sporadic or seasonal fluctuations. Generally, fish abundance was largely related to physical forces primarily associated with spatial salinity gradients of pools as well as swell conditions, with resource variability offering some, but minimal overall, contribution. These insights suggest limited top-down predatory control by fishes on macrofauna, but rather that this environment invokes the recruitment of typical marine estuarine-dependent fish species. However, abundance and growth patterns of recruits suggest that these pools are unfavourable nurseries for juvenile development and export, likely because of their limited size, reduced feeding opportunities and spatial extent, unlike typical estuaries. KEY WORDS: Ichthyofauna · Juveniles · Living microbialite · Nursery · Predator · Recruitment cues · Tidal pools Full text in pdf format PreviousNextCite this article as: Rishworth GM, Strydom NA, Perissinotto R (2017) Fishes associated with living stromatolite communities in peritidal pools: predators, recruits and ecological traps. Mar Ecol Prog Ser 580:153-167. https://doi.org/10.3354/meps12300 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 580. Online publication date: September 29, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

The relationship between habitat complexity and nursery provision for an estuarine-dependent fish species in a permanently open South African Estuary

Estuarine-dependent marine fish species rely on shallow, sheltered and food rich habitats for protection from predators, growth and ultimately recruitment to adult populations. Hence, habitats within estuaries function as critical nursery areas for an abundance of fish species. However, these habitats vary in the degree of nursery function they provide and few studies have quantitatively assessed the relative nursery value of different habitat types within estuaries, particularly in the context of habitat complexity. This study aimed to assess the nursery value of the dominant vegetated habitats, namely the submergent Zostera capensis (Setch.) (seagrass) beds and emergent Spartina maritima (Curtis) Fernald (salt marsh) beds in the Bushmans Estuary, South Africa. Biomass and stem density were sampled seasonally in order to gain insight into the vegetation dynamics of seagrass and salt marsh beds. Aerial cover, canopy height and underwater camera imagery were used to develop multiple complexity indices for prioritizing habitat complexity. The relatively consistent results of the dimensionless indices (interstitial space indices and fractal geometry) suggest that Z. capensis exhibits an overall greater degree of complexity than S. maritima, and hence it can be expected that fish abundance is likely to be higher in Z. capensis beds than in S. maritima habitats. Underwater video cameras were deployed in seagrass, salt marsh and sand flat habitats to assess the relative abundance and behaviour of the estuarine-dependent sparid Rhabosargus holubi (Steindachner 1881) in different habitats. The relative abundance of R. holubi was significantly higher in Z. capensis seagrass than S. maritima salt marsh and sand flats, whilst the behaviour of R. holubi indicated a high degree of habitat use in structured habitats (both Z. capensis and S. martima) and a low degree of habitat use in unstructured sand flat habitats.

Modelling the occurrence of postflexion stages of a marine estuarine-dependent fish in temperate South African estuaries

The movement of postflexion larvae of marine estuarine-dependent species into estuaries is critical for the survival of fishes reliant on estuaries as nurseries. However, detailed studies focused on environmental variability experienced by postflexion larvae entering a range of estuary types under varying conditions are rare. This study assessed the in situ conditions (temperature, salinity and water clarity) under which the southern African endemic fish Rhabdosargus holubi (Sparidae) recruits into estuaries. Postflexion larvae were sampled in three biogeographic regions (cool temperate, warm temperate and subtropical boundary), which included three estuary types (permanently open estuaries (POEs), temporarily open/closed estuaries and estuarine lake systems) on a seasonal basis, independent of each other. Rhabdosargus holubi larvae were more abundant in spring and summer, in POEs in the warm temperate region. Models predicted that higher larval occurrence in estuaries is a function of lower salinity (e.g. mesohaline zones of 5-17.9 salinity) and lower water clarity (e.g. 0-0.2 Kd, light extinction coefficient), particularly for warm, temperate POEs. This re-emphasizes the importance of freshwater for optimal nursery functioning, which may be compromised by impoundments, abstraction and climate change in water-short countries like South Africa.

