Sharks going down
SHARK numbers are in serious decline with the survival of some species threatened, according to a study conducted on the Great Barrier Reef by an Australian group of marine scientists.
The population modelling study conducted by Professor Sean Connolly, Mizue Hisano and Doctor William Robbins from the ARC Centre of Excellence for Coral reef Studies and James Cook University investigated shark growth, birth and mortality rates to calculate population growth rates.
“The study was only on whitetip and grey reef sharks,” Professor Connolly said.
“They are slow breeding animals often having just one or a small number of pups every other year.
“This means they do not recover from over fishing easily.”
The cause of the apparent population decline sits firmly in the lap of commercial fishing, according to Professor Connolly.
Some of the overfishing is a result of sharks being a by-catch of target species such as coral trout, and some because of shark fishing.
The main value in a shark is in shark fin and there is strong evidence of illegal fishing for shark fins within the Great Barrier Reef with finned shark carcasses being found.
“Grey reef sharks and whitetips to a slightly lesser extent are aggressive feeders, so they will attack a bait quicker than a species like coral trout,” Professor Connolly said.
“Commercial fishermen need to virtually clear out the shark population before they can start catching their target species.”
The findings of the study are supported by everyday observations by dive operators such as Aristocat that have been diving on the reefs off Port Douglas for over 20 years.
Jared Dunstan is a dive instructor on Aristocat and often dives into the waters around Agincourt Reef 12 or more times a day.
He is convinced reef shark numbers have declined dramatically in the last five years.
“Five years ago, we’d see sharks on virtually every dive. Now, we’re lucky if we see one a day,” he said.
“I used to see blacktip reef sharks regularly, but I can’t remember the last time I saw one.
“The sharks I see today are also noticeably smaller than they used to be.”
Smaller, younger sharks is indicative of a population being overfished,” said Professor Connolly.
“Sharks aren’t living long enough to grow to full size.
“There are measures that can be taken to reduce the impact on shark populations without banning all commercial fishing or dramatically extending no go zones across the entire reef.
“Firstly, enforcement authorities need greater power to stop and search boats they suspect of illegal fishing, and secondly we need to stop targeting sharks.”
Under threat: the grey reef shark