Population structure, residency, and abundance of whale sharks in the coastal waters off Nosy Be, north-western Madagascar

The whale shark (Rhincodon typus) is the world’s largest fish, growing to around 20 m in total length (Chen, Liu & Joung, 1997). The species is distributed across tropical and warm temperate marine waters worldwide (Rowat & Brooks, 2012). Although individual whale sharks are highly mobile, and capable of swimming thousands of kilometres each year (Ramírez-Macías et al., 2017; Diamant et al., 2018; Rohner et al., 2018), they often display site fidelity to areas with a predictably high density of their prey (Graham & Roberts, 2007; Rohner et al., 2020), which include a variety of zooplankton and small bait fish (Heyman et al., 2001; Robinson et al., 2013; Rohner et al., 2013a; Rohner et al., 2015a).

There are several coastal whale shark aggregations in the Western Indian Ocean, including Praia do Tofo in southern Mozambique (Cliff et al., 2007; Rohner et al., 2018), Mahe in the Seychelles (Rowat et al., 2009a; Rowat et al., 2009b), Mafia Island in Tanzania (Rohner et al., 2020), and north-west Madagascar (Diamant et al., 2018). Significant declines in whale shark sightings have also been documented from this region (Sequeira et al., 2013; Rohner et al., 2013b; Pierce & Norman, 2016; Dulvy et al., 2017). A 79% decline in whale shark sightings was observed over 2005–2011 off the Inhambane coast of Mozambique (Rohner et al., 2013b), with the population subsequently remaining low (Rohner et al., 2018), and an approximately 50% decline in peak monthly sightings was reported from the oceanic waters of the Mozambique Channel between 1991 and 2007 (Sequeira et al., 2013; Pierce & Norman, 2016). Whale sharks were seasonally common around Mahe in the Seychelles until 2009 (Rowat et al., 2009b; Rowat et al., 2011), but there was a steep decline in sightings over subsequent years (D. Rowat, pers. comm.).

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