Seagrass fragmentation impacts recruitment dynamics of estuarine-dependent fish

Seagrass beds are a key component of the estuarine landscape supporting high productivity, abundant marine life, and serve as nursery areas for many estuarine-dependent species. With increasing anthropogenic activity, there is concern about overall habitat loss via fragmentation and what effects this may have on local biotas relying on seagrasses for persistence. To examine these effects, fragmented seagrass beds (Halodule wrightii, Ascherson 1868) in two different bay systems, Corpus Christi and Aransas Bay, Texas, were delineated, quantified, and mapped, and red drum (Sciaenops ocellatus, Linnaeus 1766) was used as a model species to test for impacts of fragmentation on this common estuarine-dependent species. Red drum density, growth, and movement were measured in response to varying levels of fragmentation (i.e., High, Medium, and Low). No difference in initial recruitment of red drum density was observed among fragmentation levels for newly settled arrivals. However, there was a significant size effect; larger fish were found in non-fragmented areas. Growth rates were also compared among fragmented habitats using both RNA:DNA ratios and otolith microstructure, and no significant effect of growth among fragmentation levels was found. Migration potential was measured at the landscape-level within and among fragmented seagrass meadows by tagging and releasing 200 juvenile red drum into three separate replicated fragmented networks. Within 24h, only one fish was recaptured within the original fragmented network. The vast majority of recaptured fish were found in the nearest continuous non-fragmented seagrass bed over 50m from their release point. These results suggest a temporal transition of small newly recruited red drum, where individuals settle ubiquitously among varying levels of fragmentation but over time migrate toward more continuous beds. This study provides evidence that there may be a fragmentation “threshold” for red drum, whereby once a habitat becomes too fragmented; individuals either suffer higher mortality or more likely move to more continuous landscapes. Overall, this study suggests that habitat fragmentation and loss of more continuous seagrass meadows may have negative impacts on estuarine-dependent species using these areas as their primary recruitment habitat.

A pesca artesanal da miragaia (Pogonias cromis, Sciaenidae) no estuário da Lagoa dos Patos, Brasil

O aumento na abundí­¢ncia de peixes não residentes da zona estuarina atrai o interesse da frota pesqueira, o que determina o perí­­odo das safras de pesca na mesma época do aumento da abundí­¢ncia. No sul do Brasil a miragaia (Pogonias cromis) é um peixe estuarino-dependente, ameaçado de extinção devido í­Â sobreexplotação. No estuário da Lagoa dos Patos, a falta de dados sobre a biologia e pesca da espécie são fatores limitantes na formulação de planos de manejo. Este trabalho tem o objetivo de analisar a pesca artesanal da miragaia no estuário da Lagoa dos Patos, discutindo as variações da CPUE e as diferentes composições de juvenis e adultos desembarcados, relacionando-as com o perí­­odo de aumento da abundí­¢ncia na região. A pesca intensa é o principal responsável pelas variações na abundí­¢ncia miragaias no estuário da Lagoa dos Patos, onde as altas capturas acontecem dentro e fora do perí­­odo reprodutivo. A composição dos desembarques se modifica ao longo do ano, fêmeas e peixes adultos predominam na estação reprodutiva, durante o perí­­odo de repouso não há diferença significativa entre machos e fêmeas, e juvenis são predominantes, a média de idade também se modifica, com maiores valores no perí­­odo de desova. Conclui-se que esta pescaria pode comprometer a recuperação do estoque.

Respiratory plasticity is insufficient to alleviate blood acid-base disturbances after acclimation to ocean acidification in the estuarine red drum, Sciaenops ocellatus.

The changes in ocean chemistry stemming from anthropogenic CO2 release--termed ocean acidification (OA)--are predicted to have wide-ranging effects on fish and ultimately threaten global populations. The ability of fish to adapt to environmental change is currently unknown, but phenotypic plasticity has been highlighted as a crucial factor in determining species resilience. Here we show that red drum, a long-lived estuarine-dependent fish species native to the Gulf of Mexico, exhibit respiratory plasticity that increases CO2 excretion capacity when acclimated to OA conditions. Specifically, fish exposed to 14 days of 1000 µatm CO2 had a 32% reduction in branchial diffusion distance and increased expression of two putative CO2 channel proteins--rhag and rhcg1. No changes were observed in the erythrocyte CO2 transport pathways. Surprisingly, no significant changes in blood chemistry were observed between acclimated and acutely challenged animals; however, a non-significant 30 % drop in the magnitude of plasma C(CO2) elevation was observed. Reduced diffusion distance also comes with the cost of increased diffusive water loss, which would require greater osmoregulatory investment by the animal. OA exposure induced increased gill Na(+), K(+) ATPase activity and intestinal nkcc2 expression, supporting both the presumed osmotic stress and increased osmoregulatory investment. However, no differences in standard metabolic rate, maximum metabolic rate or aerobic scope were detected between control and OA acclimated individuals. Similarly, no differences in critical swim speed were detected between groups, suggesting the energetic cost related to respiratory plasticity is negligible against background metabolism. The current study demonstrated that red drum exhibit respiratory plasticity with only mild physiological trade-offs; however, this plasticity is insufficient to fully offset the OA-induced acid-base disturbance and as such is unlikely to impact species resilience.

