Fish assemblages in seagrass beds are influenced by the proximity of mangrove forests

Mangrove forests and seagrass beds frequently occur as adjacent habitats in the temperate waters of southeastern Australia. At low tide when fish cannot occupy mangroves they might utilise adjacent habitats, including seagrass. We first sampled small fish from seagrass beds close to and far from mangroves in the Pittwater estuary, NSW, Australia. Seagrass beds close to mangroves had a greater density of fish species than beds far from mangroves (close: mean 16.0 species net⁻¹, SE 1.0; far: 13.2, 1.3; P < 0.05). In particular, juvenile fish were in greater densities near to than far from mangroves (close: 5.3, 0.4; far: 3.1, 0.4; P < 0.05). We then sampled the mangrove forests during the high tide and seagrass beds during the low tide, in beds along a continuum of distances from mangroves. Multivariate analysis showed that fish assemblages differed with distance from mangroves, and the differences were attributed to the composition of the fish assemblage (i.e. presence/absence of fish species), not the abundances of individual species. In particular, fish that utilise mangrove forests at high tide were found in greater species densities and species richness in seagrass nearer to mangroves. A negative relationship was found between the density of mangrove-utilising fish species and the distance of the bed from mangroves (R ² = 0.37, P < 0.05). This confirms the important connectivity between mangroves and seagrass for fish in temperate Australian waters.     

Variation in assemblages of fish associated with deep and shallow margins of the seagrass Posidonia australis

Little is known about the fish fauna of deep Posidonia australis seagrass beds in Australia. We investigated associated with the deep and shallow margins of P. australis during two surveys in Jervis Bay, New South Wales. Fish associated with deep and shallow seagrass at each of two areas within a large bed of P. australis were compared over a period of 3 mo (November 1986 to January 1987). This was followed by a larger scale survey in which we compared fish found at the two depths within three large beds of P. australis in Jervis Bay over 2 yr (September 1988 to June 1990). In the smaller scale survey there were marked dissimilarities in relative abhundance and composition of species between samples from deep and shallow P. australis. There were also significant differences in abundance between deep and shallow seagrass on most occasions for the majority of the dominant fish species. As a result of the small-scale survey, we concluded that differences in species composition of fish associated with deep and shallow P. australis may have been due to the presence of species from nearby bare substrate among the sparser leaves of the deep seagrass. The results of the smaller scale survey were not confirmed by the larger survey. The relative abundances of species in samples from deep and shallow P. australis were not dissimilar for all beds. In addition, it was rare for any of the dominant species to have significantly different abundances in deep and shallow seagrass. There was little evidence that the species composition of fish associated with deep P. australis was strongly influenced by the fauna of deep bare substrate. Rather, samples from deep P. australis were far more similar to those from shallow P. australis than to a comparable set of samples collected from deep bare substrata. This study demonstrates how the results and conclusions of small-scale survey work, a feature of many environmental impact asseesments, can be misleading when applied to larger spatial and temporal scales, even within the same bay.     

The role of different types of detached macrophytes in the food and habitat choice of a surf-zone inhabiting amphipod

Allorchestes compressa is the dominant macroinvertebrate species in wrack accumulations on surf zones of south-western Australia. These amphipods were provided with a choice of macrophyte material representing brown and red algae and seagrass in a series of preference experiments in the laboratory and field. Feeding experiments showed that A. compressa exhibited a strong preference for particular types of macrophytes (P < 0.01). Amphipods primarily consumed brown algae, with 69–98% of the biomass of Ecklonia radiata and 64% of the biomass of Sargassum sp. lost over the experiments. This study has shown that the amphipod A. compressa exhibits a clear preference for brown algae over red algae and seagrass as food. In terms of habitat preference, tank experiments using a series of pair-wise comparisons showed that, in the absence of fish predators, A. compressa selected seagrass as its preferred habitat over the other types of wrack (P < 0.001). When satiated or starved, between 68 and 83 and 79 and 98% of amphipods were found in Amphibolis and Posidonia, respectively. In contrast, field-cage experiments revealed that A. compressa preferred either mixed wrack, brown algae or red algae over seagrass as a habitat (P < 0.01). The contrasts between results from the laboratory and field suggest that other factors such as the presence of predators, water flow and light could influence habitat choice in the surf zone. This study shows that allochthonous material transported to surf zones from other habitats therefore play different roles in driving secondary production in this shoreline habitat.     

