Kátya G. Abrantes

Residency and Spatial Use by Reef Sharks of an Isolated Seamount and Its Implications for Conservation

Although marine protected areas (MPAs) are a common conservation strategy, these areas are often designed with little prior knowledge of the spatial behaviour of the species they are designed to protect. Currently, the Coral Sea area and its seamounts (north-east Australia) are under review to determine if MPAs are warranted. The protection of sharks at these seamounts should be an integral component of conservation plans. Therefore, knowledge on the spatial ecology of sharks at the Coral Sea seamounts is essential for the appropriate implementation of management and conservation plans. Acoustic telemetry was used to determine residency, site fidelity and spatial use of three shark species at Osprey Reef: whitetip reef sharks Triaenodon obesus, grey reef sharks Carcharhinus amblyrhynchos and silvertip sharks Carcharhinus albimarginatus. Most individuals showed year round residency at Osprey Reef, although five of the 49 individuals tagged moved to the neighbouring Shark Reef (∼14 km away) and one grey reef shark completed a round trip of ∼250 km to the Great Barrier Reef. Additionally, individuals of white tip and grey reef sharks showed strong site fidelity to the areas they were tagged, and there was low spatial overlap between groups of sharks tagged at different locations. Spatial use at Osprey Reef by adult sharks is generally restricted to the north-west corner. The high residency and limited spatial use of Osprey Reef suggests that reef sharks would be highly vulnerable to targeted fishing pressure and that MPAs incorporating no-take of sharks would be effective in protecting reef shark populations at Osprey and Shark Reef.

Site fidelity and sex-specific migration in a mobile apex predator: implications for conservation and ecosystem dynamics

Combining movement behaviour with other ecological information of predators and their prey is essential for an adequate understanding of ecosystem dynamics. The movement patterns of broadnose sevengill sharks, Notorynchus cepedianus, were monitored with acoustic and satellite technology in coastal areas of southeast Tasmania, Australia. Individuals were tagged in two habitats (Norfolk Bay and the Derwent Estuary) for which we had ecological information such as diet, population structure and abundance. Notorynchus cepedianus showed seasonal site fidelity in the use of the coastal habitats. The general pattern was for sharks to exit coastal areas over winter and females to return the following spring and males in summer. Their movement into these coastal areas coincided with high seasonal abundance of their known prey species during summer, suggesting feeding site fidelity. Individuals tagged in two coastal areas showed low spatial and dietary overlap, suggesting localized site fidelity and fine spatial scale resource partitioning. This has rarely been reported for large mobile predators. Both satellite and acoustic methods showed that males make northerly migrations during winter to distances of at least 1000 km. The combined use of tracking, diet and abundance information demonstrated that N. cepedianus are likely to exert significant predation pressure on prey inhabiting these areas during summer. Overall, this study highlights the benefit of complementing movement data with other ecological information to understand the habitat use of large mobile predators and their potential influences on ecosystem structure and function.

Stable isotope‐based community metrics as a tool to identify patterns in food web structure in east African estuaries

Summary 1. Quantitative tools to describe biological communities are important for conservation and ecological management. The analysis of trophic structure can be used to quantitatively describe communities. Stable isotope analysis is useful to describe trophic organization, but statistical models that allow the identification of general patterns and comparisons between systems/ sampling periods have only recently been developed. 2. Here, stable isotope-based Bayesian community-wide metrics are used to investigate patterns in trophic structure in five estuaries that differ in size, sediment yield and catchment vegetation cover (C3/C4): the Zambezi in Mozambique, the Tana in Kenya and the Rianila, the Betsiboka and Pangalanes Canal (sampled at Ambila) in Madagascar. 3. Primary producers, invertebrates and fish of different trophic ecologies were sampled at each estuary before and after the 2010–2011 wet season. Trophic length, estimated based on d 15 N, varied between 36 (Ambila) and 47 levels (Zambezi) and did not vary seasonally for any estuary. Trophic structure differed the most at Ambila, where trophic diversity and trophic redundancy were lower than at the other estuaries. Among the four open estuaries, the Betsiboka and Tana (C4-dominated) had lower trophic diversity than the Zambezi and Rianila (C3-dominated), probably due to the high loads of suspended sediment, which limited the availability of aquatic sources. 4. There was seasonality in trophic structure at Ambila and Betsiboka, as trophic diversity increased and trophic redundancy decreased from the prewet to the postwet season. For Ambila, this probably resulted from the higher variability and availability of sources after the wet season, which allowed diets to diversify. For the Betsiboka, where aquatic productivity is low, this was likely due to a greater input of terrestrial material during the wet season. 5. The comparative analysis of community-wide metrics was useful to detect patterns in trophic structure and identify differences/similarities in trophic organization related to environmental conditions. However, more widespread application of these approaches across different faunal communities in contrasting ecosystems is required to allow identification of robust large-scale patterns in trophic structure. The approach used here may also find application in comparing food web organization before and after impacts or monitoring ecological recovery after rehabilitation.

