Evaluating the instance of whale shark (rhincodon typus) external injuries and healing capabilities in the Indian Ocean

Abstract

Identification of the stressors associated with marine anthropogenic activities is a challenging yet crucial component of conservation planning. By documenting external injuries and assessing wound characteristics the specific activities that certain species are exposed to can be quantified. This information, when combined with an understanding of physical trauma resilience, may improve the allocation of resources in mitigation programmes. Surface frequenting species such as the whale shark (Rhincodon typus) can be vulnerable to vessel strike, especially when traversing areas of high recreational and commercial activity. On a local scale whale shark injury incidence has been well documented, however, studies covering broader spatial scales are limited. The present study provides a regional injury incidence estimate by reviewing encounter description data from four key aggregation sites in the Indian Ocean: Djibouti, Seychelles, Maldives and Tanzania. A total of 12,299 encounters with 1134 identified individuals were reviewed from 2009 to 2017 revealing that 20.7% of whale sharks had a major injury and that 61.4% of major injuries were caused by vessel collisions. The Maldives (29.0%) and Djibouti (22.2%) aggregations had the highest incidence of injured individuals. Overall prevalence of major vessel related injuries peaked in 2016 (14.3%), which may be linked to mounting unregulated tourism in these regions. Results showed that males were more likely to have a major injury than females, as too were larger individuals when compared to smaller individuals. These findings may be explained by demographic variances in behavioural traits such as surface utilisation or evasiveness. Successional images of the same injury revealed that whale sharks, like other elasmobranch species, can tolerate major external trauma and have extraordinary healing capabilities. Information on anthropogenic injury prevalence in conjunction with injury healing rates can be used to determine high risk geographic locations where restrictions such as speed limits can be implemented and enforced.