Impact of predation-disturbance by large epifauna on sediment-dwelling harpacticoid copepods: Field experiments in a subtidal seagrass bed

The hypothesis was tested that predation-disturbance by epibenthic macrofauna affects the abundance of sediment-dwelling harpacticoid copepods. The copepods inhabited a subtidal seagrass (Zostera marina L.) bed in British Columbia, Canada. The response of the harpacticoid community was observed in controlled field experiments in which epibenthic predators-disturbers were excluded from portions of the seagrass bed. Controlled, exclusion-cage (0.8 m² area, 7-mm mesh) experiments were conducted within the seagrass bed from late March/early April to mid-June in both 1986 and 1987. Sampling was conducted biweekly. Exclusion of large epibenthic predators-disturbers had little effect on sediment-dwelling harpacticoid copepod density. Total harpacticoid numbers and abundances of dominant species generally did not increase in the exclusion treatment relative to the control. Shifts in species composition of the harpacticoid community did not occur. The treatment control was adequate in simulating the exclusion cage structure. It appears that large epibenthic predators-disturbers have little impact on the abundance of harpacticoid copepod populations at this study site.     

Patterns of selective predation by juvenile,benthivorous fish on estuarine macrofauna

Benthic feeding on macrofauna was studied in juveniles of the sparids Lithognathus lithognathus and Rhabdosargus holubi in the upper reaches of the Gamtoos Estuary, South Africa. Fish and benthic macrofauna were sampled simultaneously, and the selection of invertebrate prey assessed. Both fish species strongly selected for corophioid amphipods and consumed other benthic taxa in low numbers. R. holubi also exploited aquatic autotrophs, while L. lithognathus had a narrow prey-spectrum, feeding almost exclusively on the tube-dwelling amphipod Grandidierella lignorum. G. lignorum was the most abundant prey species, both in the benthos and the fish's diet. This species also showed prominent behavioural differences between the sexes; males were markedly more active on the sediment surface than females, who rarely left their tubes during the day. Males switched from an infaunal to epifaunal microhabitat in search of receptive females, concurrently increasing their exposure to fish predators. Consequently, L. lithognathus selected significantly more males than female amphipods, causing a marked bias towards females in the sex ratio and age-structure of the amphipod population. Juvenile amphipods were less preyed upon, presumably as a result of lower prey-detection or capture efficiency by the predators. Accepting current notions about predation as an important structuring element for benthic communities, our data also stress the prominence of size-and sex-selective predation in structuring individual prey populations.     

Sponge-feeding by the Caribbean starfish Oreaster reticulatus

The common Caribbean starfish Oreaster reticulatus (Linnaeus) feeds on sponges by everting its stomach onto a sponge and digesting the tissue, leaving behind the sponge skeleton. In the San Blas Islands, Republic of Panama, 54.2% of the 1549 starfish examined from February 1987 to June 1990 at eight sites were feeding, and 61.4% of these were feeding on sponges, representing 51 species. Sponges were fed on disproportionately heavily in comparison to their abundance, which was only 9.7% of available prey. In feeding choice experiments, 736 pieces of 34 species of common sponges from a variety of shallow-water habitats, and also 9 ind of a coral, were offered to starfish in individual underwater cages. Acceptance or rejection of sponge species was unambiguous for 31 of the 34 species, and there was a clear relationship between sponge acceptability and sponge habitat. Starfish ate 16 of 20 species that normally grow only on the reefs, but only 1 of 14 species that live in the seagrass meadows and rubble flats surrounding the reefs. The starfish live in the seagrass meadows and rubble flats, and avoid the reefs, and so the acceptable reef sponges are generally inaccessible until a storm fragments and transports them into starfish habitat. After Huricane Joan washed fragments of reef sponges into a seagrass meadow in October 1988, starfish consumed the edible species. When the seagrass meadow was experimentally seeded with tagged reef sponge fragments in June 1994, O. reticulatus consumed edible species and accumulated in the area seeded. Reef sponges that were living in a seagrass meadow, from which O. reticulatus had been absent for at least 4 yr (from 1978 to 1982), were eliminated when the starfish migrated into the area, and the sponges have been unable to recolonize up to June 1994. O. reticulatus feeding and habitat preferences appear to restrict distributions of many Caribbean reef sponge species to habitats without O. reticulatus and may have exerted significant selective pressure on defences of those sponges that live in O. reticulatus habitats.     

