Beach safety: Can drones provide a platform for sighting sharks?

CSIRO 2019 Open Access. Context: A series of unprovoked shark attacks on New South Wales (Australia) beaches between 2013 and 2015 triggered an investigation of new and emerging technologies for protecting bathers. Traditionally, bather protection has included several methods for shark capture, detection and/or deterrence but has often relied on environmentally damaging techniques. Heightened environmental awareness, including the important role of sharks in the marine ecosystem, demands new techniques for protection from shark attack. Recent advances in drone-related technologies have enabled the possibility of real-time shark detection and alerting. Aim: To determine the reliability of drones to detect shark analogues in the water across a range of environmental conditions experienced on New South Wales beaches. Methods: A standard multirotor drone (DJI Inspire 1) was used to detect shark analogues as a proxy during flights at 0900, 1200 and 1500 hours over a 3-week period. The 27 flights encompassed a range of environmental conditions, including wind speed (2-30.0 km h-1), turbidity (0.4-6.4 m), cloud cover (0-100%), glare (0-100%), seas (0.4-1.4 m), swells (1.4-2.5 m) and sea state (Beaufort Scale 1-5 Bf). Key results: Detection rates of the shark analogues over the 27 flights were significantly higher for the independent observer conducting post-flight video analysis (50%) than for the drone pilot (38%) (Wald P = 0.04). Water depth and turbidity significantly impaired detection of analogues (Wald P = 0.04). Specifically, at a set depth of 2 m below the water surface, very few analogues were seen by the observer or pilot when water turbidity reduced visibility to less than 1.5 m. Similarly, when water visibility was greater than 1.5 m, the detection rate was negatively related to water depth. Conclusions: The present study demonstrates that drones can fly under most environmental conditions and would be a cost-effective bather protection tool for a range of user groups. Implications: The most effective use of drones would occur during light winds and in shallow clear water. Although poor water visibility may restrict detection, sharks spend large amounts of time near the surface, therefore providing a practical tool for detection in most conditions

Movement and growth of the coral reef holothuroids Bohadschia argus and Thelenota ananas

Tropical sea cucumbers are among the largest mobile invertebrates on coral reefs and are widely regarded as sedentary. Mark-recapture methods provide empirical estimates of movement and growth but have often been unsuccessful with sea cucumbers. We applied a new photographic mark-recapture technique to measure rates of short-term displacement (over a few days), long-term displacement (over 2 yr) and growth of Bohadschia argus and Thelenota ananas on the Great Barrier Reef, Australia. Recapture rates were 50–100% in the short-term study and 50–77% in the long-term study. In the short-term studies in 2010 and 2012, average movement rates ranged 2–8 m d-1 for B. argus and 5–9 m d-1 for T. ananas. Long-term movement averaged 15–47 m over 2 yr, with some individuals displacing less than 5 m and several others moving more than 100 m. Our study shows that some tropical sea cucumbers can be highly mobile, and many appear to display home ranging. Growth rates were positive yet modest for smaller individuals, but many of the large individuals lost weight over the 2 yr study. Classical growth models indicated that B. argus attain an average maximum size in 15–20 yr, and the empirical data on growth show that they can lose or gain weight thereafter. Hence, longevity appears to be at least several decades. The 2 species are slow growing, and the negative growth in large individuals undermines previous estimates of growth and longevity based on size-frequency data

Age validation in the Lutjanidae: a review

The Lutjanidae (tropical snappers) are important to fisheries throughout the tropics and subtropics. Reliable age-based demographic data are essential for the sustainable management of lutjanid fisheries, and are underpinned by the selection of appropriate ageing structures and validation of increment periodicity within these. Lutjanid age validation has attracted considerable research attention over the past three decades, but lacks a recent synthesis. This paper reviews the four main approaches used in lutjanid age validation; bomb radiocarbon dating, radiometry (lead-radium dating), chemical tagging and marginal increment analysis (MIA). Bomb radiocarbon and lead-radium dating provide absolute age estimates, which can validate increment counts in calcified structures. However, bomb radiocarbon dating cannot accurately age fish older than ∼55 years, an age that some lutjanids may meet or exceed. The development of 14 C reference curves for the postbomb decline period offers potential to accurately age fish only a few years old. Technical advances and empirical verification of key assumptions over the last two decades have established the accuracy and validity of lead-radium dating. However, this approach is not uniformly applicable to all study species or areas. Mark-recapture studies using fluorochromes clarify changes in increment appearance and periodicity as fish age, but lutjanid recapture rates are often poor. Marginal increment analysis (MIA) is susceptible to bias and misinterpretation if not rigorously applied. While no single validation approach offers a complete solution, almost all studies support annual increment formation in lutjanid otoliths. Future validation studies would benefit from the development of otolith reference collections and cross-institutional collaboration. Because MIA is inexpensive and logistically simple, its use on lutjanids will inevitably continue, and we therefore provide guidelines for its rigorous application

Scaling of Activity Space in Marine Organisms across Latitudinal Gradients

Unifying models have shown that the amount of spaceused by animals (e.g., activity space, home range) scales allometricallywith body mass for terrestrial taxa; however, such relationships arefar less clear for marine species. We compiled movement data from1,596 individuals across 79 taxa collected using a continental passiveacoustic telemetry network of acoustic receivers to assess allometric scal-ing of activity space. We found thatectothermic marine taxa do exhibitallometric scaling for activity space, with an overall scaling exponentof 0.64. However, body mass alone explained only 35% of the varia-tion, with the remaining variation best explained by trophic positionfor teleosts and latitude for sharks, rays, and marine reptiles. Taxon-specific allometric relationships highlighted weaker scaling exponentsamong teleostfish species (0.07) than sharks (0.96), rays (0.55), andmarine reptiles (0.57). The allometric scaling relationship and scalingexponents for the marine taxonomic groups examined were lowerthan those reported from studies that had collated both marine andterrestrial species data derived using various tracking methods. Wepropose that these disparities arise because previous work integratedsummarized data across many studies that used differing methods forcollecting and quantifying activity space, introducing considerableuncertainty into slope estimates. Ourfindings highlight the benefitof using large-scale, coordinated animal biotelemetry networks to ad-dress cross-taxa evolutionary and ecological questions