By Stephanie Wenclawski
Here is an update on our drone project looking at how human activities may affect the distribution of marine megafauna such as sea turtles, sharks, and rays (see here and here for our previous posts). It is well known that marine organisms have evolved to blend into their environment to an extent . For example, sharks are well known for counter shading where their dorsal side is darker than their ventral side to help them blend in with the direction the sunlight is shining on them. You may have wondered, when we are using the drone, how do we know we are spotting them all? Read below for our newest investigation.
In January, we spent some time on Abaco trying to determine the detection probability of our drone under different local conditions. Our idea here was if we know how many decoy sharks we put out in a specified area, we can tell if the drone footage shows all of them. To test this, we put out approximately 10 decoy sharks made out of foam and painted them grey. We then flew the drone over the transect area where the sharks were placed. The video footage from the drone was examined to see if all 10 sharks placed appeared in the video. If some sharks were “missing”, we knew the drone footage from our previous surveys of developed and non-developed areas might not be detecting all of the sharks present.
The shark decoys were placed on the surface of the water for some trials and about half way down the water column for other trials. The sharks were placed half way down the water column in some trials because we noticed that many times in our videos sharks would we swimming at this location. Being in the field also presents challenges of its own. With quite buoyant decoy sharks, we found ourselves searching for rocks on the shoreline to get the sharks to sink into the water column. For surface and below surface trials, we varied the substrate and conditions we put the sharks on. We tested the detection probability on sand and seagrass substrates, as well as in water with clarity reduced by approximately 40%.
This was successful for us and we are looking forward to implementing our findings in our survey data. More to come soon on our findings!
Here is a photo of one of our shark decoys submersed in about 6 ft of water and Stephanie Wenclawski braving the cold water to deploy!