WHAT SHOULD WE MAKE OF THE NEW STRIPED BASS STUDY?

Ever since the Atlantic States Marine Fisheries Commission got serious about striped bass management back in the 1980s, the Chesapeake Bay fisheries have stood apart from those on the rest of the coast. 

Amendment 4 to the striped bass management plan, adopted in 1989, created a dual size-limit management approach, that allowed smaller fish to be killed in Chesapeake than could be landed out on the coast.  In justifying such approach, Amendment 4 stated

“Smaller size limits in ‘producing areas’ are necessary for the maintenance of historical fisheries on native fish which have primarily small fish available to them; in the Chesapeake Bay, most striped bass remain in the Bay for only 2 to 6 years before leaving to take part in coastal migrations; large mature fish return each year for only a brief period to spawn in the Bay, returning to coastal waters after spawning.”

The Amendment applied the same logic to fish in North Carolina’s Albemarle Sound, and in the Hudson River.

That distinction between Chesapeake Bay and the coast has endured ever since.  Amendment 5 to the management plan set a Bay size limit of 20 inches, versus a 28-inch minimum size on the coast.  The minimum size in the Bay was dropped to 18 inches in Amendment 6, and was increased back to 20 inches after Addendum IV to that Amendment reduced Bay harvest by 20.5 percent.  Maryland has since adopted a 19-inch minimum size for most of its Chesapeake Bay season, while Virginia has stuck to the 20-inch minimum on its side of the Bay.

The Chesapeake Bay is the single most important nursery area for striped bass, and comprises the majority of the coastal migratory stock.  Young fish remain in the Bay for a period of time before moving out into coastal waters, but just how long they remain is open for debate.

For a very long time, biologists believed that the fish left the bay when they were relatively young, and that most of the migratory fish were females.  A document included in the University of Maryland’s compilation, Ecosystem Based Fisheries Management for Chesapeake Bay; Striped Bass Species Team Background and Issue Briefs, expressed scientists’ prevailing opinion, saying

“The Chesapeake Bay hosts a year-round adult striped bass population that is predominantly male.  The majority of females age 3+ leave the Bay and become part of the coastal migratory stock.  This emigration may take place as early as three years of age, although there is some indication that the emigration by age is gradual with successively greater numbers of emigrants with each succeeding age…” 

The State of Maryland added some nuance to that general statement, noting that

“The striped bass stock within Chesapeake Bay is composed of pre-migratory fish, primarily ages 10 and younger, and coastal migratory striped bass range in age from age 2 to more than age 30,”

and so recognized that there was considerable age overlap between fish that entered the migratory population and those that remain in the Bay.

That makes sense in the context of striped bass behavior in other nursery areas.  The most recent benchmark stock assessment stated that

“…Results from tagging 6,679 fish from New Brunswick, Canada to the Chesapeake Bay , during 1959-1963, suggest that substantial numbers of striped bass leave their birthplaces when they are three or more years old and thereafter migrate in groups along the open coast.  These fish are often referred to collectively as the ‘coastal migratory stock,’ suggesting that they form one homogeneous group, but this group is probably, in itself, heterogeneous, consisting of many migratory contingents of diverse origin.”

Against that background comes a scientific paper that provides new insights into striped bass migration, and could have significant implications for striped bass management. 

The paper, “Differential Migration in Chesapeake Bay striped bass,” was published in the journal Plos One by a team of scientists led by David H. Secor, a biologist who has a long and very well-respected history in striped bass research.

It describes a study in which 100 striped bass of various sizes, captured in the Chesapeake Bay, were fitted with acoustic tags and tracked for four years, as they migrated within and outside the Bay.  The tagged bass were classified into four size ranges, 18-24 inches, 24-28 inches, 28-32 inches, and larger than 32 inches, with the three larger size classes of fish gill netted and tagged in the middle reaches of the Potomac River between March 30 and April 11, and the smallest pound netted and tagged at the mouth of the Potomac on October 30.

Over the course of four years, the researchers found that almost all of the striped bass larger than 32 inches, regardless of sex, joined in the coastal migration, although a few did remain resident in the Bay, while the smaller fish remained in the Chesapeake Bay throughout the year.  

Notably, the researchers found that

“Our results did not support Kohlenstein’s early hypothesis that there is a large pulse of females emigrating at 3 years of age and males remain resident throughout their lives.”

Both males and females joined the coastal migration at about the same size.  The study also revealed that the mortality rates of bass that remain in the Bay, 70.3 percent per year, was approximately twice the 36.9 percent annual mortality rate of those that leave the Bay to engage in the coastwide migration.

So that leads to the question that’s undoubtedly at the forefront of most anglers’ minds:  What does this study mean for striped bass management?

At this point,that is not completely clear.

