Temporal and spatial variability in the abundance,biomass and distribution of Palaemon peringueyi (Decapoda: Crustacea) are influenced by biological but not physico-chemical factors in the permanently open Kariega Estuary,South Africa

Biological and physico-chemical factors affecting abundance and biomass of shrimp Palaemon peringueyi were investigated in 2010–2011 in the lower, middle and upper reaches of the freshwater-deprived permanently open Kariega Estuary. Shrimp abundance and biomass ranged from 0 to 88 ind. m^?2 and 0 to 4.4 g wwt m^?2, respectively. Shrimps were most abundant in the lower reach, and highest abundances and biomasses were recorded in the lower reach in June when salinities were low. The shrimps were almost always absent in the upper reach. Juveniles were found mostly in the lower and middle reaches, whereas adults were distributed in all three reaches. Regression analyses showed no statistically significant relationship for either abundance or biomass of the shrimps with temperature, salinity and dissolved oxygen (p > 0.05). The relationship between shrimp abundance and biomass and vegetation cover was positive and statistically significant (R² = 0.109 and 0.185, respectively; p < 0.05). Vegetation cover plays an important role in determining the presence, distribution and abundance or biomass of P. peringueyi in this estuary.     

Does the flatfish community of the Mondego estuary (Portugal) reflect environmental changes?

Temporal and spatial patterns of abundance of the flatfish community in the Mondego estuary were investigated from 2003 to 2007, based on monthly beam trawl samples. During the study period there was a severe drought, with consequential reductions in river runoff. A total of eight flatfish species occurred within the estuary, with seasonal specific richness and abundance varying considerably. It was possible to identify three main groups: one composed of species present throughout the study period and in high densities: Platichthys flesus and Solea solea (maximum densities 3.7 and 4.1 Ind 1000 m^?2, respectively); a second group of less abundant but regularly present species: Scophthalmus rhombus and Solea senegalensis (maximum densities 0.2 and 0.3 Ind 1000 m^?2, respectively); and a third group of occasional species that occurred only during the drought period: Arnoglossus laterna, Buglossidium luteum, Dicologlossa hexophthalma and Pegusa lascaris. Flatfish distribution patterns varied according to the estuarine use guild: marine‐estuarine dependent fish occurred mainly in the upper reaches, while marine stragglers and marine‐estuarine opportunists occurred mostly in the downstream areas. Species with more northern latitudinal affinities were the most affected by the drought, related with a lesser extent of river plumes to the coastal area, resulting in a reduction in half of the abundance levels. However, flatfish species with more southern affinities increased in abundance during the drought, benefiting also from an increase in estuarine water temperature. Early summer salinity and precipitation values were good proxies for estimating abundance levels of P. flesus and S. solea, respectively, emphasizing the importance of hydrodynamics for the recruitment and abundance of these commercially important species.     

Effects of temperature and salinity on the standard metabolic rate (SMR) of the caridean shrimp Palaemon peringueyi

The standard metabolic rate (SMR) of the caridean shrimp Palaemon peringueyi to changes in temperature (15-30 °C), salinity (0-45‰) and a combination thereof was investigated. The rate of oxygen consumption of the shrimp was determined using a YSI oxygen meter. At a constant salinity of 35‰ the respiration rate of P. peringueyi increased with an increase in temperature and ranged between 0.260 and 0.982 μl O₂ mg wwt^− 1 h^− 1. The Q₁₀ value over the temperature range 15-25 °C was estimated at 3.13. At a constant temperature of 15 °C the respiration rate of P. peringueyi also increased with an increase in salinity and ranged between 0.231 and 0.860 μl O₂ mg wwt^− 1 h^− 1. For combination experiments the absence of any significant difference in the respiration rate of P. peringueyi at the four temperatures over the salinity range 15-35‰ suggests that the shrimp is well adapted to inhabiting environments characterised by variations in salinity and temperature such as those encountered within the middle and lower reaches of permanently open estuaries with substantial freshwater inflow. On the other hand, the total mortality of the shrimp recorded at salinities < 5‰ at all four temperatures suggests that the upper distribution of the shrimp may reflect physiological constraints. Similarly, the increase in the respiration rate of the shrimp at the four temperatures at salinities > 35‰ suggests that the shrimp may experience osmotic stress in freshwater deprived permanently open and intermittently open estuaries where hypersaline conditions may develop.     

