Flight of the rockfish: What migration taught us about Maryland's state fish

September 19, 2017

David Secor, a fisheries scientist at University of Maryland Center for Environmental Science's Chesapeake Biological Laboratory, has made a career out of understanding where fish migrate and why.

With funds and support from Atlantic States Marine Fisheries Commission, NOAA Chesapeake Bay Office, MD Department of Natural Resources, and the Potomac River Fisheries Commission, Secor embarked on a study to understand migration paths of Potomac and Atlantic Striped Bass, also known as rockfish. Primarily, the fish are born in Chesapeake Bay, but they tend to leave as they age, and Secor wanted to know where they went and why.

In this audio story, we talk to him about his Potomac and Atlantic Striped Bass Telemetry study to understand more about his motivations and approach. You can learn more about his study and watch videos referenced in the audio story on Secor's lab website.

Story transcript

Professor David Secor has spent years trying to tell the story of some of our favorite fish, such as Bluefin tuna, Atlantic sturgeon, and Maryland’s state fish, the striped bass. So much of their stories stem from where they migrate and why, and that’s what Secor has been investigating since he started with the University of Maryland Center for Environmental Science in 1990.

Secor shared a lot of the stories he had been collecting in 2015 when he published “Migration Ecology of Marine Fishes.” His book examined, in part, how tracking fish has changed over time due to technological advancements. We spoke with Secor about how some of that technology has helped him study striped bass, what he’s learned about the fish over his time at UMCES, and why this fish, once in decline, is so important to Maryland.

Our conversation starts in front of his computer in Chesapeake Biological Laboratory’s Cronin Research Complex. Secor pulls up an animation that shows striped bass movements.

When he hits play, the distinct sounds of “Flight of the Bumblebee” surge from his speakers as small dots on the screen spring to life. Each dot is a fish and each jolt signifies its movement. The dots start in the tributaries of Chesapeake Bay, and over time, some shift through the Atlantic Ocean to northern waters off the coast of Cape Cod, while others stay put.

DAVID SECOR: So these fish, you’re going to see them go up the Potomac, pretty high up into upper Potomac, Anacostia, D.C. area, and then they’re going to come back down. You’ll see some flit across the C&D canal and move into the Delaware which is kind of interesting. But you’ll also note these big fish, the yellow dots, this is April, you’re going to see those off Long Island in May and then by end of May early June you’re going to see them off Cape Cod. SO they are really booking after they spawn and getting out of the Potomac.

UMCES: Seeking cooler water? Is that why they go north?

SECOR: Well, it’s been known for some time that large fish leave the Chesapeake and the motivation for this project was to get a better handle on that and exactly what size do they leave the Bay. And is it males, females, that was the purpose of this study.

Secor was setting out to answer questions that trace to his first coming to UMCES. He wanted to know whether rockfish born in the Chesapeake Bay stay or leave, and if they leave, where do they go, when, and why? Technology would help his effort.

UMCES: Can you talk about the value that technology has had in your studies, and why some of what you’ve learned because of those advances are important to understanding what happens below the surface?

SECOR: I pioneered some of the work that allowed us to evaluate how fish move in and out of the Chesapeake Bay using a chemical record within the ear stones of fish. That work was really helped along kind of by this unique marriage of disciplines we have at CBL.

Colleagues of mine have been developing ways of looking at these ear stones to tell them about some things related to migration in salmon and I looked at that and thought, ‘Wow that would really work for striped bass.’ So I found a geologist who used what’s called an electron microprobe up at College Park and developed a technique for looking at how strontium varied in different parts of the otolith.

In other words, Secor used a special machine that can analyze the chemical makeup of something as small as a fish’s ear bone without breaking it. The otolith, or ear stone, acts like a passport for a fish. It grows in layers annually, signifying age, and alters as it incorporates minerals, such as calcium, in waters the fish has visited. Because mineral makeup is unique, an otolith can reveal where a fish was at different stages of its life.

SECOR: That variation could tell us that striped bass move in and out of the Chesapeake every year or up and down these tributaries every year to spawn and that striped bass that are larger tend to be more migratory. Those things we expected, but then we also discovered things that we didn’t expect, that some large striped bass never leave the Chesapeake and that some adult striped bass don’t spawn every year. So we started finding things that didn’t fit the model and that, in some ways, stimulated my book. That fish has more variation, striped bass and other fish, have more variation in what they do, you can’t typify them by just one behavior.

And I looked at birds as a model, so you can look at Canada geese, they don’t all migrate, right? It turns out that what’s termed partial migration, incomplete migration behaviors within populations is very common in birds and as we’ve been deploying these new technologies like otolith chemistry and electronic tags, we’re discovering it’s more and more common in marine fishes, as well, like striped bass. And striped bass, I think, it may sound a little arrogant, become kind of a model species from the research we’ve done, my students and I have done over the past couple of decades for this partial migration behavior.

