Sharks can be distinguished from other fishes by the absence of bones within their body, with all modern day sharks belonging to the clade Selachimorpha. They have been present in the oceans for over 450 million years with the modern day shark existing for about 100 million years. Over 500 species of shark can be found worldwide, coming in a diverse range of sizes and habitats.
Did you know?
The biggest fish in the sea is the whale shark which can grow up to 12.65m.
The smallest modern day shark is the dwarf lantern shark which when fully grown reaches 21cm.
Where can they be found?
Sharks can be found in all world oceans, and a wide variety of depth gradients.
The deepest any shark has been observed was a Portuguese dogfish that went to a max depth of 12,100 feet (3,688 metres). However, most species opt to stay within the Euphotic or sunlit zones.
How do they hunt?
Sharks rely on six very important senses to enable them to hunt effectively and efficiently
• Olfactory - sense of smell capable of detecting 1ppm of blood in seawater
• Hearing - directional
• Sight - very sensitive, the size of the eye is representative of the depth at which they live
• Touch - lateral line system that runs the length of the body and is used to detect changes in water pressure, motion and vibration
• Taste - not well understood, shark will take exploratory bites to figure out what something is
• Electroreception - Ampullae of Lorenzini, are a series of gel filled pores that line the snout of sharks and are able to detect minute chnages in electromagnetic fields
Photo Credit - Shin Arunrugstichai
Why are they at risk?
The problems facing these incredible fish come from man’s fear of them. In 1975 the release of the film Jaws had a huge impact on the general public’s opinion of sharks and it can still be seen today.
Sharks worldwide are in decline, it is now estimated that when comparing populations there has been a 70% decline in most shark species in the last 20-30 years and a 90% decline in some genus (Hammerhead sharks).
This decline is largely due to the fishing industry. For years fishing has gone unreported and unregulated with the number of sharks caught every year estimated to be upwards of 100 million individuals.
This removal of apex predators form ecosystems worldwide is having a devastating effect both on shark populations and marine food webs.
With the reduction in apex predators, mesopredators are now able to proliferate in number and occupy the highest level without the threat of predation. This increase then leads to a loss in their prey species and can result in the collapse of an ecosystem.
Within the Caribbean, a decline in the number of Caribbean sharks has caused an increase in the number of groupers. The higher number of groupers feeding on parrot fish has then meant that algae is beginning to take over parts of the reef as the important herbivorous grazer is being removed.
Sharks of Koh Tao
Historically the island of Koh Tao has been home to the following sharks:
- Whale sharks
- Blacktip reef sharks
- Grey reef sharks
- Bull sharks
- Leopard sharks
- Guitar sharks
However in recent years you would be hard pressed to see anything other than the first two on the list and even they are not guaranteed.
But why are they being reduced?
Just like elsewhere in the world, fishing is playing a huge part in their decline. If you were to venture to the fish markets on Koh Samui, Chumphon and Surat Thani you would come across Bull, Grey Reefs, Hammerheads and Blacktips at different life stages.
It’s sad to think about but worldwide this is now a common occurrence, between the fishing industries unregulated practice, the demand for shark products and the fear people have of sharks, those that remain are struggling to recover their numbers and now stand at risk of population collapses.
Photo Credit - Shin Arunrugstichai
What can you do?
Spread awareness about the problems facing sharks
Look for programs that can use any photos or videos you may have of sharks to help with their research
Dont buy shark fin or shark products
Reduce your consumption of seafood and reduce the levels of bycatch
New Heaven Dive School office 9am-7pm: +66 (0) 77 457 045 Conservation office: 9am-5pm: +66 (0)82 569 8570
48 Moo 3 Chalok Baan Kao
Koh Tao, Surat Thani
Copyright 2016 New Heaven Reef Conservation Program