Taking the Long View
The Fall & Rise & Fall of Stripers
& a Lot of Less-famous Fish
Striped bass, for thousands of years,
have been coming back to the great spawning rivers of the Chesapeake Bay. And scientists, for decades, have been trying to figure out why striped bass reproduce so well during certain eras and so poorly during other eras. Bob Wood (above) has come up with a new theory that may answer these old questions. Credit: top, David Harp; bottom, Michael W. Fincham.
DON'T CALL HIM ISHMAEL. You'd never find Bob Wood in a Herman Melville novel. There's no damp, drizzly November of the soul that would send him out to sea in search of a Moby Dick or even a menhaden. Wood did his boat time during graduate school: day-long cruises on the Chesapeake Bay, dragging fish trawls, hauling up small fish, counting them, tossing them back, recording data. Then doing it again. And again. And again.
He didn't like it then, he doesn't miss it now. There was the seasickness, but mostly it was a day lost to his dissertation work. And that dissertation work was where he did most of his trawling. He hauled out older fish surveys done by other people on other boats and even culled through records of long-ago fish catches by watermen who once upon a time went chasing after stripers and blues, yellow perch and white perch and menhaden.
Wood was also chasing something. He launched another kind of fishing expedition, one that carried him through archives of climate data where he began hauling up records on high-pressure systems and low-pressure systems, rain events and snow storms, high-flow years and low-flow years. He even began dredging up decades-old data on regional climate patterns with names like the Ohio Valley High, the Azores-Bermuda High, the North Atlantic Oscillation.
He was chasing a connection: a big, Moby Dick-like connection. Could there be a link between those large-scale climate forces and those sudden, unexplained boom years when big numbers of new stripers and other Bay-spawning fish come surging down the Bay's major rivers? What about other boom years that brought large hordes of ocean-spawning fish like menhaden sweeping off the continental shelf and into the estuary?
Bob Wood never had an itch to go to sea, but his long obsession with climate links eventually led him to a mysterious force lurking out there in the middle of the Atlantic Ocean. It's now called the AMO, short for the Atlantic Multidecadal Oscillation. It's a cycle lasting 65 to 75 years during which sea-surface temperatures warm up for several decades before cooling down for several decades. Wood didn't discover the AMO — but he did discover the connection between the AMO out there in the Atlantic Ocean and fish species back in the Chesapeake Bay.
And it took a while. By the time he discovered the AMO connection, he was no longer a graduate student, but the director of the Oxford Cooperative Laboratory, working for the National Oceanic and Atmospheric Administration (NOAA). He calls his latest breakthrough a teleconnection. "It means far-apart connections," he says. "If you see things in one place, it seems to affect things in another place." A famous teleconnection would be the El Niño/La Niña cycle in the Pacific. El Niño is a warming of Pacific waters that, among other effects, can bring rains to California and drought to the Midwest. La Niña is a cooling period with opposite effects. The AMO is something like that. It is a distant warming and cooling of waters in the middle of the Atlantic, and according to Bob Wood, it is the force that largely controls the rising and falling of striped bass and menhaden populations in the mainstem of the Chesapeake Bay.
And it's a force over which we have no control.
When the AMO gives us good years for new stripers, it generally gives us poor years for new menhaden. And menhaden, of course, are the fish that stripers love to feed on. So the warming of the AMO will give you a lot of stripers, but not a lot of food. And vice versa. Good times for menhaden will often be poor times for stripers. A lot of food, but not a lot of stripers.
That's a twist worthy of the old gods out of Greek myths. Every gift they ever gave us mortals carried a dark side. As mere mortals trying to manage the natural world, we instinctively try to maximize all the fish that matter most to us.
We want a Bay full of stripers and a Bay full of menhaden. But that may not be an option.
Data from the Deeps — and the Shallows
Bob Wood didn't always get seasick on boats, but when he did, he toughed it out.
