Zooplankton Availability Research Articles

Application of a Conceptual Stocking Model for Two Recreationally Important Species

ABSTRACTFish stocking continues to be an important and often‐used tool in fisheries management. Mortality of fish may be high in the first few days after stocking due to predation and a lack of appropriate food and habitat. The objective of this study was to develop a conceptual model that may identify stocking locations that can help improve the survival of stocked fish. We identified three factors—predator risk, zooplankton (food) availability and measures of habitat that could be important to the survival of stocked Walleye (Sander vitreus) and White Bass (Morone chrysops) fingerlings in Lake McConaughy, Nebraska. The spatial distribution of these factors was used to create a predictive surface to identify those stocking locations hypothesised to lead to higher survival. Final stocking maps identified the best 12.8% and 13.9% of the predicted area of Lake McConaughy for stocking Walleye and White Bass, respectively. Both maps predicted that the best locations were along the south‐central shoreline of Lake McConaughy. The maps for both species were interestingly similar. These results are likely due to the similarities in predators consuming the stocked species and the zooplankton consumed by fingerlings of both species. Habitat availability may be less important, or other aspects of habitat may need to be considered. Identifying the best locations for stocking based on predation risk and the availability of food and habitat could increase the survival of stocked fishes and help managers reach population objectives.

SPATIAL AND TEMPORAL VARIATION OF ZOOPLANKTON COMPOSITION NEAR WHALE SHARK SIGHTINGS IN PROBOLINGGO OF EAST JAVA, INDONESIA

Whale shark occurrence in Probolinggo differs from other Indonesian locales, suggesting a link to zooplankton availability. Zooplankton composition and whale shark emergence are the focus of this study. From December 2017 to November 2018, six observation points were made each month. A plankton net filters and lugol preserves water. Olympus CX23 microscope observations were repeated twice. The spatial analysis revealed varying whale shark numbers at each station (Chi-square test, X2 = 1418.6, P <0.05), with six sharks observed at station PR_5. Zooplankton numbers were similar at each location. Temporal analysis revealed significant differences in whale shark appearance each month (Chi-square test, X2 = 81.04, P <0.05), with March and November having the highest appearance among the three individuals. The amount of zooplankton varied (Chi-square test, X2 = 148.61, P <0.05), with the highest abundance in April and March. Whale shark appearance and zooplankton composition were not correlated (r = 0.01, P< 0.05) both geographically and temporally. Whale sharks were linked to zooplankton kinds. Results indicate whale sharks are particularly interested in Acartia sp. (r = 0.3, P < 0.05). This suggests that whale sharks' appearance is determined by their demand for food, not zooplankton availability.

Lipid biomarkers reveal trophic relationships and energetic trade‐offs in contrasting phenotypes of the cold‐water coral Desmophyllum dianthus in Comau Fjord, Chile

Abstract Benthic suspension feeders like corals and sponges are important bioengineers in many marine habitats, from the shallow tropics to the depth of polar oceans. While they are generally considered opportunistic, little is known about their actual in situ diet. To tackle this limitation, fatty acid trophic markers (FATMs) have been employed to gain insights into the composition of their diet. Yet, these in situ studies have not been combined with physiological investigations to understand how physiological limitations may modulate the biochemistry of these organisms. Here, we used the cold‐water coral (CWC) Desmophyllum dianthus in its natural habitat in Comau Fjord (Northern Patagonia, Chile) as our model species to assess the trophic ecology in response to contrasting physico‐chemical conditions (variable vs. stable) and ecological drivers (food availability) at three shallow sites and one deep site. We took advantage of the expression of two distinct phenotypes with contrasting performance (growth, biomass, respiration) coinciding with the differences in sampling depth. We analysed the corals' fatty acid composition to evaluate the utility of FATM profiles to gain dietary insights and assess how performance trade‐offs potentially modulate an organism's FATM composition. We found that 20:1(n‐9) zooplankton markers dominated the deep high‐performance phenotype, while 20:5(n‐3) and 22:6(n‐3) diatom and flagellate markers, respectively, are more prominent in shallow low‐performance phenotype. Surprisingly, both energy stores and performance were higher in the deep phenotype, in spite of measured lower zooplankton availability. Essential FA concentrations were conserved across sites, likely reflecting required levels for coral functioning and survival. While the deep high‐performance phenotype met with these requirements, the low‐performance phenotype appeared to need more energy to maintain functionality in its highly variable environment, potentially causing intrinsic re‐allocations of energy and enrichment in certain essential markers (20:5(n‐3), 22:6(n‐3)). Our analysis highlights the biological and ecological insights that can be gained from FATM profiles in CWCs, but also cautions the reliability of FATM as diet tracers under limiting environmental conditions that may also be applicable to other marine organisms. Read the free Plain Language Summary for this article on the Journal blog.

