When the Chesapeake Bay striped bass stock collapsed in the late 1970s, people tried to figure out why.
Recreational fishermen were quick to point fingers at the commercial sector, which was not yet burdened by significant regulation. There were no gear restrictions and no annual quotas, and the fishery accounted for a larger share of the catch than it does today. The line between commercial and recreational fishermen was badly blurred, as few if any states required commercial licenses, and successful anglers regularly sold fish that they couldn’t use.
Commercial fishermen, on the other hand, tended to blame a declining bass population on Mother Nature and the so-called “cycle,” which saw bass populations wax and wane on their own, regardless of human activity.
Both sides were sure they were right, and both were unwilling to consider the other’s position. Now, a recent paper published in the American Fisheries Society’s journal, Marine and Coastal Fisheries Dynamics, Management, and Ecosystem Science, suggests that both the recreational and commercial fishermen owned a share of the truth, but neither had a full understanding of what had occurred.
In “Perspective comes with time: What do long-term egg and juvenile indices say about Chesapeake Bay striped bass productivity?” author James H. Uphoff Jr. uses historical fisheries data to cast new light on the most recent striped bass collapse. In particular, he analyzed the relationship between the Maryland striped bass juvenile abundance index (JAI) and data relating to both the spatial and temporal distribution of striped bass eggs in the Chesapeake Bay, as determined by samples taken in plankton nets towed in the vicinity of known spawning areas.
Uphoff used the relationship between the two data sets to create what he termed an “index of relative larval survival.” If, over the years, there was a fairly constant relationship between egg distribution and the JAI, it would suggest that overfishing was the probable cause of last century’s striped bass stock collapse. On the other hand, if the relationship between egg distribution and the JAI showed significant changes over time (for example, if eggs were widely distributed in the Bay in a given year, but the JAI for such year was low), it would suggest that environmental conditions made a significant contribution to the striped bass collapse.
To further bolster any evidence of environmental conditions being a cause of stock collapse, Uphoff looked at two other species common in the tributaries of the Chesapeake Bay, white perch and yellow perch, to determine whether their patterns of changing juvenile abundance correlated to that of the striped bass.
When all of the data was analyzed, Uphoff found that while the striped bass JAI declined quickly throughout the 1970s, egg abundance didn’t show a similar decline until 1979, and only declined enough to impact striped bass recruitment between 1982 and 1988. Such pattern suggests that, since the level of eggs found in the plankton net tows remained high throughout almost all of the 1970s, the stock collapse could initially be attributed to environmental conditions that were not conducive to larval survival. However, by the early 1980s, chronic overfishing led to low egg production, and so exacerbated the stock’s collapse and delayed its recovery.
The evidence of environmental conditions being the primary driver of the last stock collapse was supported by similar movements in the JAIs of white perch and yellow perch over the relevant period. Changes in the JAI of white perch, a small, non-migratory species that belongs to the same genus as the striped bass, were closely correlated to changes in the striped bass JAI, while there was a significant, but more moderate, correlation between the JAI of striped bass and that of the estuarine yellow perch, an unrelated species.
Throughout the striped bass stock’s recovery, an increase in spawning stock biomass trailed a corresponding increase in egg production by what appears, in a graph accompanying the paper, to be an interval of five or six years; there was a similar correlation between a decline in egg production in the early 1990s and a decline in spawning stock biomass that occurred a few years later. However, beginning around the year 2000, the two values diverged, with spawning stock biomass reaching its peak around 2003 and then beginning a long decline about five years later, while egg production fluctuated, with no clear direction, within a fairly narrow range.
Such divergence again suggests that environmental conditions played a dominant role in the latest decline in spawning stock biomass.
That information can be used to guide the management response to that decline, which also seems to be driven by environmental conditions. While fishery managers have no control over the environment, the history of the past collapse, in which a muted management response led to years of overfishing and so impaired the stock’s ability to rebuild, reinforces the need for conservative management measures that set the stage for rebuilding once environmental conditions improve.
That point was reinforced by another recent paper, “Climate effects on the timing of Maryland Striped Bass spawning runs,” which was written by Angela Giuliano and also appeared in Marine and Coastal Fisheries Dynamics, Management, and Ecosystem Science. In that paper, the author investigated the relationship between warming waters in the spawning areas and the timing and length of the striped bass spawn in portions of Chesapeake Bay. She also examined the age of the fish participating in the spawn and the time at which females from various age classes completed spawning.
The Chesapeake striped bass spawn generally gets underway when water temperatures reach 14o Celsius (about 57o Fahrenheit) and ends when it rises above 20o C (68o F) and significant larval mortality occurs. The timing of the spawn differs somewhat in the various spawning areas. Giuliano found that, while the timing of the spawn’s start has not significantly changed since the 1980s, the end of the spawn in the Chesapeake Bay, defined by the date when water temperatures exceed 20o C, is now occurring earlier, shortening the spawning season. She noted that a similar, 4-day shortening of the spawning season has been observed on the bass’ Hudson River spawning grounds.
In what are probably Guliano’s most important observations about striped bass management, she wrote,
If temperatures continue to warm quicker in the latter portion of the spawning season, this could result in a reduced time period during which temperature conditions are ideal for Striped Bass survival…these temperature changes could affect the timing of larval Striped Bass relative to their zooplankton prey, a concept known as match-mismatch. Large year-classes for Striped Bass tend to occur after cold and wet winters and [research] showed a potential mechanism for this, with the rate that copepods reach the adult stage over the winter being dependent on water temperatures…
Previous literature and the present study indicate that the larger females spawn earlier in the season than smaller females and that this range of spawning dates is a result of natural selection to assure that some larval fish will encounter favorable conditions for growth and survival. With the shifting spawning window and potential changes in zooplankton availability due to rising water temperatures, it has been suggested that having a broad age range of spawning fish will make it more likely that some eggs and larvae will experience these ideal conditions. Although fisheries managers cannot directly control the water temperature that larval fish will encounter, they can consider how management actions may affect the age range of fish available in the spawning stock in addition to the size of the spawning stock. If these management goals are considered in tandem, the Striped Bass stock may be better positioned to adapt to the conditions expected under a changing climate. [emphasis added, citations omitted]
Right now, faced with environmental conditions that are causing an extended period of low recruitment, fishery managers are facing the same situation that they faced in the late 1970s. To their credit, they have, in recent years, taken actions intended to rebuild the overfished striped bass stock, including the adoption of emergency management measures in May 2023. If managers hew to their current course, overfishing won’t be allowed to hamper rebuilding, as it did in the 1980s.
However, the recent research suggests that merely maintaining the size of the spawning stock might not be enough. We must also consider the age structure of the spawning stock.
The Atlantic States Marine Fisheries Commission has already laid the foundation for such a two-pronged approach. One of the stated objectives of its striped bass management plan is to “Manage fishing mortality to maintain an age structure that provides adequate spawning potential to sustain long-term abundance of striped bass populations.”
Now, managers need to adopt measures designed to meet that objective, even if such measures are more restrictive than those needed to merely maintain stock size.
They already have the tools and the knowledge that they need to do so. Hopefully, they also have the will to put such tools and knowledge to use.
Top photo by Kyle Schaefer
