Figure 1C shows EDE’s behavior before, during and after entanglement. Notable changes in response to entanglement were no socializing, little foraging, in which we did not observe any successful foraging events such as fish catches or ‘with fish’, and an increase in traveling, particularly traveling more at faster speeds (5–6 kph) (5% before, 30% during, 3% after) and less at slower speeds (2–3 kph) (63% before, 20% during, 54% after). She also engaged in other behaviors, such as leaps, fast accelerations (fast swims), and rapid surfacings. Fast swims occurred at a rate of 5.68/h compared to 1.62/h before and 0.71/h after entanglement. Her rapid surfacing rate increased to 3.79/h, compared to 0.60/h before and 0.32/h after entanglement.
Prior to entanglement, EDE was in a group 89% of the time (61% of that with at least one maternal kin); however, during her entanglement, she was with others only 10% of the time (22% of that time with at least one maternal kin) (Fig. 1D). After she freed herself from entanglement, she spent 98% of her time in groups (71% of that time with at least one of her maternal kin) (Fig. 1D).
Entanglement in fishing gear is a major threat to nearly all marine mammals. Detailed observations of individuals throughout an entanglement event are rare but provide important insight into behavioral impacts of such events. Our longitudinal dataset on EDE enabled us to compare her behavior and social interactions before, during, and after entanglement. Several marked changes stand out. First, she spent 30–50% less time foraging and 45–60% more time traveling during entanglement than before or after. In addition to traveling more, she increased her travel speed and engaged in other fast and erratic behaviors such as fast swims, rapid surfacings and leaps. She did not socialize at all when entangled. Post-entanglement, her activity budget was similar to pre-entanglement.
Some of our observations are similar to those observed in Mann et al. (1995), where costly behaviors such as fast travel and leaping increased during entanglement. Further, the additional drag from trailing fishing gear can exacerbate energy expenditure and prevent efficient foraging. Van der Hoop et al. (2013) placed a Dtag on an entangled Northern right whale and found slower diving ascents and descents, as well as shallower dives during entanglement than for whales that were not entangled. Modeling the drag force of gear attached to the entangled right whale also showed that whales entangled in fishing gear experience a greater energetic demand. Further, there appears to be a critical duration period in which the duration of additional drag due to entanglement can be a predictor of survival. Thus, the combination of increased energetic behaviors and drag due to a fishing line may further decrease the chances of survival, especially with long durations of entanglement.
In addition to physical and behavioral impacts, there was a drastic decrease in EDE’s social interactions. She was alone during the vast majority of the entanglement period, the inverse of time in groups before and after, respectively. Similarly, Wells et al. (1998) observed an entangled female dolphin in Sarasota Florida who was alone during each sighting of her entanglement, in comparison to only 24% of sightings alone when not entangled. Weinrich (1996) observed 30–40 Atlantic white-sided dolphins (Lagenorhynchus acutus) abandon a conspecific immediately after the individual became entangled in a gillnet. They noted that not a single conspecific remained or returned to investigate or assist the entangled individual. While this observation isn’t surprising, one might expect that close kin may remain with a distressed mother, calf, or sibling. When a young calf was entangled in Shark Bay, the mother remained nearby and others joined briefly, but none were close to the calf except the mother.
There are multiple examples of dolphins pushing a dead conspecific at the surface, in what appears to be an attempt to assist the deceased. Mann and Barnett (1999) observed dolphins attempting to intervene and defend a calf from a tiger shark attack. Gibson (2006) observed a non-lethal shark attack on a calf in which there was very little response to the presence of several small to medium-sized sharks or the attack from the calf, mother or group members. Therefore, dolphins do not always flee or abandon an individual in a potentially dangerous situation, particularly when it is between a mother and calf.
This begs the question then, why have we observed a pattern of seemingly social avoidance by conspecifics following entanglement? One hypothesis is that social avoidance is a strategy to avoid danger or an unfamiliar object. Furthermore, avoidance of diseased conspecifics has been observed in social lobsters. Even EDE’s brief encounters with others were simply in passing.
Previous studies of entanglement focused on large whale species, such as right and humpback whales, which are classified as solitary. Here, we use longitudinal data to quantify the impact of entanglement in a highly social species. In addition to marked behavioral changes, we identified an additional stressor, isolation, which has received little attention with respect to entanglement. As such, the costs of entanglement (e.g., infection, injury, energetic costs, inability to forage), are likely exacerbated by social isolation. Bottlenose dolphins have highly differentiated social relationships that last for years. Thus, the effect of even short-term isolation could be substantial.
These observations highlight the need for regulating the disposal of fishing gear and decreasing pollution in marine habitats. Marine debris is an increasing global threat that impacts a wide array of species. The number of individuals entangled in marine debris is three times higher than that of the 1990s and nearly 50% more species have been reported in marine debris encounters since 1997. Shark Bay is a relatively pristine habitat, listed as a UNESCO World Heritage Site in 1991, which experiences very low anthropogenic pressures. Despite that, in addition to EDE, at least five provisioned dolphins and their calves have been entangled in fishing gear at Monkey Mia (unpublished records). Many other sites around the world experience much greater fishing pressure (both recreational and commercial) and human impacts, thus even stricter regulations should exist in these regions. While such detailed observations of entanglement are rare, they offer a glimpse into the costs of entanglement for dolphins and whales.