Declines in lake whitefish larval densities after dreissenid mussel establishment in Lake Huron

Lake whitefish (Coregonus clupeaformis) is an ecologically and commercially significant species across the Laurentian Great Lakes. Over the past 20 years, lake whitefish population abundance has substantially declined across lakes Huron and Michigan, being driven by reduced recruitment of juvenile fish into the population. However, the life stage at which the recruitment bottleneck is occurring and what factors are contributing to the declines remain unknown. One hypothesis is that dreissenid mussels reduced zooplankton availability to larval lake whitefish, leading to poor growth and survival of this critical life stage. Here, we present results of a larval fish survey conducted at the Fishing Islands spawning region in Lake Huron and examine whether declines in juvenile recruitment are linked to reduced larval fish density. Larval fish were collected annually during two time periods: (1) a historical time period before dreissenid mussel establishment (1976–1986); and (2) a contemporary time period after dreissenid mussels became established (2017–2019, 2021). We found significant declines in larval densities and growth between historical and contemporary time periods. Following dreissenid establishment, larval densities and growth were on average only 23% and 55% of historical values, respectively. Moreover, year class strength at the juvenile stage (age 4) was positively related to larval density. Several explanatory variables contributed to annual variation in larval densities, with dreissenid mussels and water levels having the most consistent effect. Our results suggest that juvenile recruitment is being limited at the larval stage, owing to overall lower larval production and potentially exacerbated by slower growth.

Variability, skipped breeding and heavy-tailed dynamics in an Antarctic seabird.

The population dynamics of many colonially breeding seabirds are characterized by large interannual fluctuations that cannot be explained by environmental conditions alone. This variation may be particularly confounded by the use of skipped breeding by seabirds as a life-history strategy, which directly impacts the number of breeding pairs and may affect the accuracy of breeding abundance as a metric of population health. Additionally, large fluctuations in time series may suggest that the underlying population dynamics are heavy tailed, allowing for a higher likelihood of extreme events than expected under Gaussian dynamics. Here, we investigated the effect of demography on time series for abundance of the Adélie penguin Pygoscelis adeliae and explored the occurrence of heavy-tailed dynamics in observed Adélie time series. We focus this study on the Adélie penguin as it is an important bellwether species long used to track the impacts of climate change and fishing on the Southern Ocean ecosystem and shares life-history traits with many colonial seabirds. We quantified the impacts of demographic rates, including skipped breeding, on time series of Adélie abundance simulated using an age-structured model. We also used observed time series of Adélie breeding abundance at all known Antarctic colonies to classify distributions for abundance as Gaussian or non-Gaussian heavy tailed. We then identified the cause of such heavy-tailed dynamics in simulated time series and linked these to spatial patterns in Adélie food resource variability. We found that breeding propensity drives observed breeding fluctuations more than any other vital rate, with high variability in skipped breeding decoupling true abundance from observed breeding abundance. We also found several Antarctic regions characterized by heavy-tailed dynamics in abundance. These regions were often also characterized by high variability in zooplankton availability. In simulated time series, heavy-tailed dynamics were strongly linked to high variability in adult survival. Our results illustrate that stochastic variability in abundance dynamics, particularly the presence of variable rates of skipped breeding, can challenge our interpretation of fluctuations in abundance through time and obscure the relationship between key environmental drivers and population abundance.

