It’s May, and that means that it’s weakfish season here on
Long Island.
Weakfish may not be as popular as striped bass, at least in
modern times, but they are arguably the signature species of Long Island’s
bays. Look at angling literature written
a century ago, and you’ll find that it is the weakfish, rather than the bass,
fluke or blues, that get most of the writers’ attention.
Unfortunately,
weakfish have fallen on hard times.
Although the species has always been known for wide swings in abundance,
a few years ago the population level dropped to record lows, with the stock
retaining only about 3% of its spawning potential. Since then, it has bounced back a little, but
the population still remains at an overfished level.
There has been a lot of debate about what caused weakfish to
decline so badly. A
stock assessment performed in 2009 pointed to increased natural mortality
that, it noted, was probably due to some sort of interaction between weakfish
and other species. While the assessment
never clearly described the nature of such interaction, it shouldn’t be much of
a surprise to learn that some folks quickly decided that all of the weakfish
were being eaten by striped bass, and offered to remove the offending predator
from the ecosystem, so that weakfish might better thrive…
“It seems to me that we’re talking about predator/prey
relationships…the reality is that it seems to me that we have a whole bunch of
fish up here eating on those little critters down here. Until we point to those fish that [are] doing
most of the damage, and either, one, I’ll use the word ‘control’ those quantities
or stock sizes—and I’m going to say the nasty [word ‘SB’], striped bass, and
now fluke and anything else that predates on juvenile weakfish, it leads me to
another question that only—well, it’s this—if we have a minimum threshold for
SSB, has anyone determined what the surplus above the SSB has and its effect,
and particularly on weakfish?”
Fortunately, fisheries managers didn’t take the bait, and
striped bass were not scapegoated for the weakfish’s decline. However, that didn’t change the fact that the
fish’s decline was being attributed to largely natural mortality.
Hearing that, some members of the fishing
community had problems with the notion of reducing weakfish harvest in order to
help slow the population’s decline and perhaps initiate a slow rebuilding.
The attitude of
“It’s not our fault, so we shouldn’t be punished” prevailed in such
discussions.
And although such attitudes were wrong, the fact that
management in so many fisheries is focused on fishing mortality makes it pretty
clear why some people might think that way.
However,
a new benchmark assessment of the weakfish population, which was just released
by the Atlantic States Marine Fisheries Commission, takes a new look at where
fishing mortality fits into the greater scheme of things. Instead of establishing reference points
based on fishing mortality alone, the new assessment recommends a more
complicated structure, in which overall mortality plays a key role.
And that’s important, because when we talk about the health
of the stock, it’s total mortality that matters, and not just the fish killed
by anglers.
Usually, natural mortality is pretty constant, so merely
addressing fishing mortality works well enough.
However, weakfish are infamous for periods of abundance punctuated by
years of scarcity, in which the fish all but disappear.
Over the years, the weakfish’s disappearance has been blamed
on an eelgrass blight that impoverished its traditional feeding grounds,
overfishing and the aforementioned predation by other species. The new stock assessment suggests that it
may, in fact, be tied to the Atlantic Multidecadal Oscillation, a periodic
variation in sea surface temperature that affects the entire North Atlantic
basin for extended periods of time.
Such environmentally-driven declines in population are
probably too large to be averted by any sort of fishery management measure; at
the same time, they make the weakfish particularly vulnerable to overfishing.
Thus, managers have to craft a management approach that is
appropriate for managing weakfish during the good times, when they are
abundant, and also provides them adequate protection when natural mortality
drives numbers down, so that the stock is in a position to recover once more
favorable conditions return.
It’s a challenge that has rarely, if ever, been addressed in
any other marine fisheries management plan, and as far as I know, it’s never
been done at ASMFC. However, it appears
that the most recent stock assessment has mapped out a path to meet that
challenge.
Instead of creating a typical control rule, in which there
are targets and threshold levels of biomass and fishing mortality, the stock
assessment proposes a more complex, two-part approach.
The first step hinges upon abundance. The assessment proposes a spawning stock
biomass threshold of 6,880 metric tons ( a little over 15 million pounds),
which is equivalent to 30% of an unfished stock, assuming average natural
mortality. Any time that the spawning
stock biomass is below that threshold (as it is today), fishing mortality
should be minimized.
Should spawning stock biomass rise above the threshold, as it
hopefully will at some point in the future, managers would then look at total
mortality. That’s when things get a
little complicated.
Since the weakfish’s wide population swings seem to be due
to natural mortality, and that natural mortality also swings from fairly low to
very high levels, managers can’t really sue fishing mortality to control the
health of the stock. Total mortality,
(represented by the letter “Z”) is a far more appropriate standard.
Again, a threshold was established, at Z=1.36 (equal to about 75% of the
fish being removed from the population each year). If total mortality exceeds that level,
fishing mortality should continue to be minimized.
Managers
also included a mortality target, Z=0.98, which is roughly equivalent to 65% of
the fish being removed from the population each year. Should total mortality fall between threshold
and target, a limited amount of fishing would be allowed. In the event that total mortality was less
than the target level, managers would fall back on traditional, fishing
mortality-based approaches, with Ftarget=0.55 (around 42% or 43% of
the population could be harvested each year).
Such approach would remain in place until total mortality again rose
above the target level.
The new
multi-stage control rule, which recognizes that total mortality is the real key
to sustainable fisheries, is particularly appropriate to the weakfish’s
variable natural mortality. However, as
waters warm and fish are confronted with various environmental stresses,
managers may discover that a similar approach will be useful in managing other
species.
Whether or
not that proves to be true, ASMFC deserves congratulations for designing a
rational and, at least in the region, novel approach to resolve a difficult
management problem.
Charlie...
ReplyDeleteI am a firm believer in natural cycles.
I do not see the type of declines in the related species.
What is the status of the other species in the "drum/Sciaenidae" family? Are they showing similar cycles?
Thanks
Greg DiDomenico
Garden State Seafood Association