The whale shark Rhincodon typus is the largest fish in the world, but little was known about its biology and ecology until the mid 1990s. In recent years, whale shark research effort has increased, particularly at coastal aggregation sites where they are accessible to scientists, but also to fishers and tourists. Whale sharks are listed as Vulnerable on the IUCN Red List of Threatened Species and a better understanding of their general biology as well as of the reasons or drivers for their visits to coastal aggregation sites is needed to inform managers and implement conservation strategies.
This thesis presents the first description of some fundamental biological characteristics of whale sharks. A new underwater measurement technique, laser photogrammetry, is described and applied to whale sharks for the first time at a coastal aggregation site at Praia do Tofo in southern Mozambique. The 122 measured individuals were 432–917 cm in total length and 74% were male. With the incorporation of additional data from stranded whale sharks, the total length of male maturity was estimated to be 907 cm, and natural growth rates were provisionally estimated to be 78 cm year-1. While the sex bias and size structure reported here is similar to other coastal whale shark aggregations around the world, almost all of which are dominated by juvenile males, the measured size at maturity was larger than that estimated elsewhere. This first estimation of natural growth rate will be useful for refining demographic models for the species.
Whale shark diet was assessed using stomach content analysis of 5 stranded individuals and signature fatty acid analysis of biopsies of 24 individuals. Whale sharks (and reef manta rays Manta alfredi from the same area) had an unusual fatty acid profile that was dominated by ω6 long-chain polyunsaturated fatty acids. This stands in contrast to the ω3- dominated profiles of surface-dwelling zooplankton, which were thought to be the main prey of whale sharks. The diet of whale sharks and manta rays is unique among large marine animals because they are both far-moving, deep-diving filter-feeders that live in the highly patchy food environment of tropical and subtropical systems. Their diet is therefore likely to be a mix of various prey groups. Stomach contents indicated that emergent zooplankton, such as mysids (82% index of relative importance) are a major part of their diet and, considering fatty acid profiles of other potential prey groups, their diet mix also likely includes deep-water plankton and fish in addition to surface zooplankton.
Drivers of the coastal aggregation of whale sharks and manta rays (reef manta ray and giant manta ray M. birostris) at Praia do Tofo were investigated with generalised linear models that included local environmental, spatial and temporal predictors. Sightings of reef manta rays were most influenced by these local variables, while sightings of the wider ranging giant manta rays and whale sharks were less predictable and may be related to larger-scale oceanographic drivers. In contrast with other whale shark aggregations, we found no clear seasonality for whale shark sightings at Praia do Tofo. A steep decline insightings of reef manta rays (88%, ANOVA F = 23.49, p = 0.002; 2003–2011) and whale sharks (79%, ANOVA F = 7.39, p = 0.042; 2005–2011) is hypothesised to be primarily driven by unsustainable fishing, with negative impacts of increased eco-tourism, and large-scale oceanographic variation, respectively, also potential factors in their decline.
A detailed oceanographic study focused on southern Mozambique showed that three upwelling mechanisms in the region could lead to increased plankton biomass on the continental shelf and hence influence whale shark sightings at Praia do Tofo. These mechanisms include shelf-edge, divergent and cyclonic lee-eddy upwelling. All are driven by mesoscale eddies that propagate southwards through the Mozambique Channel. By attaching satellite-linked tags to 15 whale sharks, we further showed the potential influence of these eddies on their movements beyond enhancing productivity on the shelf. Using randomised model tracks based on real tracking data, we showed that whale sharks spent significantly more time in coastal areas than model sharks that moved randomly. Overall, whale sharks had a step length distribution that approximates the optimal foraging strategy (μ = 2.11). A behavioural-switching state-space model indicated that most (85%) surface feeding occurred in shallow water, while over deeper water the sharks’ behavior was mostly classified as travelling.
This work has improved our understanding of the basic biology of whale sharks and has challenged the long-standing view of their diet being largely dominated by day-time surface zooplankton. Tracking data has demonstrated that whale sharks move offshore but also spend extensive time within a ~200 km area along the coast. Intense eddy-driven upwelling occurred frequently in this same region. Upwelling mechanisms enhance biomass on the continental shelf and are important drivers for whale sharks to aggregate at Praia do Tofo. They also have implications for other marine animals, such as the two species of manta rays, and help explain why this area is so biologically rich. Regional conservation efforts should therefore consider this relatively large stretch of coastline.