Food-web modification by an invertebrate predator in the Great Salt Lake (USA)

Summary During unusually wet years the salinity of the Great Salt Lake (Utah) decreased from above 100 g/L to 50 g/L. This allowed the predaceous insect Trichocorixa verticalis to invade the pelagic region of the lake and reach a mean summer density of 52/m³. Concurrent changes in the pelagic ecosystem were: a decrease in the dry biomass of the previously dominant filter-feeding brine shrimp Artemia franciscana from 720 to 2 mg/m³, the invasion of three other zooplankton taxa, a 10 × decrease in community filtration rate, a 20 × increase in chlorophyll a concentration, a 4 × decrease in water clarity and perhaps a decrease in soluble nutrients. Trichocorixa abundance was also inversely correlated with the abundance of Artemia along a salinity gradient in the lake's estuary. In a 9-d microcosm experiment Trichocorixa preyed on nauplii and decreased the total density of Artemia from 103 to 6/L. The reduction in Artemia allowed protozoans to increase 10–100 ×. Changes in chlorophyll and clarity were consistent with those observed in the lake. These results suggest that invertebrate predation may be an important factor structuring simple food webs such as those found in moderately saline lakes.     

The composition and community structure of the ichthyofauna of the upper Scheldt estuary: synthesis of a 10-year data collection (1991–2001)

Fish collections on the cooling water filter screens of the Doel power plant were used to analyse temporal trends in estuarine fish populations of the upper Scheldt estuary, Belgium. Between 1991 and 2001, 62 species shared the estuary, with 15 species present at any one time. Identified were 23 marine stragglers, 16 freshwater species and four diadromous species, but their contribution to the total catch was insignificant. In terms of numbers, marine estuarine opportunists (17 species) mainly occurring as juveniles and estuarine residents (two species) were the dominant life cycle categories. Six fish species contributed to >97% of the total catch: three Pomatoschistus species [P. minutus (32%), P. microps (30%) and P. lozanoi (6%)], two Clupeidae [Clupea harengus (16%) and Sprattus sprattus (8%)] and Syngnathus rostellatus (5%). Abundance of almost all species in the estuary was highly seasonal and characterized by pronounced abundance peaks. Ordination of the data showed that variability in the temporal structure of estuarine fish community was first determined by differences in annual recruitment and then by predictable, cyclical patterns of species abundance.     

Niche overlap between juvenile flatfishes, Platichthys flesus and Solea solea, in a southern European estuary and adjacent coastal waters

The distribution and abundance patterns as well as the feeding ecology of two flatfishes, Platichthys flesus (Linnaeus, 1758) and Solea solea (Linnaeus, 1758), were studied in the Douro estuary and adjacent coastal waters (Portugal) in order to evaluate the niche overlap between these species. Fish were collected during sampling surveys conducted every 2 months between November 2000 and March 2002 using a 12‐m otter‐trawl with 18 mm stretched mesh size, at 12 sites within the estuary and six sites in the adjacent coastal area. The highest densities of P. flesus were registered in the middle estuary; a peak density of 9.12 ind 1000 m^?2 was obtained in October 2001. Solea solea densities were low compared with P. flesus; the highest values were also recorded in the middle estuary in October 2001. For both species, the proportion of juveniles was above 90% within the estuary. Highest densities of P. flesus and S. solea were recorded at sites with low water depth and high densities of crustaceans and polychaetes, the main food items in the diet of these species. High values of spatial and diet niche overlap were found between these two flatfishes (0.97 and 0.92, respectively).     

Copepod biomass in an estuarine and a stagnant brackish environment of the S. W. Netherlands

In the mesohaline zone of the Westerschelde estuaryEurytemora affinis is the dominant copepod, demonstrating large biomass values nearly throughout the year. In the meso-polyhaline Lake VeereAcartia tonsa is abundant, mainly during summer. In spring a small population ofEurytemora americana is found. The tidal estuary harboured far denser copepod populations throughout the year than the stagnant brackish lake water. The average yearly copepodid+adult biomass in the Westerschelde estuary was approx. 850 mg/m³ (wet weight), in Lake Veere approx. 130 mg/m³. Temporarily low oxygen values did not influence negatively the copepod populations in the Westerschelde estuary. The seasonal distribution of the dominant copepods in both areas is explained in the light of recent litterature data. PerennialEurytemora affinis abundance in the Westerschelde estuary must be mainly caused by large concentrations of nannodetritus particles, bacteria included, throughout the year.Acartia tonsa in Lake Veere has to thrive mainly on nannophytoplankton. Communication no. 141 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherland.     