Fine-Scale Movements of the Broadnose Sevengill Shark and Its Main Prey, the Gummy Shark

Information on the fine-scale movement of predators and their prey is important to interpret foraging behaviours and activity patterns. An understanding of these behaviours will help determine predator-prey relationships and their effects on community dynamics. For instance understanding a predators movement behaviour may alter pre determined expectations of prey behaviour, as almost any aspect of the preys decisions from foraging to mating can be influenced by the risk of predation. Acoustic telemetry was used to study the fine-scale movement patterns of the Broadnose Sevengill shark Notorynchus cepedianus and its main prey, the Gummy shark Mustelus antarcticus, in a coastal bay of southeast Tasmania. Notorynchus cepedianus displayed distinct diel differences in activity patterns. During the day they stayed close to the substrate (sea floor) and were frequently inactive. At night, however, their swimming behaviour continually oscillated through the water column from the substrate to near surface. In contrast, M. antarcticus remained close to the substrate for the entire diel cycle, and showed similar movement patterns for day and night. For both species, the possibility that movement is related to foraging behaviour is discussed. For M. antarcticus, movement may possibly be linked to a diet of predominantly slow benthic prey. On several occasions, N. cepedianus carried out a sequence of burst speed events (increased rates of movement) that could be related to chasing prey. All burst speed events during the day were across the substrate, while at night these occurred in the water column. Overall, diel differences in water column use, along with the presence of oscillatory behaviour and burst speed events suggest that N. cepedianus are nocturnal foragers, but may opportunistically attack prey they happen to encounter during the day.

Importance of terrestrial subsidies for estuarine food webs in contrasting East African catchments

Little is known on the degree to which terrestrial organic matter delivered to tropical estuaries contributes to estuarine consumers. Here, stable isotope analysis is used to constrain this contribution for contrasting east African estuaries whose catchments differ in relative C3/C4 vegetation cover. As these two types of vegetation differ strongly in δ13C, we anticipated that terrestrial subsidies would be reflected in a gradient in estuarine consumer δ13C values, following the relative importance of C3 (characterised by low δ13C) vs. C4 (characterised by high δ13C) cover. Five estuaries were sampled for aquatic biogeochemical parameters, primary producers and consumers of different trophic ecologies: the Zambezi (catchment with a C3/C4 cover of 61/39%) in Mozambique, the Tana in Kenya (36/64%) and the Betsiboka (42/58%), Rianila (85/15%) and Canal des Pangalanes (C3-dominated) in Madagascar. Sampling was done before and after the 2010/2011 wet season. There were positive relationships between the proportion of C4 cover in the catchment and turbidity, δ13CDIC, δ13CDOC, δ13CPOC and δ15NPN. There were also significant positive relationships between δ13CPOC and consumer δ13C and between δ15NPN and consumer δ15N for all consumer trophic guilds, confirming the incorporation of organic material transported from the catchments by estuarine consumers, and implying that this material is transported up to high trophic level fish. Bayesian mixing models confirmed that C4 material was the most important source for the highly turbid, C4-dominated estuaries, contributing up to 61–91% (95% CI) to phytodetritivorous fish in the Betsiboka, whereas for the less turbid C3-dominated estuaries terrestrial subsidies were not as important and consumers relied on a combination of terrestrial and aquatic sources. This shows that the ecology of the overall catchment affects the estuaries at the most basic, energetic level, and activities that alter the turbidity and productivity of rivers and estuaries can affect food webs well beyond the area of impact.

