POPULATION ECOLOGY OF WHALE SHARKS (Rhincodon typus)

OVERALL OBJECTIVES

The whale shark is the largest fish in the world, and this harmless, charismatic species sustains major ecotourism industries in several countries. Unfortunately, whale sharks are highly migratory and protection in some countries provides no respite from exploitation elsewhere in its range. Targeted fisheries for whale sharks have decimated populations, leading to fears about the sustainability of both tourism and the species itself. Key questions about their biology and ecology such as the size of the remaining population, the presence or absence of migration pathways and the identification of critical habitats such as breeding and key feeding areas remain unanswered.
Southern Mozambican waters contain a large, migratory population of whale sharks, and the burgeoning diving and snorkelling industry provides much-needed employment in an extremely poor country. Our project works in cooperation with dive operators to research whale shark ecology while assisting tourism providers to implement international best-practice standards in sustainable ecotourism.

PHOTOGRAPHIC IDENTIFICATION

Each whale shark possesses a distinct ‘fingerprint’ of spots on their flank, enabling the photographic identification of individual sharks using the Interactive Individual Identification System (a free software package available online). Around 300 sharks have been identified to date. The Mozambican population shows pronounced sexual segregation, with close to 80% of sharks being male. Most sharks present of either sex are immature, with an average length of around six metres. By tracking the proportion of new to resighted sharks we are estimating the size of the population visiting Mozambican waters. Initial results show a high resighting rate of almost 30%, but low average residency times for individual sharks. Thus, it appears that whale sharks visit Mozambique for a quick seafood buffet before continuing on to places unknown.

BROAD-SCALE MOVEMENT PATTERNS

To accurately estimate population size at a local and regional scale we need to understand the geographical dispersal of whale sharks after they leave our study area. Using a combination of photo-ID sharing, satellite tagging and dietary isotope work we are measuring the extent that various locations across the Indian Ocean are ’sharing’ sharks. These projects aim to provide an estimate of the number of whale sharks in the Indian Ocean, while providing improved information on migratory routes and critical habitat areas.

QUANTIFICATION OF BODY SCARRING

By classifying scarring on whale sharks, and identifying the origin of scars where possible, we hope to approximate the relative contribution of natural versus human-induced mortality to whale sharks of different sizes and across different locations. Around 30% of Mozambican whale sharks possess distinct scars originating from a variety of sources ranging from shark bites to entangled fishing nets.

TOURISM IMPACTS

A sustainable whale shark tourism industry has to balance visitor satisfaction against minimising impacts on the sharks. Certain swimmer behaviours towards sharks, such as touching, lead to sudden avoidance behaviours resulting in both stress to the shark and a poor experience for swimmers. By quantifying whale shark responses to boats and swimmer behaviours we can generate appropriate interaction guidelines, while testing the effectiveness of existing codes of conduct at maximising the quality and duration of encounters.