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.
-----
This essay
first appeared in “From the Waterfront,” the blog of the Marine Fish
Conservation Network, which can be found at http://conservefish.org/blog/
No comments:
Post a Comment