Thursday, March 31, 2016
COOKING THE BOOKS ON MENHADEN
Earlier this month, a report entitled “The Fate of an Atlantic Menhaden Year Class” was distributed by the Menhaden Fisheries Coalition.
That report concluded that fishing mortality is a very small part of overall menhaden mortality, and implied that further landings reductions will have little benefit for the stock. It’s just the sort of conclusion that one would expect in a report commissioned by a group commissioned by the menhaden industry, which is trying hard to defeat conservationists’ efforts to further regulate menhaden harvest.
The menhaden report is interesting, because the data it presents isn't wrong. It was lifted from the most recent menhaden stock assessment, which determined that the stock was not overfished and not subject to overfishing.
But that’s not the point.
It wasn’t the data, but how it was used, that ought to raise eyebrows.
The hot issue in menhaden management right now is the so-called “BERP,” Biological and Ecological Reference Points. It’s cutting-edge stuff, and scientists are still trying to figure out the best way to figure things out.
In traditional, single-species fisheries management, the key concept is “sustainable yield,” with “yield” being the operative term. Biologists calculate the size of the stock that will allow the greatest volume of fish to be harvested over the long term, and then designate that stock size as Bmsy—the biomass that produces maximum sustainable yield. It usually becomes the stock rebuilding target. Harvest levels are then set at or, more typically, modestly below maximum sustainable yield.
The definitions can differ from species to species, but ½ Bmsy is usually the threshold that defines an overfished stock.
Although that’s not exactly how menhaden are managed—the key metric isn’t biomass, but rather fecundity, or the number of eggs produced by the spawning stock—the principal is the same, with the overfishing occurring when fecundity is one-half of the fecundity target.
However, that definition only considers one parameter, the ability of the stock to sustain current harvest levels without declining in abundance. What it doesn’t consider at all is whether a stock size limited to Bmsy will provide adequate support for the greater ecosystem in which it exists.
That’s important in the case of menhaden, because they’re one of the most important forage fish on the East Coast. At some time or another during their lives, everything from the humble sea robin to the great whales will feed on menhaden at least some of the time.
The Menhaden Fisheries Coalition’s report doesn’t consider BERP at all. It tries to argue, using questionable logic, that menhaden harvest removes such a small percentage of fish from each year class that current harvest levels are fine. By taking that approach, it also implies that the conservationists’ efforts are, at the least, misguided.
Although the document is structured to resemble a scientific paper, it’s not. It is an industry propaganda piece, and needs to be read with that in mind. For example, the paper notes that
“The analysis depicts the magnitude of losses for the year class as it ages, keeping the losses of natural mortality and both types of fishing mortality, reduction and bait, in the proper perspective. [emphasis added]”
Yet who is to determine what the “proper perspective” should be? Giving that responsibility to the menhaden industry permits it to bias the reader in favor of its own analysis.
That alone should raise warning flags.
And those warning signs are certainly justified by the way the Menhaden Fisheries Coalition tried to address the issue.
Earlier, I noted that the report ignored BERP. However, it also ignored fishing’s impact on the fecundity of the stock—the very measure that biologists use to determine the health of the menhaden population.
Instead, the report compares removals from fishing activities, over the life of a typical year class, to the size of that year class at the beginning of its very first year of life--before the juvenile menhaden begin experiencing any sort of predation or environmentally-related loss. Based on that comparison, the author assures us that fishing has relatively little impact on the stock, accounting for a meager 6.4% of all mortality throughout the life of the year class.
Does that even make sense?
All species of fish probably experience their highest levels of mortality, in absolute terms, during their first year, when the juveniles are small enough to be vulnerable to just about anything that swims. The size of a typical Age 0 year class of menhaden, when measured in individual fish, is so large that a comparison to the number of older fish harvested would, of necessity, make that harvest appear surpassingly small.
When losses from fishing mortality are calculated in that manner, the report is completely correct when it states that
“Whatever way you wish to calculate the impact of fishing, both reduction and bait, on the size of the standing stock [measured in individual fish], the overall percentage remains remarkably small.”
But there’s another, equally correct statement that deserves attention, and that’s when the report notes that
“with the greatest majority of natural mortality occurring on age zero and 1 year old fish, it is a fair assumption to identify predation as a major contributor of these small fish.”
That sentence is important for a couple of reasons. First, is an acknowledgement of how important menhaden are as a forage fish; second, because it must be read in conjunction with a sentence from the benchmark stock assessment, which states
“Throughout the time series, age-2 and age-3 fish have produced most of the total estimated number of eggs spawned annually; however, in more recent years, ages-4+ have contributed more significantly to the overall number of eggs.”
Thus, when fecundity is the measure of a stock’s health, the measure used by the Menhaden Fisheries Coalition to measure harvest’s impact is completely inappropriate. What matters isn’t the number of fish harvested compared to the original size of a year class.
What matters is the number of fish harvested compared to the overall abundance of the older fish that produce the eggs.
And when we use the report’s data to tell that particular story, fishing mortality rises to a lot more than 6.4% of the stock.
It breaks down like this:
At age 2, about 23% of the year class is removed. At age 3, the number drops a bit to 16%.
After that, the number of fish begins to drop off quickly, since the reduction fleet concentrates on age-2 and age-3 menhaden, the most reproductively important age classes in the population.
Once the fishery is dominated by the bait fishery, percentages drop sharply, to 9% at age-4, 2% at age-5 and effectively 0% at ages-6+.
The removals of the reproductively imporant menhaden total about 19%, roughly triple the 6.4% removal figure touted in the report. The removals of the most important spawners—the age-2s and age-3s—is a little higher yet, about 21%.
Maybe that's still low enough to avoid overfishing, but it sure doesn’t tell the whole story, for the critical question isn’t just how many menhaden may be taken out of the water, but also how many must be left in to provide adequate forage for restored stocks of fish. Everything from the summer flounder that might snatch a peanut (age-0) or two when they swim past an inlet sandbar to adult bluefin tuna that slam into schools of adult fish in winter off North Carolina, regularly feed on menhaden, and there must be enough left in the water to fulfill their needs.
And that’s before we even begin to consider the ospreys, humpback whales and everything else that make menhaden part of their diet.
We won’t even come close to knowing how many menhaden they all will need until the BERP calculations are done.
In the meantime, we need to keep viewing the issue from the “proper perspective.” And that’s the perspective that we deem to be proper, and not the one that the industry tells us to choose.