Exploited species impacts on trophic linkages along reef-seagrass interfaces in the Florida Keys

The removal of fish biomass by extensive commercial and recreational fishing has been hypothesized to drastically alter the strength of trophic linkages among adjacent habitats. We evaluated the effects of removing predatory fishes on trophic transfers between coral reefs and adjacent seagrass meadows by comparing fish community structure, grazing intensity, and invertebrate predation potential in predator-rich no-take sites and nearby predator-poor fished sites in the Florida Keys (USA). Exploited fishes were more abundant at the no-take sites than at the fished sites. Most of the exploited fishes were either omnivores or invertivores. More piscivores were recorded at no-take sites, but most (approximately 95%) were moderately fished and unexploited species (barracuda and bar jacks, respectively). Impacts of these consumers on lower trophic levels were modest. Herbivorous and smaller prey fish (< 10 cm total length) densities and seagrass grazing diminished with distance from reefs and were not negatively impacted by the elevated densities of exploited fishes at no-take sites. Predation by reef fishes on most tethered invertebrates was high, but exploited species impacts varied with prey type. The results of the study show that, even though abundances of reef-associated fishes have been reduced at fished sites, there is little evidence that this has produced cascading trophic effects or interrupted cross-habitat energy exchanges between coral reefs and seagrasses.     

Diel variation in the abundance of epifauna associated with seagrasses of the Indian River,Florida, USA

Day-night comparisons of the motile epifauna in the canopy of seagrass beds in the Indian River lagoon, Florida revealed that abundances were not stable over the diel period. Collections were made between February and June, 1982. Higher densities of epifauna occurred on seagrasses at night. For both of the numerically-dominant gastropod and crustacean groups, four of the five top ranked species followed this diel pattern. Crustaceans showed particularly large changes in abundance, averaging a twofold increase in nocturnal samples. Faunal similarity between day and night collections remained high, however, indicating that community structure was preserved during diel abundance changes. Equating epifauna collected from seagrass blades with those associated with the seagrass habitat in general therefore may be erroneous, particularly if collections are made during daylight. Changes in prey availability due to the diel pattern are likely to extend to important community trophic functions.     

Conservation Strategies for Species Affected by Apparent Competition

Apparent competition is an indirect interaction between 2 or more prey species through a shared predator, and it is increasingly recognized as a mechanism of the decline and extinction of many species. Through case studies, we evaluated the effectiveness of 4 management strategies for species affected by apparent competition: predator control, reduction in the abundances of alternate prey, simultaneous control of predators and alternate prey, and no active management of predators or alternate prey. Solely reducing predator abundances rapidly increased abundances of alternate and rare prey, but observed increases are likely short‐lived due to fast increases in predator abundance following the cessation of control efforts. Substantial reductions of an abundant alternate prey resulted in increased predation on endangered huemul (Hippocamelus bisulcus) deer in Chilean Patagonia, which highlights potential risks associated with solely reducing alternate prey species. Simultaneous removal of predators and alternate prey increased survival of island foxes (Urocyon littoralis) in California (U.S.A.) above a threshold required for population recovery. In the absence of active management, populations of rare woodland caribou (Rangifer tarandus caribou) continued to decline in British Columbia, Canada. On the basis of the cases we examined, we suggest the simultaneous control of predators and alternate prey is the management strategy most likely to increase abundances and probabilities of persistence of rare prey over the long term. Knowing the mechanisms driving changes in species’ abundances before implementing any management intervention is critical. We suggest scientists can best contribute to the conservation of species affected by apparent competition by clearly communicating the biological and demographic forces at play to policy makers responsible for the implementation of proposed management actions. Estrategias de Conservación para Especies Afectadas por Competencia Aparente     

