By Sea Grant Communications, 541-737-2716 SOURCE: Steve Berkeley 541-867-0135
CORVALLIS - Research into reproductive success of different age groups of black rockfish has yielded a disconcerting finding - the percentage of older mature females caught on the Oregon coast appears to have declined rapidly in recent years.
That could mean that the species is being overfished, although Oregon State University researcher Steven Berkeley says his data is not a stock assessment.
With funding from Oregon Sea Grant, Berkeley began studying the fish at the Hatfield Marine Science Center in Newport on an intuition-based hunch that their long life cycles served some evolutionary purpose. So far, he's been unable to find any truly older fish to confirm his hypothesis, but he has discovered enough to support it and is concerned about the reproductive health of the species.
Black rockfish can live into their 30s, which makes them relative youngsters in the rockfish family - some species of rockfish live well into their 100s. Black rockfish spawn hundreds of thousands of tiny, translucent live young that spend their first weeks in the plankton layer near the surface of the ocean, later settling out into bays, estuaries and nearshore reefs. It takes a black rockfish five years or more to become a sexually mature adult.
"There is a phenomenally high mortality rate for the larvae," Berkeley said, which appears to be higher for the offspring of younger adults.
Prior to the spawning season, Berkeley and his graduate students collected every mature female black rockfish carcass they could find from charter boats and docks up and down the coast - about 1,000 over three years. They used the otoliths, or growth rings in their ear bones, to find their ages, and examined their ovaries to determine when they would have spawned that spring. They found that the older fish spawned earlier and over a longer period of time than the younger adults.
In the initial year, the only one for which they currently have complete data, most of the surviving juveniles had been born early or in the middle of the season, before younger adults begin to spawn, Berkeley said. That indicated the offspring of older fish were more likely to survive to become juveniles.
Then there was the experiment that worried him.
The researchers caught 30 live females of a variety of sizes, each full of larvae. They held the females alive until they were ready to spawn and then milked the larvae from the females, analyzing some and raising the rest in batches with excess, adequate and near-starvation levels of food. Results already show that the offspring of older fish have a significantly higher lipid content, which should give them a better chance of surviving.
But older was relative. Berkeley wanted to compare the offspring of truly older females - those 15, 20, maybe 25 years old. But when the earbones were checked, not one of the 30 females was over the age of nine.
"The point of the experiment was to compare the offspring of old fish, and we had none," he said. He went back to his carcass data, and didn't like what he found.
In 1996, the first year they collected carcasses, 38 percent of the mature females were 10 years old or older. In 1997, only 20 percent were older than 10. And in 1998, the proportion dropped to less than 8 percent.
"It indicated to me that there may be a problem developing in the fishery," Berkeley said.
Oregon Fish and Wildlife did a species, size and age comparison of fish from lightly and heavily fished reefs this year, Berkeley said. "They show the same age structure. It looks like this is not just a local phenomenon. Our sample goes to Port Orford with the same percentage."
"Traditional" species management wisdom allows a take of roughly 65 percent of the estimated spawning population, by weight, leaving from 35 to 40 percent to reproduce, Berkeley said. But if his early results about age distribution, reproductive success and spawning season hold true, the reproductive potential of the species has been reduced further than expected.
"If the larvae of older fish are more fit, you're losing the fish most capable of effective reproduction," he said.
In similar situations, Berkeley said, a slot limit has been used in which only fish of intermediate size can be taken. But that's difficult with rockfish. By the time they are pulled to the surface their air bladders are so extended they die if directly returned. Berkeley has shown charter boat fishermen how to vent the bladder using a holding tank and a hypodermic needle. It's a technique he has used with great success while researching live fish.
"Or if you don't really know what you're doing and you want to protect a species, you set aside large areas, functional ecosystems, for no harvest. That way you protect a segment of the population with all age classes. But that's a much different policy battle than a slot limit."