A new look at past reveals lost generations of rockfish


March 29, 1994|By PETER BAKER

SOLOMONS -- David Secor reaches in his shirt pocket, pulls out what might pass for a white pebble. The object is an ear stone from a 44-inch rockfish caught during the 1992 spring trophy season, and, the Chesapeake Biological Laboratory scientist says, it and others like it are providing insights to the life histories of Maryland's most popular fish.

Secor calls himself a fish demographer, and by using the ear stones, or otoliths, he can replot where, when and to a degree why rockfish populations flourish or founder.

"It is important that we have generations supporting other generations in our population so that Social Security will continue to be viable, for example," said Secor, 33. "And the same is true of the striped bass. . . . It is important that we have generations out there to support reproduction."

Secor's work with ear stones, which are removed from ear canals and sectioned, has created a method of aging older rockfish that is much more accurate than the traditional method of using the fish's scales.

"Think about sectioning an onion. If we cut a thin slice out of an onion, we can see rings on the inside," Secor said. "We do the same thing with the otolith, and . . . you can count the years."

Among the traditional methods of sizing up the year classes, which Secor calls generations, is the annual net survey taken of juvenile striped bass, which can be used to loosely forecast how many adults can be caught several years later.

But the juvenile catch is variable year to year and subject to changing environmental conditions.

"Striped bass aren't very good at predicting when [spawning] conditions are going to be best for their offspring," Secor said. "Sometimes they hit it, sometimes they don't. It is just like the lottery."

During the past 24 years there have been four years in which the juvenile index has ranged from good (1982) to very good (1989) to excellent (1970) to extraordinary (1993). But in many of the other years the index has been mediocre or worse.

What Secor found in a study of ear stones from 1992 trophy fish was a large generation gap.

"I got one group of fish that were at least 36 inches to 40 inches, HTC and these had to be 8- to 10-year-old fish," Secor said. "Then there was another group -- 42 to up to nearly 52 inches -- and these fish were much older, 21 to 30 years old.

"When I aged them, I could see there was really a dramatic, missing segment of the population. There is about 10 years of missing generations, corresponding to generations between 1972 and 1981."

So through the 1970s and well into the 1980s, the rockfish population was riding the backs of the population's elder citizens.

Part of the reason for the scarcity of spawning age fish from the missing generations was a 12-inch minimum size limit, which allowed rockfish to be taken from the population long before they reached sexual maturity. The large spawners were still around, Secor said, because a 36-inch maximum size limit required them to be released if caught.

"It was these big fish that saved the population," Secor said, and the 1970 generation that created the spawns of the early 1980s that were the first targeted for protection by the five-year moratorium initiated in 1985.

"Had the moratorium not been in place to protect this group," Secor said, "[the 1970 generation] would have continued to be the major producers, but this group is now so rare that you would have expected reproduction eventually to fail."

Given the restrictions of the moratorium and size and season limitations that have existed in the years since, the rockfish population no longer is threatened by the absence of those 10 generations.

"Then this year [1993] we have a 100-year event in juvenile production," Secor said. "Striped bass are busting out all over. There is hardly enough room in the rivers for all the juveniles produced this year."

So the present is promising. But Secor, with further study, will look to the past to predict the future.

"Every fish has its own story . . . ," Secor said. "My interest is to try to figure out what the individual stories of fish are rather than to look at all the fish at once."

A new technique developed by Secor will allow him to use the Chesapeake Bay's gradient of salinity -- from freshwater in the upper tributaries and the upper bay to full marine salinity at Cape Charles -- to track movements and season patterns of striped bass in the estuary.

"We can measure salinity in the otolith and tell something about where it was in different seasons and at different ages in its life," Secor said.

Among the early discoveries Secor has made is that there indeed may be adult striped bass in the Chesapeake Bay that never have left fresh water. The normal life cycle is for mature fish to join the coastal migratory stocks and return to the bay only each spring to spawn.

"That is real interesting because a lot of fishermen and scientists have long suspected that there may be a resident population in the upper bay or the upper parts of tributaries that never left freshwater," Secor said. "But no one has been able to show that until now."

Baltimore Sun Articles
Please note the green-lined linked article text has been applied commercially without any involvement from our newsroom editors, reporters or any other editorial staff.