Fish Responses to Experimental Fragmentation of Seagrass Habitat

Abstract: Understanding the consequences of habitat fragmentation has come mostly from comparisons of patchy and continuous habitats. Because fragmentation is a process, it is most accurately studied by actively fragmenting large patches into multiple smaller patches. We fragmented artificial seagrass habitats and evaluated the impacts of fragmentation on fish abundance and species richness over time (1 day, 1 week, 1 month). Fish assemblages were compared among 4 treatments: control (single, continuous 9‐m² patches); fragmented (single, continuous 9‐m² patches fragmented to 4 discrete 1‐m² patches); prefragmented/patchy (4 discrete 1‐m² patches with the same arrangement as fragmented); and disturbance control (fragmented then immediately restored to continuous 9‐m² patches). Patchy seagrass had lower species richness than actively fragmented seagrass (up to 39% fewer species after 1 week), but species richness in fragmented treatments was similar to controls. Total fish abundance did not vary among treatments and therefore was unaffected by fragmentation, patchiness, or disturbance caused during fragmentation. Patterns in species richness and abundance were consistent 1 day, 1 week, and 1 month after fragmentation. The expected decrease in fish abundance from reduced total seagrass area in fragmented and patchy seagrass appeared to be offset by greater fish density per unit area of seagrass. If fish prefer to live at edges, then the effects of seagrass habitat loss on fish abundance may have been offset by the increase (25%) in seagrass perimeter in fragmented and patchy treatments. Possibly there is some threshold of seagrass patch connectivity below which fish abundances cannot be maintained. The immediate responses of fish to experimental habitat fragmentation provided insights beyond those possible from comparisons of continuous and historically patchy habitat.     

Environmental determinants of distribution and foraging behaviour of cormorants (Phalacrocorax spp.) in temperate estuarine habitats

The distribution and behaviour of cormorants in estuarine environments was examined on the central coast of New South Wales, Australia, with respect to habitat associations at different spatial scales. No consistent variation in abundance was found for four species of cormorants (great Phalacrocorax carbo, pied P. varius, little black P. sulcirostris, and little pied P. melanoleucos) with state of tide (high and low) and time of day (early, middle, and late) at five estuarine locations. Differences in abundance were found among locations that were not confounded by short-term temporal variation (i.e. time of day and tide). Differences in abundance were detected among habitats (e.g. bays, creeks, and headlands) separated by hundreds of metres to kilometres in different estuaries. Cormorants of all species were rare on the open coast and near the entrance of estuaries. Abundances of cormorants varied greatly within and among creeks, bays, and river channels. Presence of seagrass beds explained much of this variation and most of the cormorants swimming and feeding were found near seagrass. Mapping of seagrass beds and the positions of cormorants at scales of metres to hundreds of metres showed a close relationship between the presence of swimming and roosting beds and the presence of seagrass beds for P. melanoleucos and for P. carbo. We argue that cormorants make decisions to visit particular estuarine habitats, especially those with seagrass, where many types of prey (e.g. fish and crustaceans) are probably most abundant. These choices must be interpreted in the context of decisions that cormorants make on scales of hundreds to thousands of kilometres during periodic excursions to the interior of Australia. Further, environmental threats to seagrass beds could impinge on these mobile visitors to the same extent as on more permanent residents. Received: 14 February 2000 / Accepted: 17 July 2000     

Predation on meiofauna by juvenile fish in a Western Mediterranean flatfish nursery ground

The authors investigated the gut contents of juveniles of three flatfish species (Buglossidium luteum, Arnoglossus thori and A. laterna) collected from Banyuls Bay, Western Mediterranean, from January 1981 through June 1982. Young solenettes, B. luteum, appear to be dependent on meiofauna, particularly harpacticoid copepods, as prey during their first year of benthic life. By comparison, the scaldfish, especially A. laterna, prey primarily upon peracarids from the beginning of their benthic life. Evidence of prey selection exists for the genera Photis (amphipod) and Longipedia (harpacticoid copepod). By regrouping these results with those of Bodiou and Villiers (1978/1979) on the goby Deltentosteus quadrimaculatus, and comparing them with other data from the literature, it appears that the impact of predation by juvenile fish is significant for some meiofaunal species, providing that they are actively selected by the predators, as is Longipedia scotti. Generally, however, predation impact on the meiofauna as a whole is weak.     