Gone with the wind – Wind speed affects prey accessibility for a High Arctic zooplanktivorous seabird, the little auk Alle alle

Foraging ecology of chick rearing seabirds is affected mainly by the food availability on feeding grounds, but it can be also modulated by environmental conditions during the foraging trip, in that wind force. Considering predicted strengthening of surface winds over the Arctic Ocean, this factor may have a growing impact on the foraging performance of Arctic seabirds. Here, we studied how wind speed could affect prey accessibility for the High Arctic zooplanktivorous seabird, the little auk Alle alle breeding in Svalbard in 2015–2019. First, we estimated availability of its preferred prey, a cold water copepod Calanus glacialis, based on wider-scale mesozooplankton biomass model and environmental conditions. Then we estimated prey accessibility by including wind speed, the factor affecting the flapping flight performance of little auks commuting from/to the colony. Finally, we compared reproductive performance of the little auks (chick diet, growth rate and survival and duration of foraging flights of adults) between the studied years differing in wind and food availability conditions. We found that wind speed could affect significantly food accessibility for a zooplanktivorous seabird. Despite high spatial and temporal variability in prey availability and accessibility in shelf waters of SW Spitsbergen, interannual differences in duration of foraging flights and chick growth rate, little auks were able to sustain high breeding success confirming their capacity to buffer suboptimal foraging conditions. Our multidisciplinary work, combining multi-year remote sensing of oceanographic conditions, zooplankton availability and accessibility modelling, little auks diet composition and chick growth and survival emphasizes the importance of including wind conditions in the studies of foraging ecology of seabirds.

Differential Effects of Food Restriction and Warming in the Two-Spotted Goby: Impaired Reproductive Performance and Stressed Offspring

Climate change is a growing threat to marine organisms and ecosystems, and it is already modifying ocean properties by, for example, increasing temperature and decreasing pH. Increasing water temperature may also lead to an impairment of primary productivity and an overall depletion of available zooplankton. Understanding how the crossover between warming and zooplankton availability impacts fish populations has paramount implications for conservation and mitigation strategies. Through a cross factorial design to test the effects of ocean temperature and food availability in a temperate marine teleost, Pomatochistus flavescens, we showed that hindered feeding impacted sheltering and avoidance behaviour. Also, low food availability impaired fish reproduction, particularly male reproduction, as the expression of cyp11b1, a gene with a pivotal role in the synthesis of the most important fish androgen, 11-ketotestosterone, was significantly reduced under a low food regime. In contrast, temperature alone did not affect reproductive success, but offspring showed increased saturated fatty acid content (embryos) and increased lipid peroxidation (larvae). Altogether, food availability had a stronger effect on fitness, showing that coping with elevated temperatures, an ability that may be expected in shallow-water fish, can be indirectly impacted, or even overwhelmed, by the effects of ocean warming on primary productivity and downstream ecological processes.

Saprolegniasis on the eggs of the common carp (Cyprinus carpio L.) with the occurrence of micropredators at Al-Wahda fish hatchery, south of Baghdad

The aim of the present study is to find out if the occurrence of the zooplankton with the incubated eggs of the common carp (Cyprinus carpio L.) could increase the fungal infection causing saprolegniasis in the main fish hatchery in Iraq during the artificial propagation from April to June 2009. Four eggs treatments were used, the highest hatching rate was 85.8 % in treatment no. 2 in which filtered water was used to incubate the eggs and lowest hatching rate was 21 % in the negative control according to the availability of zooplankton and the absence of fungicidal treatment. Crustacea identified were: Bosmonia, Cyclops, Daphnia, Moina, Nauplii and aquatic insects were: Chironomas, Notonicta, Culex.

