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.
Highlights
Large marine animals (.1 t in weight) feeding on small prey (,1 g) need to consume vast amounts of food to sustain their energy demands
Whale sharks off Mafia Island, Tanzania, target zooplankton patches characterized by a higher biomass, larger mean size and dominated by macrozooplankton compared with non-feeding areas
Whale sharks were feeding in zooplankton patches with a mean of 25 mg of dry mass per m3, 10 times the biomass found in background samples
Summary
Large marine animals (.1 t in weight) feeding on small prey (,1 g) need to consume vast amounts of food to sustain their energy demands. Whale sharks range through a variety of habitats in tropical and subtropical waters, from bathypelagic depths to the coastal surf zone (Brunnschweiler et al, 2009; Hueter et al, 2013), predictable feeding aggregations are known only from a small number of locations (Rowat and Brooks, 2012). These aggregations are typically dominated by juvenile sharks from 3 to 9 m in total length We hypothesized that zooplankton in whale shark feeding areas would: (i) have a higher biomass, (ii) have a larger mean size of zooplankton and (iii) comprise mainly macrozooplankton, in comparison with adjacent areas in which whale sharks were not feeding at that time
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