Lending an Ear
White perch and striped bass tell us about their travels
Like the stump of a fallen tree, this cross-section of a striped bass otolith, which has been meticulously cut and polished, gives us a window into the fish's life. Count the rings on this mineral structure and you can see the striper's age. Looking even closer, scientists can spot clues to where this fish had swum and when. Credit: David Secor.
DAVID SECOR KEEPS A COLLECTION OF EAR BONES. The inner-ear appendages, called otoliths, are tucked away on a shelf in his laboratory in Solomons, Maryland. He has samples from bluefin tuna, white perch, and a dozen other fish species. There's even one from the largest striped bass ever caught in the state. But right now, he's holding an otolith taken from a golden tilefish. It's white and about the size and shape of a small seashell.
"Aren't they beautiful" says Secor, a fisheries ecologist at the Chesapeake Biological Laboratory of the University of Maryland Center for Environmental Science (UMCES). "See how sculpted they are?"
And, yes, this otolith — which, like a seashell, is actually a mineral deposit, not a true bone — is notched all around with small bumps and rivulets. For Secor, however, the real beauty here lies in the information this otolith carries.
Like a tree, each otolith contains internal rings circling around its core. Count them, and you can see how old this tilefish was when it died. More important, chemical clues hidden within Secor's otoliths can also help scientists like him trace the paths the fish took as they migrated.
On the Chesapeake Bay, the migrations of white perch and striped bass are likely as old as the estuary itself. Each year, they swim tens of miles, and sometimes hundreds of miles, from the rivers where they were born to salt water and back again.
But recent studies, and Secor's own otoliths, show that these migration patterns may be more complicated than previously thought. The prevailing view had been that entire stocks of these fish always migrate together — if some go, all go. But Secor and colleagues have found that a subset of white perch living in the Bay never migrate. Instead, they spend their entire lives in their freshwater rivers. What's more, the long-term survival of many fish populations could depend on such unexpected behaviors, Secor says. His work adds to a growing body of research that suggests that, when it comes to understanding and conserving fish, diversity counts.
"We get insights into how fish differ that are very much more sensitive and very much more sophisticated than fisheries science had 120 years ago," says Tom Miller, director of the Chesapeake Biological Laboratory. "But I do think it remains to be seen about what role they have in fisheries management."
Birds Do It, Fish Do It
Migration heretics — animals whose travels don't match conventional wisdom — are far from a new concept in biology. The sight of geese flying south en masse may signal the start of autumn for many. But scientists have long known that birds don't always migrate like they should. Small subsets of birds from many traveling populations, in fact, don't migrate at all, instead staying behind in the territories where they were born.
The phenomenon is called "partial migration" — because only part of the population migrates in any one year. It's been recorded in a number of bird species, including red-tailed hawks and European robins. But, until recently, no one had looked for it in marine fish.
That's because, for many decades, fisheries scientists largely glossed over diversity within fish populations. Instead, they treated those populations — which in reality are made up of different types of individuals with unique behaviors — as a single, unified lump. In fisheries-speak, such a homogeneous group is called a stock. And stocks, for the purpose of scientists, migrate as one and spawn as one. No partial migration allowed.
Such a concept made it possible for scientists to do the sorts of calculations that allowed them to set quotas and fisheries seasons, Secor explains. But it stuck. "The stock became the population," he says. "And that view was fairly rigid for many decades."
But some, like Secor, weren't content with that. Before moving to Maryland in 1991, Secor had spent a year studying red sea bream, rabbitfish, and other aquaculture fish in Japan. "It wasn't as interesting to me looking at a tank with a fish in it as it was trying to look out here," he says, indicating his office window with a view of the Bay. "What's intrigued me is what's hidden."
Like the lives of fish, he says. Take white perch and striped bass. They famously spawn in rivers around the Bay, such as the Potomac and Patuxent, then swim to saltier waters as they begin to mature. The Bay's white perch remain in the estuary, but striped bass venture farther, eventually leaving to roam the Atlantic coast as far north as Canada. Each species of fish returns each year around springtime to spawn. But those migrations take place underwater and over many miles.
