However, recently-published research suggests that, at least
in the years 2023 and 2024, there were more than enough zooplankton around to
support a successful striped bass spawn; even so, the
Maryland juvenile abundance indices for those years were 1.02 and 1.98,
respectively, far below the long-term average of 11.0.
The
paper “Influence of feeding on zooplankton on Striped Bass postlarval
mortality, growth, and year-class success in the Choptank River, Maryland,
during the 1980s and 2023-2024” was published on December 22, 2025, in the journal
Marine and Coastal Fisheries.
The lead author, James H. Uphoff, Jr. is an experienced and very
well-respected biologist employed by the Maryland Department of Natural
Resources, who is well-known for his striped bass-related research.
The paper casts doubt on the suggestion that a mismatch
between the timing of the striped bass spawn and the availability of zooplankton
is responsible for the poor striped bass spawns of the past seven years, observing
that
“High feeding incidence of first-feeding Striped Bass
postlarvae on zooplankton and low mortality did not always translate to better
year-class success during the 1980s and 2023-2024. A prominent role of poor larval feeding
success on zooplankton was not suggested for continuous poor year-class success
during 2019-2024.”
In the Introduction to the paper, the authors note that
“Year-class success of Upper Bay Striped Bass is largely
determined within the first 3 weeks of life in early spring and is mainly a
product of environmentally influenced, highly variable survival of eggs and
larvae.”
They go on to state that
“Water temperature and flow are important environmental influences
on year-class success of Striped Bass.
Survival of eggs and prolarvae is negatively affected by low (<11-12oC)
and high (>21oC) water temperatures. A large portion of spawning may occur over a
few days early in the season, starting at low—sometimes lethal—temperatures in
a gamble that subsequent conditions will favor offspring survival. Temperature may also indirectly affect
survival via its effect of the timing of zooplankton blooms for first-feeding
larvae (match-mismatch hypothesis), while flow has been associated with
zooplankton dynamics, nursery volume, location of the nursery, advection from
the nursery, water quality, and toxicity of contaminants. [citations omitted]”
And that
“Successful or unsuccessful initial feeding of larvae is one
of the foundational hypotheses explaining variations in year-class success of
fishes, but a strong connection has not been universally supported by
studies. Growth and mortality in larval
fish may be linked processes connected to feeding success and expressed as
variable sizes at age, stage durations, and stage-specific mortality, and as
highly variable year-class success. The
concept that larval feeding and size-specific growth and mortality rates
positively interact predicts that survival of a cohort is directly related to
feeding and growth rates during the prerecruitment period. [citations omitted]”
With those things in mind, the researchers set about
capturing striped bass postlarvae (defined as larvae that had absorbed their
yolk sacs) to determine whether poor feeding success might have led to the poor
2023 and 2024 year classes. They then
compared the feeding success of those samples with the feeding success of
postlarvae during seven years during the 1980s (1981-1986 and 1989), a period
that included the worst years of the last stock collapse, when spawning success
was, at times, as poor as it was in 2023 and 2024 (Maryland JAIs of 1.98 in
1980, 1.22 in 1981, and 1.37 in 1983).
The researchers acknowledged such poor spawning years, saying
“The 1980s were generally a period of poor year-class success
and lower postlarval survival in the Choptank River, but several years of
higher postlarval survival were present, as were a moderate year class (1982
[JAI 8.45]) and a strong year-class (1989 [JAI 25.20).”
In comparison, the highest Maryland JAI for the period
2019-2025 was last year’s 4.04.
Once the postlarval striped bass were obtained, the contents
of their guts were sampled, to determine if they contained food and, if they
did, what the nature of that food was. Food
was broken down into three categories, 1) cladocerans (tiny crustaceans
popularly known as “water fleas”), 2) copepods (another group of tiny
crustaceans), and 3) miscellaneous.
The researchers then scored the presence or absence of food using
a “feeding incidence” value, calculated by dividing the number of postlarval striped
bass with guts containing food items by the total number of postlarval bass
sampled. The resultant feeding
incidence, or “FI” could be limited to a
specific category of food items, or could be broadened to include cladocerans,
copepods, and miscellaneous food and non-food items.
During the 1980s, three years (1981, 1982, and 1989) had
high FIs for cladocerans, exceeding .50 in all cases, while the remaining years
had cladoceran FIs of less than 0.35.
Copepod FIs were high (over 0.35) in 1985 and 1990, moderate (0.22-0.26)
in 1982, 1984, and 1989, and below 0.11in the remaining years. While the cladoceran FI seemed to have little
correlation with postlarval survival, years when the copepod FI was less than
0.11 saw relatively high levels of postlarval mortality; relatively low
mortality rates occurred in years when the copepod FI exceeded 0.38, which years
also saw higher growth rates in the postlarval bass.
