Natural or Artificial? Habitat-Use by the Bull Shark, Carcharhinus leucas

BACKGROUND: Despite accelerated global population declines due to targeted and illegal fishing pressure for many top-level shark species, the impacts of coastal habitat modification have been largely overlooked. We present the first direct comparison of the use of natural versus artificial habitats for the bull shark, Carcharhinus leucas, an IUCN ‘Near-threatened’ species - one of the few truly euryhaline sharks that utilises natural rivers and estuaries as nursery grounds before migrating offshore as adults. Understanding the value of alternate artificial coastal habitats to the lifecycle of the bull shark is crucial for determining the impact of coastal development on this threatened but potentially dangerous species. METHODOLOGY/FINDINGS: We used longline surveys and long-term passive acoustic tracking of neonate and juvenile bull sharks to determine the ontogenetic value of natural and artificial habitats to bull sharks associated with the Nerang River and adjoining canals on the Gold Coast, Australia. Long-term movements of tagged sharks suggested a preference for the natural river over artificial habitat (canals). Neonates and juveniles spent the majority of their time in the upper tidal reaches of the Nerang River and undertook excursions into adjoining canals. Larger bull sharks ranged further and frequented the canals closer to the river mouth. CONCLUSIONS/SIGNIFICANCE: Our work suggests with increased destruction of natural habitats, artificial coastal habitat may become increasingly important to large juvenile bull sharks with associated risk of attack on humans. In this system, neonate and juvenile bull sharks utilised the natural and artificial habitats, but the latter was not the preferred habitat of neonates. The upper reaches of tidal rivers, often under significant modification pressure, serve as nursery sites for neonates. Analogous studies are needed in similar systems elsewhere to assess the spatial and temporal generality of this research

Reef-fidelity and migration of tiger sharks, Galeocerdo cuvier, across the Coral Sea

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in PLoS One 9 (2014): e83249, doi:10.1371/journal.pone.0083249.Knowledge of the habitat use and migration patterns of large sharks is important for assessing the effectiveness of large predator Marine Protected Areas (MPAs), vulnerability to fisheries and environmental influences, and management of shark–human interactions. Here we compare movement, reef-fidelity, and ocean migration for tiger sharks, Galeocerdo cuvier, across the Coral Sea, with an emphasis on New Caledonia. Thirty-three tiger sharks (1.54 to 3.9 m total length) were tagged with passive acoustic transmitters and their localised movements monitored on receiver arrays in New Caledonia, the Chesterfield and Lord Howe Islands in the Coral Sea, and the east coast of Queensland, Australia. Satellite tags were also used to determine habitat use and movements among habitats across the Coral Sea. Sub-adults and one male adult tiger shark displayed year-round residency in the Chesterfields with two females tagged in the Chesterfields and detected on the Great Barrier Reef, Australia, after 591 and 842 days respectively. In coastal barrier reefs, tiger sharks were transient at acoustic arrays and each individual demonstrated a unique pattern of occurrence. From 2009 to 2013, fourteen sharks with satellite and acoustic tags undertook wide-ranging movements up to 1114 km across the Coral Sea with eight detected back on acoustic arrays up to 405 days after being tagged. Tiger sharks dove 1136 m and utilised three-dimensional activity spaces averaged at 2360 km3. The Chesterfield Islands appear to be important habitat for sub-adults and adult male tiger sharks. Management strategies need to consider the wide-ranging movements of large (sub-adult and adult) male and female tiger sharks at the individual level, whereas fidelity to specific coastal reefs may be consistent across groups of individuals. Coastal barrier reef MPAs, however, only afford brief protection for large tiger sharks, therefore determining the importance of other oceanic Coral Sea reefs should be a priority for future research.Funding was provided by the the Agence Francaise de Développement (http://www.afd.fr), French Pacific Fund, the CRISP program (www.crisponline.info) and QLD Fisheries

Future research directions on the "elusive" white shark

White sharks, Carcharodon carcharias, are often described as elusive, with little information available due to the logistical difficulties of studying large marine predators that make long-distance migrations across ocean basins. Increased understanding of aggregation patterns, combined with recent advances in technology have, however, facilitated a new breadth of studies revealing fresh insights into the biology and ecology of white sharks. Although we may no longer be able to refer to the white shark as a little-known, elusive species, there remain numerous key questions that warrant investigation and research focus. Although white sharks have separate populations, they seemingly share similar biological and ecological traits across their global distribution. Yet, white shark’s behavior and migratory patterns can widely differ, which makes formalizing similarities across its distribution challenging. Prioritization of research questions is important to maximize limited resources because white sharks are naturally low in abundance and play important regulatory roles in the ecosystem. Here, we consulted 43 white shark experts to identify these issues. The questions listed and developed here provide a global road map for future research on white sharks to advance progress toward key goals that are informed by the needs of the research community and resource managers

Habitat features influence catch rates of near-shore bull shark (Carcharhinus leucas) in the Queensland Shark Control Program, Australia 1996–2012

