Predicting Geographic Ranges of Marine Animal Populations Using Stable Isotopes: A Case Study of Great Hammerhead Sharks in Eastern Australia

© Copyright © 2020 Raoult, Trueman, Kingsbury, Gillanders, Broadhurst, Williamson, Nagelkerken, Booth, Peddemors, Couturier and Gaston. Determining the geographic range of widely dispersed or migratory marine organisms is notoriously difficult, often requiring considerable costs and typically extensive tagging or exploration programs. While these approaches are accurate and can reveal important information on the species, they are usually conducted on only a small number of individuals and can take years to produce relevant results, so alternative approaches may be preferable. The presence of latitudinal gradients in stable carbon isotope compositions of marine phytoplankton offers a means to quickly determine likely geographic population ranges of species that rely on productivity from these resources. Across sufficiently large spatial and temporal scales, the stable carbon isotopes of large coastal or pelagic marine species should reflect broad geographic patterns of resource use, and could be used to infer geographic ranges of marine populations. Using two methods, one based on a global mechanistic model and the other on targeted low-cost latitudinal sampling of fishes, we demonstrate and compare these stable isotope approaches to determine the core population geography of an apex predator, the great hammerhead (Sphyrna mokarran). Both methods indicated similar geographic ranges and suggested that S. mokarran recorded in south-eastern Australia are likely to be from more northern Australian waters. These approaches could be replicated in other areas where coastlines span predictable geographic gradients in isotope values and be used to determine the core population geography of highly mobile species to inform management decisions

Effect of lactation stage and concurrent pregnancy on milk composition in the bottlenose dolphin

Although many toothed whales (Cetacea: Odontoceti) lactate for 2–3 years or more, it is not known whether milk composition is affected by lactation stage in any odontocete species. We collected 64 pooled milk samples spanning 1–30 months postpartum from three captive bottlenose dolphins Tursiops truncatus. Milks were assayed for water, fat, crude protein (TN × 6.38) and sugar; gross energy was calculated. Ovulation and pregnancy were determined via monitoring of milk progesterone. Based on analysis of changes in milk composition for each individual dolphin, there were significant increases (P<0.05) in fat (in all three dolphins) and crude protein (in two of three), and a decrease (P<0.05) in water (in two of three) over the course of lactation, but the sugar content did not change. In all three animals, the energy content was positively correlated with month of lactation, but the percentage of energy provided by crude protein declined slightly but significantly (P<0.05). At mid-lactation (7–12 months postpartum, n=17), milk averaged 73.0±1.0% water, 12.8±1.0% fat, 8.9±0.5% crude protein, 1.0±0.1% sugar, 1.76±0.09 kcal g−1 (=7.25 kJ g−1) and 30.3±1.3% protein:energy per cent. This protein:energy per cent was surprisingly high compared with other cetaceans and in relation to the growth rates of calves. Milk progesterone indicated that dolphins ovulated and conceived between 413 and 673 days postpartum, following an increase in milk energy density. The significance of these observed compositional changes to calf nutrition will depend on the amounts of milk produced at different stages of lactation, and how milk composition and yield are influenced by sampling procedure, maternal diet and maternal condition, none of which are known

Biology of angel sharks (Squatina sp.) and sawsharks (Pristiophorus sp.) caught in south-eastern Australian trawl fisheries and the New South Wales shark-meshing (bather-protection) program

Two species of angel shark (Squatina australis, S. albipunctata) and two species of sawshark (Pristiophorus nudipinnis, P. cirratus) are frequently caught in south-eastern Australia. Little is known of the biology of these elasmobranchs, despite being caught as secondary target species in large numbers. The present study collected morphometric and reproductive data from sharks caught in shark-control nets, commercial fishing trawlers and research trawlers in south-eastern Australia. All four species had female-biased sexual size dimorphism, but growth curves between sexes did not differ. Male S. australis individuals were fully mature at ~800-mm total length, male P. nudipinnis at ~900 mm, and male P. cirratus at ~800 mm. Anterior pectoral margins could be used to determine total length in all species. No morphometric measurement could reliably separate Squatina spp. or Pristiophorus spp., although S. albipunctata over 1000-mm total length had larger eyes than did S. australis.1 page(s