When those 1980s results were applied to the survey findings
from 2023 and 2024, the researchers found that for “first-feeding” larvae—those
between 5 and 7 millimeters in length—the copepod FI in 2023 was “well above”
the 0.38 level that resulted in favorable growth and mortality rates during the
1980s, and was slightly above that level in 2024 as well; both the 2023 and 2024
copepod FIs were higher than all but one copepod FI measured previously. The 2023-2024 copepod FIs for postlarval bass
between 8 and 10 millimeters in length were significantly higher than the FIs
for the 5 to 7 millimeter category.
The researchers noted that the strongest year class of the
1980s, 1989, resulted from a high initial abundance of first-feeding larvae, a
moderate copepod FI, and a high cladoceran FI; in the next-highest year class, 1982,
the copepod and cladoceran FIs were similar to those of 1989, but the initial
abundance of first-feeding larvae was lower.
Yet the 2024 year class, which also had similar, if not higher, FI
values for both cladocerans and copepods, was poor.
That was a surprising result, for based solely on FI values,
the 2023 and 2024 year classes should have been at least moderately successful,
if not markedly strong.
But that was not the case.
So, the researchers went on to comment on water temperatures,
observing that
“The period when postlarvae predominated in the Choptank
River occurred earlier in 2023-2024 than during the 1980s. Postlarval collections began on April 13 in
2023 and on April 18 in 2024. Postlarval
periods in the years 1980-1988 were estimated to begin as early April 22 [sic] and
as late as May 18…Postlarval periods for 1989 and 1990 were estimated to have
started on May 3 and May 1, respectively…
“Temperatures rose very quickly in 2023-2024 during the
1-week interval from what we interpreted as peak spawn to when the sampling of
feeding larvae started…
“Analyses of temperatures in long-term spawning surveys of
eggs or adult spawners have indicated a shortening of the spawning season since
around 2000. In general, spawning temperature
milestones for the beginning and end of spawning in the Choptank River,
Nanticoke River, Potomac River, and Head of Bay indicated that spawning was not
starting much earlier, but it peaked and ended earlier. Shortening of the spawning season would be
reflected by earlier postlarval periods.
[citations omitted]”
The researchers concluded by writing,
“Our investigation of Striped Bass postlarval feeding success
in 2023-2024 did not indicate that FIs on major zooplankton prey were too low,
and our proxy indicator of [the mortality rate] did not indicate high
postlarval mortality. Our feeding
investigation did not encompass the entire recent 6-year drought in year-class
success, but the findings for 2023-2024 did not indicate a consistent,
prominent role for feeding success…”
Most other factors that limited spawning success in the past
do not seem to be playing that role today, so
“This leaves changes in temperature during spawning and early
larval development as a hypothesis warranting investigative emphasis.”
The new paper is creating something of a stir among those
who follow and try to predict striped bass spawning success, as the
match-mismatch hypothesis had many proponents and seemed to explain the year-to-year
variability of striped bass spawning success.
To that point, the paper does not directly challenge the match-mismatch
hypothesis; it may well have impacted striped bass spawning success in the
past, even though it doesn’t seem to be responsible for the poor spawns of 2023
and 2024.
Could it have been responsible for other poor spawns over
the past seven years?
Right now, we don’t have the data to know, but we do have
the data to show that water temperatures in the Chesapeake tributaries where
striped bass spawn is rising, and the rising water temperatures are shortening
the period when conditions are right for a successful spawn.
So those rising temperatures may very well be a prime
contributor to the current spawning drought.
But maybe we are trying too hard to find a single culprit. Maybe the bass are falling victim to multiple
adverse conditions, not all of which need to occur in a single year.
“hypothesizes that it is easier to predict poor recruitment rather
than good recruitment because an environmental variable effects recruitment
only when its value is extreme (fatal); otherwise, the value maythe be benign
and not influence recruitment. Thus,
good recruitment necessitates all environmental conditions not be harmful and
for some to be especially favorable; poor recruitment, however, requires only
one environmental variable to be extreme.”
Looking at the past seven years in that perspective, it is
possible, but not necessarily true, that a match-mismatch situation contributed
to some of the bad spawns over the past seven years, while high water temperatures
and/or a shortened spawning season contributed to others. And maybe some yet-unidentified factor also
played some role.
The only thing that seems likely right now is that the poor
2023 and 2024 year classes did not result from a zooplankton/larval striped
bass mismatch, and that something else—perhaps higher water temperatures—caused
the dismal spawns in those years.
Whether that “something else” caused all of the poor spawns
that have occurred since 2019, and might also cause further poor spawns in the
immediate future, is something that the scientists still need to figure out.
Until they do, managing striped bass with the assumption
that the current very low recruitment will continue well into the future seems
to be the right thing to do.
No comments:
Post a Comment