Secor has taken his migration research to another level using a system he likens to EZ Pass. Around 2014, he implanted 100 Potomac River striped bass with thumb-sized transmitters built to last two and a half years. As each fish was tagged, Secor and his team took notes of that fish’s weight, length, sex, and the code on each transmitter before releasing it back into the water. Those transmitters interact with acoustic receivers that Secor or other scientists already positioned in the water along the Chesapeake and as far north as Cape Cod. When a tagged fish swims near a receiver, the receiver records it. When the scientists retrieved their receivers to scour the data, they could see how migration routes of fish of different sizes and sexes varied.

SECOR: One thing we’ve learned is that striped bass tend to be fairly conservative. They want to go back to the same areas year after year to feed. We see concentrations in Mass Bay off Cape Cod that are fairly consistent year after year suggests that they’re exhibiting what’s called site fidelity, moving to these same areas year after year.

Now we’re only seeing the fish where receivers are. Animal mark recapture experiments are only as good as their design, so we try to be strategic about answering specific questions based on what we tag, when we tag and where we tag. That’s our ability to control the experimental design.

The dream of this is that we begin to fill out these regions with EZ Pass systems so that we can better track these fish. We have 20 receivers extending 12 to 25 miles off Ocean City and our colleagues have impressive receiver assets off their coasts, off Delaware and New York and in Gulf of Maine and within these estuarine systems, and I just feel like we can’t tag enough fish.

Like an EZ Pass can only count the number of cars using that tolled roadway, the acoustic monitors and transmitters can only offer a sample of a population’s movements, but it does give scientists enough to start to tell a story. This study showed, for example, backed up the idea that smaller fish tend to be resident while larger fish are migratory. It also helped Secor visualize how much the Chesapeake Bay contributes to fisheries in this region as well as in the mid-Atlantic and New England coastal waters.

UMCES: So life starts for striped bass predominantly in Chesapeake Bay?

SECOR: Absolutely, freshwater, tidal areas of the Chesapeake. I think it still holds, it’s always been believed and there’s some evidence, that Chesapeake Bay is the major producer for the coastal fisheries, at least in regions from southern New England through Cape Hatteras where most striped bass are harvested.

Striped bass are Maryland’s state fish, after all.

Also called a rockfish, a reference to its association with oyster reefs, which watermen call rock, the fish earned its state designation in 1965. The General Assembly prefaced the law by noting, in part, “In the judgment of the members of the General Assembly of Maryland, it is a simple act of justice and of equity that this fine old Maryland fish should be honored by being designated as the official fish of the State of Maryland...”

The striped bass has other unique characteristics, as well.

UMCES: Can you tell me a little about how big do they get? What do they look like?

SECOR: I always think of them as a very extravagant temperate fish. It’s like a tropical fish trying to be a temperate fish. So many of our temperate fish are kind of boring, they don’t have stripes. They’re fairly unique in being striped and being a temperate fish.

Their sizes can be quite large, over 90 pounds. And they can get quite old. When I first arrived here, I began aging them using otoliths or ear stones and I was amazed. At that time, we had huge striped bass. They weren’t harvesting bigger fish as they do now. So I encountered a fish that was older than I was; it was kind of an epiphany. It was like a 33-year-old fish. Wow, imagine what that fish had encountered overall those decades of its life to survive. It’s just amazing.

I encountered these fish at the time because we had a hatchery program, which was part of the whole restoration program for striped bass in the 80s and 90s when they stocked a million striped bass per year. To see the prodigious amount of eggs these large females could produce was really something.

UMCES: Were rockfish on a decline at one point? is that why there was a hatchery?

SECOR: Yeah, there was. In the 70s, we had what is termed some really strong year class or maybe a couple strong year classes and that means for various reasons, these are spring spawning fish and in most years, they don’t produce a lot of offspring but every once in a while, maybe once every 10 years, there’s a banner crop of striped bass juveniles. The environment just aligns in the right way with spawning and you get huge year classes.

And so they had one of these moving through the Chesapeake and areas like the Potomac and Choptank and other rivers, and at that time, striped bass were heavily targeted by both recreational and commercial fisheries as a pan fish.

So this pan fishery means that fish that were 1 and 2 years old, especially males, which tend to be more resident in these systems, they were heavily harvested. There were gill nets bank to bank, and in the 70s increased affluence released a huge cohort of anglers onto the scene and so estimates were that 90 percent of a year class was being removed each year. So maybe a very strong year class juvenile production could support that kind of fishing, maybe, but think then all the subsequent year classes that were smaller, they couldn’t withstand that kind of fishing mortality. So the population really plummeted and then we had a spate of really dry years for year classes, just the environment was not conducive for a strong year class through the 70s and so the bottom dropped out of the fishery and the resource.

So there was a lot of worry about the potential for overfishing, but also with the environmental legislation and increased awareness of what was going on in urban estuaries like around DC and Baltimore, there was increased attention to the idea that it could be pollution that was diminishing striped bass. You had these major, competing hypotheses, and they finally figured out that you couldn’t really rest those two things apart.

