70%? Really?

Bay Journal, Mortality rate of Bay’s striped bass twice that of those in the Atlantic

Striped bass that stay year-round in the Chesapeake Bay are dying at nearly twice the rate of those that migrate each year to the Atlantic Ocean, a new study has found. The cause or causes aren’t clear, but the lead researcher said that it needs to be addressed to right a troubling decline in the prized finfish.

An electronic tagging study led by scientists from the University of Maryland Center for Environmental Science found that large mature striped bass leave the Bay every year to roam coastal waters until the next spring. Those smaller, younger fish that remain in the Chesapeake died off at the rate of 70% a year.

“The mortality rate is alarming,” said Dave Secor, a professor at the UMCES Chesapeake Biological Laboratory in Solomons. “If fish are dying at greater than 50% or 60% a year, that’s a problem.”

 At 70% mortality each year, the percentage of fish that survive to become members of the sea going migratory stock is almost negligable. A 32 inch fish is thought to be about 7-8 years old, and the survival to 8 years would be 0.02%. I don't even see how that's possible, or how you could even test it with 100 fish (see below)

The study, published Thursday in the journal PLOS One, appears likely to draw further attention to disease and overfishing, two suspects in the decline of striped bass, also known as rockfish, which are among the most sought-after fish in the Chesapeake and along the Atlantic coast.

UMCES scientists implanted acoustic transmitters in 100 striped bass from the Potomac River and tracked their movements over four years. Signals emitted by their tags were picked up as they swam by receivers stationed in the Bay and along the coast — a kind of E-Z Pass network for fish, as Secor described it.

The telemetry data revealed that fish 32 inches and longer left the Bay as soon as spawning ended in spring and migrated as a group north to near-shore waters off Massachusetts, where they are a favorite target of recreational anglers. They headed south in late fall, returning to the Potomac by the next spring to spawn.

I always thought that the migratory fish left a little earlier, more like in the mid 20 inch range. And still, every summer, if you look in the right places at the right time, you can find fish well over the 32 inch number. I know; I've caught 40+ inch fish in mid summer, when all the migrants are supposed to be up around Long Island and Cape Cod.

That migratory pattern is well known, but the tagging study helped scientists get a firmer handle on which fish migrate, which do not and where they wind up.

“The scale of this is unique,” Secor said, because it relied on collaboration with a number of partners. “We couldn’t have done this study otherwise.”

While it’s been thought that only large female striped bass migrate out of the Bay, Secor said the study found that larger tagged male fish also joined the exodus.

Over time, the receiver network stopped detecting signals from some fish, which the scientists took as evidence that they had died. The much lower 37% mortality rate seen for migratory striped bass suggests something is happening to those fish staying in the Bay.

I've seen previous work, probably by Secor's lab, which suggest that somewhere as they mature, a lot of fish make a "choice" to either become Bay resident fish, or to become migratory. Clearly, migratory fish do well. You wonder if the amount of available food triggers the "decision."

The Wombat has Rule 5 Sunday: Kayleigh “Honey Badger” McEnany up on time and within budget at The Other McCain.