Big fish seen surviving in depleted oceans

An international team of scientists concluded that a far wider diversity of creatures was likely to survive — from predators such as sharks and tuna to tiny molluscs and algae — in a possible reprieve for the diversity of marine life.

Writing in the journal Nature, they said an influential study in 1998 gave rise to a widely used yardstick indicating that over-fishing would lead to “fishing down marine food webs”, shifting to ever smaller species as big ones were depleted.

But checks of stock abundance and other data indicated that the measure gave inaccurate readings from the Gulf of Thailand to seas off Alaska, according to the scientists in the United States, Canada, Australia and Britain.

“If you are fishing down the food web, the terminal state of the oceans would be jellyfish and plankton,” lead author Trevor Branch, assistant professor of aquatic and fishery sciences at the University of Washington in Seattle, told Reuters.

“What we see now is that everything is going to be reduced but we still have predators and prey and everything in between.”

There were exceptions, such as a collapse of northern cod or South American sardine stocks linked to over-fishing, he said.

The study did not assess whether the seas were in a better or worse state overall than previously believed. A related study in the journal Science in 2009, however, found that some steps to curb over-fishing were showing success.

Branch said the findings were not necessarily good news for larger fish. “It means there are some out there but we are fishing them harder than ever before. It’s good news and bad news,” he said.

Tuna, oysters

The report said the 1998 “mean trophic level” (MTL) gauge of catches used a flawed rating scale — algae 1, herbivores 2, carnivores 3 or 4 or more. A catch of bigeye tuna might rate 4.5, for instance, while a net of American oysters rated 2.0.

A trend towards lower MTL numbers in catches worldwide led to the conclusion that over-fishing was stripping the oceans of predators. But a review of data and new evidence revealed no overall decline in the balance of marine species, he said.

In the Gulf of Thailand, for instance, the MTL number had risen, which should indicate an improvement in marine stocks. But it was up simply because local fishermen who used to catch mussels and shrimps had turned to larger fish.

“If you use the rising MTL, the Gulf of Thailand is in the best state it has ever been in,” Branch said. But Gulf of Thailand stocks of almost all species are about 10 percent of levels in the 1950s in one of the worst ecosystem collapses.

“Half the time you get the right answer and half the time you get the wrong answer,” he said of MTL, saying it was like flipping a coin. Better yardsticks were the use of surveys to measure the abundance of fish as well as catch data.