Evidence that copepod biomass during the larval period regulates recruitment of Lake Erie walleye

Walleye (Sander vitreus) is an economically and culturally important species in Lake Erie that has experienced large interannual variability in recruitment. We examined the importance of prey biomass during the larval period to walleye recruitment while also considering the importance of temperature. Using nine years of field data over a 22-year period (1994–2016) for larval walleye and zooplankton, we found that strong recruitment events occurred in years when the biomass (dry µg L-1) of copepods (e.g., calanoids, cyclopoids) was greater during the spring larval period. Conversely, the biomass of cladocerans and mean spring water temperatures were poor predictors of walleye recruitment. Our results highlight the need to consider zooplankton availability during the larval period when seeking to understand the recruitment dynamics of freshwater fish populations such as Lake Erie walleye.

Crustacean zooplankton available for larval walleyes in a Lake Michigan embayment

Adequate densities of zooplankton prey are critical for growth and survival of larvae of many fish species. Little information exists on the density of zooplankton in Great Lakes inshore areas during early spring, when larvae of important fishes rely on zooplankton. Reduced age-0 walleye recruitment and the absence of data on zooplankton availability for larval walleyes in northern Green Bay, Lake Michigan, led us to assess zooplankton densities during this critical spring period. We conducted biweekly vertical plankton tows in 2014–2016 near reefs and river plumes used by spawning walleyes for periods when larval walleyes were expected to be relying on zooplankton prey. Densities of zooplankton were well below literature values identified for good growth and survival of larval walleyes, averaging 1.5 individuals L−1 for all taxa and 0.12 individuals L−1 for large-bodied taxa across all sites and sampling dates. Various factors could contribute to the low density of zooplankton observed. We found low but significantly higher densities of cyclopoid copepods, nauplii, Bosmina, and total zooplankton at river mouth sites compared to open water sites. These results suggest that food availability for larval walleye in our study area was severely limiting which is consistent with the paucity of strong year classes observed since 2000. We suspect northern Green Bay has limited potential for producing strong year classes of walleyes under such conditions. Fishery managers working in unproductive waters should consider assessing the zooplankton community during critical periods to identify potential bottlenecks to reproductive success and larval fish survival.

Dynamic spawning patterns in the California market squid (Doryteuthis opalescens) inferred through paralarval observation in the Southern California Bight, 2012–2019

AbstractThe California market squid is ecologically and economically important to the California Current Ecosystem and coastal communities. However, the population undergoes periodic and large‐scale fluctuations on the order of magnitudes, largely as a result of warm ocean temperature and reduced ocean productivity related to cycles in El Niño Southern Oscillation. These fluctuations can lead to cascading trophic effects on the ecosystem, market uncertainty, and substantial revenue losses for fishing communities. In order to investigate the role that spawning behavior may play in mitigating the detrimental environmental effects on the population as well as allowing the species to capitalize during favorable ocean conditions, surveys for market squid paralarvae were conducted three times per hatching season from 2012 to 2019. The specific objectives were to (a) assess whether there was a large‐scale synchronous peak in paralarval abundance each season, (b) investigate whether market squid paralarvae were found at certain locations in similar abundances throughout the duration of a hatching season, and (c) to understand the influence of oceanographic parameters, such as zooplankton availability, sea surface temperature, and chl‐a on paralarvae abundance during each hatching season. Adaptable spawning strategies that varied over time were observed. Market squid displayed large‐scale synchronous spawning during cool and productive oceanographic conditions; protracted spawning was observed during warm and oligotrophic conditions. No overall site fidelity was observed across all effort, but during two seasons where habitat compression was possible, market squid consistently spawned in certain areas. Generalized additive models were used to explore the effects of a small set of oceanographic variables on paralarval density on a seasonal basis. Sea surface temperature and geographic variables were generally the most important variables. These findings indicate that market squid spawning behaviors and habitat are adaptable and dependent on oceanography and population density.

