Abstract
The life history of the whale shark (Rhincodon typus), including its reproductive ecology, still remains largely unknown. Here, we present results from the first whale shark population study around Darwin Island, Galapagos Marine Reserve. Following a diversified approach we characterized seasonal occurrence, population structure and size, and described habitat use of whale sharks based on fine scale movements around the island. Whale shark presence at Darwin Island was negatively correlated with Sea Surface Temperature (SST), with highest abundance corresponding to a cool season between July and December over six years of monitoring. From 2011 to 2013 we photo-identified 82 whale sharks ranging from 4 to 13.1 m Total Length (TL). Size distribution was bimodal, with a great majority (91.5%) of adult female individuals averaging 11.35 m±0.12 m (TL±SE), all but one showing signs of a potential pregnancy. Population dynamics models for apparently pregnant sharks estimated the presence of 3.76±0.90 (mean ± SE) sharks in the study area per day with an individual residence time of 2.09±0.51 (mean ± SE) days. Movement patterns analysis of four apparently pregnant individuals tracked with acoustic tags at Darwin Island revealed an intense use of Darwin's Arch, where no feeding or specific behavior has been recorded, together with periodic excursions around the island's vicinity. Sharks showed a preference for intermediate depths (20–30 m) with occasional dives mostly to mid-water, remaining the majority of their time at water temperatures between 24–25°C. All of our results point to Darwin Island as an important stopover in a migration, possibly with reproductive purposes, rather than an aggregation site. Current studies carried out in this area to investigate regional scale movement patterns may provide essential information about possible pupping grounds for this enigmatic species.
Highlights
The whale shark, Rhincodon typus Smith 1828, has a widespread global distribution, occurring throughout the tropical and sub-tropical seas [1]
Mean Sea Surface Temperature (SST) around Darwin Island was significantly different between months between 2007 and 2013 (Oneway ANOVA, p,.0001)
Four of them corresponded to the months of May and June, while the rest of the records (207) were made between July and December, with no whale sharks recorded between January and April (Fig. 2)
Summary
The whale shark, Rhincodon typus Smith 1828, has a widespread global distribution, occurring throughout the tropical and sub-tropical seas [1]. Rowat and Brooks [3] described an aggregation as a site with more than 10 individuals in less than 1 km. Sequeira et al [19] suggested that large-scale whale shark migrations have different patterns depending on sex and size, and that females may display natal philopatry, given that the longest satellite tracks recorded to date all belong to large females. If this was the case, mature females would carry out long migrations to the same breeding grounds where they were born, so studies focusing on pregnant whale sharks could provide essential information to corroborate this hypothesis
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