REBUILDING FISH STOCKS: WHAT DOES THE "TARGET" REALLY MEAN?

 

Over the past few years, there has been a lot of talk about rebuilding the Atlantic striped bass population.

The talk heated up in 2019, after a benchmark stock assessment found bass to be overfished, but it had been going on at more muted levels for quite a few years before that. 

Arguably, the first mention may have occurred at the May 2009 meeting of the Atlantic States Marine Fisheries Commission’s Atlantic Striped Bass Management Board when, in response to some members’ efforts to increase the commercial quota, Ritchie White, the Governor’s Appointee for New Hampshire, observed that

“You have issues in the northern range that are trending down substantially.  Maine and New Hampshire, especially Maine, a good chunk of those anglers didn’t see any striped bass this year.  I guess I don’t see things quite as positively as some, and I think it is time for caution.”

While Mr. White didn’t call for a reduction in landings, he certainly didn’t support an increase, and strongly suggested that the bass might be facing a troubling future.

That suggestion proved prophetic three years later, when a stock assessment update indicated that the striped bass would become overfished by 2017.  However, after briefly discussing measures to halt the stock’s decline, and perhaps begin recovery, the Management Board, at its November 2011 meeting, ultimately opted to eschew any remedial action, since the striped bass stock had not yet declined badly enough to trip any of the so-called “management triggers” in the management plan.

A 2013 benchmark assessment found that the striped bass stock was still waning, and its findings did trip two management triggers, but the Management Board still did not undertake a formal rebuilding plan, probably relying on advice from the ASMFC’s former striped bassa fishery management plan coordinator, who advised that the stock would rebuild at some undetermined point in the future if the fishing mortality rate was reduced to its target level.

That rebuilding never occurred, the stock became overfished, and the Management Board is now hoping that striped bass are on track to rebuild by 2029, although recent calculations by ASMFC’s Striped Bass Technical Committee strongly suggest that’s not going to happen at the current fishing mortality rate.

But all of that begs the question of just what “rebuilding” the striped bass stock means.

Amendment 7 to the Interstate Fishery Management Plan for Atlantic Striped Bass states that

“The 1995 estimate of female [spawning stock biomass] is used as the SSB threshold because many stock characteristics, such as expanded age structure, were reached by this year, and this is also the year the stock was declared recovered.  The female SSB target is equal to 125% of the female SSB threshold.”

But that really doesn’t explain very much. 

On one hand, the stock was declared “recovered” in 1995, and had developed many desirable characteristics by that time; based on that, it’s reasonable to ask why the 1995 female spawning stock biomass, rather than 125% of that level, isn’t the standard for a fully rebuilt stock.

On the other hand, for most East Coast fisheries, the threshold defining an overfished stock is 50% of the target level; if that were to hold true for striped bass, then a fully-rebuilt stock should stand at 200%, not merely 125%, of the threshold spawning stock biomass.

Why is 125% the magic number?

The 2018 stock assessment tries to explain it, saying

“[Spawning potential ratio]-based reference points…were associated with unrealistic equilibrium female SSB levels.  For example, fishing at [a fishing mortality rate that would produce a spawning potential equal to 40% of that of an unfished stock] resulted in an equilibrium female SSB approximately two times the highest female SSB estimated in the time series…More reasonable equilibrium female SSB results were associated with lower maximum spawning potential ratios…the fishery has generally operated at or above these levels since approximately 1995.  The [stock assessment subcommittee] was not able to fully explain the dynamics associated with the [spawning potential ratio]-based reference points and therefore only considered empirical reference points associated with female SSB levels.”

To put it more simply, perhaps setting the spawning stock biomass target at 200% of the 1995 level is theoretically correct, but such a target would not be realistically achievable.  On the other hand, setting a lower target would be inconsistent with the spawning stock biomass levels that the bass have consistently achieved for most of the past 30 years.  Thus, instead of trying to use a mathematical model to determine the biological reference points, managers opted for so-called “empirical” reference points that reflect the striped bass stock’s observed performance over the past three decades.

And from a practical standpoint, it works.

Few fishermen would be unhappy if striped bass abundance returned to the levels of the mid-2000s, when fish of all sizes were readily available, and there were plenty of older, larger females in the spawning stock.  At the same time fishermen, and particularly recreational fishermen, were displeased with the lower abundance of bass that they experienced in the mid- to late 2010s, when recruitment was generally below average and many ages and sizes of fish were missing from the population.

So when we talk about a “rebuilt” striped bass population, we are talking about a spawning stock biomass that is large enough, and structured well enough, to support robust, sustainable recreational and commercial fisheries, but not necessarily either a population that meets clear biologically-defined goals nor a population that approaches the abundance exhibited by an unfished stock.

In federal fisheries, the biomass target, and thus the rebuilding target, is much more clearly defined.  It is the biomass—typically measured in terms of spawning stock biomass—that will allow the stock to produce maximum sustainable yield.  Such level is typically determined in a management-track stock assessment, and may be expressed in various ways.  Most common is probably the approach that failed with striped bass—expressing the target in terms of spawning potential, as compared to an unfished stock. 