Secor and a generation of scientists like him, however, began using new research methods, including new ways of looking at otoliths, to open up that underwater world.
Still, figuring out what to do with that new understanding is complicated. "We're getting a wealth of information now," says Steve Cadrin, who studies new ways of assessing the health of stocks at the University of Massachusetts Dartmouth. "But we need to sort out this wealth of information. What of these new complexities are really important to us and which of them do we need to consider to do a better job with our fishery management?"
You could call him the otolith collector: David Secor admires a striped bass otolith. The inner-ear structures — which help fish sense their motion and orientation, much like our own inner ears help us to balance — come in all shapes and sizes. Some are about as long as a guitar pick, while others, like those from bluefin tuna (bottom left), are much smaller. To analyze otoliths, you first have to slice horizontally to obtain a thin section (top left). Photographs: above, Daniel Strain; left top, illustration by Bob Jones, produced for the Center for Quantitative Fisheries Ecology, Old Dominion University; and left bottom, David Secor.
To begin to answer those questions, Secor turned to white perch (Morone americana). These silver and spiny-finned fish are an angler's dream in the Chesapeake. They're abundant and never hard to find. Secor calls them the white lab rats of the Chesapeake. "I love white perch," he says with enthusiasm.
So just over ten years ago, Secor and his graduate students at the time, Richard Kraus and Lisa Kerr, began collecting and examining otoliths from perch caught on the Patuxent River. Because of the way they're made, otoliths absorb some of the chemical signatures that are unique to particular ecosystems, such as freshwater or saltwater habitats. And those cues can give scientists hints to where a fish has been and when.
One of the newest windows to those secrets comes from oxygen atoms. Every water body carries two major types of this basic element, Secor explains, a lighter version and a heavier version. Salt water, however, tends to bear a lot more heavy oxygen atoms than fresh water does. So think of them like a ship's logbook. If Secor sees a lot of heavy oxygen atoms in a particular otolith, for instance, he can be pretty sure his fish had spent time in salty water.
Using this log, Secor found that perch had more in common with birds than their scales would suggest. Most Patuxent perch did make the circuitous trip from the upper Patuxent River toward the Chesapeake and back again on a yearly basis — just what you would read in a Bay nature guide. But others, about three percent, stayed where they were. Secor calls these exclusively freshwater fish, which looked like any other white perch, "residents." What's more, that strategy seemed to get locked in for the fish's life. Once a perch became a resident or a migrant, it usually stayed a resident or a migrant.
Drawing from that study, Secor expanded his research to other major rivers on the Chesapeake, from the mouth of the Susquehanna south to Virginia's James River. And in each, his team found similar groups of residents mixed in with migrating fish. How many residents the researchers found depended on the river in question and what the weather was like that year. Residents, for instance, were common in the upper Bay but rarer in Virginia. Migrants were most abundant during wet years.
Secor had discovered his case of marine partial migration.
Scientists had previously known that certain species of salmonlike fish, such as brook trout around Quebec, showed similar behavior. But Secor's perch study was one of the first to discover an example of partial migration in a non-salmon fish. He and his colleagues, whose research was funded in part by Maryland Sea Grant, published their results in a number of journals, most recently in 2012 in Estuaries and Coasts.
And Secor, at least, wasn't inclined to pass his findings off as an accident. "You could say 'Well, that's just an anomaly,'" he says. But "it may be that these minority behaviors are prevalent in other marine fishes and...have some function in the ecosystem and the population."
In other words, weird behaviors do matter. To understand why, you need to first understand what the triggers for partial migration are.
And that, Secor says, may come down to the classic dilemma posed by the British punk band The Clash: should I stay or should I go? If you get all the food you need living in a river, for instance, there's no reason to leave. But if a river's crowded and food is scarce, you'd want to migrate, even if that exposes you to predators. And, in fact, he and his colleagues found that perch born later in the year — or those most likely to face crowding and food scarcity — usually become migrants. Fish born earlier, however, tend to be residents.