Distribution, abundance, and roles of freshwater clams (Bivalvia, Unionidae) in the freshwater tidal Hudson River

1. An extensive series of PONAR grabs was used to determine the distribution and abundance of unionid clams in the freshwater tidal Hudson River. 2. The five species of unionids collected were distributed very unevenly within the river. Mean river-wide density and biomass of unionids were 8.0m^?2 and 6.2 g DM m^?2 (shell-free)., respectively. 3. The environmental variables that we measured (water depth, distance from shore, sediment granulometry and organic content, presence or absence of macrophytes, and the chlorophyll a and particulate organic matter content of the water) explained little of the variation in abundance of unionids. 4. The distributions of the various species of clams did not differ significantly with respect to the environmental variables measured. 5. We estimate that unionids filter a significant amount of water (0.14m³ m^?2 day^?1, on average) in the Hudson River estuary, roughly equivalent in magnitude to downstream flushing. 6. We project that unionids will serve as a major substratum for the settlement of the zebra mussel (Dreissena polymorpha), which is now invading the estuary. We emphasize that unionids may play important non-trophic roles in large river ecosystems.     

Biological availability and humic properties of dissolved organic carbon in Lake Balaton (Hungary)

The nursery role of the Mondego estuary for marine fish species was studied between June 2003 and May 2004. The spatial and temporal distribution and abundance patterns of 0-group Dicentrarchus labrax (Linnaeus, 1758), Platichthys flesus (Linnaeus, 1758) and Solea solea (Linnaeus, 1758) were analyzed based on monthly sampling surveys in five stations along the estuarine gradient. Fishing took place during the night at low water of spring tides, using a 2 m beam trawl. The spatial patterns of estuarine colonization were different according to species. D. labrax showed a wider distribution, but the main nursery ground was the same as for S. solea. Highest densities of S. solea juveniles were found in oligohaline areas, with muddy bottoms and high benthic invertebrates availability, while P. flesus occurred mainly in the sandy uppermost areas. D. labrax was found in both these areas. Fish abundance in the estuary mainly reflected seasonal changes.     

The food web in the lower part of the Seine estuary: a synthesis of existing knowledge

The Seine estuary illustrates the alterations to estuaries due to human activities heavy releases of pollutants of various origins and significant morphological changes beginning in the middle of the 19th century. The intertidal mudflat surface has been seriously reduced (< 30km²) since the channels of the Seine River came under management. While the role of the Seine estuary in the dynamics of the eastern English Channel ecosystem is recognized as important, the biological characteristics of the estuary remained relatively unknown until the 1990s. Biological diversity was progressively impoverished from the polyhaline zone to the oligohaline zone. In spite of a heavily contaminated environment, the macrobenthic and planktonic fauna of the Seine estuary remains similar to those of other northeastern Atlantic estuaries. The fauna exhibit clear contrasts between areas with very high abundance and others with very low abundance. The pelagic fauna, especially the copepod Eurytemora affinisand the shrimp Palaemon longirostris, are more abundant in the Seine estuary than in other estuaries. Diversified and abundant, Abra alba-Pectinaria koreni and Macoma balthica benthic communities occur, respectively, in the outer and inner parts of the estuary. In subtidal flats, benthic fauna is especially poor in terms of specific richness, abundance and biomass. Paradoxically, considering the high abundance of prey, fish are particularly scarce. Two food webs have been identified. In the oligohaline zone, where turbidity is maximum, the food web is exclusively planktonic, due to dredging that prevented benthic fauna from settling. In the polyhaline zone, fish populations that feed particularly on benthic fauna benefit from low turbidity and high oxygen concentrations. So, in spite of heavy organic and metallic contamination and human activities, the Seine estuary remains a highly productive ecosystem, which provides a nursery for marine fish and feeding grounds for migratory birds. A global management plan appears to be necessary in order to guarantee that the Seine estuary continues to function as it currently does.     

