Inter-annual feeding ecology of resident whale sharks from Mafia Island, Tanzania
Authors: Prebble Clare E.M. , Rohner Christoph A. , Pierce Simon J. , Trueman C. 
Published: July 2019

Background Mostdirectobservations of whale sharks occur at coastal feeding aggregation sites. The dominant food source in a feeding aggregation can often be quickly identified. However, there is a risk that short-term studies provide an unrepresentative snapshot of the sharks' long-term diet. Stable isotope analysis enables researchers to examine time-integrated results. Whale sharks at Mafia Island show a high degree of site fidelity. This provides a rare opportunity to study consistency, and changes over time, in individual trophic level and diet over a multi-year timescale. Approach Skinbiopsies were collected from free-swimming whale sharks between October 2011 and December 2014. Each shark was individually identified, sexed, and had their length estimated visually and/or with laser photogrammetry. Surface plankton was collected to measure the baseline isotopic signal. Additional samples from other local species from varying trophic levels were taken, including other planktivorous fishes, jellyfish, and piscivorous fishes. Targeted hand tows of the dominant food source in season, the sergestid shrimp Lucifer hanseni, were collected to investigate the longer term importance of this food source. The stable isotope composition of carbon and nitrogen were determined for all samples. Results Wewill be presenting preliminary results from this study. Throughout the sampling period we collected 223 skin biopsies from 79 individual sharks, 75% of which were male. Total lengths of all sampled sharks ranged from 4.5 to 8 m. One hundred and seventy two samples of zooplankton and other local species were collected. Overall there were no highly significant effects of the year of sampling of the sharks on δC13 (ANOVA p = 0.041) or δN15 (p > 0.05). Mean whale shark δN15 value (9.3 ± 0.07%) was similar to the other local planktivorous fishes sampled (Rastrelliger.k 10.79 ± 0.33%, and Hemiramphus sp. 10.84 ± 0.42%). There was considerable overlap of points in the δC13~δN15 biplot. The δC13 values of the sharks (–17.04 ± 0.07%) were, however, more enriched than the L. hanseni dominated surface zooplankton prey (–26.9 ± 0.38%). Whale sharks exhibited a wide range in δC13 values (–19.62 to 14.68%), and δN15 values (7.29 – 11.64%). Conclusions Thenumber of individuals sampled represents 60% of the identified population at Mafia Island, and as such this provides a unique opportunity to assess diet across a broad spectrum of the sharks that are routinely present. Initial results suggest no significant differences in feeding preference of the sharks over the three year study period. The broad range of both δC13 and δN15 is wider than seasonal changes reported in other elasmobranchs, so likely represents varied individual diets. The sharks appear to be feeding at the same trophic level as the other planktivorous fishes within this ecosystem. We will support these results by calculating an estimate of the trophic position of the sharks. The proximity and overlap of the whale shark isotope results compared to local planktivores suggests a high level of residency over the study period, which is further supported by photo-ID and acoustic telemetry. To corroborate this result we will be calculating the isotopic niche width using Bayesian ellipses framework in the SIBER package in R. Though the sharks may be highly resident, the enriched δC13 values of the sharks with respect to the surface zooplankton suggests they also have a more benthic or mesopelagic food source in their diet along with the seasonally abundant L. hansenii.