By Jackie Reuder, Intern, Shark Angels
Scientific Study: Diel habitat use patterns of a marine apex predator (tiger shark, Galeocerdo cuvier) at a high use area exposed to dive tourism by Hammerschlag, N., Gutowsky, L.F.G., Gallagher, A.J., Matich, P., Cooke, S.J.
Background
Apex predators (any animal that is at the top of the food chain, land or water) usually need a large area to live since the majority of apex animals are large. Since apex predators can travel long distances what they do in those areas are important to understanding their life and the life around them, including their prey. It can also be important to understand where they mate, give birth, or where their young are for conservation purposes. Tiger sharks are known to be more active at night than during the day, but we are unsure where they go during the day and if they are coming in contact with humans around this area. Sharks generally move around for food, to mate, or to find a place they like. Recently, there are a number of studies looking into how humans can influence animal behavior and if it changes because us humans are more present than normal. Currently, it is still unsure if human presence influences where a shark goes. This study focuses on the daily activity (also called diel movement) of tiger sharks around a known tiger hotspot in the Bahamas, called Tiger Beach.
Methods
This study took place in the Bahamas around a frequently visited beach called Tiger Beach. Here, there are several ecotourism practices including snorkeling and scuba diving. Up to four scuba diving charters can be in an area at any given time of approximately 1.5km2. In order to track where tiger sharks are going, they are attached with tags and tracked using stationary underwater listening stations. Each shark has a tag surgically implanted under their skin and the underwater listening stations record where and when the shark passes the station. Researchers can then dive to the listening stations and download the information recorded. In total, 42 tiger sharks were tagged between May 2014 and November 2014 and 32 listening stations were placed throughout the 1.5km2 area, including around the high ecotourism sites. Researchers were specifically looking for: 1. areas of the most activity, 2. centers of the most activity, 3. how many were detected each day, and 4. residency (who and how many stayed in the area for an extended period of time).
Findings & Results
Listening stations started recording June 2014 and information was downloaded from the stations May 2015, ending recording. 55,847 detections were recorded from 39 of the 42 sharks that were tagged, and 37 sharks showed some sort of residency in the area. The daily average of sharks present varied from season to season, but there was an overall decrease during the summer months. Generally, the tagged sharks showed a significant amount of time in the NE quadrant of the area, but there was no difference in time spent there during the night or day. 34 sharks were detected the most in the area, but no difference in the number of times they were detected day or night. The NW and NE quadrants of the area were the highest places sharks were detected, and the NE tended to be slightly higher at night.
Conclusions
This study found that there were no differences in day and night habitat use from the tagged sharks. They did find, though, that there are places in the area that the tiger sharks liked to stay. They also found that most sharks moved in and out of the area, except during the summer when there were very few sharks present. Other studies have shown that tiger sharks go to southeast Florida during the summer, so finding few in the summer supports this theory. Finally, and maybe most importantly, there is no evidence to suggest that ecotourism like snorkeling and scuba diving affects the behavior of the tiger sharks around Tiger Beach, Bahamas.