Seasonal aggregation of juvenile whale sharks (Rhincodon typus) at Christmas Island, Indian Ocean
KeywordsCoral Reef Whale Shark Alternate Food Source Crab Population Crab Larva
Whale sharks form seasonal feeding aggregations at particular coral reef locations (Colman 1997) that are critical to the life cycle of this vulnerable species. Our year-round observations at Christmas Island (10°30′S, 105°40′E) from 1996–2008 revealed that whale sharks aggregate in summer (>90% observed between December and March). During the 2007–2008 summer aggregation (November 28–April 30), we used under- and above-water observations to estimate total length (TL; to nearest 0.25 m) and the presence of people or boats alongside the shark was used as a reference. Photographs also confirmed length estimates. All observed individuals were probably juveniles (mean TL = 4.60 m; range = 2.5–8 m; n = 82), given maturation occurs over 9 m (Colman 1997). Swimming under the sharks and searching for claspers indicated a sex ratio of 1:3.3 male:female (n = 13), although male biased sex ratios have also been observed (M. Meekan, personal communication)
This seasonal aggregation of juvenile whale sharks coincides with the annual mass spawning of red land crabs (Gecarcoidea natalis). Whale sharks feed on the crab larvae (Meekan et al. in press), however, in summer 2007–2008, we also observed feeding (horizontal and vertical suction) as late as 74 days after the larvae had emerged from the sea. Numerous other species spawn during summer and these alternate food sources may become increasingly important due to the recent decline of the red crab population.
Thanks to Wet ‘n’ Dry Adventures, Parks Australia, Christmas Island divers, Linda Cash, Gunter Noack.
- Meekan MG, Jarman SN, McLean C, Schultz MB (in press) DNA evidence of whale sharks (Rhincodon typus) feeding on red crab (Gecarcoidea natalis) larvae at Christmas Island, Australia. Mar Freshw ResGoogle Scholar