Dolphin Research in The Bahamas
Why are we studying these dolphins?
A devastating category 5 (the strongest category) hurricane, Dorian, hit Abaco in 2019, with the eye of the hurricane hitting Marsh Harbour for 12 hours. Sadly, this hurricane impacted the Sea of Abaco, and northern Abaco mainland and barrier islands dramatically.
There is a coastal population of bottlenose dolphins (Tursiops truncatus) in the Sea of Abaco which were predicted to have been displaced during the hurricane. Therefore, since then, there has been an increased effort to monitor the population of bottlenose dolphins in the Sea of Abaco, and the population off Southern Abaco, particularly the waters surrounding Sandy Point, where Charlotte and Diane (BMMRO) are based. BMMRO and their collaborators are collecting data to estimate the population size of these populations since the hurricane.
These populations are monitored using photo ID and passive acoustic monitoring. The photo ID programme has been running since 1992 and has resulted in a catalogue of dorsal fin ID pictures for all individuals within this area. These dolphins collect nicks and scratches throughout their lives which are unique to them, so photo IDs can be used to identify animals. These marks appear as the animals grow older, typically the more boisterous males will have more marks. Charlotte and Diane have also used underwater mounted hydrophones in different areas of the Sea of Abaco to understand the soundscape of this region, over the past several years. Passive acoustic monitoring is when underwater recordings are made via a hydrophone and the data can then be collected and analysed. Using both measures, the coastal population has been monitored successfully in the wake of Dorian.
After the hurricane initial sightings and acoustic detections appeared to show a decline in dolphin presence. This was alarming as bottlenose dolphins in this area are top predators and therefore play an important role in maintaining the biodiversity in these areas. As more time passes since Dorian the dolphins sightings increased. This may have been because of the dramatic decrease in boat traffic immediately after Dorian due to 90% of boats in Marsh Harbour being destroyed. In 2020, the boat traffic remained low due to Covid-19 restrictions. The increase in dolphins in this area may have been due to either of these reductions in traffic.
Little Bahama bank surrounds Great Abaco and Grand Bahama Islands and is an area of shallower waters, which is surrounded by an immediate abrupt drop in water depth. These shallow waters have high levels of biodiversity, with many different ecosystems, such as mangroves, coral reefs, and sea grass beds. These bottlenose dolphins rely on the Sea of Abaco for their livelihoods. If their habitat is altered by natural disasters such as hurricanes this can have knock on effects on their ability to survive in this area as they did before. Therefore, if the dolphin populations have decreased or relocated, it may indicate that their habitat has been damaged.
Additionally, as Abaco is within hurricane alley in the Bahamas, it frequently gets hit by hurricanes in back-to-back years. It has been found using population models that these dolphins have a higher risk of mortality if they are faced by two large hurricanes in two consecutive years. As there has not be a large hurricane since Dorian in 2019, the increase in dolphins within the coastal waters of Abaco may be explained by their time to recover without another storm.
Written by Emily McCloskey (University of St. Andrews)