Prey selection by the scyphomedusan predator Aurelia aurita

We describe feeding behavior of Aurelia aurita (Linnaeus) using gut content analyses of field-collected specimens and a mesocosm experiment. The field studies were conducted in Narragansett Bay, Rhode Island, USA from March to April 1988, and the mesocosm studies were done at the Marine Ecosystems Research Laboratory at the University of Rhode Island. Patterns of prey selection changed with medusa diameter. Smaller medusae (12 mm diameter) consumed mostly hydromedusan prey whereas larger medusae (up to 30 mm diameter) ingested greater numbers of copepod prey. While larger medusae did feed on copepods, their diet also contained more barnacle nauplii and hydromedusae than expected from the relative abundances of these prey types in plankton samples. A marginal flow mechanism of feeding by A. aurita provided an explanation for the patterns of prey selection we observed in medusae of different sizes and among widely divergent prey types. Our data indicated that large prey, with escape speeds slower than the marginal flow velocities around the bell margins of A. aurita, made up a substantial fraction of the daily ration when they were available. Such prey species may be more important to nutrition than the more abundant copepods and microzooplankton. Successful development of young medusae may depend upon an adequate supply of slowly escaping prey.     

Predation risk assessment by green frog (<Emphasis Type="Italic">Rana clamitans</Emphasis>) tadpoles through chemical cues produced by multiple prey

Many prey assess predation risk through predator chemical cues. Numerous studies have shown that (1) prey sometimes respond to chemical cues produced by heterospecifics and (2) that many species are capable of associative learning. This study extends this research by focusing on predation risk assessment and antipredator behavior in environments containing chemical cues produced by multiple prey species. The results show that green frog (Rana clamitans) tadpoles (1) assess risk from the chemical cue produced during predation by a heterospecific (gray tree frog, Hyla versicolor, tadpoles) and (2) can exhibit similarly strong behavioral responses to a mix of conspecific and heterospecific cues compared to conspecific cue alone, depending on their conditioning environment. I then discuss how the prey choice of the predators and the relative abundances of the prey species should influence the informational value of heterospecific cues.     

Herbivory and detritivory among gammaridean amphipods from a Florida seagrass community

The gammaridean amphipods Cymadusa compta (Smith), Gammarus mucronatus Say, Melita nitida Smith and Grandidierella bonnieroides Stephensen from a seagrass community in the Indian River estuary of Florida (USA) fed variously upon large drift algae, small algae epiphytic on seagrasses and seagrass leaf debris and detritus. Consumption was measured in the laboratory using an index (CI) equivalent to mg ingested mg^-1 amphipod day^-1. Observations revealed that the amphipods fed by macrophagy, an attack upon large algae and seagrass debris, and by microphagy, small particle detritus feeding and scraping of plant surfaces for diatoms and other epiphytic algae. C. compta was a macrophagous feeder with a generalized diet of algae and seagrass debris, preferring epiphytic algae and drift algae at mean rates of 1.10 and 0.87 CI, respectively. Gammarus mucronatus fed upon epiphytes and seagrass debris equally at mean rates of 0.90 and 0.97 CI, respectively. The diet of M. nitida condisted primarily of epiphytes, consumed at an average rate of 1.05 CI. Grandidierella bonnieroides fed in a specialized microphagous manner, grooming plant surfaces for small particle detritus and diatoms at an approximate CI rate of 1.45. Assimilation of plants ingested, as reflected by carbon-14 uptake, varied similarly among the 4 amphipods. Epiphytic algae appeared to be most useful as food, providing means of 41 to 75% carbon-14 uptake as ingesta. Drift algae and seagrass debris were of less value, with means varying between 11 and 24 % of carbon-14 uptake by the amphipods. The data show a pattern of feeding which resembles resource partitioning of food both by size and kind. Other evidence, however, including population limitation by predators and an apparent overabundance of food, indicate that resource partitioning as seen may be an artifact, and one which has no co-evolutionary basis among the present species.Contribution No. 102 of Harbor Branch Foundation, Inc.     

