Exodus! Large-scale displacement and social adjustments of resident Atlantic spotted dolphins (Stenella frontalis) in the Bahamas

Abstract

Over the last 20 years, significant habitat shifts have been documented in some populations of cetaceans. On Little Bahama Bank (LBB) there are sympatric communities of resident Atlantic spotted dolphins (Stenella frontalis) and bottlenose dolphins (Tursiops truncatus), monitored since 1985. The size and social structure (three clusters: Northern, Central, Southern) have been stable among the spotted dolphin community with little immigration/ emigration, even after large demographic losses (36%) following two major hurricanes in 2004. In 2013 an unprecedented exodus of over 50% (52 individuals) of the spotted dolphin community was documented. The entire Central cluster and a few Northern and Southern individuals relocated 161 km south to Great Bahama Bank (GBB), also home to two sympatric resident communities of spotted dolphins and bottlenose dolphins. During the late summer of 2013 and the summers of 2014 and 2015 both sites were regularly monitored but no former LBB dolphins returned to LBB. Uncharacteristic matriline splits were observed. Social analyses revealed random associations for those spotted dolphins and very little integration between spotted dolphins that moved to GBB (MGBB) and those dolphin resident to GBB (RGBB). Male alliances among spotted dolphins were present, with some altered patterns. On LBB, the operational sex ratio (OSR) was reduced (.40 to .25). OSR for MGBB and RGBB dolphins were similar (.45 and .43). A significant steady decrease in sea surface temperature and chlorophyll a (a proxy for plankton production) occurred on LBB leading up to this exodus. Similar trends were not present over the same period on GBB. The sudden large-scale shift of spotted dolphins from LBB to GBB in association with the gradual decline in certain environmental factors suggests that a possible "tipping point" was reached in prey availability. This study provides a unique view into social and genetic implications of largescale displacement of stable dolphin communities.

Surface behaviors correlate with prey abundance and vessels in an endangered killer whale (Orcinus orca) population

“…Resident killer whale movement patterns are partially driven by density and distribution of their preferred prey species (Ford & Ellis, 2006; Heimlich‐Boran, 1986, 1988; Nichol & Shackleton, 1996). Therefore, a decline in Chinook availability at the magnitude observed may result in large‐scale spatial displacement and behavioral shifts of the SRKWs, a pattern recently reported in local populations of other marine predators including spotted dolphins ( Stenella frontalis ; Herzing et al., 2017). Among other factors (i.e., environmental contaminants and anthropogenic disturbance), declining salmon abundance is resulting in the destruction and loss of SRKW critical foraging habitat (Ford et al, 2017).…”

Diversity and Consequences of Social Network Structure in Toothed Whales

“…This often leads to the sexes segregating, either spatially or socially (Ruckstuhl, 2007). In our review, studies have reported social segregation between the sexes in both species of bottlenose dolphins (e.g., Kent et al, 2008;Mann et al, 2012), Australian humpback dolphins (Hawkins et al, 2019;Hunt et al, 2019), Risso's dolphins (Hartman et al, 2007), Atlantic spotted dolphins (e.g., Herzing et al, 2017;Danaher-Garcia et al, 2019), and northern bottlenose whales (Gowans et al, 2001). In contrast, studies have found no evidence for sexual segregation in the association networks of killer whales (Baird and Whitehead, 2000;Williams and Lusseau, 2006;Tavares et al, 2017), pilot whales (Augusto et al, 2017), and spinner dolphins (Karczmarski et al, 2005).…”

“…The application of social network theory to conservation problems generally has the potential to improve outcomes in endangered populations (Snijders et al, 2017), and social structure has been proposed as a key determinant of population dynamics in cetaceans (Wade et al, 2012). Several studies have utilized social networks to help define management units (Alves et al, 2013;Wang et al, 2015;Esteban et al, 2016b) or to better understand populations' response to mortality and removal events (Williams and Lusseau, 2006;Herzing et al, 2017;Busson et al, 2019), however, we feel there are additional roles for social network analysis in conservation generally, and in toothed whales specifically. In many toothed whale populations, one large source of mortality is fisheries bycatch, which can simultaneously and suddenly remove entire social units.…”

Common bottlenose dolphin (Tursiops truncatus) social structure and distribution changes following the 2008 Unusual Mortality Event in the Indian River Lagoon, Florida

“…Localized UMEs within the IRL (2001,2008,2013) are all suspected to be nonepizootic with the majority of strandings occurring in the northern IRL regions (Mosquito Lagoon, northern Indian River, and Banana River;Marine Mammal Commission 2003, 2009, 2010 which are often subjected to harmful algal blooms (Lapointe, Herren, Debortoli, & Vogel, 2015), seagrass die-offs (Kamerosky, Cho, & Morris, 2015;Lapointe, Herren, Brewton, & Alderman, 2019), and periodic fish kills (Gobler et al, 2013). Changes to primary production and prey availability due to negative environmental factors have the potential to cause drastic habitat-use shifts in cetaceans quickly (e.g., within a year; Herzing, Augliere, Elliser, Green, & Pack, 2017) or over several decades (Weinrich, Martin, Griffiths, Bove, & Schilling, 1997). Further, spatial variability in environmental conditions can affect group size (Sueur et al, 2011), cluster segregation (Busson et al, 2019;Wiszniewski, Allen, & Möller, 2009), and habitat use (Herzing et al, 2017).…”

“…Changes to primary production and prey availability due to negative environmental factors have the potential to cause drastic habitat‐use shifts in cetaceans quickly (e.g., within a year; Herzing, Augliere, Elliser, Green, & Pack, 2017) or over several decades (Weinrich, Martin, Griffiths, Bove, & Schilling, 1997). Further, spatial variability in environmental conditions can affect group size (Sueur et al, 2011), cluster segregation (Busson et al, 2019; Wiszniewski, Allen, & Möller, 2009), and habitat use (Herzing et al, 2017).…”