Epiphytes of the St. Lucia Estuary and their response to water level and salinity changes during a severe drought

St. Lucia is the largest estuary in South Africa with extensive areas of submerged macrophytes that fluctuate rapidly in response to water level and salinity changes. Epiphytes associated with submerged macrophytes were sampled during a severe drought between November 2004 and October 2005 when very low water level and high, variable salinity characterised the estuary. Potamogeton pectinatus and Ruppia cirrhosa were the dominant submerged macrophytes throughout the estuary, with P. pectinatus occurring at relatively low salinity (∼10 ppt) and R. cirrhosa at higher salinity (9–33 ppt). Zostera capensis, normally the other dominant submerged macrophyte, was conspicuously absent under the prevailing conditions. Epiphytic biomass, estimated as chlorophyll a, varied greatly between sites and over the 12 month sampling period, ranging from 10.9 to 71.7 mg Chl a m⁻² leaf area for P. pectinatus and 16.9–165.0 mg Chl a m⁻² leaf area for R. cirrhosa. Epiphytic biomass was twice as high in the Southern Lake where R. cirrhosa occurred, probably because the dominant epiphytes were macroalgae. An assessment of the diatom species composition of the epiphytic community indicated the dominance of only six species throughout the estuary. Neither epiphytic biomass nor diatom species composition showed strong statistical relationships with the environmental variables measured and it is believed that biological factors may be more important than the physico-chemical environment in determining epiphyte biomass distribution. Because epiphyte biomass is dependent on the presence of host surface area it will only contribute substantially to overall system biomass and productivity when submerged macrophyte area cover is high. In the St. Lucia Estuary this will occur when the water level is high and the upper level of the salinity gradient does not increase above that of seawater.     

Species composition and seasonal dynamics of water mites (Hydracarina) in a mountain stream (Steina,Black Forest,southern Germany)

The species composition and seasonal dynamics of water mites were studied in a small softwater stream in southern Germany from October 1986 to November 1988. On average water mites contributed 5.5% by abundance and 1.8% by biomass to the total invertebrate community. Annual densities and biomasses averaged 623–1057 (mean 905) individuals M^–2 and 45.9–75.6 mg (mean 64.0) dry mass m^–2, respectively. 41 species were identified, Torrenticola elliptica (Torrenticolidae) being the most abundant. Nearly every taxon showed a distinct and consistent seasonality, with maximum abundance and biomass in summer and minimum values in winter. Both abundance and biomass of water mites were significantly correlated with water temperature (p < 0.001).     

Correlation of the distribution ofMesopodopsis slabberi (Crustacae,Mysidacea) with physico-chemical gradients in a partially-mixed estuary (Tamar,England)

The physical and chemical processes operating in the River Tamar Estuary (south-west England) have been comprehensively described and reported in the literature. There are well-established gradients of salinity, suspended sediment and oxygen which vary both on short-term (tidal) and long-term (seasonal) cycles. Freshwater runoff, the main factor determining salinity distribution, is also the cause of the high variability in suspended sediment concentrations. The biological processes are less well studied and information on the link between the benthic and pelagic systems is particularly lacking. Mysids, through their role as detritivores and as a major component in the diet of some fish, provide this link. Of the four species of mysid distributed longitudinally in the Tamar Estuary, the most abundant isMesopodopsis slabberi which occurs between 5 and 25 km from the estuary head. Observations over an annual cycle have shown marked seasonal changes in both abundance and distribution in the estuary. During winter and spring, densities remained generally low (<50 m⁻³) but, as water temperatures increased, the density increased and reachedca 1200 individuals m⁻³ in July. There was a shift in the longitudinal distribution ofM. slabberi in response to changes in the position of the salinity gradient. Adults comprised the majority of the population in salinities less than 10‰ whereas juveniles and immature animals were distributed over a wider area than the adults and occurred in water of higher salinity than the main adult distribution.M. slabberi appears to utilise the two-layered estuarine circulation to maintain its position in the estuary.     