The researchers suggest that

“size at emigration for Chesapeake Bay striped bass has been stable in recent decades.  This supports a key assumption of the ASMFC’s spatially explicit stock assessment, one that specifies differential migration and holds considerable advantage in specifying the productivity of estuarine and ocean segments of the fishery to better support regional allocation tactics.”

That sounds like, in the next amendment to the management plan and/or after the next benchmark stock assessment, Maryland may succeed in getting the two-stock model it has been seeking, and isolating its Bay fishery from that of the coast.

The researchers also noted that

“Emigration by striped bass, referenced here as differential migration, represents a type of partial migration, influencing population production, resilience, and stability.  Spatial buffering against regional differences in exploitation, pollution, and food web conditions can occur when contingents within populations vary in their migration patterns as documented here for striped bass…the capacity of differential migration to contribute to stability in the overall population and to convey key trophic roles across its range will have much to do with managing exploitation and other sources of mortality in both the Chesapeake Bay and the shelf regions.”

Thus, the applicability of the study’s findings to all, or at least a substantial majority, of the bass spawned in Chesapeake Bay could make a big difference in how striped bass are managed in the future.

So, do the great majority of striped bass remain in the Chesapeake Bay until they’re 32 inches long?

Call me a skeptic, but I have my doubts.  Very serious doubts.

In the early 1970s, just a couple of years after the big 1970 year class showed up in Maryland's juvenile abundance survey.  Ever since then, I’ve observed swarms of little striped bass enter northeastern waters soon after a big Maryland year class was produced in the Chesapeake Bay.  That pattern has continued right through the present day, as a surge of small fish started appearing in local waters a few years after to good 2015 year class was spawned. 

It has worked the other way, too.  Back in the late 1970s, as recruitment began to fall off in the run up to the striped bass stock’s collapse, we noticed a marked lack of small bass in western Long Island Sound, even though big fish were still very abundant and Hudson River recruitment remained more or less on track.  More recently, we saw the number of school fish in the northeast fall off sharply after recruitment declined in for the period 2004-2010.

And when I was actively involved in the American Littoral Society’s fish tagging program, I tagged a number of sub-32-inch bass in western Long Island Sound that were caught within the next year in the Choptank, Potomac and Rappahannock rivers, which all flow into the Chesapeake Bay, which would seem to show some movement of smaller bass between Bay and southern New England.

While the old saw that “correlation doesn’t necessarily indicate causation,” still holds true, it’s also true that when you see a pattern hold for nearly fifty years, you can probably start believing that some sort of relationship might exist.

The researchers recognize that there are a lot of small bass moving along the coast, and admit that they don’t know where they come from, saying

“Another migration behavior not observed [in the study] but well documented is the occurrence of age 1-3 striped bass in shelf waters.  Recent telemetry work has focused on aggregations of these small fish in the shelf waters off NJ and Southern New England, but because they were not subsequently tracked during spawning tributaries, inferences on where they came from are speculative.  Early research noted high abundances of small striped bass in Southern New England waters and attributed them to strong recruitment years in the Chesapeake Bay.  The provenance of young ocean striped bass—whether regional spill over from natal systems or the result of longer ocean migrations, remains unknown…”

If I were to speculate—a formal way to describe making a wild-ass guess—I’d argue that the results of the survey might have been biased by collecting the smallest size contingent, the 18 to 24-inch fish, in late October, when the fall striped bass migration in the northeast was well underway.  

It is possible—and I’m not arguing that this is true, but merely that it’s possible—that the bass collected and tagged in October were all resident fish, because the striped bass of that size that had already joined the coastwide migration were somewhere between New Jersey and Rhode Island at the time.

And because those migrating bass were still too young to spawn, perhaps they wintered in the ocean, as most striped bass do, and never entered the Bay.  That would explain why small bass acoustically tagged off New Jersey and New England didn't show up in the spawning tributaries of the Chesapeake Bay.

For the same reason, the larger striped bass tagged in the spring in the Potomac River could also have been resident fish, which is why they, too, remained in the Bay until they reached the 32-inch size that triggered migration, while other fish the same size were wintering off the Virginia/North Carolina line and migrating north in the spring.

Right now, that’s just a guess, one that doesn’t even rise to the level of a legitimate hypothesis--though in all honestly, I think that it’s true.

Yet that doesn’t mean that there’s any flaw in the new study, which represents good, peer-reviewed work and makes its own real and significant contribution to the knowledge we have of striped bass.

It’s just that science rarely moves forward in giant steps.  

Every study adds an incremental bit of knowledge, teaching us something that we did not know before.  And when the researchers tell us that the small bass on the coast represent

“a topic that would be well engaged through genetic markers or additional telemetry research,”

they’re telling us something that we should already realize:

We will never know it all.  Science advances by degrees.  More research will always be needed.