Like most graduate students in fisheries science, he had to take his turn working the trawl surveys that the Virginia Institute of Marine Science has been running every year since 1955. Unlike most students, he didn't love being out on boats.
Up and Down
Striped bass inhabit coastal waters and are commonly found in bays but may enter rivers in the spring to spawn. Some populations are landlocked. The U.S. East Coast migratory population is composed of three major stocks: Hudson, Chesapeake, and Roanoke.
Distribution: On the Atlantic coast, these fish range from the St. Lawrence River, Canada, to the St. Johns River, Florida, although they are most prevalent from Maine to North Carolina.
Key distinguishing markings: The striped bass is a silvery fish that gets its name from the seven or eight dark, continuous stripes along the side of its body.
Size: Striped bass can grow as long as 60 inches.
Source: Maryland Department of Natural Resources
Summer trips were usually the worst for Wood, a tall slender student with dark hair, a dark beard, and a delicate stomach. When the trawl boat would make a haul down near the mouth of the Bay, the deckhands would dump the catch on the big, sloshing culling table, and Wood would go to work sorting fish — with ocean swells rolling under the boat, with diesel fumes hanging over the deck, with jellyfish tentacles slapping at his face. Picking through the flopping fish, he'd try to figure out which were larval anchovies or alewives, yellow perch or white perch, which were white mullet, satinfish shiner, bigeye scad, or bighead sea robin.
When it got bad, he'd go over to the side of the boat and throw up. Then he'd lie down on the deck, summer or winter, and wait until the next trawl was done. When the net came up, he'd scramble up and take his place at the table again. He always went back to the table. When it got worse, when he got dehydrated and went greenish in the face, the captain put him ashore. He left him on a dock down near Norfolk and called the lab to come pick him up. This only happened once, but it made Wood a legend around the lab.
Back in his office, however, grinding away on his dissertation, Wood learned to love the rich load of fish data the trawl survey hauled home. The survey hit all the salinity levels of the estuary, covering the lower Bay, motoring up to the freshwater reaches of its large rivers, and recording all the fish species it caught: who's coming on strong, who's not, who's looking healthy, who's looking sick. Begun in 1955, the VIMS survey is now the oldest ongoing trawl survey in the country. It hits 1,224 stations a year and over the decades that adds up to more than 240 species, 41,000 net hauls, and 20 million fish samples.
Wood also had fish data from the shallows of the Bay. Both Maryland and Virginia had long-running net seine surveys designed to trap young stripers swimming near the shore. Maryland began its survey back in 1954, Virginia in 1967. Both states focus on striped bass, the Bay's most popular gamefish and, for decades, one of its most profitable commercial catches. But in each state the surveys collect data on dozens of other species as well.
Data from the deeps and data from the shallows, piling up decade after decade. The key questions, the raisons d'etre for all the surveys were these: who's having a good year for offspring, who's having a poor year, and what does that tell us about how many fish are coming next year?
The Seesaw Signal
How many striped bass could be coming next year has befuddled scientists for decades. Their sudden and unpredictable boom years can turn out twice as many offspring as the year before, sometimes three times as many, sometimes 10 times as many. More than 30 years ago, biologists Don Heinle and Joe Mihursky came up with a clue: cold, wet winters bode well for a striped bass boom year.
Bob Wood came at the issue from a different angle. Before he was a fisheries scientist, he was a climatologist who spent a lot of time looking at huge, noisy data sets jammed with multiple variables. If certain weather patterns brought on boom years for stripers, perhaps those same patterns were also bringing boom years for other species at the same time. "I thought the patterns in nature are not one fish at a time," says Wood. "If there is an environmental signal, it is probably not going to pick out a single fish."
To probe all his data, Wood tried a statistical technique called Principal Component Analysis. Designed to dig out patterns buried in the data, this analytic tool uncovered an unexpected connection: whenever fish that spawned in the Bay did well, fish that spawned in coastal waters did poorly. And vice versa: whenever coastal spawners did well, Bay spawners did poorly.
