Fish Nursery Areas Research Articles

Are vegetated areas more attractive for juvenile fish in estuaries? A comparison in a tropical estuary

Seagrass beds are some of the essential nursery areas for fish, due to the great structural complexity found in these environments. Despite their importance, they are among the most threatened coastal ecosystems and their influence on fish assemblages is still poorly studied. Therefore, this study aimed to compare the composition and structure of fish assemblages found in vegetated and unvegetated areas in a tropical estuary. Samples were taken in two areas with seagrass (Halodule wrightii) and two where this vegetation is absent (tidal flats), observing variations in the hydrological regime between the dry and rainy seasons of 2014. A total of 86 species were recorded, of which 11 occurred only in vegetated areas and 31 only in the unvegetated regions. No difference was found in diversity between vegetated and unvegetated areas, but the values of abundance and biomass were higher in unvegetated areas. Thus, the environmental characteristic of unvegetated areas proved to be a major factor in determining the biomass and richness patterns. Analysis of the abiotic data set indicated that temperature and dissolved oxygen are the variables that act as environmental filters in the temporal differentiation of the structure of fish assemblages, while salinity and total dissolved solids are related to variation between seagrass beds and tidal flats. In addition, our results show that these areas are used by the species in different stages of their ontogenetic development, which may be linked to differences related to the availability of food resources and the possibility of refuge from predation.

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

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

Using spatial data analysis for delineating existing mangroves stands and siting suitable locations for mangroves plantation

Mangroves protect shorelines from waves and floods, help prevent coastal erosion by stabilizing sediments with their tangled root systems, serve as valuable nursery areas for fish and invertebrates. In addition, mangroves can play an important role in protecting the land from future seawater rise.Small mangrove stands are distributed along the Red Sea coast in Egypt as small patches, rarely exceed few square kilometers. These stands have been rapidly destroyed in recent years due to the rapid development tourist activities and old running problem of over grazing and using the trees as fuel.The current research aimed at delineating the existing mangroves stands within the area between El Quseir – Marsa Alam and finding the most suitable locations to plant mangroves trees. Sharm El-Bahari was found to be the healthiest mangroves stands in the area. By interpreting the satellite images and field survey the four environmental factors controlling the growing of mangroves: physical and chemical properties of seawater, soil properties and coastal geomorphology. These factors were measured at Sharm El-Bahari site and considered the best environmental boundaries suitable for mangroves growing in the study area. With these values in mind, the sites suitable for mangroves plantation were located where these environmental requirements were met. Six sites suitable for mangroves plantation were identified; these sites were ranked using Analytic Hierarchy Process (AHP).

The role of seagrass meadows, mangrove forests, salt marshes and reed beds as nursery areas and food sources for fishes in estuaries

