Warming Waters May Change the Timing of Striped Bass Migrations

Fish Could Begin their Spring Treks up the Chesapeake Bay Earlier

Climate change could give striped bass a head start in one of the Chesapeake Bay’s most exciting and consequential races.

That race is the spring trophy season. For several weeks beginning in April, recreational anglers from around the region try their hand at catching female striped bass (Morone saxatilis) as they migrate up the estuary to spawn. But warming waters may give the stripers an edge in this sprint, a new study funded by Maryland Sea Grant suggests. The warmer the Bay’s waters get in the spring, researchers say, the sooner females begin their migrations. That may allow more fish to make it to their spawning grounds where they can reproduce before the fishing season begins.

The study shows that climate change can have large impacts on the behavior of fish species, even influencing the timing of their migrations, says Thomas Miller, a fisheries scientist. Natural resource managers will have to adapt to those changes as they regulate fisheries, adds Miller, a researcher at the Chesapeake Biological Laboratory of the University of Maryland Center for Environmental Science -- or get left behind.

Scientists have known that striped bass keep a precise schedule when it comes to their yearly migrations. After entering the Chesapeake from the Atlantic Ocean in the winter, female striped bass wait in the estuary’s deeper channels until the conditions for spawning are just right. That is, until spring comes.

Striped bass typically head to their spawning grounds in the freshwater portions of the estuary when water temperatures in the Bay reach around 54 to 57° F. That way, females can match their egg laying to the rapid growth of plankton, the favorite type of food for young fish -- something that only happens during a narrow window in the spring. “You have to match the production in the rivers just right,” Miller says.

But because migration timing depends on water temperatures, climate change could play havoc with the fish’s schedules. To investigate the possibility, Miller and his former graduate student Adam Peer pored through monitoring data on striped bass collected by the Maryland Department of Natural Resources between 1985 and 2010. The Bay was already warming up during that period: between the 1960s and 1990s, average water temperatures in the estuary sizzled higher by nearly two degrees Fahrenheit. 

And sure enough, the duo found evidence that females were starting their spring journeys several days earlier on average than they used to. Those departures, however, varied wildly from year to year based on spikes or dips in hot weather. The fish seemed to shift their departure date by around three days for every 1.8 degrees Fahrenheit change in water temperature from year to year, Miller says. The team published its results in the North American Journal of Fisheries Management.

Those findings could have big implications for recreational fishing, Miller says. The striped bass trophy season was scheduled for mid May when it was first instituted around the Bay in 1991. But more than a decade ago, managers moved that kickoff date to the third Saturday in April -- to better match the migration of striped bass, Miller says.

Between 2002 and 2010, trophy hunters reeled in around 29 to 63 percent of the females that are able to spawn in the Bay, surveys suggest. Many are thrown back, but there is a possibility that the stress of being pulled to the surface may cause some females to drop their eggs or otherwise delay spawning. Miller’s data shows that anglers are more successful in some years than others. Those years tended to fall during periods when the estuary was cooler than usual, or when the fish got late starts on their swims up the Bay. That change in timing seemed to increase the overlap between the fishing season and the fish’s migrations. 

What impact the recreational fishery has on the ups and downs of striped bass populations is hard to determine, Miller says. But he suspects that it could be sizable. The numbers of stripers plummeted in the Chesapeake in the late 1970s and early 1980s because of overfishing, but later recovered, hitting record highs about a decade ago. Since then, the abundance of the fish has declined, but not dramatically.

In the future, however, striped bass may get up and go even earlier in the year as the Bay’s waters continue to warm. According to scientific projections, water temperatures in the estuary may warm up by another three-and-a-half to nine degrees Fahrenheit by the end of the century. That means that migrations could begin one to two weeks earlier than they do today. But Miller says that fisheries managers may also need to be more flexible about when they schedule the trophy season, adjusting the timing regularly to better protect female stripers.    

“You’ve got all this natural variability in the wild system with sometimes I’m moving up [the Chesapeake] early, and sometimes I’m moving up later,” he says of the fish. “But you’ve got this trophy season that’s a fixed date.” Even as the average temperature of the Bay’s waters continue to warm, cold snaps will still hit the Bay, he says, potentially helping anglers to catch a lot more scaly trophies in those years.

Striped bass aren’t unique in their sensitivity to temperature changes, either, says Janet Nye, a fisheries ecologist at Stony Brook University in New York and another former graduate student of Miller’s.

She points to the case of Maine lobsters. Throughout 2012, an ocean heat wave hit the Atlantic Ocean, forcing lobsters to start their summer migrations from the deep ocean to their spawning grounds in shallower waters about a month too early. As a result, commercial fishers caught a lot more lobsters than normal, with potential long-term consequences for the fishery’s size and sustainability.

To address such effects from climate, Nye, who was not involved in Miller’s study, agrees that fisheries managers should consider more-flexible management strategies and regulations. She says that striped bass managers on the Bay “could positively impact the fishery just by delaying the opening of the fishing season when it’s cold so that more of those females can get up the river and spawn.”

So for all those fish out there: on your mark, get set, go.

Photo: A recreational angler holds up a trophy fish that he caught on the Chesapeake Bay. Credit: George Halt

This story was modified from its original version to reflect that there is large uncertainty over whether the stress of being caught and released can cause female striped bass to delay spawning.