Wood discovered another surprise in the data: these patterns lasted for several decades. Boom years for stripers, for example, seemed to come in bunches, and so did bust years. And the pattern affected a lot of fish: The Bay spawners include species like alewives, blueback herrings, white perch, yellow perch, shad, and, of course, stripers. The coastal spawners who come in from the continental shelf include spot, croaker, hardhead, weakfish, drum, and, of course, menhaden.
Down and Up
Atlantic menhaden are one of the most abundant fish species in estuarine and western coastal Atlantic waters. Native Americans in pre-colonial America called the fish "munnawhatteaug," which means, "fertilizer," and menhaden are probably the fish that the indigenous tribes urged the Pilgrims to plant along with their corn.
Distribution: Nova Scotia, Canada, to Central Florida.
Key distinguishing markings: Menhaden are silvery in color with a distinct black shoulder spot behind their gill opening. They also have a variable number of smaller spots on their sides. Their caudal (tail) fin is deeply forked.
Size: The maximum size of Atlantic menhaden is approximately 15 inches.
Source: Maryland Department of Natural Resources
"I did not expect to see what I saw," says Wood, who quickly gave his discovery a name: the CBASS recruitment pattern, short for Chesapeake Bay Anadromous, Shelf-spawning Species. That's a mouthful, perhaps helpful to scientists. It's a Chesapeake seesaw: when one fish group goes up, the other goes down.
Was the seesaw signal real? His finding was so unexpected Wood went back to the table again, searching through other data sets, looking for more evidence of the seesaw pattern. Baltimore Gas and Electric Company, for example, had records of how many fish were sucked into their intake pipes at the Calvert Cliffs Nuclear Plant. For that data Woods had to pull mildewed paper reports out of old file cabinets in a musty basement and then copy the data by hand, a job he managed to find fascinating. "I actually got to see the pattern emerge, watch it grow and develop," he says. And wherever he looked, in every data set he surveyed, the same seesaw appeared: when Bay spawners go up, coastal spawners go down — and vice versa.
His big surprise set him off on another search: what could be causing these alternating ups and downs? His first instinct was to look for some kind of climate force just as Don Heinle, Joe Mihursky, and others had done decades before. They tied boom years for stripers to cold, wet winters and late springs. The melting of ice and snow, they suggested, helped scour more detritus off the land, feeding the zooplankton that in turn feed tiny, newly spawned stripers. Timing is crucial: if that surge of water and food comes late and lasts deeper into spring, then plenty of food will be in the rivers just when stripers are spawning. Survival chances for their offspring can skyrocket.
For Wood, weather patterns were only a starting point. His inner climatologist told him there might be larger climate forces that create cold, wet winters and late springs. The key, he suspected, could be regional patterns in atmospheric pressure, always measured as pressure at sea level. "You can interpret everything from sea-level pressure," says Wood. Pressure fronts create huge air masses and move them around. They tell the wind which way to blow, they send us low-pressure zones that bring gray skies and big storms, they give us high-pressure zones that bless us with calm sunny weather. They could also be bringing us boom and bust years for fish species.
Wood had to go trawling again, this time in a sea of climate data. He hauled up records of regional sea-level pressures during every spawning day for 32 separate springtime Bay spawning seasons. It was another climatology approach never tried before in fish-stock studies. And it paid off. Wood was able to identify two regional pressure patterns, the Ohio Valley High and the Azores-Bermuda High, that seemed to control boom years and bust years for stripers and other Bay spawners.
If the Ohio Valley High dominates the mid-Atlantic during March, then the seesaw lifts Bay spawners. Cold and wet winters last longer, loading the rivers with more food for new fish. But if the Azores-Bermuda High shifts westward and dominates the Mid-Atlantic during March, then the seesaw lifts up coastal spawners. A warm, dry spring arrives early, setting up wind patterns that help carry more menhaden and coastal fish across the continental shelf and into the Bay.
