To improve understanding of bull shark Carcharhinus leucas reproductive biology, we analysed reproductive traits from 118 bull sharks caught along Reunion Island coasts (Western Indian Ocean), ...including 16 gravid females. Specific microsatellite loci were used to investigate the frequency of multiple paternity. Males and females reached maturity at c. 234 cm and 257 cm total length (LT), respectively, and litter sizes ranged from 5 to 14 embryos. Analysis of the 16 litters collected in various months of the year indicated that parturition occurs between October and December, with a size at birth c. 60–80 cm LT and that the gestation period is probably c. 12 months. Assuming a 1 year resting period and a period of sperm storage (4–5 months) between mating (in June–September) and fertilisation, the reproductive cycle of bull sharks at Reunion Island would be biennial. At least 56.25% of the litters investigated were polyandrous, sired by 2–5 males. Several males that each sired several litters conceived during the same or distinct mating seasons were detected, suggesting both a seasonal aggregation of sharks to mate and some male fidelity to mating site. Altogether, these findings provide valuable information for both shark risk management and conservation of the species in the Western Indian Ocean.
In response to a spate of shark attacks in Reunion Island in the southwest Indian Ocean since 2011, local authorities developed an experimental shark-control program based on those conducted for ...decades in Australia and South Africa. In order to greatly reduce, if not eliminate, the impact of such a shark fishing program on bycatch and undersized target shark species, the use of conventional “drumlines” was improved by the addition of an innovative “Catch-A-Live”® system, making the drumlines ‘SMART’ (Shark Management Alert in Real Time). This is a real-time strike alert system based on an adapted triggering mechanism, which links the fishing line to a GPS buoy connected to the Iridium satellite. This system alerts fishers on duty via a computer-based communication system within just a few minutes, enabling immediate intervention. Off Reunion Island, up to 20 SMART drumlines (SDLs) were deployed along the west and southwest coast in coastal waters to target bull (Carcharhinus leucas) and tiger sharks (Galeocerdo cuvier) in trials conducted between 2014 and 2017. During 58,770 h of fishing there were 269 catches of more than 14 species, of which 86.9% were retrieved alive. There were marked differences in survival among species. While the most fragile species were the giant trevally (Caranx ignobilis), scalloped hammerhead sharks (Sphyrna lewini) and small carcharhinid species, most of the other bycatch species (stingrays Dasyatis sp., giant guitarfish Rhyncobathus djiddensis, and tawny nurse sharks Nebrius ferrugineus) were generally found alive and in a condition suitable for tag and release. Of the target species, 94.8% of all individuals were found alive. These survival rates are far higher than those of other programs using conventional drumlines in KwaZulu-Natal (South Africa) and Queensland (Australia). There were strong diurnal and lunar catch patterns. This information is invaluable in planning fishing operations to maximise the catch of the target species, while reducing the chances of killing the bycatch. These results highlight the potential for use of SDLs in research programs aimed at tagging large sharks capable of tripping the trigger, especially in situations where catch rates are so low that it is impractical for the fishing vessel to remain at sea for the duration of each fishing operation.
In coastal ecosystems, top predators are exposed to a wide variety of nutrient and contaminant sources due to the diversity of trophic webs within inshore marine habitats. Mercury contamination could ...represent an additional threat to shark populations that are declining worldwide. Here we measured total mercury, carbon and nitrogen isotopes, as well as mercury isotopes, in two co-occurring shark species (the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier) and their potential prey from a coastal ecosystem of the western Indian Ocean (La Réunion Island). Our primary goals were to (i) determine the main trophic Hg sources for sharks and (ii) better characterize their diet composition and foraging habitat. Hg isotope signatures (Δ199Hg and δ202Hg) of shark prey suggested that bull sharks were exposed to methylmercury (MeHg) produced in offshore epipelagic waters, while tiger sharks were exposed to offshore mesopelagic MeHg with additional microbial transformation in slope sediments. Δ199Hg values efficiently traced the ecology of the two predators, demonstrating that bull sharks targeted coastal prey in shallow waters while tiger sharks were mainly foraging on mesopelagic species in the deeper waters of the island slope. Unexpectedly, we found a positive shift in δ202Hg (>1‰) between sharks and their prey, leading to high δ202Hg values in the two shark species (e.g. 1.91 ± 0.52‰ in bull sharks). This large shift in δ202Hg indicates that sharks may display strong MeHg demethylation abilities, possibly reflecting evolutionary pathways for mitigating their MeHg contamination.
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•Mercury isotopes were analyzed in bull and tiger sharks from the Indian Ocean.•Hg MIF signature revealed that bull sharks target coastal prey in shallow water.•Δ199Hg values showed that tiger sharks forage in deeper habitat on the island slope.•δ202Hg shift between prey and predators may indicate MeHg demethylation in sharks.
