Whale sharks target dense prey patches of sergestid shrimp off Tanzania

Abstract

Large planktivores require high-density prey patches to make feeding energetically viable. This is a major challenge for species living in tropical and subtropical seas, such as whale sharks Rhincodon typus. Here, we characterize zooplankton biomass, size structure and taxonomic composition from whale shark feeding events and background samples at Mafia Island, Tanzania. The majority of whale sharks were feeding (73%, 380 of 524 observations), with the most common behaviour being active surface feeding (87%). We used 20 samples collected from immediately adjacent to feeding sharks and an additional 202 background samples for comparison to show that plankton biomass was ∼10 times higher in patches where whale sharks were feeding (25 vs. 2.6 mg m−3). Taxonomic analyses of samples showed that the large sergestid Lucifer hanseni (∼10 mm) dominated while sharks were feeding, accounting for ∼50% of identified items, while copepods (<2 mm) dominated background samples. The size structure was skewed towards larger animals representative of L.hanseni in feeding samples. Thus, whale sharks at Mafia Island target patches of dense, large, zooplankton dominated by sergestids. Large planktivores, such as whale sharks, which generally inhabit warm oligotrophic waters, aggregate in areas where they can feed on dense prey to obtain sufficient energy.

Long-Term Photo-Identification Reveals the Population Dynamics and Strong Site Fidelity of Adult Whale Sharks to the Coastal Waters of Donsol, Philippines

“…Whale shark abundance appears to peak c. 1 month prior to these plankton communities' highest yearly densities in the area, and coincides with the dry season in the Philippines resulting in reduced water output from the rivers into Donsol waters (Lapitan-Tandang, 2010). Plankton tows were conducted in the general area yearround, but not in close proximity to feeding whale sharks as recommended by (Rohner et al, 2015a). The high density of phytoplankton is likely supporting complex zooplankton compositions, but the links remain unclear.…”

“…The largest of the shark species, whale sharks are highly mobile (e.g., Wilson et al, 2006;Sleeman et al, 2010;Berumen et al, 2014;Robinson et al, 2017), but form predictable seasonal aggregations in hotspots around the world, predominantly associated with the presence of food (e.g., Motta et al, 2010;Robinson et al, 2013;Rohner et al, 2015a). Some whale sharks display a degree of site fidelity on an annual and inter-annual basis (Graham and Roberts, 2007;Holmberg et al, 2008;Fox et al, 2013;Araujo et al, 2017), and this predictability makes the whale shark an ideal target species for wildlife tourism (Catlin and Jones, 2010;Rowat and Brooks, 2012).…”

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…5 (continued) be an increased reliance on high-biomass prey patches that are sparsely distributed and often dominated by 1 or a few prey species (e.g. Rohner et al 2015, Armstrong et al 2016. For example, in Peru, where strong equatorial up welling leads to overall high surface primary productivity, zooplankton prey is likely to be abundant.…”

“…Between 2013 and 2014, 25% of 790 re corded fishing trips captured more than 1 species of mobulid in a single net (J. M. Rambahiniarison unpubl.). Whale sharks, another filter-feeding elasmobranch, also rely on dense and often monospecific prey patches to survive in oligotrophic regions (Rohner et al , 2015, while sympatric rorqual whales exhibit trophic niche separation in highly productive polar foraging grounds (Santora et al 2010, Gavrilchuk et al 2014. It is possible that prevalent theories of niche overlap (May & Mac Arthur 1972, Pianka 1974, 1981 do not adequately describe the trophic dynamics of sympatric marine filter-feeders due to the prey density thresholds they require to meet energetic demands.…”

Young humpback whale Megaptera novaeangliae feeding in Santa Catarina coastal waters, Southern Brazil, and a ship strike report

“…3) (Burkenroad 1945 . Dense patches of P. petrunkevitchi are likely to be ideal for large planktivore whales to feed (Nicol 2006) since sergestid shrimps are already known to support the energetic demands of other large planktivorous animals such as whale sharks (Rohner et al 2015). Because A. americanus and P. petrunkevitchi inhabit the pelagic realm, they probably present equivalent potential to be preyed on by whales in the coastal waters of southern Brazil.…”