SPATIAL ECOLOGY AND STABLE ISOTOPE COMPOSITION OF CETACEANS IN THE BOHOL SEA
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Studies on Philippine cetaceans (i.e., whales and dolphins) began in the early 1980s, and have resulted in the location and identification of species in Philippine waters. However, the population estimates of most of the cetacean species are uncertain, which is important in evaluating whether the populations are stable, decreasing, or increasing in relation to potential threats and conservation measures. I conducted line-transect surveys between 2010 and 2019 in the Bohol Sea, Philippines. A total of 2009.5 km survey efforts were made, which covered an area of 8041 km2 throughout the 5-year survey period. Distance sampling methods estimated the total abundance for four delphinid species: Risso’s dolphin (Grampus griseus) = 525 individuals (95% Confidence Intervals (CI), 279–986), Fraser’s dolphin (Lagenodelphis hosei) = 2064 individuals (95% CI, 1224–3480), melon-headed whale (Peponocephala electra) = 2288 individuals (95% CI, 1310–3994), and long-snouted spinner dolphin (Stenella longirostris) = 3260 individuals (95% CI, 1658–6409). A power analysis showed that multiple surveys per year are necessary for the next 5 decades to detect 3–20% rate of decline in the population. The Bryde’s whale (Balaenoptera edeni) in the Bohol Sea had been the target of small-scale whale fishery and yet little is known about the population it belonged to. Unresolved taxonomy of this whale species makes it important to find other suitable and simple techniques to improve identification. I utilized light microscopy to examine the ultrastructure of the Bryde’s whales’ baleen bristles. Ninety-five percent of the bristle samples that were studied showed a prominent micro-pattern containing bright oval-shaped spots on the outer layer of the bristle. This characteristic has not been reported previously for Bryde’s whales or other baleen whales. The data are preliminary yet this micro-pattern can be a feature useful to distinguish a possible subspecies of whale. Bristles were 278.1 ± 42.9 μm wide whereas diameter of the medullae were 77.7 ± 16.7 μm. Observations of marine mammals do not always represent foraging regions for these highly-mobile aquatic animals, hence, stable isotope analysis was applied to investigate the foraging habits of Bryde’s whales. Stable carbon and nitrogen ratios derived from 24 baleen plates of Bryde’s whales showed intra-individual variations whereas consistency was found in baleen plates known to come from the same whale individual. Mean baleen δ15N value was 9.05 ± 0.7 ‰ whereas δ13C value was -17.06 ± 0.6‰. Isotopic data also suggest that the Bohol Sea Bryde’s whales foraged in the same habitat or region for several months/years prior to capture. The oscillation of the δ13C values were not defined, therefore δ15N values were used in a Generalized Additive Model (GAM) to improve the accuracy of the estimates of baleen growth rates. A mean (± SD) rates of baleen growth was 7.8±2.7 cm yr-1 for Bryde’s whale, a first report for this whale species. The data presented here provide some critical data gaps facing policy, management, and conservation efforts for cetaceans in Philippine waters.