Migrants Vs. Residents
To bring in the Bay's bounty, like these striped bass sold at Captain White's Seafood City in Washington, D.C. (top), Chesapeake watermen follow fish as they migrate upstream and downstream each year. But sometimes that gets tricky. Most white perch caught in the lower Chesapeake from 2005 to 2006 tended to migrate as expected, according to estimates by David Secor and his colleagues (table). But, more often than not, perch from the upper Bay never left the freshwater rivers where they were born. Table source: Kerr and Secor, 2012; photograph, Daniel Strain.
Secor suspects that the fish aren't genetically programmed to be one or the other — they're merely reacting to the conditions they're facing. In fact, there's no evidence to suggest that residents only reproduce with residents or migrants with migrants. Instead, when it comes time to spawn, they mix.
But Secor says that the perch population as a whole may need different kinds of individuals, some that stay and others that go. Think of them as the tortoise and the hare from the nursery tale. The migrants are the hares. They grow fast and reproduce a lot, thanks to the usually abundant supplies of food in the Bay's mainstem. So if you want your population to expand as quickly as possible, they're your guys.
But migrants aren't dependable. A simple disturbance, such as a year of bad weather, could wind up eliminating much of the Bay's prime food sources and, by extension, a whole season's worth of migrants. Residents, however, live in a more stable environment, which allows them to continue to chug along during both good years and bad. They're your tortoises, and the offspring they produce sustain the population over time. Each strategy, in other words, has something to add to the overall population's chances of winning the race for survival.
Secor dubbed this success through a diverse set of behaviors a "portfolio effect." In stock markets — the Wall Street kind — you never want to put all your money into one company. Likewise, in the case of fish, a population shouldn't depend on only one strategy (also called a life history) for succeeding in a threatening world. "When you have diverse life histories . . . what you end up with is resilience," says Graham Sherwood, a research scientist at the Gulf of Maine Research Institute in Portland, Maine.
Sherwood studies similar behaviors in Atlantic cod, and he wagers that partial migration may be much more common in fish than many expect.
"The more you look at this, the more ubiquitous it is," Sherwood says. "Pretty much every [animal] species does this to some degree or another."
Conserving Diverse Fish
That includes striped bass in the Bay, a favorite among watermen, says Secor, who's been examining migration diversity in these fish, too. For stripers, who have recovered from a steep decline in the 1970s brought on by overfishing, partial migration isn't simple. Preliminary results from Secor's otolith analyses suggest that juvenile stripers don't have just two migration strategies — staying or going. They have many. Some young bass, for instance, begin migrating downriver as expected, then weeks later, for reasons that aren't clear, make a U-turn and come back.
Because each of those migration strategies may be important to the long-term success of striped bass, Secor argues that it's important to conserve that diversity — protecting fish across an array of Bay habitats.
These new research findings surrounding the diversity of fish stocks could one day influence how fisheries managers set their policies. But diversity shouldn't be considered in fishery regulations simply for diversity's sake, says Steve Cadrin of the University of Massachusetts. First, researchers will need to fill in the emerging picture of multi-faceted fish populations with additional details. Some behaviors, for instance, may be more critical than others for sustaining a flagging population. And figuring out which are which could become one of the bigger challenges facing fisheries scientists over the next few decades. "We don't want [management] to be so complex it's impractical," he says. "But we don't want it so simple that it's not effective."
Some scientists are working to figure out new ways of quantifying the importance of fish diversity. Cadrin, for instance, collaborated with Secor and Kerr to use mathematical analyses to investigate which of the Patuxent's perch, residents or migrants, might be most important for the populations. As expected, their results show that while migrants boost the fishery's sheer numbers, residents can keep the population from collapsing during bad years. Those results were published in 2010 in the journal Ecological Applications.
Regardless, Secor says, there's no turning back now. With modern analyses, fisheries science has entered a new era — one in which you can't ignore the diversity within fish stocks. "Really the statement that I believed when I was growing up, that the life of sea animals is hidden, is really no longer true," he says.
Which is a big revelation from a few little otoliths.