Each year millions of larval and 0+ juvenile fishes are recruited into estuarine fish populations around the world. For several decades the roles of littoral aquatic and emergent macrophyte habitats as nursery areas for many of these species have been studied and debated at length. This review attempts to collate the published literature and provide a synopsis of the varying, and sometimes conflicting, views on this topic. A large number of studies have shown that a range of species and an abundance of juvenile fishes are associated with littoral macrophytes in estuaries, some of which are found almost exclusively within particular plant habitats. Other studies have shown the movement of certain juvenile fishes from one type of littoral plant habitat to another as they grow and develop new feeding strategies and dietary requirements. Overall, it would appear that seagrass beds and mangrove forests are particularly favoured by fishes as nursery areas in both estuaries and the nearshore marine environment, and that the loss of these habitats leads to a decline in juvenile fish diversity and abundance. Salt marshes and reed beds generally have a lower diversity of fishes than seagrass and mangrove habitats, possibly due to the more temperate location of salt marshes and the dense structure of some reed beds. Stable isotope studies in particular are providing increasing evidence that carbon assimilated by juvenile fishes in mangrove, marsh and reed habitats is not primarily derived from these macrophytes but comprises a mixture of these sources and a diverse range of macro- and microalgae, particularly epiphytic, epipsammic, epipelic and epilithic diatoms and algae found in these areas. The closest trophic link between the macrophyte food chain and associated fishes occurs in seagrass habitats where a significant portion of the overall macrophyte leaf biomass often consists of epiphytic algae and diatoms. Structurally, mangrove forests, salt marshes and reed beds provide more substantial and complex habitats for juvenile fish refuge, but some of these habitats are constrained with regard to nursery provision by being fully exposed at low tide. Under such circumstances the small fish are sometimes forced into creeks and channels where larger piscivorous fishes are often present. Overall, in terms of a broad ranking of the four habitats as potential fish nursery areas, seagrass meadows are ranked first, followed by mangrove forests, salt marshes and then reed beds. This ranking does not imply that the lower ranked habitats are unimportant, since these plants perform a myriad of ecosystem services that are not related to the provision of fish nursery areas, e.g. bank stabilization. It is also emphasized that the protection of specific plant species should not be encouraged because it is important to have an ecosystem approach to conservation so that the diversity of habitats and their connectivity for fishes is maintained.

Seasonal composition, abundance and biomass of the subestuarine fish assemblage in Solís Chico (Río de la Plata estuary, Uruguay).

The large estuaries can present long narrow branches called subestuaries or tidal creeks. These types of subsystems are distributed along the Uruguayan coast of the Río de la Plata estuary and are very important as nursery and refuge areas for fish. For the first time, the seasonal composition and abundance of the fish community of the Solís Chico subestuary was studied by using beach and gill nets. Fourteen species, mainly euryhaline (86%) presented a significant representation of juvenile stages. The fish community was dominated by Odontesthes argentinensis, Platanichthys platana, Mugil liza, Brevoortia aurea, Micropogonias furnieri and Paralichthys orbignyanus, similar to adjacent subestuaries. While Micropogonias furnieri and B. aurea were the most abundant species, some other species were rarely caught. A seasonal variation of the fish assemblage abundance was detected, with higher values in autumn showing a positive correlation with temperature. Species that complete their life cycle in the Río de la Plata estuary, some of which are relevant to fisheries (64% of the analyzed species) were captured in the Solís Chico subestuary. The importance of this environment as a transitional system for some estuarine fish species is advised.

Habitat partitioning by juvenile fishes in a temperate estuarine nursery, South Africa

Multiple habitats were investigated in a known fish nursery area to further understand habitat partitioning among juveniles in the lower reaches of the warm temperate, permanently open Swartkops Estuary, South Africa. Fishes were collected using a 30-m seine net with a mesh size of 10 mm in sand, mud, creek and vegetated habitat types. Each habitat type was sampled in two locations twice per season from February 2013 to January 2014. Shallow-water creeks and vegetated habitats with coverage of Zostera capensis and Spartina maritima were found to be important use areas for numerous solely estuarine and marine estuarine-dependent species. This was evidenced by the high species diversity, abundance and size range per species occurring in these habitats. Seasonal trends were similar to those in previous studies worldwide, where higher abundances of juveniles of marine estuarine-dependent species coincided with summer recruitment into estuarine nurseries. However, recruitment appears to begin as early as late winter in some species, a phenomenon probably linked to a warming climate. Both resident species and those utilizing the area as a nursery area show a large degree of plasticity in habitat use in the lower reaches of the estuary, which became apparent when multiple habitats were compared. The drivers of these patterns involve a complex interaction of species, habitat type, behaviour, feeding, predator avoidance and physico-chemical factors occurring in the estuary.