Fish fauna of dry tropical and subtropical estuarine floodplain wetlands

Estuarine floodplain wetland pools occur adjacent to marine coasts and estuaries throughout the world. In Australia’s dry tropics and sub-tropics, low and irregular rainfall means estuarine wetland pools are isolated for much of the time, resulting in varied within-pool conditions, with chemistry ranging from fresh to hypersaline, depending on the balance between freshwater and marine inputs and the time between connections. Varied physical conditions and irregular connectivity provide the potential for substantial faunal difference among pools. The present study compares the compositions and structures of the fish fauna of a broad cross section of estuarine wetland pools adjacent to the estuary of the Fitzroy River, one of the largest rivers in Australia’s dry tropical/subtropical zone. Ten pools were sampled between February 2004 and May 2005. The total species richness was low, with the 6123 fish recorded over the study, comprising only 44 species. This low species richness was reflected at the individual pool level, with a maximum total richness of 25 species in a single pool. Different pools had faunas that differed as a function of the proportion of the community comprised of marine spawned, compared with freshwater spawned, species. This was a reflection of the extent of connectivity to freshwater and marine systems, which determined both the physical nature of pools and the sources of faunal supply. Despite faunal differences among pools, at a functional level pool fish faunas were dominated by detritivores, regardless of pool type, size, season or connectedness.

Long-term changes in species composition and relative abundances of sharks at a provisioning site.

Diving with sharks, often in combination with food baiting/provisioning, has become an important product of today’s recreational dive industry. Whereas the effects baiting/provisioning has on the behaviour and abundance of individual shark species are starting to become known, there is an almost complete lack of equivalent data from multi-species shark diving sites. In this study, changes in species composition and relative abundances were determined at the Shark Reef Marine Reserve, a multi-species shark feeding site in Fiji. Using direct observation sampling methods, eight species of sharks (bull shark Carcharhinus leucas, grey reef shark Carcharhinus amblyrhynchos, whitetip reef shark Triaenodon obesus, blacktip reef shark Carcharhinus melanopterus, tawny nurse shark Nebrius ferrugineus, silvertip shark Carcharhinus albimarginatus, sicklefin lemon shark Negaprion acutidens, and tiger shark Galeocerdo cuvier) displayed inter-annual site fidelity between 2003 and 2012. Encounter rates and/or relative abundances of some species changed over time, overall resulting in more individuals (mostly C. leucas) of fewer species being encountered on average on shark feeding dives at the end of the study period. Differences in shark community composition between the years 2004–2006 and 2007–2012 were evident, mostly because N. ferrugineus, C. albimarginatus and N. acutidens were much more abundant in 2004–2006 and very rare in the period of 2007–2012. Two explanations are offered for the observed changes in relative abundances over time, namely inter-specific interactions and operator-specific feeding protocols. Both, possibly in combination, are suggested to be important determinants of species composition and encounter rates, and relative abundances at this shark provisioning site in Fiji. This study, which includes the most species from a spatially confined shark provisioning site to date, suggests that long-term provisioning may result in competitive exclusion among shark species.

