Thursday, May 12, 2016
ASMFC TAKES NOVEL APPROACH TO WEAKFISH MORTALITY
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.