This Chesapeake seesaw pattern was his first discovery, and it paid off in other ways: a Ph.D. in 2000, a post-doctoral appointment at the Chesapeake Biological Laboratory, then fairly quickly a job with the NOAA Chesapeake Bay Office. Shortly thereafter he was appointed director of NOAA's Cooperative Oxford Laboratory. It was an amazing rise, said another scientist. Wood was a graduate student sorting fish on trawl surveys, and four years later he was in charge of a federal marine research laboratory.
Amid his fast rise, however, big questions still lingered about his research. The Ohio Valley High and the Azores-Bermuda High seemed to be driving those fish populations — but what was driving those regional climate patterns? The big fish was still out there.
The Roller Coaster
In the year 2000, Bob Wood got his doctorate and the Atlantic Multidecadal Oscillation (or AMO) got its name. This ocean cycle brings several decades of warming waters followed by several decades of cooling waters in the Atlantic basin. A dozen years after it was named, the AMO remains loosely described and its effects widely debated.
The temperature swings can be small, but the cycle seems to have far-reaching effects. An earlier warm phase of the AMO has been tied to the Dust Bowl of the 1930s and the droughts of the 1950s. Since the early 1990s, the AMO has been in a warm, positive phase — and we've seen twice as many big hurricanes, including Isabel, Ivan, Katrina, and Sandy. We've also seen some boom years for new stripers.
When Bob Wood began reading about the AMO, his inner climatologist came alive again. "When I saw that it had cycles, I said 'Wow!' Then I looked at the statistical correlations," he says, "and it was amazing." The recent ups and downs of the AMO seemed to correlate with the ups and downs of fish populations in the Chesapeake.
Fish populations are driven, in part, by regional-scale weather patterns, and those patterns are driven, in turn, by larger-scale climate forces with daunting names like the Atlantic Multidecadal Oscillation, the North Atlantic Oscillation, the Azores- Bermuda High, and the Ohio Valley High. more . . .
Proving an AMO connection, however, took some more digging. Wood's fish data went back 60 years, but the data on the AMO ocean temperatures goes back 150 years, with some tree ring studies tracing the AMO some 400 years into the past. To extend his fish data, Wood went to fishing reports in old newspapers and anecdotes in histories of defunct sportsmen's clubs. His breakthrough, however, was close at hand. On the library shelves at his own Oxford laboratory, he turned up an old book, published in 1964, that listed all the U.S. fish harvests all the way back to the 1880s.
That gave an opening for Wood to figure out how many new fish were entering the estuary in decades past. Before long he had striper and menhaden data stretching back 120 years. In his new data he found his old seesaw pattern: when stripers were up, menhaden were down.
And in the AMO records, he had his teleconnection. Striper numbers were rising during warm decades of the AMO and sagging during cool decades. And menhaden were doing the opposite. It was the AMO that seemed to be driving the Ohio Valley High, the Azores-Bermuda High, and the fish populations of the Chesapeake Bay. "For 30 to 35 years, things will start getting better and then go down again," says Wood. "And then we'll start the roller coaster ride all over again. You go up the hill and down the hill, up the hill and down the hill" (see graph below).
What Wood finally produced was a big-picture theory, a picture that can be pretty "fuzzy," he admits, but one that carries a good deal of explanatory power. During a warm phase, air masses off the ocean collide with cold fronts off the land, and the clash creates winter coastal storms. Nor'easters, sometimes called "white hurricanes," pull moisture off the ocean, creating late-winter rain and snow across the Chesapeake watershed. The end result during springtime snowmelt and runoff is higher river flow, more fish food in the rivers during spawning, and an expanded nursery zone for tiny new stripers. Voila, a boom year for stripers.