Nursery ground value of an endangered wetland to juvenile shrimps

Although urban development impacts wetlands around the world, until now there have been no studies of its effects on coastal wetlands in tropical regions of developed countries such as Australia. In fact the ecological value of such wetlands is poorly understood. This study provides an initial step in evaluating the ecological value of urbanised, tropical coastal wetlands by determining (a) the extent to which a remnant wetland, in a highly urbanised estuary in northern Australia, is used by juvenile commercial penaeid shrimps, and (b) the extent to which the shrimps rely on food chains based on wetland plants versus marine based food chains. Juvenile penaeids were abundant in the 11 wetland pools sampled. Catches included 5 commercial penaeids with two species, Fenneropenaeus merguiensis and Metapenaeus bennetae, comprising half the catch. Densities in the wetland pools were usually substantially higher than in adjacent estuarine habitats. Stable isotope analysis indicated that much of the nutrition of juvenile shrimps was supplied by marine primary producers (phytoplankton, epiphytic and epilithic algae, microphytobenthos, green filamentous algae) however the locally abundant saltmarsh grass Sporobolus virginicus was also a major contributor. In contrast, there was little indication of nutritional support from mangrove carbon. The lack of importance of mangrove carbon is surprising because the catches of F. merguiensis are often closely linked to the area or extent of mangroves, suggesting that aspects of mangrove systems other than the supply of mangrove carbon may determine the distribution of juvenile F. merguiensis.

Sources of nutrition supporting juvenile penaeid prawns in an Australian dry tropics estuary

Prawn fisheries are among the main sources of income in several tropical countries, where juveniles of many species inhabit estuarine wetlands. Although plants in these wetlands are considered to be essential food sources for juvenile prawns, some studies suggest that wetland producers are of limited importance. In the present study, δ13C and δ15N were used to identify differences in diet between penaeid species and size classes, and to determine if terrestrial wetland producers are important for nutrition. Two estuarine floodplain pools were sampled: one surrounded by mangroves and one surrounded by salt marsh. There were differences in diet between species and size classes. As mangrove δ13C (–29.7 to –26.3‰) was very different from salt marsh δ13C (–16.3 to –15.4‰), the importance of these producers was examined by comparing the isotopic composition of the prawns between sites and by using the IsoSource model. Although aquatic sources were the most important, salt marsh grass was also a significant contributor, supporting the hypothesis that these wetland producers are important for juvenile prawn nutrition. There was no evidence that mangrove material was of major importance for any species, suggesting that mangrove productivity is not the primary reason for the occurrence of penaeid prawns in mangrove habitats.

Are Large Herbivores Vectors of Terrestrial Subsidies for Riverine Food Webs

The tropical savannas of Africa have witnessed a dramatic reduction in native large mammalian herbivore populations. The consequences of these changes for terrestrial-aquatic food-web linkages are poorly documented. We used natural abundances of stable carbon and nitrogen isotopes (δ13C, δ15N) to determine spatial and temporal patterns in the importance of herbivore-mediated subsidies for consumers in the Mara River, Kenya. Potential primary producers (terrestrial C3 and C4 producers and periphyton) and consumers (invertebrates and fish) were collected during dry and wet seasons from different sites along the river, representing a gradient from forested highlands to natural savanna grasslands with high herbivore densities across mixed agricultural and livestock-dominated zones. Bayesian mixing models were used to estimate the relative contributions of terrestrial and algal sources of organic carbon supporting consumer trophic groups. Organic carbon sources differed for consumer groups and sites and with season. Overall, periphyton was the major energy source for most consumer groups during the dry season, but with wide 95% confidence intervals. During the wet season, the importance of terrestrial-derived carbon for consumers increased. The importance of C3 producers declined from 40 and 41% at the forested upper reaches to 20 and 8% at river reaches receiving hippo inputs during the dry and wet seasons, respectively. The reciprocal increase in the importance of C4 producers was higher than expected based on areal cover of riparian vegetation that was mainly C3. The importance of C4 producers notably increased from 18 and 10% at the forested upper reaches to 33 and 58% at river reaches receiving hippo inputs during the dry and wet seasons, respectively. This study highlights the importance of large herbivores to the functioning of riverine ecosystems and the potential implications of their loss from savanna landscapes that currently harbor remnant populations. Although the importance of C4 terrestrial carbon in most river systems has been reported to be negligible, this study shows that its importance can be mediated by large herbivores as vectors, which enhance energetic terrestrial-aquatic linkages in rivers in savanna landscapes.