Thursday, May 19, 2016
WHEN THE NUMBERS DON'T TELL THE WHOLE STORY
Scientists have been modeling fish populations for quite a few years. Even so, such modeling didn’t play much of a role in fisheries management here on the East Coast until 1995, when a “virtual population analysis” developed by Maryland biologists was used to determine whether the Atlantic striped bass stock had recovered from a precipitous collapse.
In Amendment 5 to the Interstate Fishery Management Plan for Atlantic Striped Bass, the Atlantic States Marine Fisheries Commission announced that
“personnel from Maryland Department of Natural Resources developed a model to estimate the total weight of sexually mature striped bass females in Chesapeake Bay and along the Atlantic Coast…The model can also estimate future relative population size under given regulatory programs.
“The predictive abilities of the [spawning stock biomass] model are its true utility. Managers can examine the effects of proposed regulations (e.g. minimum sizes, F rates) and biological factors (e.g. recruitment variability) on the adult female population. Since female biomass is the currency of reproductive potential in the population, the model’s output describes the past, current, and future ability of the stock to replenish itself through annual reproduction. Additionally, the comparison of current SSB estimates to historical high reference levels (estimated from 1960-1972) allows managers to evaluate the relative health of the population and its rate of recovery (or decline).”
ASMFC’s strong endorsement of the Maryland population model was a bit premature; time has demonstrated that many changes had to be made before it truly reflected the state of the striped bass population. Yet today, a little more than 20 years after Amendment 5 was released, ASMFC’s words do ring true. The current version of the model now provides a powerful tool that can be relied upon when managing one of the most popular, and certainly the most scrutinized, fisheries on the East Coast.
While using a virtual population analysis to manage striped bass seemed novel in 1995, such model-driven management seems to be the norm today. Thanks to the success of the striped bass model, we sometimes tend to accept the conclusions of all of the population models without too much question. However, some models have proven to be better than others, and some haven’t worked at all.
The question managers must then ask is, what do you do when a model fails?
Such failure isn’t as unusual as one might believe. Some fish have life histories that make them difficult to model, some data used in models can be ambiguous and some models can be internally flawed.
The mid-Atlantic stock of black sea bass has proven notoriously difficult to model. There are a number of reasons, including the fact that the fish is a protogynous hermaphrodite, with most fish beginning life as females but each having the potential to transition into a male at some point in their lives. The stock also appears to be comprised of three separate sub-stocks that inhabit distinct areas of the coast during the summer, with little or no mixing, but frequently mix on the offshore grounds during the winter.
Such factors were apparently not adequately addressed in the most recent stock assessment, which was completed in late 2011. As a result, a peer review panel found such assessment inadequate for management purposes, leaving the Mid-Atlantic Fishery Management Council and state fisheries managers in a quandary as to how to manage the stock.
They opted for prudence, adopting a constant-harvest approach that they recognized was overly conservative but, given the lack of good data, was needed to assure that overfishing would not occur. At the August 2015 Mid-Atlantic Council meeting, biologists presented a new approach that, while still conservative, allowed managers to increase harvest levels without putting the stock at risk.
Some fishermen complained that even the new approach was far too conservative, claiming that there was an abundance of black sea bass in local waters and that the stock could easily sustain a higher harvest.
They may be right.
However, there is also a chance that they may be wrong; given the choice between harvesting too few fish because regulations don’t permit fishermen to fully exploit an abundant resource, or harvesting too few fish because the stock has been overexploited and the fish just aren’t there any more, I’d prefer the former situation every time.
It’s always best to err on the side of caution.
Of course, that’s not always what managers do.
In 2008, the National Marine Fisheries Service conducted an assessment of Gulf of Maine cod. Taken on their face, the numbers looked good, and strongly suggested that the stock was on its way toward recovery.
But when folks looked a little deeper, things weren’t so clear. The evidence of growing abundance wasn’t reflected in all of the trawl samples used to evaluate the stock. Instead, many such samples showed very meager abundance, while just one or two caught an abundance of fish.
At that point, biologists had to make a decision. Did the samples that caught very few cod reflect the true state of the stock, and were the two trawls loaded with fish merely an anomaly? Or should the results of all of the trawls be considered together, and the combined result used to evaluate the stock.
In the end, biologists chose the latter option, and time proved that they had erred. For in 2011, NMFS conducted an update to the 2008 assessment, and discovered that the Gulf of Maine cod population hadn’t been increasing as they believed; instead, it had declined significantly.
Managers’ failure to make the more risk-averse assumptions had come back to haunt them, and to severely stress the New England groundfishing community.
Today, fishery managers along the South Atlantic coast are facing another perplexing situation. This time, it is arising out of ASMFC’s most recent benchmark stock assessment for red drum.
The initial assessment was performed through the SEDAR process in late 2014. A prior assessment, conducted in 2009, found that
“abundance of young fish for both the northern (NJ-NC) and southern (SC-FL) stock complexes have remained relatively stable since 2000. The stock assessment concluded that sufficient numbers of young fish are surviving to move offshore and join the adult spawning population, indicating that overfishing is probably not occurring.”
However, the data which went into such assessment was limited. The 2014 SEDAR report noted that
“The last benchmark stock assessment…was able to provide reliable estimates of spawning potential ratio and escapement, but estimates of abundance and biomass were considered too uncertain for advice to manage the two red drum stocks.”
Thus, in preparing the 2014 benchmark assessment, a new model was used, which seemed better suited to the data available. However, in practice, the new model did not yield stable results. Reliable runs that accurately reflected the state of the two stocks could not be produced. Instead of analyzing the suitability of the assessment for fishery management purposes, SEDAR’s review workshop could only recommend steps for improving the modeling effort.
In response, Addendum II to the benchmark stock assessment was drafted, and presented to ASMFC earlier this month. Addendum II showed both stocks to be at lower abundance levels than previously believed; the spawning potential of each was below such stocks’ threshold levels.
Some anglers had already grown concerned about the health of the red drum stock, and were seeking additional protections even before ASMFC released the updated assessment.
Other anglers characterized the assessment as “junk science,” or fell back on conspiracy theories, calling the assessment an effort to reduce the harvest in Georgia, which has the most liberal regulations anywhere on the East Coast.
ASMFC’s South Atlantic State/Federal Fisheries Management Board had enough doubts about the accuracy of the assessment that, in a unanimous vote, it refused to accept the stock assessment for management purposes, and instead tasked its Technical Committee to answer a series of questions and present additional data related to its conclusions.
And that leaves red drum in an uncomfortable place.
If the stock assessment is, in fact, correct, both the northern and southern stocks of red drum are badly overfished, and in immediate need of tougher management measures. By delaying such action, the Management Board may have put both stocks at risk of even more serious depletion. Red drum could follow the path of Gulf of Maine cod.
On the other hand, if the stock assessment is flawed, the delay will have prevented imposing unnecessarily harsh restrictions on recreational harvest, in effect preventing red drum anglers from experiencing the same sort of frustration that black sea bass fishermen in the mid-Atlantic are now having to bear.
So the question ultimately comes down to one of weighing the risks.
Is it better to place the fish stock in jeopardy, and perhaps cause it to fall to levels that will require a long and painful rebuilding process?
Or is it better to frustrate a few fish-hungry anglers, forcing them to go home with somewhat lighter coolers until the issues can be resolved, at which point they will be able to go back to enjoying a healthy and abundant fish stock?
I would have chosen the latter course. ASMFC’s management board chose the former.
We can only hope that they were right.