Understanding shark habitat use is vital for informing better ecological management of coastal areas and shark populations. The Queensland Shark Control Program (QSCP) operates over ∼1800 km of Queensland coastline. Between 1996 and 2012, catch, total length and sex were recorded from most of the 1992 bull shark (Carcharhinus leucas) caught on drum lines and gill-nets as part of the QSCP (sex and length was not successfully recorded for all individuals). Gear was set at multiple sites within ten locations. Analysis of monthly catch data resulted in a zero-inflated dataset for the 17 years of records. Five models were trialled for suitability of standardising the bull shark catch per unit effort (CPUE) using available habitat and environmental data. Three separate models for presence-absence and presence-only were run and outputs combined using a delta-lognormal framework for generalized linear and generalized additive models. The delta-lognormal generalized linear model approach resulted in best fit to explain patterns in CPUE. Greater CPUE occurred on drum lines, and greater numbers of bull sharks were caught on both gear types in summer months, with tropical sites, and sites with greater adjacent wetland habitats catching consistently more bull sharks compared to sub-tropical sites. The CPUE data did not support a hypothesis of population decline indicative of coastal overfishing. However, the total length of sharks declined slightly through time for those caught in the tropics; subtropical catches were dominated by females and a large proportion of all bull sharks caught were smaller than the size-at-maturity reported for this species. These factors suggest that growth and sex overfishing of Queensland bull shark populations may be occurring but are not yet detectable in the available data. The data highlight available coastal wetlands, river size, length of coastline and distance to the 50 m depth contour are important for consideration in future whole of lifecycle bull shark management. As concerns for shark populations grow, there is an increasing requirement to collate available data from control programs, fisheries, ecological and research datasets to identify sustainable management options and enable informed stock assessments of bull shark and other threatened shark species

THE FILE ASSIGNMENT PROBLEM

Climate and weather-based drivers of shark movement are poorly known, yet vital for determining measures for effective conservation of shark populations and the management of shark-human interactions at different time scales. The bull shark, Carcharhinus leucas, an IUCN ‘Near-threatened’ species, is globally distributed in subtropical to tropical regions and is implicated in many attacks on humans because of its euryhaline habitat-use. However, drivers determining rapid transitions of this species among habitats along the freshwater-estuarine-marine continuum are yet to be fully understood. To identify triggers for movement by this species into beach areas we used conditional (binomial and gamma) generalised linear modelling (CGLM) of historical bull shark catches from the Queensland Shark Control Program (QSCP) collected from 1996 to 2007 across 1783 km of the Queensland coastline, Australia. We then compared catches before and after key weather events (such as floods) between 2006 and 2014 and used passive long-term acoustic telemetry to monitor movements of bull sharks into beach areas to test the model predictions. The CGLM showed that bull shark catch (occurrence) in beach areas is driven by rainfall and further influenced by sea surface temperature. Our model suggests that ≥100 mm total rainfall in the catchment associated with each beach area is significantly correlated with increased bull shark catch 1–8 days after the rainfall, a relationship also confirmed by the movements of acoustic tagged sharks between estuarine and beach areas. These trends provide the first predictive relationship for identifying increased risk of bull shark-human interactions in beach areas. They also suggest that the activity patterns of bull sharks are correlated with rainfall and this makes them particularly susceptible to localised, short-term changes in weather and long-term climate change. Keywords: Weather, Climatic variables, Acoustic tags, Human interactio

Proportions of time (± SE) bull sharks were detected in differing salinity (A) and habitats (B).

<p>Proportions of time (± SE) bull sharks were detected in differing salinity (A) and habitats (B).</p

Total length and ontogenetic stage of bull sharks in the Gold Coast System.

<p>Mean (±SE) total length (TL) and range of <i>C. leucas</i> of different ontogenetic stages (N = neonate, J = juvenile, SA = subadult, A = adult) caught on rod and line (n = 8), longlines (n = 49) and in gillnets (n = 9) in the upper and lower areas of the Nerang River (natural habitat) and the upper and lower canals (man-made habitat) from January 2006 to December 2008. (RU = river upper, CU = canal upper, RL = river lower, CL = canal lower). Note: rod and line fishing done sporadically across the entire system and gillnets were only deployed in the upper reaches of the Nerang River. Note: A^# = a putatively pregnant female 300 cm TL.</p

Gold Coast Canal system.

<p>Map illustrating the location of the Nerang River (natural habitat) and Broadwater (in black) and adjoining canal systems (artificial habitat) (in white). Green and grey section of main map illustrates the approximate delineation of low (grey) and high (green) saline areas and upper (grey) and lower (green) river and canals. Numbers indicate the locations of longline surveys (LL), acoustic receivers (AR- highlighted in yellow) and conductivity temperature and depth meters (CTD). Lined area indicates freshwater separated from the artificial and natural habitats by a weir and lock.</p

Catch per unit effort (CPUE) (± SE) for quarterly longlines surveys for bull sharks in the Gold Coast System.

<p>Quarterly surveys for 2006–2008 in natural and artificial habitats. RU = river upper, CU = canal upper, RL = river lower, CL = canal lower.</p