A boom year is not the only benefit: the AMO can bring boom decades. Since the AMO's current warm phase heated up in the early 1990s, striper counts in young-of-the-year surveys have jumped strongly in five years — with fewer bust years in between. They shot up in 1993, a year that brought a heavy mid-March snow storm in the Mid-Atlantic region. They shot up after the blizzard of 1996, after the President's Weekend storm of 2003, and once more after the late January blizzards of 2011. We'll see what the "Snowquester" storm of March 2013 yields.
Wood's theory can't tell you whether next year will bring a lot of stripers. "The AMO is a general tendency," says Wood. It can tell you the probability that a warm decade will bring more big storms and those storms will bring more boom years for stripers.
And it can tell you why good years for stripers can lead to poor years for menhaden. All those storms and wind patterns that supply food for stripers can scatter the offshore larvae of menhaden and other coastal spawners, making it more difficult for them to move off the ocean and into the estuary.
When the AMO shifts into a cool phase, menhaden do much better. Cooler temperatures create frequent high-pressure fronts, leading to fewer storms, calmer weather, and easier passage for fish moving out of shelf waters and into the estuary. The menhaden picture is still fuzzy, in part because it's difficult to monitor the offshore migrations of all those shelf-spawning fish. "I'm not sure we've nailed down how most of these critters make it into the Bay," says Wood. That will take a lot of old-fashioned, on-the-water sampling cruises out in the rolling waters of the coastal ocean. Wood has no plans to be aboard.
Stripers and menhaden ride the roller coaster: the Atlantic Multidecadal Oscillation (AMO) is an ongoing series of long-duration rises and drops in sea-surface temperatures in the North Atlantic as measured against a mean sea surface temperature (designated above as zero on the vertical axis). Several decades of warmer waters (red zones above) will be followed by several decades of cooler waters (blue zones above). Instrument-based evidence for the AMO goes back 150 years, but studies of paleoclimates find the AMO signal reaching back 400 years. Research by Bob Wood (illustrated in this graph) suggests that the warm phase also brings more frequent jumps in striped bass reproduction, while the cool phase brings better years for menhaden reproduction. Credit: Maryland Sea Grant figure using an AMO graph plotted with data on sea-surface temperatures developed by Alexey Kaplan et al. (adjusted to remove warming associated with human activities)
The Downhill Ride
If Bob Wood is on the right track, then fisheries managers in Maryland and Virginia will have to rethink their options. The current warm phase of the AMO gave us a good run of boom years, but that run may be winding down. According to several reports, the warming seems to be waning, turning downhill towards a relatively cooler phase. The Chesapeake may soon see fewer boom years for stripers.
And a lot of fisheries scientists, at least, think Wood is on the right track. The AMO, nearly unknown a decade ago, is drawing a lot of attention in recent years, according to Ed Houde, a prominent fisheries biologist at the Chesapeake Biological Laboratory. "It's a powerful factor that influences fish production, particularly the reproductive dynamics of fish," says Houde. "And Bob's work is probably the best of it here in the Chesapeake region."
Wood is no Cassandra crying doom, he is a scientist trying to forecast the future and he hopes managers will listen. To preserve striper populations, fisheries managers will not have as many boom years to boost fish stocks. But they will have options.
One of them, if you buy Wood's theory, would be reducing fishing pressure on stripers fairly early. That option does more than avoid collapse. Boom years can still pop up during the cool era — albeit less frequently — and keeping a good number of stripers in the spawning rivers can magnify those year classes. And there may be more menhaden around for stripers to feed on.
"We want to get back to this ideal good time with lots of striped bass and lots of menhaden," says Wood. "When you were a kid, your father caught all those fish. And when you grow up, you expect even more of that. The answer is: it may not be obtainable."
Trying to get the both of best worlds at the same time, a Bay full of stripers and a Bay full of menhaden, sounds a lot like old-fashioned hubris. You remember hubris: in the old Greek dramas that was always the fatal flaw. That's what got the gods laughing.