A low-cost drone based application for identifying and mapping of coastal fish nursery grounds

Acquiring seabed, landform or other topographic data in the field of marine ecology has a pivotal role in defining and mapping key marine habitats. However, accessibility for this kind of data with a high level of detail for very shallow and inaccessible marine habitats has been often challenging, time consuming. Spatial and temporal coverage often has to be compromised to make more cost effective the monitoring routine. Nowadays, emerging technologies, can overcome many of these constraints. Here we describe a recent development in remote sensing based on a small unmanned drone (UAVs) that produce very fine scale maps of fish nursery areas. This technology is simple to use, inexpensive, and timely in producing aerial photographs of marine areas. Both technical details regarding aerial photos acquisition (drone and camera settings) and post processing workflow (3D model generation with Structure From Motion algorithm and photo-stitching) are given. Finally by applying modern algorithm of semi-automatic image analysis and classification (Maximum Likelihood, ECHO and Object-based Image Analysis) we compared the results of three thematic maps of nursery area for juvenile sparid fishes, highlighting the potential of this method in mapping and monitoring coastal marine habitats.

Eight decades of sampling reveal a contemporary novel fish assemblage in coastal nursery habitats.

In order to adequately monitor biodiversity trends through time and their responses to natural or anthropogenic impacts, researchers require long time series that are often unavailable. This general lack of datasets that are several decades or longer makes establishing a background or baseline of diversity metrics difficult - especially when attempting to understand species composition changes against a backdrop of climate and ecological variability. Here, we present an analysis of a community of juvenile nearshore fishes based on nearly 8 decades of highly standardized Norwegian survey records. Using multivariate statistical techniques, we: (i) characterize the change in taxonomic community composition through time, (ii) determine whether there has been an increase in warm-water affinity species relative to their cold water affinity counterparts, and (iii) characterize the temporal change in the species' functional trait assemblage. Our results strongly indicate a shift toward a novel fish assemblage between the late 1990s and 2000s. The context of changes within the most recent two decades is in stark contrast to those during the 1960s and 1970s, but similar to those during the previous warm period during the 1930s and 1940s. This novel assemblage is tightly linked to the warming temperatures in the region portrayed by the increased presence of warm-water species and a higher incidence of pelagic, planktivorous species. The results indicate a clear influence of ocean temperature on the region's juvenile fish community that points to climate-mediated effects on the species assemblages of an important fish nursery area.

Seasonal variation in species composition and abundance of demersal fish and invertebrates in a Seagrass Natural Reserve on the eastern coast of the Shandong Peninsula, China

Seagrass habitats are structurally complex ecosystems, which support high productivity and biodiversity. In temperate systems the density of seagrass may change seasonally, and this may influence the associated fish and invertebrate community. Little is known about the role of seagrass beds as possible nursery areas for fish and invertebrates in China. To study the functioning of a seagrass habitat in northern China, demersal fish and invertebrates were collected monthly using traps, from February 2009 to January 2010. The density, leaf length and biomass of the dominant seagrass Zostera marina and water temperature were also measured. The study was conducted in a Seagrass Natural Reserve (SNR) on the eastern coast of the Shandong Peninsula, China. A total of 22 fish species and five invertebrate species were recorded over the year. The dominant fish species were Synechogobius ommaturus, Sebastes schlegelii, Pholis fangi, Pagrus major and Hexagrammos otakii and these species accounted for 87% of the total number of fish. The dominant invertebrate species were Charybdis japonica and Octopus variabilis and these accounted for 98% of the total abundance of invertebrates. There was high temporal variation in species composition and abundance. The peak number of fish species occurred in August–October 2009, while the number of individual fish and biomass was highest during November 2009. Invertebrate numbers and biomass was highest in March, April, July and September 2009. Temporal changes in species abundance of fishes and invertebrates corresponded with changes in the shoot density and leaf length of the seagrass, Zostera marina.

