A “Wicked Problem” Reconciling Human-Shark Conflict, Shark Bite Mitigation, and Threatened Species

Conservation measures often result in a “wicked problem,” i.e., a complex problem with conflicting aims and no clear or straightforward resolution without severe adverse effects on one or more parties. Here we discuss a novel approach to an ongoing problem, in which actions to reduce risk to humans, involve lethal control of otherwise protected species. To protect water users, nets are often used to catch potentially dangerous sharks at popular bathing beaches, yet in Australian waters one of the targeted species, the white shark (Carcharodon carcharias) is listed as Vulnerable, while bycatch includes the Critically Endangered grey nurse shark (Carcharias taurus). Recent, highly publicised, shark attacks have triggered demands for improved bather protection, whilst welfare and conservation organisations have called for removal of lethal measures. This leaves management and policy makers with a wicked problem: removing nets to reduce impacts on threatened species may increase risk to humans; or leaving the program as it is on the premise that the benefits provided by bather protection are greater than the impact on threatened and protected species. We used multivariate analysis and generalised additive models to investigate the biological, spatial-temporal, and environmental patterns influencing catch rates of threatened and of potentially dangerous shark species in the New South Wales shark nets over two decades to April 2019. Factors influencing catches were used to develop a matrix of potential changes to reduce catch of threatened species. Our proposed solutions include replacing existing nets with alternative mitigation strategies at key beaches where catch rate of threatened species is high. This approach provides stakeholders with a hierarchy of scenarios that address both social demands and threatened species conservation and is broadly applicable to human-wildlife conflict scenarios elsewhere.

Nation-wide assessment of the distribution and population size of the data-deficient nurse shark (Ginglymostoma cirratum)

The study presents the first national assessment of a nurse shark (Ginglymostoma cirratum) population, conducted using a combination of transect surveys and baited remote underwater videos (BRUVs). Density of nurse sharks in Belize was found to be higher in reefs than in lagoons, and in the atolls furthest away from the mainland and human settlements. Only large and old protected areas were found to have a positive impact on nurse shark abundance. Absolute abundance of nurse sharks was estimated using distance sampling analysis, giving a total nurse shark population in the range of 3,858 to 14,375 sharks. Thanks to a vast area of suitable habitat for nurse sharks in the country and legislation already in place for the safeguard of the species, Belize could represent an important hotspot for nurse sharks in the Western Atlantic. The data presented here hence offers a baseline for the long-term monitoring of the Belizean nurse shark population and improves our understanding of nurse shark abundance and distribution in the wider Caribbean basin.

Species Distribution Model Predictions of the Critically Endangered Grey Nurse Shark in Australia

Species distribution models (SDMs) are commonly used to forecast how threatened species are influenced by climate change. The grey nurse shark (Carcharias tauras) is a critically endangered species inhabiting both the east and west coasts of Australia, with negligible genetic interchange between the two populations. I used Generalized Linear Models (GLM), Maximum Entropy (MaxEnt) models and Boosted Regression Trees (BRT) to predict the distribution of the grey nurse shark. The data were a sample of presence-only data, derived from the known grey nurse shark sighting locations, from the east coasts of Australia, with pseudo-absences generated and bootstrapped from a restricted background. I verified these models using leave-one-out cross validation and model metrics including AICc, BIC, percentage of deviance explained, leave-one-out cross-validated R2, AUC, maximum Cohen’s Kappa, specificity and sensitivity. Cross-validated R2 was used as an overall comparison method across model types. I performed out-of-source validation by comparing model projection with the distributional range of the ragged tooth shark (Carcharias taurus) in South Africa. The prediction of the selected model was consistent with the current distributional range of the ragged tooth shark.

Fossil dermal denticles reveal the preexploitation baseline of a Caribbean coral reef shark community

Preexploitation shark baselines and the history of human impact on coral reef–associated shark communities in the Caribbean are tpoorly understood. We recovered shark dermal denticles from mid-Holocene (∼7 ky ago) and modern reef sediments in Bocas del Toro, Caribbean Panama, to reconstruct an empirical shark baseline before major human impact and to quantify how much the modern shark community in the region had shifted from this historical reference point. We found that denticle accumulation rates, a proxy for shark abundance, declined by 71% since the mid-Holocene. All denticle morphotypes, which reflect shark community composition, experienced significant losses, but those morphotypes found on fast-swimming, pelagic sharks (e.g., families Carcharhinidae and Sphyrnidae) declined the most. An analysis of historical records suggested that the steepest decline in shark abundance occurred in the late 20th century, coinciding with the advent of a targeted shark fishery in Panama. Although the disproportionate loss of denticles characterizing pelagic sharks was consistent with overfishing, the large reduction in denticles characterizing demersal species with low commercial value (i.e., the nurse shark Ginglymostoma cirratum) indicated that other stressors could have exacerbated these declines. We demonstrate that the denticle record can reveal changes in shark communities over long ecological timescales, helping to contextualize contemporary abundances and inform shark management and ecology.

Distribution and ontogenetic habitat shifts of reefassociated shark species in the northeastern Caribbean

Understanding how reef-associated sharks utilise coastal reef habitats throughout their ontogeny is essential for their effective management. In this study, we assessed the distribution of sharks in the northern Caribbean Netherlands (Saba, Saba Bank and St Eustatius) and ontogenetic shifts in habitat and depth use of the 2 most observed species: Caribbean reef shark Carcharhinus perezi and nurse shark Ginglymostoma cirratum. We used stereo baited remote underwater video (stereo-BRUV) surveys at 376 sites. A total of 126 shark sightings (6 species) were recorded, with C. perezi (n = 72) and G. cirratum (n = 42) most frequently observed. The probability of recording at least 1 shark per deployment ranged from 0.19 to 0.37 and is comparable with shark occurrences in the small number of other stereo-BRUV studies in the Wider Caribbean Region. Habitat type was the most important factor driving reef-associated shark occurrences, with the highest probability of observing C. perezi and G. cirratum in soft-coral habitat. Additionally, occurrences of C. perezi were significantly influenced by the management zone, with highest probabilities of occurrence in no-fishing zones. Almost all observed reef-associated sharks (95.6%) were juveniles, indicating that the study area may be a nursery area. Overall, both species were observed in deeper waters with increasing size, indicating ontogenetic shifts in depth use. Our findings imply that protected areas should not be limited to a single habitat or depth, but focus on protecting a large area with the range of habitats and depths necessary for reef-associated sharks to complete their life cycle.