Title:

Risky business? A note on repeated live strandings of common bottlenose dolphins (Tursiops truncatus) while foraging in a shallow water environment

Publication Year:
2018
Abstract:

Here we describe several live stranding events that are thought to be related to foraging in a shallow water environment by common bottlenose dolphins inhabiting Namibian waters. The total population consists of fewer than 100 individuals that occur exclusively in very shallow waters (all encounters in <30 m depth) within a narrow strip of coastline stretching approximately 200 km north and 400 km south of Walvis Bay (Elwen, Leeney, and Gridley, 2019). The dolphins are regularly seen in a very shallow lagoon environment, where 59% of their observed behaviors were linked to foraging or possible foraging (Namibian Dolphin Project [NDP], unpublished data) and they can become stranded unintentionally on a receding tide. We discuss the significance of these stranding events for such a small, coastal population. We report on resightings of stranded individuals, by comparing photographic records from strandings data to those taken during shore and vessel-based photo-identification (photo-ID) surveys. In addition, we examined the role of human intervention in strandings events and provide information that is relevant to the broader development of optimal foraging theory. The NDP has collected photo-ID, acoustic recordings, behavioral observations and stranding information for cetaceans in Walvis Bay, Namibia (22°55' S 14°28' E), since 2008. The NDP currently coordinates the Namibian cetacean strandings network and records. Walvis Bay is a square-shaped bay roughly 10 × 10 km with a maximum depth of approximately 30 m across the mouth (Figure 1). At the southern end of Walvis Bay "First Lagoon" covers approximately 9 km² (Figure 1). First Lagoon (hereafter referred to as "the lagoon") is very shallow and characterized by extensive mudflats between deeper channels (Figure 2), but the maximum depth does not exceed 4 m. The mean tidal range in Walvis Bay is 1.42 m, with a maximum of 1.97 m (South African Navy, 2007). As the tide recedes, the combination of wide mud flats and channels means large areas of ground can be rapidly exposed, creating a high stranding risk for dolphins (Figure 2). Shore-based observations on the western side of the lagoon are difficult due to lack of road access. However, there is good accessibility to the shore on the northern and eastern banks where a large volume of people, including wildlife-oriented tours, pass throughout the day. Therefore, dolphins stranded in this area are highly likely to be reported. Walvis Bay is at the approximate center of the dolphin's known range and is the only embayment of significant size therein. Bottlenose dolphins are encountered year-round within the bay with approximately half the population resident year-round and the other half seen predominantly during winter months (Elwen et al., 2019).

Publication Title:

Marine Mammal Science

Volume:
36
Issue:
1
Pages:
305-314
Item Type:
Journal Article
Language:
en

EIS custom tag descriptions