Bayesian spatio-temporal approach to identifying fish nurseries by validating persistence areas

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 528:245-255 (2015) - DOI: https://doi.org/10.3354/meps11281 Bayesian spatio-temporal approach to identifying fish nurseries by validating persistence areas Iosu Paradinas1,*, David Conesa1, M. Grazia Pennino2, Facundo Muñoz3, Angel M. Fernández4, Antonio López-Quílez1, José María Bellido4 1Departament d’Estadística i Investigació Operativa, Universitat de València, C/ Dr. Moliner 50, Burjassot, 46100 Valencia, Spain 2Institut de Recherche pour le Développement (IRD), UMR EME 212 (IRD/Ifremer/Université Montpellier 2), Centre de Recherche Halieutique Méditerranéenne et Tropicale, Avenue Jean Monnet, B.P. 171, 34203 Sète cedex, France 3Institut National de la Recherche Agronomique (INRA) - Centre Val de Loire Unité Amélioration, Génétique et Physiologie Forestières, 2163 Avenue de la Pomme de Pin, CS 40001 ARDON, 45075 Orleans Cedex 2, France 4Instituto Español de Oceanografía, Centro Oceanográfico de Murcia, C/ Varadero 1, San Pedro del Pinatar, 30740 Murcia, Spain *Corresponding author: paradinas.iosu@gmail.com ABSTRACT: Spatial and temporal closures of fish nursery areas to fishing have recently been recognized as useful tools for efficient fisheries management, as they preserve the reproductive potential of populations and increase the recruitment of target species. In order to identify and locate potential nursery areas for spatio-temporal closures, a solid understanding of species-environment relationships is needed, as well as spatial identification of fish nurseries through the application of robust analyses. One way to achieve knowledge of fish nurseries is to analyse the persistence of recruitment hotspots. In this study, we propose the comparison of different spatio-temporal model structures to assess the persistence of a spatial process. In particular, we apply our approach to a 2-stage Bayesian hierarchical spatio-temporal model that describes both the occurrence and the abundance of European hake Merluccius merluccius recruits in the western Mediterranean Sea. Results clearly show areas of high occurrence and abundance, mainly along the shelf break and the upper slope of the Spanish Mediterranean coast. Understanding the distributional patterns associated with key life stages such as recruitment is essential for appropriate spatial management, including the implementation of Fisheries Restricted Areas and/or Marine Protected Areas that improve the management of fishery resources. KEY WORDS: Nursery areas · Distribution pattern · Ecosystem approach · Spatial fisheries management · Spatio temporal analysis · Bayesian hierarchical modelling · Merluccius merluccius Full text in pdf format PreviousNextCite this article as: Paradinas I, Conesa D, Pennino MG, Muñoz F, Fernández AM, López-Quílez A, Bellido JM (2015) Bayesian spatio-temporal approach to identifying fish nurseries by validating persistence areas. Mar Ecol Prog Ser 528:245-255. https://doi.org/10.3354/meps11281 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 528. Online publication date: May 28, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

The seascape of demersal fish nursery areas in the North Mediterranean Sea, a first step towards the implementation of spatial planning for trawl fisheries.

The identification of nursery grounds and other essential fish habitats of exploited stocks is a key requirement for the development of spatial conservation planning aimed at reducing the adverse impact of fishing on the exploited populations and ecosystems. The reduction in juvenile mortality is particularly relevant in the Mediterranean and is considered as one of the main prerequisites for the future sustainability of trawl fisheries. The distribution of nursery areas of 11 important commercial species of demersal fish and shellfish was analysed in the European Union Mediterranean waters using time series of bottom trawl survey data with the aim of identifying the most persistent recruitment areas. A high interspecific spatial overlap between nursery areas was mainly found along the shelf break of many different sectors of the Northern Mediterranean indicating a high potential for the implementation of conservation measures. Overlap of the nursery grounds with existing spatial fisheries management measures and trawl fisheries restricted areas was also investigated. Spatial analyses revealed considerable variation depending on species and associated habitat/depth preferences with increased protection seen in coastal nurseries and minimal protection seen for deeper nurseries (e.g. Parapenaeus longirostris 6%). This is partly attributed to existing environmental policy instruments (e.g. Habitats Directive and Mediterranean Regulation EC 1967/2006) aiming at minimising impacts on coastal priority habitats such as seagrass, coralligenous and maerl beds. The new knowledge on the distribution and persistence of demersal nurseries provided in this study can support the application of spatial conservation measures, such as the designation of no-take Marine Protected Areas in EU Mediterranean waters and their inclusion in a conservation network. The establishment of no-take zones will be consistent with the objectives of the Common Fisheries Policy applying the ecosystem approach to fisheries management and with the requirements of the Marine Strategy Framework Directive to maintain or achieve seafloor integrity and good environmental status.