Crustacean epifauna of seagrass and macroalgae in Apalachee Bay,Florida, USA

Epifaunal crustaceans on turtlegrass (Thalassia testudinum) and five dominant macroalgae (Anadyomene stellata, Digenia simplex, Halimeda incrassata, Laurencia poitei and Penicillus lamourouxii) were quantitatively sampled bimonthly over a one-year period from September 1979 to September 1980 in a subtropical seagrass meadow in Apalachee Bay, Florida (northeastern Gulf of Mexico). These plant species exhibited a wide range of morphologies, with surface area-to-biomass ratios differing by over 2.5 times. A similar suite of crustaceans occurred on all macrophytes despite differences in shape or architecture among plant species. Relative abundances of many crustaceans, however, varied among plant hosts. Similarity analysis indicated that the epifaunal associates of T. testudinum were distinct from those of the macroalgae. Species richness was generally higher on turtlegrass than on any of the macroalgae. Abundances of total crustaceans per plant biomass or per plant surface area, on the other hand, were greater on all macroalgal species compared to the seagrass. Abundances (per plant biomass or plant surface area) of 14 of the 16 numerically dominant epifaunal species differed significantly among macrophytes. Twelve of the 16 species had greater abundance on one or more macroalgae, while only two species were more abundant on T. testudinum. Almost half of the dominant species had greatest abundances on the branching red alga L. poitei. Although abundances per plant biomass and plant surface area were greater on macroalgae relative to turtlegrass, densities (individuals per meter square of bottom) of animals associated with T. testudinum were significantly greater than those associated with macroalgae, primarily because of the greater abundance of turtlegrass in the grass bed. Both surface area-to-biomass ratios and degree of branching were poorly correlated with epifaunal abundance and number of species. Neither structural feature is an adequate predictor of faunal abundance and species richness among plant species, especially when macrophytes with very different morphologies are compared.     

Differences in the structures of fish assemblages inThalassia testudinum beds in Guadeloupe,French West Indies,and their ecological significance

The structures of fish assemblages in twoThalassia testudinum beds in Guadeloupe, French West Indies, one adjacent to mangroves and the other adjacent to coral reefs, were compared between January 1983 and May 1984. The aim of the study was to compare the influences of mangroves and coral reefs on the utilization of seagrass beds by fishes through examination of species composition, catch rate, size of fishes and temporal changes. The two fish assemblages were similar in terms of the number of species they had in common (nearly 44% of the total number of species collected) and the great abundance of juveniles. They both comprised species that usually inhabit other habitats, i.e., estuaries, open waters or coral reefs. Estuary-associated species (e.g. Gerreidae) were the most abundant species in the seagrass bed near the mangroves, while small pelagic species (e.g. Clupeidae) were the most abundant species in the seagrass bed near the coral reefs. The seagrass bed near the mangroves was preferentially utilized as a nursery area by small juveniles of various species (e.g. Clupeidae, Sparidae, Gerreidae, and at least one coral reef species,Ocyurus chrysurus). The abundance of these species varied frequently, suggesting successive arrivals and departures of juveniles over time. The seagrass bed near the coral reefs was characteristically utilized by fishes that are more able to avoid predation, i.e., fishes that forage over seagrass beds at night and shelter in or near the coral reefs during the day (large juveniles of coral reef species and adults of schooling pelagic species, respectively). The constant migrations of these fishes between the coral reefs and seagrass beds explained the relative stability of the structure of the fish assemblage in the seagrass bed over time. Thus, the two seagrass beds were not equivalent habitats for fishes. The distinct ecological influences of the mangroves (as a nursery for small juveniles) and coral reefs (as a shelter for larger fishes) on the nearby seagrass beds was clearly reflected by the distinct utilizations of these seagrass beds by fishes.     