Assessment of permanent magnets and electropositive metals to reduce the line-based capture of Galapagos sharks, Carcharhinus galapagensis

Sharks possess anterior electrosensory pores (ampullae of Lorenzini), which allow them to detect very weak electromagnetic fields. Powerful magnetic fields may overwhelm this sense, and repel sharks, even in the presence of an attractant. Using underwater video, we tested seven rare earth magnet configurations, two ferrite magnet configurations and two rare earth electropositive metals as means to reduce the rate at which Galapagos sharks (Carcharhinus galapagensis) depredated baited lines. Configurations of three 50. mm diameter rare earth magnet discs showed the most potential, with a vertical configuration of magnets alongside the bait reducing depredation by 50%, and a stacked configuration of the same magnets above the bait also producing significantly more aborted investigations of the bait prior to depredation. No other magnetic or electropositive metal configuration produced significant reductions in depredation rates, time taken to strike, or number of prior investigations. Our study showed that the overriding factor determining Galapagos shark behaviours towards baits was conspecific density. The number of sharks present increased as trials progressed, with a corresponding decrease in their time to depredate baits. This effect was particularly apparent when three or more animals were present. These higher shark densities diminished the effectiveness of our experiments as individuals engaged in non-selective " mob" rushes towards the closest bait. Although our results showed that social interactions between sharks outweighed individual responses to depredation-mitigation devices, magnetic deterrents have high potential for reducing shark bycatch for species that occur in lower densities, or which interact less vigorously with conspecifics than Galapagos sharks. © 2011

Hooked on fishing? Recreational angling interactions with the critically endangered grey nurse shark Carcharias taurus in eastern Australia

The grey nurse shark Carcharias taurus is critically endangered in eastern Australia. Although fully protected, instances of recreational hooking persist in this population, with potentially fatal consequences. Here we used in situ underwater video to quantify the rates at which C. taurus interacts with a range of proximately deployed recreational fishing gears, and we suggest appropriate management changes to limit such interactions. Bottom-set baits elicited strong responses, with 15 to 43% of whole and filleted mackerel baits depredated within 5 min. Smaller Australian sardine (pilchard) and squid baits were taken by C. taurus at a significantly lower, yet appreciable rate of 3 to 15%. These smaller baits were depredated more by recreationally important teleosts, although this relationship was not significant for sardine baits. There was no consistent diel influence on shark bait depredation, although C. taurus was the only nocturnal bait depredator. Trolled gears posed no direct threat to C. taurus at any time, even when trolled at depth. Benthic-oriented jigs were rarely snapped at by C. taurus, yet may still pose a foul-hooking risk as sharks showed a propensity to rub against these jigs at depth. Vertical jigs elicited little response by C. taurus, although foul-hooking was also a risk as jigs contacted sharks in 5% of proximate drops, with near misses or line-only interactions occurring in a further 6% of cases. Our findings suggest that restricting bottom-set baits and benthic-oriented gears such as jigs around C. taurus aggregations would be a feasible and enforceable strategy to minimise recreational fishing interactions. © Inter-Research 2013

Genetic data show that Carcharhinus tilstoni is not confined to the tropics, highlighting the importance of a multifaceted approach to species identification

This study shows a range extension for the Australian blacktip shark Carcharhinus tilstoni, which was believed to be restricted to Australia's tropical waters, of >1000 km into temperate waters, revealing its vulnerability to a wider commercial fishery.8 page(s

Estimates of abundance and undercounting of bottlenose dolphins off northern Natal, South Africa

Twenty-eight transects were flown along an 81 km section of the N Natal coast. The probability of seeing a Tursiops truncatus group was estimated at 0.31 (0.15; 0.46), implying that most groups were not seen. -from Authors</p