Pools and rapids as spawning and nursery areas for fish in a river stretch without floodplains

This study aimed to evaluate the importance of two environments situated in the main channel of the Peixe River (a tributary of the upper Uruguay River) on fish reproduction and initial growth. Ichthyoplankton, macrozooplankton, and zoobenthos collections were taken on a monthly basis from October 2011 to March 2012, sampling a rapids and a pool environment. The instrument used for the capture of the ichthyoplankton in both environments was a light trap. In total, 795 eggs and 274 larvae were captured. The species that presented higher abundance and occurrence frequency out of the total captured in both environments were Leporinus obtusidens, Bryconamericus iheringii, and Bryconamericus stramineus. The evaluation of the feeding activity reveals a major repletion degree of the larvae in more advanced stages in the pool. The pool environment presented a higher abundance of larvae in more advanced development stages. We conclude that the channel of the Peixe River is important for the reproduction and initial growth of fish and that each river environment seems to fulfill a different role in the life cycle of the ichthyoplankton community.

Assessing the nursery role for marine fish species in a hypersaline coastal lagoon (Mar Menor, Mediterranean Sea)

The shallow inshore areas of coastal lagoons function as nursery grounds for the early life stages of marine fish. Multi-species approaches and the assessment of several population variables are of great value in studies focused upon the ecological importance of estuarine systems as fish nursery areas. Therefore, the objective of this study was to analyse the spatial differences in the abundance, biomass, growth and condition indexes of juveniles from three marine species: Sparus aurata, Liza aurata and Liza ramada, in order to assess the nursery role of the Mar Menor coastal lagoon for these species. Results showed high spatial variability of the studied variables, with higher abundance and biomass values at those sites closest to the inlet connecting the lagoon with the Mediterranean Sea. Moreover, there were significant relationships between the growth and condition of the fish with hydrographical circulation patterns within the lagoon, indicating the suitability of the Mar Menor as a nursery area for the three studied fish species.

Reproductive biology and recruitment of the deep-sea fish community from the NW Mediterranean continental margin

Temporal patterns in deep-sea fish reproduction are presently unknown for the majority of deep continental margins. A series of seasonal trawling surveys between depths of 300 to 1750m in the Blanes submarine canyon and its adjacent open slope (NW Mediterranean) were conducted. The bathymetric size distributions and reproductive cycles of the most abundant species along the NW Mediterranean margin were analyzed to assess the occurrence of (i) temporal patterns in reproduction (i.e., spawning season) along a bathymetric gradient and (ii) preferential depth strata for recruitment. The fish assemblages were grouped in relation to their bathymetric distribution: upper slope, middle slope and lower slope species. Middle-slope species (i.e., 800–1350m) showed short (i.e., highly seasonal) reproductive activity compared to the upper (300–800m) and lower (1350–1750m) ones. Our results, together with those previously published for megabenthic crustacean decapods in the area, suggest a cross-phyla depth-related trend of seasonality in reproduction. In the middle and lower slope species, the reproductive activity reached a maximum in the autumn–winter months and decreased in the spring. The observed seasonal spawning patterns appear to be ultimately correlated with changes in the downward transport of organic particles and with seasonal changes in the physicochemical characteristics of the surrounding water masses. The distribution of juveniles was associated with the bathymetric stratum where intermediate nepheloid layers interact with the continental margins, indicating that this stratum acts as a deep-sea fish nursery area.