There’s no smell like home: how sensory ontogeny plays a role in the early life history of estuarine-dependent fishes

There’s no smell like home: how sensory ontogeny plays a role in the early life history of estuarine-dependent fishes

Osmoregulation and branchial plasticity after acute freshwater transfer in red drum, Sciaenops ocellatus

Red drum, Sciaenops ocellatus, is an estuarine-dependent fish species commonly found in the Gulf of Mexico and along the coast of the southeastern United States. This economically important species has demonstrated freshwater tolerance; however, the physiological mechanisms and costs related to freshwater exposure remain poorly understood. The current study therefore investigated the physiological response of red drum using an acute freshwater transfer protocol. Plasma osmolality, Cl−, Mg2+ and Ca2+ were all significantly reduced by 24h post-transfer; Cl− and Mg2+ recovered to control levels by 7days post-transfer. No effect of transfer was observed on muscle water content; however, muscle Cl− was significantly reduced. Interestingly, plasma and muscle Na+ content was unaffected by freshwater transfer. Intestinal fluid was absent by 24h post-transfer indicating cessation of drinking. Branchial gene expression analysis showed that both CFTR and NKCC1 exhibited significant down-regulation at 8 and 24h post-transfer, respectively, although transfer had no impact on NHE2, NHE3 or Na+, K+ ATPase (NKA) activity. These general findings are supported by immunohistochemical analysis, which revealed no apparent NKCC containing cells in the gills at 7days post transfer while NKA cells localization was unaffected. The results of the current study suggest that red drum can effectively regulate Na+ balance upon freshwater exposure using already present Na+ uptake pathways while also down-regulating ion excretion mechanisms.

Diurnal and water salinity-dependent metabolic activity of juvenile white steenbras Lithognathus lithognathus

The euryhaline white steenbras Lithognathus lithognathus is an important estuarine-dependent fish species in the South African marine and estuarine environment. Its tolerance to water quality fluctuations, in particular its metabolic rate responses to salinity changes, has not been studied. In order to contribute to ecophysiogical data on this species, oxygen consumption by white steenbras was determined to quantify metabolic activity. White steenbras were most active during daylight hours when oxygen consumption was 28% higher than during the scotophase. Standard, routine and active metabolic rate estimates were 0.04, 0.09 and 0.12 mg O2 g−1 h−1, respectively. In juveniles (body mass [W] = 19.8–85 g fish−1), the metabolic rate (MR; mg O2 g−1 h−1) was estimated as MR = 0.62W−0.526. The MR was reduced in fish exposed to a fast drop in salinity within 10 min from 35 to 5, but not in fish exposed to a fast drop to 25 or a slow drop within 3 or 9 h to 25 or 5, respectively. At a salinity of 5, oxygen consumption rate was 51% lower than at salinities of 25 and 35.

Site fidelity and condition metrics suggest sequential habitat use by juvenile common snook

The common snook Centropomus undecimalis is an estuarine-dependent fish that relies on landward wetlands as nursery habitat. Despite its economic importance, portions of the snook's early life history are poorly understood. We compared habitat use of young-of-the-year (YOY) snook in 2 geomorphic mesohabitats (tidal pond and tidal creek) along an estuarine gradi- ent (upstream vs. downstream) within a single wetland during fall recruitment. We used abun- dance, length, condition indices, and stable isotopes to assess ontogenetic mesohabitat use and site fidelity. We found that (1) YOY snook were more abundant within the upstream creek and ponds; (2) the smallest snook were found only in ponds; (3) snook from ponds had lower condition (Fulton's K and hepatosomatic index); (4) snook began moving from ponds to the creek at ~40 mm standard length; and (5) snook from the 2 mesohabitats were isotopically distinct, indicating high site fidelity at rather small spatial scales. Collectively, these data identified sequential use of meso- habitats, wherein seaward-spawned YOY snook moved landward and recruited to pond habitats, where they dedicated energy to growth (as length) before making an ontogenetic habitat shift to the creek. Once in the creek, YOY snook condition improved as they approached maturity and started the downstream return towards seaward locations. The wetland network that was previ- ously viewed as generalized nursery habitat instead consists of mesohabitats that support differ- ent life stages in sequence. This represents ontogenetic habitat complementation, in which lower availability of a required mesohabitat type may limit the entire wetland's contribution to the adult population.