Date Published: August 9, 2017
Publisher: Public Library of Science
Author(s): Denise L. Herzing, Bethany N. Augliere, Cindy R. Elliser, Michelle L. Green, Adam A. Pack, Ulrike Gertrud Munderloh.
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 large-scale displacement of stable dolphin communities.
Emigration and immigration patterns can greatly influence the origin and structure of social groups [1–2] as well as effect changes to previously stable groups, e.g. . Very little is known about the effects of large-scale emigration events in social animals involving stable social clusters of individuals, where researchers have knowledge and long-term tracking of both the population from which immigrants arise and the population into which they join. Usually immigration events are at the individual, or small group level, where the association choices made by resident individuals can strongly affect the acceptance of immigrants into the population [1,4], ultimately affecting the grouping patterns and social structure . Research on immigration events, whether large- or small-scale can be challenging due to the logistics involved in following individuals  and the fact that even for well-studied species immigration events are often rare . This is particularly true for many cetacean species in which individuals typically spend most of their time underwater making them often difficult to track, and where some individuals (usually males) from distinct communities may pay occasional short-term visits to adjacent communities for purposes of mating and not immigration. Here, we describe the large scale and relatively rapid emigration of more than 50% of the community of long-term resident Atlantic spotted dolphins (Stenella frontalis) from Little Bahama Bank (LBB) to Great Bahama Bank (GBB) where another community of Atlantic spotted dolphins is resident.
Between Fall 2012 and Spring 2013, a total of 52 spotted dolphins (25 males = 15 adults 5 juveniles, 5 calves and 27 females = 16 adults, 8 juveniles, 3 calves) that had been resident (in three distinct clusters) on LBB up to the Fall of 2012, moved to GBB. We compared changes in their group size and describe the interactions between the displaced dolphins and resident dolphins on GBB.
Dispersal can have profound effects on the structure and stability of a population  and subsequent demographic and/or environmental factors can help shape the future social structure [3,14,37–43]. For 28 years the resident community of Atlantic spotted dolphins suggested a stable association scenario with long-term social clusters, high social differentiation and preferred companions  until sometime between fall 2012 and spring 2013 when a major distribution shift of 50% of the stable spotted dolphin community occurred, throwing both the remnant community, and the shifted community, into new situations. Responses to demographic changes may differ between populations, with varying degrees of alterations in population and/or social structure as they adapt to changing conditions , which can profoundly affect the survival of the individual, and structure at the community and population levels. We discuss the social and genetic implications of the unprecedented emigration event described here and the possible factors that may have contributed to the move.