Scarring instance and healing capabilities of whale sharks and possible implications
Authors: Womersley Freya C. , Leblond Savinien T. , Rowat David R.L. 
Published: July 2019

Background With increasing levels of tourism and other anthropogenic activities around whale shark aggregations globally there is an increased risk of physical damage to sharks from boat collisions. As such, documenting the occurrence of injuries to sharks can be a useful method of recording the impacts of tourism and other marine based activities as well as the effectiveness of management efforts. The seasonal whale shark aggregation off Djibouti has seen an increase in both physical scarring and tourism and is used here as a case study. Approach Scarring records and photos in the Djibouti whale shark database from 2009 to 2015 were reviewed to establish instance and effects of injury on the viability of whale sharks. To calculate the level of injury, in-water sighting data, which recorded scarring, were sorted into major or minor scar categories after the methods of Speed et al. (2008). Major scars were classified as being potentially life threatening and include fin amputation and lacerations penetrating the sub-dermal layer. The probable origin of the scars was also attributed where possible. To avoid overestimation of population scarring and to make results comparable across aggregations, minor scars (such as abrasions, small bites and nicks) were omitted. To quantify medium-term effects of injury, photographic evidence from returning whale sharks was reviewed to estimate healing rates. Healing rates were obtained using the maximum number of years between observing a fresh injury and a fully healed scar. A fully healed scar was defined as an injury with no non-skin/sub-dermal tissue visible. Results The incidence of major scarring recorded in the Djibouti aggregation varied during the study period from a minimum of 15.6% in 2011 to 27.3% in 2015. Of these, 57.9% were attributable to boat strikes in 2011 compared to 40% in 2015. Lacerations to fins showed the quickest healing rates. Sharks observed with fresh lacerations to the first dorsal fin indicative of propeller strikes showed scar-tissue growth on subsequent re-sighting and fully healed scars within a year. Lacerations to the main body of the sharks showed healing rates of a maximum of two years. Amputations showed a maximum healing time of one year but little capacity for tissue regeneration. However, one case of a partially amputated first dorsal fin showed full re-growth of the fin after five years, an instance that is exceptional and has not been recorded before. Conclusions When reviewing scar photos it is clear that whale sharks have the ability to tolerate major injuries through their extraordinary regenerative capabilities. This suggests that whale sharks can recover from boat strikes; however, sharks that did not survive strikes will not be re-recorded and would show as a loss from the dataset. Similarly, the long-term hindrances to normal behavior that are caused by major injuries remain largely unknown. The study reinforces the need for a series of regulations including speed limits to be implemented and enforced within the study site, Arta Marine Protected Area, Djibouti. This approach of recording scars can be applied to other shark populations to improve understanding of shark healing capabilities and the species as a whole.