Feeding habits of the Pacific Bluefin tuna (Thunnus orientalis) larvae in two nursery grounds based on morphological and metagenomic analyses

The feeding ecology of Pacific Bluefin tuna (PBT: Thunnus orientalis) larvae collected from two nursery grounds, namely the Japan Sea and the Nansei area (western North Pacific), was evaluated based on gut content analyses using morphological and metagenetic techniques. The PBT larvae were collected at the surface layer (≥30 m depth) in 2016 and 2017, and 172 and 114 individuals from the Japan Sea and Nansei area, respectively, were analyzed. A generalized additive model applied revealed that the number of prey in the gut increased with growth, in waters above 24 °C and in subsurface (~30 m depth) waters, during daytime. The total dry weights of prey per gut calculated from the size of copepods and cladocerans also increased during daytime and with growth. Both the morphological and metagenetic analyses indicated that the taxonomic composition of gut contents was influenced by the food selectivity of PBT larvae and by zooplankton (prey) availability in the ambient water. Podonidae were positively selected as PBT larvae prey in both areas, and copepodites and adult Corycaeidae and Pontellidae were positively selected in the Nansei area only. A metagenetic approach indicated Calanidae copepods and soft zooplankton such as appendicularians as important preys in both areas. Other copepods were not positively selected using this approach, but Clausocalanidae and Oithonidae were frequently detected in the Japan Sea. This might be related to the higher abundances of these families in the Japan Sea than in the Nansei area. On the other hand, the cladoceran Penilia avirostris and the copepods Echinostomatidae and Oncaeidae were rarely observed in the guts of PBT larvae although were abundant in the water column. Overall, PBT larvae feed abundantly and grow fast in warm and long-day waters rich in Podonidae, Corycaeidae, Pontellidae, and larval copepods, which contributes to their survival and recruitment in the Japan Sea and Nansei area.

Feeding increases the number of offspring but decreases parental investment of Red Sea coral Stylophora pistillata.

Successful reproductive output and recruitment is crucial to coral persistence and recovery following anthropogenic stress. Feeding is known to alter coral physiology and increase resilience to bleaching.The goal of the study was to address the knowledge gap of the influence of feeding on reproductive output and offspring phenotype.Colonies of Stylophora pistillata from the Northern Gulf of Aqaba (Red Sea) were fed an Artemia diet or unfed for 5 months during gametogenesis, fertilization, and brooding. In addition, time to settlement and mortality of planulae were assessed at water temperatures ranging from winter temperature (22°C) to three degrees above average peak summer temperature (31°C). A range of physiological parameters was measured in parents and offspring.In brooding parents, feeding significantly increased protein concentration and more than tripled the number of released planulae. Planulae from unfed colonies had higher chlorophyll per symbiont concentration and concomitantly higher photosynthetic efficiency compared to planulae from fed parents. In settlement assays, planulae showed a similar thermal resistance as known for this Red Sea adult population. Mortality was greater in planulae from unfed parents at ambient and 3°C above ambient temperature despite higher per offspring investment in terms of total fatty acid content. Fatty acid profiles and relative abundances were generally conserved between different fed and unfed colonies but planulae were enriched in monounsaturated fatty acids relative to adults, that is, 16:1, 18:1, 20:1, 22:1, and 24:1 isomers.Ultimately the availability of zooplankton could influence population physiology and recruitment in corals.

Fertilization of Fresh Water Fish Ponds

This document outlines the principles and practices of fertilizing freshwater fish ponds to enhance their productivity. Fertilization, akin to its use in land-based agriculture, can significantly boost the production of small natural food-consuming fish species such as bluegill and golden shiner by increasing the availability of algae and zooplankton. This process not only supports small fish but also benefits larger predatory fish, like largemouth bass, by improving their food supply. Phosphorus is identified as the most critical nutrient, often supplemented with nitrogen for better results. The document discusses the forms and application methods of fertilizers, emphasizing liquid fertilizers for their rapid effectiveness. Practical guidelines for determining the timing and frequency of fertilization based on water temperature and clarity are provided, alongside techniques for applying both liquid and granular fertilizers efficiently. The document also addresses common challenges such as muddy water, aquatic weeds, water exchange, and low alkalinity, and offers solutions for each. Finally, it highlights the importance of a committed, long-term fertilization program to avoid inefficiencies and aquatic weed issues. First published June 1990.

