Nearly 2000 sharks spotted over decade

2015-06-23 06:00

Shark Spotters have released a report following trends and shark movement over the last ten years.

Nasief Manie/ Foto24

Shark Spotters have released a report following trends and shark movement over the last ten years. PHOTO: Nasief Manie/ Foto24

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Cape Town boasts the second largest aggregation of white sharks in the world and the largest aggregation on the doorstep of a major city.

Having completed ten years of shark research, Shark Spotters recently released a report mapping trends in shark movement in False Bay over the last ten years.

This season saw fewer sharks visiting the waters than previous years, with 142 sighted. Over the last decade, almost 2000 sharks have been spotted. “There is significant annual variation in the number of shark sightings and while we don’t yet fully understand the drivers of shark distribution it is likely a set of environmental variables such as water temperature or biological variables such as prey availability,” explains Shark Spotters research manager Alison Kock.

The average shark sighting lasts 17 minutes, with 77% of the sharks swimming in one direction as they pass through the area. The remaining 23% patrol the area, making at least one circle.

The research has identified over 440 by photo identification. Over 130 have been tagged for scientific purposes.

“It is only through long-term monitoring that we can better understand the drivers of occurrence and distribution of sharks,” says Kock.

“White sharks can live up to 70 years of age and have different prey, use different habitats and have different habits over their lifetime, most notably from when they are born and feed primarily on fish and other sharks to when they start to consume marine mammals. Furthermore, there are long-term environmental cycles which may influence distribution and habitat use,” she says.

The shark exclusion net has been deployed 240 times since the trial period began in March 2013. Forty sharks have been sighted when the net was out of the water and another 39 when the net was in the water.

On the whole, sharks remain more than 50 metres from the net. On two occasions sharks have come within five metres of the net. One shark swam towards the net and then turned away sharply. The other swam alongside the net before veering away.

White sharks are present in the bay all year round, but in the winter months male and female sharks tend to aggregate around Seal Island to prey on young Cape fur seals. In summer, female sharks typically aggregate inshore, preying on other sharks and fish.

False Bay is home to a large proportion of the Southern Africa white shark population that depend on the sea life here for food, explains Koch. “Seal Island provides a stable source of food for them in an environment that can be harsh and difficult to find food in. In summer months there is also food such as schools of yellowtail, geelbek, soupfin and smooth hound sharks. A large white shark needs to eat one seal pup or the equivalent every three days just to stay alive; more if they want to grow and reproduce,” she explains.

White sharks feed at the top of the food chain on a variety of prey, ranging from squid, to fish and dolphins and even scavenge on dead whale, says Kock.

“They have a direct impact on prey abundance and indirectly they impact prey behaviour. For example, prey will spend a lot of time trying to avoid being eaten, and the combination of these two effects means that white sharks play a regulatory role in our bay, influencing ecosystem structure and function,” she says.

The presence of sharks, especially those at the top of the food chain, is a sign that there is enough food to support them, which suggests a relatively healthy ecosystem, Kock says.

She adds education is key to conserving sharks and the ecosystem. “Educate your friends and family about sharks. One of the biggest problems is fear and misconception, which are obstacles in getting people to care about sharks. There are many species of sharks and rays in Cape Town, especially False Bay. Some of them are commercially exploited, others are caught in recreational catch and release and others are important for eco-tourism. These are therefore all impacted by people on some level,” she says.

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