Alexandre Wiefels

Bio: Alexandre Wiefels is an academic researcher from University of La Réunion. The author has contributed to research in topics: Caranx & Galeocerdo. The author has an hindex of 1, co-authored 1 publications receiving 6 citations.

Dynamics of marine predators off an oceanic island and implications for management of a preventative shark fishing program

Abstract: Oceanic islands are productive ecosystems, and so have higher densities of many marine predators. We investigated the dynamics of elasmobranch and teleost predators in coastal waters off Reunion Island, Indian Ocean, using fisheries-independent data from a preventative shark fishing program from January 2014 to December 2019. We developed a moonlight index that calculates exact moonlight through incorporating lunar azimuth, elevation angle and island topography. We quantified spatial–temporal and environmental drivers of occurrence using zero-inflated mixed models and assessed species-specific catchability in the program. A consistent segregated pattern was observed with higher occurrence of all species at dusk and after-dusk associated with lower luminosity. Scalloped hammerhead sharks (Sphyrna lewini) and giant trevally (Caranx ignobilis) were found to patrol coastal waters earlier in the day than the other species. Tiger (Galeocerdo cuvier) and bull (Carcharhinus leucas) sharks showed high spatial segregation, potentially reducing competition. Teleost predators were found more frequently inside the coral reef environment of the Marine Protected Area but there was no clear pattern for sharks. Seasonality was observed for giant trevally, stingrays, bull sharks, and giant guitarfish (Rhynchobatus australiae), with higher presence during early winter periods related to turbidity, photosynthetically available radiation, and temperature. Inter-annual variation in catch rates suggested that juvenile tiger sharks might be replacing bull sharks in nearshore habitats, and the consequences for mitigation of shark bite hazard are discussed. Operational alternatives are proposed to enhance reducing the impacts of preventative shark fishing upon critically endangered species, improve their conservation and ensure local ecosystem balance.

Forecasting intraspecific changes in distribution of a wide-ranging marine predator under climate change

Abstract: Globally, marine animal distributions are shifting in response to a changing climate. These shifts are usually considered at the species level, but individuals are likely to differ in how they respond to the changing conditions. Here, we investigate how movement behaviour and, therefore, redistribution, would differ by sex and maturation class in a wide-ranging marine predator. We tracked 115 tiger sharks (Galeocerdo cuvier) from 2002 to 2020 and forecast class-specific distributions through to 2030, including environmental factors and predicted occurrence of potential prey. Generalised Linear and Additive Models revealed that water temperature change, particularly at higher latitudes, was the factor most associated with shark movements. Females dispersed southwards during periods of warming temperatures, and while juvenile females preferred a narrow thermal range between 22 and 23 °C, adult female and juvenile male presence was correlated with either lower ( 23 °C) temperatures. During La Nina, sharks moved towards higher latitudes and used shallower isobaths. Inclusion of predicted distribution of their putative prey significantly improved projections of suitable habitats for all shark classes, compared to simpler models using temperature alone. Tiger shark range off the east coast of Australia is predicted to extend ~ 3.5° south towards the east coast of Tasmania, particularly for juvenile males. Our framework highlights the importance of combining long-term movement data with multi-factor habitat projections to identify heterogeneity within species when predicting consequences of climate change. Recognising intraspecific variability will improve conservation and management strategies and help anticipate broader ecosystem consequences of species redistribution due to ocean warming.

Environmental Drivers of Fine-Scale Predator and Prey Spatial Dynamics in Sydney Harbour, Australia, and Adjacent Coastal Waters

Abstract: Greater Sydney is the largest coastal city in Australia and is where bull sharks (Carcharhinus leucas) are present every summer and autumn. A decade of acoustic telemetry data was used to identify drivers of space use for bull sharks and their potential prey, according to standardised 6-h intervals using dynamic Brownian bridge movement models. Influences of environmental, physical, and biological variables on the areas of space use, location, and predator–prey co-occurrence were investigated with generalised additive mixed models. Rainfall in the catchment affected space use for all animals (i.e. teleost species and both sexes of sharks), with varying temporal responses. Male sharks responded most promptly to high rainfall moving upstream in 30 m). Shark size influenced overlap between sexes, with smaller females less likely to co-occur with larger males (~ 50 cm). Variability in space use suggests spatial segregation by sex and size in bull sharks, with individuals targeting similar prey, yet either in different areas or at different times, ultimately enabling them to exploit different resources when in the same habitats.

The challenges of independence: ontogeny of at-sea behaviour in a long-lived seabird

Abstract: The transition to independent foraging represents an important developmental stage in the life cycle of most vertebrate animals. Juveniles differ from adults in various life history traits and tend to survive less well than adults in most long-lived animals. Several hypotheses have been proposed to explain higher mortality including that of inadequate/inferior foraging skills compared to adults, young naive individuals combining lack of experience and physical immaturity. Thus a change in behaviour, resulting in an improvement of skills acquired from growing experience, is expected to occur during a period of learning through the immaturity phase. Very few studies have investigated the ontogeny of foraging behaviour over long periods of time, particularly in long-lived pelagic seabirds, due to the difficulty of obtaining individual tracking data over several years. We investigated the foraging behaviour, through activity patterns, during the successive life stages of the endangered Amsterdam albatross by using miniaturized activity loggers on naive juveniles, immatures and adults. Juvenile naive birds during their first month at sea after leaving their colony exhibited lower foraging effort (greater proportion of time spent sitting on water, higher duration and more numerous bouts on water, and lower duration and less numerous flying bouts). Juveniles reached similar activity values to those of immatures and adults as early as the 2nd-3rd months since independence, suggesting a progressive improvement of foraging performances during the first two months since fledging. We found support for the body-size hypothesis with respect to sex differences in activity parameters according to time elapsed since departure from the colony and month of the year, consistent with the important sexual dimorphism in the Amsterdam albatross. Whatever the life stage considered, activity parameters exhibited temporal variability reflecting the modulation of foraging behaviour possibly linked to both extrinsic (i.e. environmental conditions such as variability in food resources or in wind) and intrinsic (i.e. energetic demands linked to plumage renew during moult) factors.