Bottom-up drivers of global patterns of demersal, forage, and pelagic fishes

Large-scale spatial heterogeneity in fisheries production is predominantly controlled by the availability of zooplankton and benthic organisms, which have a complex relationship with primary production. To investigate how cross-ecosystem differences in these drivers determine fish assemblages and productivity, we constructed a spatially explicit mechanistic model of three fish functional types: forage, large pelagic, and demersal fishes. The model is based on allometric scaling principles, includes basic life cycle transitions, and has trophic interactions between the fishes and with their pelagic and benthic food resources. The model was applied to the global ocean, with plankton food web estimates and ocean conditions from a high-resolution earth system model. Further, a simple representation of fishing was included, and led to moderate matches with total, large pelagic, and demersal catches, including re-creation of observed variations in fish catch spanning two orders of magnitude. Our results highlight several ecologically meaningful model sensitivities. First, coexistence between forage and large pelagic fish in productive regions occurred when forage fish survival is promoted via both favorable metabolic allometry and enhanced predator avoidance in adult forage fish. Second, the prominence of demersal fish is highly sensitive to the efficiency of energy transfer to benthic invertebrates. Third, the latitudinal distribution of the total catch is modulated by the temperature dependence of metabolic rates, with increased sensitivity pushing fish biomass toward the poles. Fourth, forage fish biomass is suppressed by strong top-down controls on temperate and subpolar shelves, where mixed assemblages of large pelagic and demersal fishes exerted high predation rates. Last, spatial differences in the dominance of large pelagics vs. demersals is strongly related to the ratio of pelagic zooplankton production to benthic production. We discuss the potential linkages between model misfits and unresolved processes including movement, spawning phenology, seabird and marine mammal predators, and socioeconomically driven fishing pressure, which are identified as priorities for future model development. Ultimately, the model and analyses herein are intended as a baseline for a robust, mechanistic tool to understand, quantify, and predict global fish biomass and yield, now and in a future dominated by climate change and improved fishing technology.

The vertical distribution of maraena whitefish (Coregonus maraena) early juveniles in different times of day in a newly created oligotrophic lake

Diel vertical distribution of strictly pelagic juvenile (23–47 mm total length) maraena whitefish Coregonus maraena (Bloch, 1779) was repeatedly investigated in spring primarily using hydroacoustics in the artificial post-mining Most Lake in the Czech Republic. At the same time, an ichthyoplankton trawl was used to identify acoustical targets. During the day, fish performed extensive shoaling behaviour in depths between 2 and approximately 40 m and were not accessible for trawling. By evening, with decreasing light intensity, shoals started to disintegrate and at night fish were relatively homogeneously distributed in the water column from the surface down to a depth of 40 m. Juvenile maraena whitefish could be caught by trawl as the only fish species at night. Shoaling behaviour started again approximately 1.5 h before sunrise. The data showed steep decreases in fish density between the two surveys in spring which indicates significant mortality of early juvenile coregonids as a result of poor availability of zooplankton in a highly oligotrophic post-mining lake.