Recent studies have uncovered mosaic patterns of allometric and isometric growth underlying ontogenetic shifts in the body form of elasmobranch species (shark and rays). It is thought that shifts in ...trophic and spatial ecology through ontogeny drive these morphological changes; however, additional hypotheses relating to developmental constraints have also been posed. The bull shark (Carcharhinus leucas) is a large‐bodied coastal shark that exhibits strong ontogenetic shifts in trophic and spatial ecology. In this study, we utilise a large data set covering a large number of morphological structures to reveal ontogenetic shifts in the body form of C. leucas, stratifying analyses by sex and size classes to provide fine‐scale, more ecomorphologically relevant results. Our results indicate shifts in functional demands across the body through ontogeny, driven by selective pressures relating to trophic and spatial ecology driving the evolution of allometry. We also find significant differences in scaling trends between life stages, and between the sexes, highlighting the importance of utilising large, diverse datasets that can be stratified in this way to improve our understanding of elasmobranch morphological evolution. Ultimately, we discuss the implications of these results for existing ecomorphological hypotheses regarding the evolution of specific morphological structures, and pose novel hypotheses where relevant.
Bull sharks display shifts in functional demands across the body through ontogeny. Morphological scaling differs between life stages and sexes. These results highlight the importance of large sample sizes in studies of shark morphometry.
Blood and feathers are the most targeted tissues for isotopic investigations in avian ecology, primarily because they can be easily and non-destructively sampled on live individuals. Comparing blood ...and feather isotopic ratios can provide valuable information on dietary shifts, trophic specialization and migration patterns, but it requires a good knowledge of the isotopic differences between the two tissues. Here, δsup.13C and δsup.15N values of whole blood (in blood cells of a few species) and simultaneously grown body feathers were measured in seabird chicks to quantify the tissue-related isotopic differences. Seabirds include 27 populations of 22 wild species that were sampled in 2000-2008, and a review of the literature added 8 groups (including adult birds) to the analysis. The use of a large data set that overall encompasses wide δsup.13C and δsup.15N ranges allowed us to depict for the first time accurate relationships between blood and feather isotopic ratios across avian taxa. Blood was impoverished in sup.13C and generally in sup.15N compared with feathers. Both mean δsup.13C and δsup.15N values of feathers and blood were highly positively and linearly related feather δsup.13C = 0.972 (±0.020) blood δsup.13C + 0.962 (+ or -0.414), and feather δsup.15N = 1.014 (±0.056) blood + 0.447 (±0.665), respectively; both P < 0.0001. The regressions should be applied to mathematically correct feather or whole blood δsup.13C and δsup.15N values when comparing isotopic ratios within and between ecological studies on birds.
Two hypotheses have been proposed to explain sex-related differences in foraging in sexually monomorphic seabirds. The ‘intersexual competition’ hypothesis suggests that parents are in competition ...and that the dominated sex must adapt its behaviour to avoid competing with the dominant one. The ‘energetic constraint’ hypothesis suggests that differential energetic requirements lead to different foraging behaviour. We examined sexual differences in foraging behaviour of a sexually monomorphic tropical seabird, the Barau’s petrel, Pterodroma baraui, throughout the breeding period. We found sexual differences in foraging habitats and activities, but these were not consistent throughout the breeding period. During the prelaying exodus, males foraged further from the colony, in waters with greater surface chlorophyll concentration, and were more active than females. Males systematically took the first incubation shift, which was always longer than the others. However, no sex-related differences in foraging behaviour were observed during the chick-rearing period, both sexes sharing parental duties equally. We suggest that the sexual differences observed early in the breeding period are due to the specific needs of males and females. Females need to restore their body condition as quickly as possible after laying, which forces males to take the first long incubation shift at the nest. This may explain why males forage more actively during the prelaying exodus, to prepare for this long fasting period. Our results support for the first time the ‘energy constraint’ hypothesis to explain sexual differences in behaviour of a small, monomorphic procellariiform seabird.
► We found sexual differences in foraging habitats and at-sea behaviour. ► Sexual differences seem to result from sex-specific needs. ► Life history strategies would have forced males to invest more during incubation. ► The energy constraint hypothesis could explain sexual differences in behaviour.
Describing the trophic structure and interactions of demersal elasmobranch assemblages is fundamental to understanding food web dynamics and developing ecosystem-based management approaches. Stomach ...content analysis (SCA) and stable isotope ratios (SIA) of carbon (δ
13
C) and nitrogen (δ
15
N) from muscle were used to examine the dietary habits and intra- and interspecific trophic ecology of three sympatric batoid species (
Dasyatis pastinaca, Raja clavata,
and
Raja maderensis
) from the Azores, Northeast Atlantic. Data were analyzed with respect to sex and maturity stages. SCA showed that
D. pastinaca
feeds mostly on crustaceans, whereas
R. clavata
and
R. maderensis
prey almost exclusively on teleosts, but not on the same species.