Thus, the summer flounder target is the spawning stock biomass that will provide 35% of the spawning potential of an unfished stock, the biomass target for Atlantic pollock is based on a 40% spawning potential ratio, etc.  For other species, the biomass target may be based on other criteria; for example, the ASMFC uses fecundity—the number of eggs that can theoretically be produced by the population—to set the target for Atlantic menhaden.

It’s probably important to note that the biomass target represents the minimum level of abundance that will provide optimum benefits from the managed fishery.  Such benefits can be measured in different ways; the Magnuson-Stevens Fishery Conservation and Management Act, which governs all federally-managed fisheries, describes “optimum” yield as

“…the amount of fish which will provide the greatest overall benefit to the Nation, particularly with respect to food production and recreational opportunities, while taking into account the protection of marine ecosystems…  [internal formatting omitted],”

while the ASMFC’s Atlantic menhaden management plan defines the [Ecological Reference Point] target as

“the maximum fishing mortality rate (F) on Atlantic menhaden that sustains Atlantic striped bass at their biomass target when striped bass are fished at their F target,”

and the [Ecological Reference Point] fecundity target as

“the long-term equilibrium fecundity that results when the population is fished at the ERP F target.”

However the target is measured, the important thing to note is that it represents a floor, not a cap, on optimized abundance.  People, including too many people who ought to know better, seem to ignore that fact, and treat stocks that rise above their target levels as some sort of problem in need of correction, rather than an added benefit.

Thus, at the August 2008 meeting of the Atlantic Striped Bass Management Board, we saw Pat Augustine, then the Governor’s Appointee for New York, calling for increased striped bass harvest because the stock had exceeded its target level.  In support of his position, Mr. Augustine argued that

“I’ve often asked the question as how many more striped bass do we have to have in the ocean and do the surplus, quote-quote, above the threshold—and there are some folks that are not going to like what I say, but the reality is what kind of damage are those fish doing to the sub-species below them, including the forage fish that other species are feeding on…

“The bottom line is they’re opportunists, whatever is there they’re going to eat, so to speak.  It just seems to me until we make a quantum move to look into ecosystem management for striped bass, bluefish and weakfish together, it just seems to me we’re limited to single-species management.

“The question that still remains open and unanswered is what are the extra fish above and beyond the threshold doing to the other sub-species?  I’m not trying to start a fight with anybody.  I’m just saying it is a question.  Look at what happened to winter flounder.  We blame weather conditions and water conditions, lack of eelgrass, lack of phytoplankton, zooplankton, et cetera, on that end, and yet what is eating them?”

Apparently, the possibility that the correct answer to that last question was “people”—flounder were, after all, badly overfished for decades—never entered the speaker’s mind, as he made the usual arguments for fishing down an abundant stock in the name of protecting other fish populations, rather than seeking to reduce fishing mortality on depleted stocks so that they could return to more sustainable biomass levels.

Leaving aside the obvious problems with that approach—keep it up long enough and we’ll end up with no healthy stocks at all—it’s hard to argue that maintaining a striped bass stock somewhere between 20% and 40% of its unfished level, which is where the stock seems to have been at the time, is going to threaten ocean ecosystems.  When you think about it, maintaining a stock at well below half of its potential abundance doesn’t seem like it’s creating any sort of real “excess” at all. 

Such a misunderstanding of biomass targets might be understandable coming from someone like Mr. Augustine, who is not a professional fisheries manager, but it is far less forgivable when expressed by someone who is.  Yet, in the National Marine Fisheries Service’s comments accompanying the recently adopted “Harvest Control Rule” that will be used to manage the recreational bluefish, summer flounder, scup, and black sea bass fisheries, we see the agency stating

“Scup and black sea bass are stocks in the ‘very high’ bin, meaning the biomass is over 150 percent of their respective biomass targets—the level of biomass associated with maximum sustainable yield.  In plain language, stocks in this bin are at least 1.5 times larger than is ideal for maximizing long-term benefits…  [emphasis added]”

Again, we see a statement missing the obvious point that a biomass producing maximum sustainable yield is the minimum level that will “maximiz[e] long-term benefits.”  Higher levels of biomass will return greater benefits—the same fishing mortality rate will yield larger harvests, and greater recreational opportunities, when applied to a stock at 150% of B_msy than it will at a stock that hovers right at the target level—although, if biomass at maximum sustainable yield was calculated correctly, such greater benefits will only be transient, as the stock should eventually return to its target level.

But the point that needs to be made is that an above-target biomass, far from being a threat or problem of any kind—is something to be celebrated and enjoyed for as long as it lasts.

For biomass targets are, in the end, a practical approach to maintaining sustainable fisheries.

Far from being a theoretical cap on fish populations, they are just what the name suggests—a target for stock rebuilding that, if reached and maintained, should ensure that the managed fish stock will remain healthy and sustainable for the foreseeable future.

Not rebuilding to target will have a cost in lost opportunities for harvest, recreation and, perhaps, ecological benefits.  Rebuilding beyond the target, on the other hand, merely provides more opportunities for everyone.