Neustonic zooplankton in the northeastern Persian Gulf

The Persian Gulf is an important area in terms of ecological values and fisheries resources. It also provides a supportive spawning and nursery areas for the finfish and shellfish communities, whose survival depends on the availability of the neustonic zooplankton. Neuston composition, abundance, and distribution were studied during two oceanographic cruises in autumn (Nov. to Dec.) 2012 and summer (Aug.) 2013 aboard R/V Ja Nay Band from 26 stations along the northeast Persian Gulf. Samples were collected in duplicate using a rectangular frame (300μm mesh net). Copepods (including copepodites) were the dominant constituents of the zooplankton and accounted for 56.7% and 69.2% of the total zooneuston populations during autumn and summer cruises, respectively. Among zooneuston, copepod biodiversity was relatively high with thirty-four copepod species in twenty two genera present. The other major groups of zooplankton in term of abundance were fish larvae and eggs 15.2%, urochordates 10.1%, decapods and other crustaceans 6.2%, cladocerans 3.8%, cnidarians 2.7% and chaetognaths 2.4% during the autumn cruise. In contrast, during the summer cruise, the other most abundant zooneuston were urochordates 7.5%, decapods 6.8%, cnidarians 6.1%, chaetognaths 3.8% and fish larva and eggs 1.5%. Zooneuston composition differed slightly in different cruises. Zooneuston abundance was slightly higher during autumn (mean of 295 ± 61 ind. m−3) than summer (231 ± 61 ind. m−3). Total zooneuston abundances were not significantly different (p > 0.05) in both spatial and temporal scales, but their copepod abundances were spatially significant (p < 0.05). Among marine insects, Halobates sp. was the new record for the Persian Gulf.

Prey selection by larval northern pike (Esox lucius) exposed to different zooplankton assemblages representing seasonally flooded wetland and nearshore bay habitats

AbstractA prey selection experiment was designed to investigate whether northern pike larvae, at swim‐up and advanced stages, select similar zooplankton prey in seasonally flooded wetlands and nearshore littoral zones, which represent primary St. Lawrence River northern pike spawning habitats. At first exogenous feeding, naïve swim‐up larvae exposed to both wetland and nearshore zooplankton were generalist consumers, feeding primarily on cyclopoid copepods and small cladocerans. No positive selection was observed for any prey taxa. Advanced larvae began to exhibit positive selection for large cladocerans. Significantly larger‐sized prey were consumed by advanced larvae given wetland vs. nearshore zooplankton. Large cladocerans made up a greater proportion of the assemblage originating from seasonally flooded wetlands vs. nearshore habitats, and may explain the difference in size selectivity. Our laboratory experiment suggests pike hatched in wetland environments have greater access to preferred, large cladoceran prey, and differences in spatial and temporal spawning distributions may have important implications for zooplankton availability and hence food consumption during the critical larval period.

Early life history of fishes and zooplankton availability in a Neotropical floodplain: predator–prey functional relationships

Early life history of fishes and zooplankton availability in a Neotropical floodplain: predator–prey functional relationships

River flow, zooplankton and dominant zooplanktivorous fish dynamics in a warm-temperate South African estuary.

The possible links between river flow, zooplankton abundance and the responses of zooplanktivorous fishes to physico-chemical and food resource changes are assessed. To this end, the seasonal abundance, distribution and diet of the estuarine round-herring Gilchristella aestuaria and Cape silverside Atherina breviceps were studied in the Kariega Estuary. Spatio-temporal differences were determined for selected physico-chemical variables, zooplankton abundance and zooplanktivorous fish abundance and distribution. Results indicated that, following a river flood event in winter (>30 m3 s-1 ), altered physico-chemical conditions occurred throughout the estuary and depressed zooplankton stocks. Abundance of G. aestuaria was highest in spring, with this species dominant in the upper and middle zones of the estuary, while A. breviceps was dominant in summer and preferred the middle and lower zones. The catch per unit of effort of both zooplanktivores also declined significantly following the flooding, thus suggesting that these fishes are reliant on zooplankton as a primary food source for healthy populations. Copepods dominated the stomach contents of both fish species, indicating a potential for strong interspecific competition for food, particularly in the middle reaches. Temporal differences were evident in dietary overlap between the two zooplanktivorous fish species and were correlated with river flow, zooplankton availability and fish distribution. The findings of this study emphasize the close trophic linkages between zooplankton and zooplanktivorous fishes under changing estuarine environmental conditions, particularly river flow and provide important baseline information for similar studies elsewhere in South Africa and the rest of the world.