All in a day’s work for Cape Town researchers
A group of Cape Town-based marine mammal scientists had an unusual shark encounter while scouting for dolphins in Mossel Bay just before Easter. The team from Sea Search, who use underwater sound equipment to research whales and dolphins, were checking on one of their hydrophones attached to a buoy close to Mossel Bay’s Seal Island. They found the buoy line tangled up with a length of thick fishing line which, to their surprise, offered fierce resistance. “I looked down and just saw something glistening in the water, and realised that we had driven over this really thick fishing line,” explained Sea Search co-director Tessa Gridley.
“The line was wrapped around the engine and around the buoy, which is why we suspected it was attached to something else, and there was something putting strain on it.”
The crew held their breath, intermittently pulling in the thick fishing line or fastening it to the boat to get closer to the unseen giant — clearly tired out from trying to get free. They presumed it to be a large shark, and their suspicions were confirmed when the two crew members spotted a 2.5 – 3m Great White pass within meters of the boat. Video footage of the encounter shows curious crew members pulling in hundreds of meters of line in an attempt to free the animal. Eventually the line snapped and the shark dived away – to continue life with a few meters of steel trace dangling from its mouth.
“Luckily the team has had quite a lot of experience working with Great White sharks, and from that they think that it was a guy angling from shore who caught a Great White,” said Gridley who filmed much of the action on her cell phone. “Either their spool ran out or they had to cut the line because you’re not allowed to land a Great White.”
“It is very lucky that we coincidentally bumped into it because it had almost 1000m of line attached to it and it would have tangled onto lots of different things in the bay – there are many moorings or natural obstacles such as rocks and kelp which could have entrapped it. The chances of it surviving would have been very low because sharks need to be able to forage and move to get oxygen over their gills.”
“We think probably it was caught the day before and it is lucky that we managed to at least get it free and just leave the 3m of trace. That’s not ideal but animals with fishing line and traces attached to them is quite a familiar sight. Unfortunately it is part of the reality of human wildlife contact,” she said.
Sea Search has led the way in marine acoustic research in South Africa, with several published studies of marine life including dolphins and Humpback Whales. Researchers use hydrophones to capture underwater sound and then analyse their data back in the office. A recent study in False Bay showed that visiting humpbacks spend up to 17 hours a day singing to each other, a sound that can be heard up to 30km away.