48 research outputs found
Estimating the value of tropical coastal wetland habitats to fisheries: caveats and assumptions
Habitat valuation can provide an objective basis for the prioritisation of conservation and restoration
actions. The attribution of fisheries production to particular habitat units is an important
measure of value, but is difficult to estimate. Using the case study of habitat use by
juvenile banana prawns in a tropical estuary, we assessed the potential to produce valid
value estimates at two spatio-conceptual scales: estuary reach and whole estuary. Additionally,
we also explore the potential to produce meaningful value estimates at the scale of
whole estuary contribution to the offshore fisheries stock. A diversity of potential and actual
sources of error and logical problems means that quantification at any scale is at best of
uncertain validity and produces estimates that are likely to produce unreliable results if
treated as quantitative inputs to production models. Estimates for the whole estuary were
the most viable, although still requiring substantial assumptions that may or may not be reasonable
in particular situations. Estimates for individual habitats required the unreasonable
assumption of limited prawn movement, while estimates of contribution of an estuary to the
fishery required difficult-to-obtain and usually unavailable information. Because low occupancy
habitats can have trophic value, we also used stable isotope analysis to assess the
importance of mangroves and saltmarshes to prawn nutrition. No particular habitat was of
critical trophic importance, again suggesting that the habitat-production link is most usefully
assessed at the whole-of-estuary scale. While valuable and required to support targeted
ecosystem management and protection and restoration efforts, valid estimates of the contribution
of particular units to fisheries are likely to be unachievable in many situations
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
A global perspective on the trophic geography of sharks
Carbon isotopic analysis reveals global biogeographic traits in shark trophic interactions, and sheds light on the diverse foraging behaviour of sharks
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 predator's movement behaviour may alter pre determined expectations of prey behaviour, as almost any aspect of the prey's 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
A global perspective on the trophic geography of sharks
Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.This research was conducted as part of C.S.B.’s Ph.D dissertation, which was funded by the University of Southampton and NERC (NE/L50161X/1), and through a NERC Grant-in-Kind from the Life Sciences Mass Spectrometry Facility (LSMSF; EK267-03/16). We thank A. Bates, D. Sims, F. Neat, R. McGill and J. Newton for their analytical contributions and comments on the manuscripts.Peer reviewe
Intrapopulation variations in diet and habitat use in a marine apex predator, the broadnose sevengill shark Notorynchus cepedianus
Intrapopulation differences in diet and/or movement are important for understanding the role mobile predators play in different systems. However, ecological studies traditionally overlook individual differences. δ13C and δ15N were used in conjunction with diet and movement information to identify intrapopulation differences in diet and movement patterns of the apex predator broadnose sevengill shark Notorynchus cepedianus in southeast Tasmania. Sevengill samples from 3 inshore and 3 offshore sites were collected, and δ13C and δ15N compared between sites, sizes and sexes. Individuals captured offshore had lower δ15N than those captured inshore, indicating some degree of spatial segregation. Sevengills also had variable δ13C and δ15N within coastal habitats, suggesting intrapopulation differences in diet or migration schedules. In comparison to their main prey, most individuals had δ15N lower than expected for a top predator, also suggesting that they do not reside permanently in these areas, as their tissue was not in isotopic equilibrium with their known prey. This is in agreement with tracking data that showed seasonal use of coastal areas, with most animals leaving for the colder months but returning the following year. There was also a group of females with relatively high δ13C that suggests greater association to coastal habitats, again in agreement with tracking data, as some tagged females remained in the coastal areas over winter. Overall, together with diet and tracking information, results indicate that there are differences in movement and possibly diet in this sevengill population. This multi-methods approach allowed a better understanding of sevengill ecology than the use of any one of the techniques alone
Correction:Â Estimating the value of tropical coastal wetland habitats to fisheries: Caveats and assumptions.
[This corrects the article DOI: 10.1371/journal.pone.0215350.]
Deluge Inlet, a pristine small tropical estuary in north-eastern Australia
Deluge Inlet is a small, tide-dominated estuary on Australia's north-east tropical coast, located in the central part of the Hinchinbrook Island National Park, Australia's largest island National Park. It is situated in Australia's humid tropical zone, and experiences an intense summer wet season and regular impacts of tropical cyclones. Protection by National Parks, World Heritage and Wild Rivers legislation means it remains in near pristine condition. Deluge Inlet sports substantial biodiversity in the form of extensive mangrove forests, seagrass beds, and complex marine mammal, reptile, fish and invertebrate assemblages, all supported by a mosaic of highly interconnected habitat types. The mix of habitats and rich biodiversity makes Deluge Inlet an important nursery for many species, and supports complex food webs. Current threats are from increasing fishing and boating pressure, and effective governance will be needed to ensure Deluge Inlet remains in near-pristine condition into the future
You are what you eat: examining the effects of provisioning tourism on shark diets
Wildlife tourism is a growing industry, with significant conservation and socio-economic benefits. Concerns have however been raised about the possible impacts of this industry on the long-term behaviour, health and fitness of the animal species tourists come to see (the target species), particularly when those species are regularly fed to improve the tourism experience. Information on the contribution of food rewards to the diet of the target species at feeding sites is critical to assess the dependency on handouts and to identify possible health/fitness problems that might be associated, if handouts become the main part of animals' diets. Here, we use stable isotopes (δ13C and δ15N) to evaluate the importance of handouts for a marine predator, the bull shark (Carcharhinus leucas), at a feeding site (Fiji) where shark feeds occur 5 days/week and sharks (up to 75 individuals/dive) are fed ~200 kg of tuna heads/day. There was no evidence of incorporation of food provided, even for individuals that regularly consume food rewards. Results, when combined with those from previous studies on bull shark movements and feeding rates at our study site, show that current levels of provisioning likely have no long-term impacts on bull shark diet or behaviour. This study also demonstrates the applicability of stable isotope analysis to assess and monitor the contribution of food rewards to wildlife, and highlights the benefits of using multi-sources of information to gain a holistic understanding of the effects of provisioning predators
Habitat-specific food webs and trophic interactions supporting coastal-dependent fishery species: an Australian case study
Coastal ecosystems such as estuaries, tidal wetlands and shallow coastal waters are often highly productive and provide important habitats to many recreationally and commercially important fish and invertebrates that use these areas as nursery, feeding and/or reproduction grounds. The diversity of coastlines found worldwide results in differences in types of provisioning and function, and in community structure and trophic organisation. Since almost all coastal fishery species require particular components of the seascapes during specific stages of their life-cycles, it is important to understand the way fish use different habitats throughout ontogeny. Access to rich feeding environments is a key contributor to habitat value, and so knowledge on food webs and feeding relationships, and how these vary over space and time, is central to understanding the importance of the different coastal environments. However, the functional roles of the different habitats in supporting fishery species are still not well understood for most regions. In this study, we review and discuss the available literature to identify key knowledge gaps in the understanding of habitat- and context-specific food webs and trophic interactions supporting fisheries species relying on coastal ecosystems. We use Australia and Australian fisheries species as a case-study, as Australia's extensive coastline encompasses many of the coastal ecosystems and habitats found globally. Given the ever increasing transformation of coastal landscapes by either direct human action or by sea level rise and changing climate, these knowledge gaps need to be urgently addressed for appropriate management and mitigation of various impacts