Dasyatis pastinaca
displayed higher δ
13
C and lower δ
15
N values compared to
R
.
clavata
and
R
.
maderensis
. Trophic niche breadth was variable,
D
.
pastinaca
and
R. clavata
had the broadest and the narrowest trophic breadth, respectively. Relative trophic position categorized
D
.
pastinaca
as a mesopredator, while
R
.
clavata
and
R. maderensis
occupied higher trophic positions. With size,
R
.
clavata
and
R
.
maderensis
shifted from small prey such as crustaceans to larger prey such as teleosts, and they also exhibited significant increases in δ
15
N with size. Dietary and isotopic overlap was overall low among species, but it was higher between
R. clavata
and
R. maderensis
, suggesting more similarity in diet and habitat use between them than with
D. pastinaca
. This study depicts trophic interactions and functional roles of three co-existing batoid species in the Azorean food webs. In addition to presenting new information on the trophic ecology of
D. pastinaca
and
R. clavata
, the present study provides, to our knowledge, the first description of the diet composition and trophic level of the Macaronesian endemic batoid
R. maderensis
.
This study investigates eddy induced transport processes that affect the sources and sinks of organic matter in the Mozambique Channel (MC). Eddies generated in the MC interact with the continental ...shelf and have been hypothesized to act as conveyor belts that entrain coastal production offshore. A combination of natural isotope tracers, elemental composition and physico-chemical variables allowed distinguishing two sources of organic matter within the eddy dipole. Near the surface, coastal POM was entrained at the anti-cyclonic boundary and further transferred into the eddy. Anti-cyclonic surface POM containing coastal POM was downwelled into deeper layers where decomposition greatly affected its composition. In contrast, cyclonic water promoted new production, by upwelling nutrient-rich deep waters into the euphotic zone. This production circulated through the mixed layer of the cyclone and was possibly advected into the boundary. Boundary F(max) and surface waters shared characteristics with the cyclone and anti-cyclone respectively as the result of both import of primary production of cyclonic origin occurring at F(max) and entrainment of coastal biological material at the surface. In the overall low productivity environment of the MC, it is likely that the continuous migration of mesoscale eddies along the shelf plays an important ecological role both in enhancing pelagic production and transporting coastal production offshore.
► We study eddy processes affecting sources of POM in the Mozambique Channel. ► Coastal POM is entrained at the surface of boundary and transferred to the anti-cyclone. ► POM is affected by decomposition during downwelling of anti-cyclone surface waters. ► Cyclone promotes new primary production through upwelling of nutrient-rich deep water. ► Continuous chains of eddies along the shelf produce outburst of pelagic production.
Eutrophication by human-derived nutrient enrichment is a major threat to mangroves, impacting productivity, ecological functions, resilience, and ecosystem services. Natural mangrove nutrient ...enrichment processes, however, remain largely uninvestigated. Mobile consumers such as seabirds are important vectors of cross-ecosystem nutrient subsidies to islands but how they influence mangrove ecosystems is poorly known. We assessed the contribution, uptake, cycling, and transfer of nutrients from seabird colonies in remote mangrove systems free of human stressors. We found that nutrients from seabird guano enrich mangrove plants, reduce nutrient limitations, enhance mangrove invertebrate food webs, and are exported to nearby coastal habitats through tidal flow. We show that seabird nutrient subsidies in mangroves can be substantial, improving the nutrient status and health of mangroves and adjacent coastal habitats. Conserving mobile consumers, such as seabirds, is therefore vital to preserve and enhance their role in mangrove productivity, resilience, and provision of diverse functions and services.
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•Mangroves are enriched by oceanic nutrients delivered by nesting seabirds•Uptake of seabird-derived nutrients reverses mangrove nutrient limitations•Trophic transfer of seabird-derived nutrients enriches mangrove-associated fauna•Tidal flow exports seabird-derived nutrients from mangroves to adjacent habitats
Biogeochemistry; Aquatic science
Stomach contents and stable carbon and nitrogen isotope analyses (δ
13
C and δ
15
N) were used to investigate the trophic ecology of two apex predators, tiger sharks (Galeocerdo cuvier) and bull ...sharks (Carcharhinus leucas), from Reunion Island to describe their dietary habits at both the population and individual levels. In this oceanic island, the tiger and bull sharks were more piscivorous and teutophagous than noted in previous research from other localities. The δ
13
C values suggested that bull sharks depended on more neritic organic matter sources than tiger sharks, confirming a coastal habitat preference for bull sharks. Moreover, the total length of the bull shark influenced δ
13
C values, with smaller individuals being more coastal than larger individuals. All indicators suggest that there is a higher degree of similarity between individual tiger sharks compared with the more heterogeneous bull shark population, which is composed of individuals who specialize on different prey. These results suggest that the two species have different functions in these coastal habitats, and thus, they must be considered independently in terms of conservation and management.
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![Differences in δ[sup.13]C a...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0025-3162/small "Differences in δ[sup.13]C and δ[sup.15]N values between feathers and blood of seabird chicks: implications for non-invasive isotopic investigations")