Distribution patterns of the fish assemblage in the Mamanguape River Estuary,North-eastern Brazil

The Mamanguape River Estuary was studied along a continuum ranging from shallow sandstone reefs adjacent to the river mouth up to the limit of influence of seawater, the upper portions of the estuary. Fish samples were gathered using three types of nets along 17 sample sites, grouped in four regions according to salinity range (reefs and low, middle and upper estuaries), to seasonality (dry and rainy seasons) and to habitat usage (marine visitor, marine estuarine-opportunist, marine estuarine-dependent, estuarine resident and estuarine &; marine). Differences were found in the fish assemblages along the estuarine-reef gradient, with most species (n?=?30) being considered marine, estuarine-opportunists or estuarine-dependent, according to its abundance and distribution. A low number of species (n?=?11) were considered estuarine residents. Some species exhibited significant differences in spatial distribution pattern, in which juveniles and adults predominated in different portions of the estuary, suggesting an ontogenetic migration both in relation to the adjacent reef area and across estuarine regions. Several species were newly recorded in the Mamanguape Estuary: Anchoa spinifer, Halichoeres poeyi, Hyporhamphus roberti, Scomberomorus cavalla, Sphyraena barracuda and Ocgocephalus vesperilio.     

The Structure of a Macroinvertebrate Community in a Northern German Lake Outlet (Lake Belau,Schleswig-Holstein) with Special Emphasis on Abundance,Biomass and Secondary Production

The structure of macroinvertebrate communities was studied at I I sampling sites of the outlet of Lake Belau in the lowlands of northern Germany. To describe the structures of macrobenthic animal communities three different units were examined: abundance, biomass, and secondary production. 112 taxa were collected from the entire stream. The numbers of species ranged from 31 (fine sand) to 70 (submerged macrophytes). For the stream, average macroinvertebrate density was 18.400 ind. M⁻¹. Density was highest at the macrophytes amounting to 35,630 individuals per m², and lowest in the pure sand with only 3,900 ind. M⁻². Average biomass (dry mass) was 194 g DM m⁻² varying from 9.8 (peat) to 381 g DM m⁻² (gravel with mollusk shells near the upstream lake). For the stream, average annual production was 129 g DM m⁻² varying from 15 (peat) to 286 g DM m⁻² (macrophytes). The highest values for each unit were found in stream sections with gravel and submerged macrophytes. Lower values occured in sections that contained peat and sand. Usually, a single structure of the macroinvertebrate community was dominated by less than ten taxa, which varied at each sampling site depending on the units observed.     

BIOMASS AND DYNAMICS OF GROWTH OF ULVA SPECIES IN PALMONES RIVER ESTUARY1

During the last decade, the Palmones River estuary has undergone severe eutrophication followed by a green tide episode; two species of Ulva, rotundata Blid. and Ulva curvata (Kütz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P)of Ulva species, as well as some physicochemical variables in the estuary. Maximum biomass (up to 375 g dry wt·m^?2 in some spots, corresponding to a thallus area index of nearly 17 m²Ulva·m^?2 sediment) were observed in June and December. However, the biomass varied among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates observed. Growth rates were estimated directly in the field. The rates were generally higher in Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The difference between both estimates was used to quantify the importance of the processes causing loss of biomass, which were attributable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme self-shading. Maximum chlorophyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water. However, the absolute concentrations of tissue N and P were always higher than the critical levels for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estuary. In addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass.     

Can Chara control phosphorus cycling in Lake Łuknajno (Poland)?

Waters of Lake uknajno (623 ha, 3 m max. depth postglacial basin in Masurian Lakeland) are of meso-eutrophic type in spite of an intensive land impact and substantial phosphorus loading. The reason of this peculiarity is an abundance of 4 species of charophytes (Chara aculeolata, C. aspera, C. contraria and C. tomentosa) which dominate the submerged vegetation of the lake and yielded 417 g m^-2 dry biomass in summer 1991. The plants have high potential for phosphorus uptake and accumulation as was checked in laboratory experiments. Due to this potential Chara can outcompete planktonic algae in mixed cultures at different mineral P supplies.     

Zooplankton Dynamics in the Hyper‐Eutrophic Nakdong River System (Korea) Regulated by an Estuary Dam and Side Channels

Zooplankton in the main channel of the Nakdong River and in three tributaries was sampled from June 1994 to September 1995. Planktonic rotifers (Brachionus spp., Keratella spp., and Polyarthra spp.), cyclopoid nauplii and small cladocerans (Bosmina longirostris) were numerically dominant. There was considerable longitudinal variation of zooplankton biomass in the main channel as well as spatial heterogeneity among the major tributaries. In the middle region of the main channel, between river kilometer (RK) 170 and 150 above the estuary dam, total zooplankton abundance sharply increased from less than 100 ind. L^—1 to more than 1,000 ind. L^—1. In a downstream direction toward the estuary dam, phytoplankton biomass increased while total zooplankton biomass decreased. However, as shown by the increasing transport of zooplankton biomass, zooplankton was diluted in the reach of the estuary dam. Advective effects from major tributaries appear to be the contributory factor for the higher zooplankton biomass in the middle region. Overall, rather the external factors (flushing, retention) than internal factors (e.g., phytoplankton) appear to be responsible for changes in zooplankton abundance toward the river mouth.     

