Dynamics of Whale Shark Occurrence at Their Fringe Oceanic Habitat

Abstract

Studies have shown that the whale shark (Rhincodon typus), a vulnerable large filter feeder, seasonally aggregates at highly productive coastal sites and that individuals can perform large, trans-boundary migrations to reach these locations. Yet, the whereabouts of the whale shark when absent from these sites and the potential oceanographic and biological drivers involved in shaping their present and future habitat use, including that located at the fringes of their suitable oceanic habitat, are largely unknown. We analysed a 16-year (1998–2013) observer dataset from the pole-and-line tuna fishery across the Azores (mid-North Atlantic) and used GAM models to investigate the spatial and temporal patterns of whale shark occurrence in relation to oceanographic features. Across this period, the whale shark became a regular summer visitor to the archipelago after a sharp increase in sighting frequency seen in 2008. We found that SST helps predicting their occurrence in the region associated to the position of the seasonal 22°C isotherm, showing that the Azores are at a thermal boundary for this species and providing an explanation for the post 2007 increase. Within the region, whale shark detections were also higher in areas of increased bathymetric slope and closer to the seamounts, coinciding with higher chl-a biomass, a behaviour most probably associated to increased feeding opportunities. They also showed a tendency to be clustered around the southernmost island of Santa Maria. This study shows that the region integrates the oceanic habitat of adult whale shark and suggests that an increase in its relative importance for the Atlantic population might be expected in face of climate change.

Whale Shark (Rhincodon typus) Seasonal Occurrence, Abundance and Demographic Structure in the Mid-Equatorial Atlantic Ocean

“…Past studies on the ecology and biology of whale sharks have suggested a relationship between their movements and environmental features, including sea surface temperature, chlorophyll a concentrations, bottom relief and ocean currents [3–12]. Commonly, environments with an optimal scenario for whale shark occurrence provide suitable conditions for primary and secondary productivity, as the main known purpose for whale shark aggregation is foraging [2,13–15].…”

“…Adult whale sharks are infrequently observed, and the majority of sightings have been recorded at oceanic locations, such as the Galapagos Islands [36], Baja California Sur [18,37], the Azores [12], St. Helena Island [38] and ASPSP ([25]; present study). However, complete information on the distribution of mature whale sharks, and on the location of mating and nursery grounds, if any, remains lacking, despite being crucial for the conservation of the species.…”

Persistent Enhancement of Micronekton Backscatter at the Summits of Seamounts in the Azores

“…This region has one of the highest densities of seamounts in the Northeast Atlantic (Morato et al, 2013). Several of these seamounts host commercially valuable pelagic and demersal fish species important for local fisheries (Morato et al, 2008b;Menezes et al, 2013) and act as foraging posts for sea turtles, seabirds, marine mammals, and large pelagic fishes (Santos et al, 2007;Morato et al, 2008b;Silva et al, 2013;Afonso et al, 2014b;Tobeña et al, 2016). However, with few exceptions (e.g., Hargreaves, 1975;Martin and Nellen, 2004), previous studies on acoustic backscatter distribution in the Azores have focused only in open ocean areas (e.g., Moore, 1950;McElroy, 1974;Smailes, 1976;Wade and Heywood, 2001).…”

Overview of the Ocean Climatology and Its Variability in the Azores Region of the North Atlantic Including Environmental Characteristics at the Seabed

“…The Azores represent an important habitat for a wide range of migrating marine fauna ranging from seabirds (Monteiro et al, 1996), cetaceans , elasmobranchs (Vandeperre et al, 2014), or tuna (Pham et al, 2013) whose migrations are tightly linked to environmental factors (Amorim et al, 2009;Afonso et al, 2014;Prieto et al, in press;Tobeña et al, 2016;Druon et al, 2017). All these studies found SST and primary productivity or chlorophyll-a concentrations as the main environmental factors explaining the spatial distribution of large migratory species.…”

“…All these studies found SST and primary productivity or chlorophyll-a concentrations as the main environmental factors explaining the spatial distribution of large migratory species. Additionally, geomorphologic characteristics and features such as bathymetric slope or distance to seamounts (Amorim et al, 2009;Afonso et al, 2014) have also been referred as important drivers of the distribution of large pelagic animals, highlighting the usefulness of the comprehensive review of the geomorphology of the Azores region (Perán et al, 2016).…”