So what a modeler did, he said well it could be either, but we can only do something about fishing, and if we do something about fishing it can compensate even if we’re having a pollution effect. And that was the real, I guess, break through. And that stimulated management, that directed management to really restrict harvest so much so that in Maryland there was a moratorium. Maryland took the bold step of banning fishing and the reason they did it was because there was because there was a mediocre, moderately strong year class in like ’79, and it was decided they should protect that year class, protect it from harvest, let it grow to adult size class so that it could then have a chance to produce subsequent strong year classes. That strategy absolutely worked.

The moratorium, the fishing ban, is used as one of the great examples of effective fishery management up and down the coast and actually worldwide. It brought about I think stronger regulations and suddenly there was greater confidence that science-based assessments could recover a population of fish, improve fisheries, this startlingly rapid recovery. I’ve argued the part of the reason for that recovery is that you had this store of unmolested large striped bass females that were ready for the right environmental conditions. There weren’t a lot of them, but they were big. They were able to really catalyze. So there’s very strong evidence from the aging stuff I’ve done, from assessment work, from patterns and juvenile recruitment, all that aligns to the idea that preserving adult age structure really saved the day for striped bass.

UMCES: How’s their population today?

SECOR: They’re pretty healthy. I mean, we have an indicator of advanced conditions of striped bass and that’s this juvenile survey that occurs throughout the Chesapeake and we look for an average number of juveniles caught per seine haul as a future indicator of the health of the stock, and it’s actually used in assessment framework in terms of guiding whether we need to be more cautionary. We’ve had mediocre years for juvenile production in recent years and it may have nothing to do with pollution or overfishing. It’s kind of what nature gives us.

When we have conditions where it warms in the spring and then suddenly get a cold front, what happens is that warming dupes the striped bass. They love spawning. They spawn in huge events – billions of eggs released during warming trends over a certain range of temperature as spring warms. Then you get a cold front and actually the temperatures become lethal.

And also, when we find that we have milder winters, it’s not a good set up for striped bass, they tend to do better following colder, wetter, snow-ridden winters.

UMCES: Does climate change not bode well then for striped bass?

SECOR: Yeah, it could be a problem. The thing with climate change in the Chesapeake is yeah, we’re going to get warming, but we aren’t going to become suddenly, you know, North Carolina or South Carolina. A lot of people think you’re just going to shift the Chesapeake down latitude. There will be residual stuff going on in the Chesapeake. For instance, we may still have very cold winters. But as we transition from season to season in the spring, it’s just a huge transition period, and you have these striped bass that are migrating up. We’re looking at this as well as the kind of cues and timing of when they’re migrating up the Potomac. Then they spawn and they’re using temperature cues, but as you transition from one kind of thermal regime in the winter to another in the summer, that could be affected by climate change and warming.

The uncertainty of matching your eggs to good conditions becomes problematic. That’s the whole game is they’re trying to put their eggs and larvae in a good situation and already it’s such a, kind of ephemeral thing, about these cold fronts moving in. It only becomes more uncertain when you layer in climate change. They may adapt. Certainly these populations aren’t uniform. Some large striped bass will stay put and most will migrate. That kind of diversity allows populations to adapt to new conditions.

While climate change could pose a threat to striped bass that can’t adapt to the changing conditions, there’s another factor that could alter the fish’s future in the Bay.

SECOR: Since the moratorium, the fishery’s dramatically changed. Now there’s high desire to catch these very big striped bass. We have our trophy season in the Chesapeake, which is supposedly after spawning, but it’s hard to time that correct, especially with climate change. Then we have these big, big fisheries off New England and we know our fish are going up there and supporting those recreational fisheries and they are selecting the biggest fish.

Everyone wants to catch the biggest fish. As a result, we don’t find 20- and 30-year-old fish out there that we once saw. Finding a fish over 10 is very rare and this is a fish that may spawn initially between 6 and 8 years, so we’re diminishing the opportunity for these striped bass to sample across years that are highly variable in terms of the outcome is for their offspring. That’s a risky proposition.

There are some that are arguing—and I’m one—that we need to not only just manage this fishery for sustainability in terms of yield and the traditional reference points, but also we need to make sure we have age structure in population. We need to have some 15-, 20-year-old striped bass out there because we may come across another period of time like we did in the 70s when we had 10 years of really poor juvenile productions. We don’t have what we had in the bank back then.

But there are people like Secor who are trying to make sure history doesn’t repeat itself.

SECOR: If we look at a population-specific basis, we can inform improved assessments and it’s been very difficult historically to incorporate migration behaviors into stock assessments. Generally, what we want to do is we talk about stocks. So we think of fish in stockyards where we have a nice fence around everything so we build a boundary and say we’re just going to assess what’s in that boundary.

But it turns out that, we can do a better job if we know something about migration behaviors of fish and where they were born, we might be able to look at smaller fenced in areas and better tailor our understanding of biology in our assessments to management needs. So can you fish harder in this area or should you be more conservative in this area? We’re building that information base.