Investigating the association of fish abundance and biomass with cold-water corals in the deep Northeast Atlantic Ocean using a generalised linear modelling approach

Cold-water corals (CWC) can form complex three-dimensional structures that can support a diverse macro- and megafaunal community. These reef structures provide important biogenic habitats that can act as refuge, feeding, spawning and nursery areas for fish. However, quantitative data assessing the linkage between CWC and fish are scarce. The North Atlantic Ocean is a key region in the worldwide distribution of Lophelia pertusa, which is thought to be the most widespread frame-work forming cold-water coral species in the world. This study examined the relationship between fish and CWC reefs in the northeast Atlantic Ocean by means of video and remotely sensed data from three different CWC communities (Rockall Bank, Hatton Bank and the Belgica Mound Province). Using a tethered camera system, 37 transects were recorded during a period of 8 years. Fish–coral association was investigated using a generalised linear modelling (GLM) approach. Overall, Lepidion eques was the most abundant fish species present (143ind.ha−1). Other common species were Sigmops bathyphilus (17ind.ha−1), Synaphobranchus kaupii (15ind.ha−1), Helicolenus dactylopterus (16ind.ha−1) and Mora moro (7ind.ha−1). The highest fish biomass was measured for Lophius piscatorius (26.3kgha−1). Other species with a high biomass were Helicolenus dactylopterus (4.3kgha−1), Lepidion eques (13.2kgha−1) and Mora moro (7.8kgha−1). Overall, no significant difference in fish abundance and biomass was found at coral framework habitats compared to non-coral areas. The relationship between fish and coral framework varied among fish species and study site. Fish count and length modelling results showed that terrain variables explain a small proportion of the variation of our data. Depth, coral-framework and terrain rugosity were generally the most important explanatory variables, but this varied with species and study site.

Identification of fish nursery areas in a free tributary of an impoundment region, upper Uruguay River, Brazil

This study aims to determine the importance of different environments of the Ligeiro River (upper Uruguay River, Brazil) in fish reproduction. For this purpose, three environments (sampling sites) were selected: rapids, a pool, and the mouth of the Ligeiro River. Ichthyoplankton, zooplankton, and benthos were sampled six times per month from September, 2006 to March, 2007. Zooplankton and ichthyoplankton samples were collected early in the evening with plankton nets (64 µm and 500 µm, respectively). Benthos samples were also collected early in the evening with a Van Veen dredge. Local abiotic variables (temperature, dissolved oxygen, pH, electrical conductivity, water speed, alkalinity, water hardness, and water transparency) were measured simultaneously with the biotic data sampling and were complemented by regional variables (water flow and precipitation). A total of 43,475 eggs and 2,269 larvae were captured. Of these larvae, 80.1% were in the pre-flexion and larval yolk stages. Digestive tract content showed that the greatest degree of repletion among the larvae in more advanced phases occurred in the pool environment. Water speed was the main characteristic used to differentiate the river's rapids and mouth from the pool. The abundance of zooplankton and benthos was not related to the distribution of densities among the different components of the ichthyoplankton. A greater abundance of eggs and larvae with yolk was found in the rapids and river mouth. Ordination analyses showed a connection between the advanced stage larvae and the pool environment. In conclusion, the rapids and river mouth of the Ligeiro River's are important locations for fish reproduction, particularly in regard to spawning and drifting of the ichthyoplankton's initial stages, whereas the pool represents a nursery place for larval growth.

