Marine heatwave brings sharks

Soaring water temperatures off the WA coast have wiped out several lucrative fisheries and could have caused a spate of shark attacks, the State Government has claimed.

The Department of Fisheries said yesterday it was investigating whether a so-called marine heatwave was behind a series of events that have been wreaking havoc on fish stocks.

The heatwave, first documented during the summer of 2010-11, resulted in water temperatures rising up to 5C above normal along the west coast, particularly off the Gascoyne and Mid West.

Water temperatures hit almost 30C off some parts of the Mid West, while near Augusta in the South West they were up to 24C.

Senior WA marine scientists Alan Pearce and Ming Feng, in a Journal of Marine Systems article published this year, said the event was “unprecedented”.

Having forced the full or partial closure of fisheries in Shark Bay and Kalbarri, there are fears rising temperatures could also be affecting the $200-million-a-year western rock lobster fishery – WA’s biggest. Since 2008, there has been a dramatic decline in the number of larval-stage rock lobsters in many areas, leading to commercial catch limits for the prized species to be halved to 5500 tonnes.

Fisheries director-general Stuart Smith said the department initially thought the 2010-11 heatwave was an aberration but now believed it may be part of broader environmental change.

He said that although some of the heatwave’s effects were obvious, such as the collapse of abalone stocks off the Mid West coast, others were less clear.

One was the extent to which higher water temperatures were influencing lobster numbers. Another was whether a spike in great white attacks and sightings in the past two years, in which five people have been killed by sharks, was linked to the changing temperatures.

Great white sharks prefer cooler water and Mr Smith believes the animals may have been pushed closer to beaches as the warmer tropical water flowed south, forcing the cooler water towards the coastline. He said that along with increased surveillance, the theory was one of the only plausible explanations for the rise in great white activity given the animals took 15 years to reach sexual maturity.

“It may be the sharks are coming in with the colder water or it may be that the things they’re feeding on are coming in with the colder water and the sharks are following them,” he said.

WA Marine Science Institution chairman Peter Rogers, a former Fisheries boss, said he had never known an event like the 2010-11 marine heatwave. He said its effects were still being measured.