Dispersal of plant fragments in small streams

1. Streams are subject to frequent natural and anthropogenic disturbances that cause sediment erosion and loss of submerged vegetation. This loss makes downstream transport and retention of vegetative propagules on the streambed very important for re‐establishing vegetation cover. We measured dispersal and retention of macrophyte stem fragments (15–20 cm long) along 300 m long reaches of four small to medium sized Danish lowland streams. 2. The number of drifting stem fragments declined exponentially with distance below the point of release. This finding makes the retention coefficient (k, m⁻¹) in the exponential equation a suitable measure for comparisons among different macrophyte species, and between stream reaches of different hydrology and vegetation cover. 3. Buoyancy of macrophyte tissue influenced retention. Elodea canadensis stems drifted below the water surface, and were more inclined to be retained in deeper water associated with submerged plants and obstacles in the streambed. Ranunculus peltatus stems were more buoyant, drifted at the water surface, and were more inclined to be trapped in shallow water and in riparian vegetation. 4. The retention coefficient of drifting stems increased with the relative contact between the flowing water and streambed, bank and vegetation. Thus, the retention coefficients were highest (0.02–0.12 m⁻¹) in shallow reaches with a narrow, vegetation‐free flow channel. Here there were no significant differences between E. canadensis and R. peltatus. Retention coefficients were lowest (0.0005–0.0135 m⁻¹) in deeper reaches with wider vegetation‐free flow channels. Retention of E. canadensis was up to 16 times more likely than retention of R. peltatus. 5. Overall, the longitudinal position in the stream system of source populations of species capable of producing numerous stems, the species‐specific retention coefficients of stems, and the retention capacity of stream reaches should be important for species distribution in perturbed stream systems. Retention of stems is probably constrained in headwaters by the small downstream flux of stem fragments because of the restricted source area, and constrained in downstream reaches by small retention coefficients. Macrophyte retention may, consequently, peak in medium‐sized streams.     

Active feeding of downstream migrating juvenile pink salmon (Oncorhynchus gorbuscha) revealed in a large Barents Sea river using diet and stable isotope analysis

The recent, rapid spreading of non-native pink salmon Oncorhynchus gorbuscha in the North Atlantic area has raised concerns about their possible negative impacts on native salmonid species. Potential interactions include competition for food resources during the short freshwater phase of juvenile O. gorbuscha, but little is known about their feeding behavior in the newly occupied North Atlantic rivers. Using stable isotope and stomach content analyses, patterns of freshwater feeding of non-native O. gorbuscha fry were studied in a large Fennoscandian river, the Teno, that discharges to the Barents Sea. Changes in stable isotope values (δ¹³C, δ¹⁵N, δ³⁴S) and stomach contents from the period of emergence (April to mid-May) to estuarine entry (late May/June) were examined and provided both temporally integrated and short-term indicators of freshwater feeding dependency. In addition, the occurrence of juvenile O. gorbuscha and changes in their length and weight during their emergence/migration period were investigated. Juvenile O. gorbuscha were at the spawning grounds from April through to mid-May with abundance peaking in mid-May. Fish moved to the estuary by late May and their abundance decreased toward June, and their body size increased concurrently. Stomach analyses indicated no feeding activity in April–early May in the spawning areas, but the stomach fullness indices increased markedly in fish sampled in the estuary in May and June. The most important prey items in stomachs were Chironomidae and Ephemeroptera larvae. Significant changes in all analysed stable isotopes were detected among sample periods, with a peak in mid-May and June showing significantly lower values than other sample periods. A change from the higher values reflective of parental marine feeding to the lower values reflective of freshwater feeding indicated active in-river feeding by juveniles during the study period. The documented active freshwater feeding of non-native juvenile O. gorbuscha suggests potential resource competition with native fluvial fishes, particularly salmonids.