Influences of wave fetch, tidal flow and ocean colour on subtidal rocky communities

Community assemblages on subtidal rock change markedly along gradients of wave energy, tidal flow, and turbidity. The importance of these assemblages for rare and delicate spe- cies, for shellfish, as nursery areas for fish, and for their contribution to ecosystem functioning in coastal areas has prompted much conservation effort in many countries. I applied a rapid method of calculating a large high-resolution (200 m scale) map of wave exposure 40% cover had a median of 22 species. Laminaria hyper- borea, and the most diverse communities, is found in areas of estimated low chlorophyll concen- trations and in the most wave-exposed environments, which are often but not always in areas of high tidal flow.

Diet of largemouth bass, Micropterus salmoides (Centrarchidae), an invasive alien in the lower reaches of an Eastern Cape river, South Africa

Largemouth bass (Micropterus salmoides) have been introduced to many South African river systems where they become invasive and pose a threat to native biota. The diets of small (32–138 mm TL) and large (192–448 mm TL) sized bass were analysed and compared in a marine fish nursery area in the lower Kowie River on the warm temperate coast of South Africa over a one-year period from March 2009 to February 2010. Dietary differences were detected between the two size groups. Amphipod sp. (% index of relative importance (IRI) = 69.2) and dipterans (Insecta) (%IRI = 21.9) dominated gut contents of small bass while larger bass preyed mostly on odonates (Insecta) (%IRI = 16.3) and the brachyuran Potamonautes sidneyi (%IRI = 80.0). Fish prey was of low importance during this study but comparisons with previous work on the lower Kowie River showed that when the river is flowing, young marine fish recruiting into the freshwater from the estuary become important prey items. These data suggest that in the lower Kowie River bass utilize invertebrate prey at low fish prey abundance and opportunistically feed on migrant fish when these are available.

Diet of Largemouth Bass,Micropterus salmoides(Centrarchidae), an Invasive Alien in the Lower Reaches of an Eastern Cape River, South Africa

Largemouth bass (Micropterus salmoides) have been introduced to many South African river systems where they become invasive and pose a threat to native biota. The diets of small (32–138 mm TL) and large (192–448 mm TL) sized bass were analysed and compared in a marine fish nursery area in the lower Kowie River on the warm temperate coast of South Africa over a one-year period from March 2009 to February 2010. Dietary differences were detected between the two size groups. Amphipod sp. (% index of relative importance (IRI) = 69.2) and dipterans (Insecta) (%IRI = 21.9) dominated gut contents of small bass while larger bass preyed mostly on odonates (Insecta) (%IRI = 16.3) and the brachyuran Potamonautes sidneyi (%IRI = 80.0). Fish prey was of low importance during this study but comparisons with previous work on the lower Kowie River showed that when the river is flowing, young marine fish recruiting into the freshwater from the estuary become important prey items. These data suggest that in the lower Kowie River bass utilize invertebrate prey at low fish prey abundance and opportunistically feed on migrant fish when these are available.

Food Web Structure and Habitat Connectivity in Fish Estuarine Nurseries—Impact of River Flow

River flow variability is known to influence estuarine production, yet knowledge on its effect upon estuarine food webs dynamics is still scarce. Stable carbon and nitrogen isotopes were used to assess the effect of river flow in the connectivity and food web interactions between the two main fish nursery areas of the Tagus estuary. The aims of the present work were to investigate the seasonal variation in food web structure and the exchange rate of individuals of marine juvenile fish among estuarine nurseries, to compare the spring of a rainy year (2001) with that of an average year (2000), and to investigate the impact of the winter floods of 2001. A low level of connectivity was observed for the fish species that use these areas as nurseries. In low river flow conditions, two isotopically distinct food webs were established in each nursery area. These food webs were very sensitive to small variations in the freshwater input. Winter floods seem to disrupt the localized food webs that are established in low river flow periods, leading to the re-establishment of a wider food web. While in rainy years this wide food web is maintained until spring, in average years the food web undergoes fragmentation into two localized and isotopically distinctive food webs. The increase in frequency of droughts due to climate change should lower the connectivity of the estuarine fish nurseries food webs, causing habitat fragmentation and consequent loss in complexity and resilience.