The contribution of seagrass beds (<Emphasis Type="Italic">Zostera noltii</Emphasis>) to the function of tidal flats as a juvenile habitat for dominant,mobile epibenthos in the Wadden Sea

Since the substantial loss of subtidal eelgrass (Zostera marina L.) in the 1930s, seagrass beds in the Wadden Sea are limited to the intertidal zone and dominated by Z. noltii Hornem. This study deals with the effect of vegetated tidal flats on quantities of mobile epifauna and proves empirically the function of seagrass canopies as a refuge for marine animals remaining in the intertidal zone at ebb tide. Drop-trap samples were taken in the Sylt-Rømø Bight, a shallow tidal basin in the northern Wadden Sea, on vegetated and unvegetated tidal flats during July and August 2002, and during the entire growth period of Z. noltii from May to September in 2003. The species composition in Z. noltii and bare sand flats showed minor differences since only two isopod species (Idotea baltica and I. chelipes) occurred on Z. noltii flats exclusively. Juvenile shore crabs (Carcinus maenas L.), brown shrimps (Crangon crangon L.) and common gobies (Pomatoschistus microps Krøyer) were also found abundantly on bare sand flats. However, the results showed significantly higher abundances and production of these dominant species on vegetated tidal flats. Additionally, the analyses of faunal size classes indicated higher percentages of small individuals in the seagrass bed during the entire sampling period. Despite drastic diurnal fluctuations of dissolved oxygen at low tide, faunal density in the residual water layer remaining in seagrass canopies at ebb tide was found to be consistently higher than that found in artificially created tide-pool units. Although species composition of mobile epifauna did not basically differ between vegetated and unvegetated tidal flats, Z. noltii beds are considered to contribute quantitatively to the function of tidal flats, as an extended juvenile habitat for some of the most important species of the Wadden Sea food web.Communicated by O. Kinne, Oldendorf/Luhe     

Erratum to: Prey selection by resident common bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida

Prey selection was investigated in wild, resident common bottlenose dolphins, Tursiops truncatus, during the summer months in Sarasota Bay, Florida, USA. Stomach content analyses of 15 dolphins with extensive sighting histories and well-documented distributions were used to determine prey use. Prey availability was assessed by purse seine surveys. We compared the relative abundances of prey available to estimates of prey use at closely matching spatial and temporal scales. G-tests determined that dolphins in this study significantly selected for prey at the species, family, and soniferous/non-soniferous prey levels (G _adj  = 753.98–1,775.93, df = 1–21, p ≤ 0.01). While comprising only 6.3% of the total available prey, soniferous fishes accounted for 51.9% of the total prey consumed. Manly’s standardized forage ratios and 95% Bonferroni confidence intervals determined significant positive selection for soniferous prey and against non-soniferous prey (β^S = 0.9461 vs. β^NS = 0.0539). Dolphins selected against Gerridae, Clupeidae, and Sparidae (β ≤ 0.0014), as well as against all the species within those families (β ≤ 0.0190). It is likely that passive listening for soniferous prey provides an ecological or energetic advantage to cetaceans utilizing this specific foraging technique.     

Relative impact of a seagrass bed and its adjacent epilithic algal community in consumer diets

The aim of this work was to identify and compare, using nitrogen and carbon stable isotope data, the food sources supporting consumer communities in a Mediterranean seagrass bed (Gulf of Calvi, Corsica) with those in an adjacent epilithic alga-dominated community. Isotopic data for consumers are not significantly different in the two communities. Particulate matter and algal material (seagrass epiflora and dominant epilithic macroalgae) appear to be the main food sources in both communities. Generally, the δ¹³C of animals suggests that the seagrass Posidoniaoceanica (L.) Delile represents only a minor component of their diet or of the diet of their prey, but the occurrence of a mixed diet is not excluded. P. oceanica dominates the diet of only of few species, among which holothurians appear as key components in the cycling of seagrass material. Received: 30 July 1999 / Accepted: 17 January 2000