STRIPED BASS AMENDMENT 7--NAVIGATING THE PID: PART II, BIOLOGICAL REFERENCE POINTS

 

In the last edition of One Angler’s Voyage, I provided a brief overview of the Public Information Document for Amendment 7 to the Interstate Fishery Management Plan for Atlantic Striped Bass (PID), along with a discussion of the Goal and Objectives that ought to guide the new amendment.

In today’s blog, we’ll look at biological reference points, and why choosing the right reference points is critically important to the long-term health of the striped bass stock.

Let’s start by addressing one statement that occurs on page 7 of the PID, which never should have made it into the document.  It says

“the current reference points may be unattainable given the current objectives for fishery performance.”

You need to understand, before reading any farther, that such statement was made without any scientific or other factual support; it is merely an editorial comment that made its way into the PID at the behest of John Clark, a Delaware fishery manager, who is doing everything that he can to increase Delaware’s commercial striped bass quota. 

The reference points used to manage the striped bass fishery are “empirical” reference points derived from observations of the striped bass stock at different points of time; for whatever reason, the stock assessment model has been unable to calculate appropriate biological reference points for the striped bass.  

The most recent benchmark stock assessment reveals that

“The reference points currently used for management are based on the 1995 estimate of female [spawning stock biomass].  The 1995 female [spawning stock biomass] is used as the SSB threshold because many stock characteristics (such as an expanded age structure) were reached by this year and the stock was declared recovered.”

That benchmark assessment, which represents the best available scientific information about the striped bass stock, also says that

“To estimate the [fishing mortality] threshold, population projections were made using a constant [fishing mortality rate] and changing the value until the [spawning stock biomass] threshold value was achieved…

“For this assessment…[the spawning stock biomass] threshold was estimated at 91,436 [metric tons] (202 million pounds), with [a spawning stock biomass] target of 114,295 [metric tons] (252 million pounds).  The [fishing mortality] threshold was estimated at 0.240, and the [fishing mortality] target was estimated at 0.197.”

In other words, what the science tells us—contrary to the language in the PID—is that the current reference points are obtainable.  But to rebuild female spawning stock biomass to the biomass target, the Management Board must first reduce fishing mortality to the fishing mortality target and keep it there long enough for the stock to rebuild.

That’s something that the Management Board has never been willing to do.

But never doubt that the only reason that the current reference points might appear to be “unattainable” is because the Management Board has, to date, lacked the moral courage and political will to do what’s required pursuant to the explicit language of the striped bass management plan.

Language in the PID that talks about “management stability” and “flexibility” is merely an effort to condone the Management Board’s failure to maintain the health of the striped bass stock.

That observation is very relevant to the PID’s Issue 2, Biological Reference Points, as there are number of Management Board members who are seeking to escape the burdens of rebuilding the stock by reducing the biomass target; they want to increase landings in the short term, even if such landings would place the long-term health of the stock in greater jeopardy.  To accommodate such Management Board members, the PID says that

“other empirical-based reference points could be considered, such as the estimate of [spawning stock biomass] in a year other than 1995 as the [spawning stock biomass] threshold…For example, the [Atlantic Striped Bass Technical Committee] discussed 1993 as a possible alternative proxy year because the [spawning stock biomass] was lower than in 1995 but still produced a strong year class.”

Citing 1993 as an alternative proxy year “because the SSB was lower than in 1995 but still produced a strong year class” is a red herring of the first order.  Striped bass spawning success isn’t directly linked to the size of the female spawning stock biomass.  That biomass peaked in 2003, when the Maryland striped bass juvenile abundance survey returned a young-of-the-year index of 25.75, which was well above average, but not much different from the 1989 index of 25.20, despite the fact that, in 1989, a much smaller striped bass stock was still clawing its way back from its collapse in the decade before.  1993 did produce a strong year class—the Maryland index was 39.76 that year--that was more than four times larger than the below-average 9.27 produced by the fully-recovered spawning stock in 1995.

Trying to correlate the size of any given year class of striped bass with the size of the spawning stock at the time is an exercise in futility.  Unless the spawning stock biomass has fallen so low that it is physically incapable of producing a large year class, spawning stock size does not predict spawning success.  A small spawning stock can still produce a large year class, as it did in 1989.  And a large spawning stock can, and often does, produce below-average spawns, as was the case in 2006—just three years after the biomass peaked—when the Maryland index was a dismal 4.25.

What really matters are the environmental conditions in the spawning rivers, which are dictated by the weather each year.  Cold winters and wet springs tend to produce successful spawns and large year classes of juvenile striped bass, while warm winters and dry springs lead to poor spawning success and small year classes of juvenile fish.

As Dr. Michael Armstrong, Assistant Director of the Massachusetts Division of Marine Fisheries, noted during an American Sportfishing Association-sponsored webinar last July,

“Recruitment is striped bass is highly variable…When you have a series of lows…we start seeing spawning stock biomass eroding, and that’s exactly what has caused the [current] erosion of spawning stock biomass, it’s these poor year classes.  It’s primarily not fishing, it’s primarily environmental causes.  And the primary cause is…the water regime in Chesapeake Bay.  When you have flood springs, you get bad recruitment.  When you get really dry springs, you get bad recruitment.  When you get nice cool, wettish springs, you get big year classes…”

The problem, of course, is that no one can predict, a year or even years in advance, what the environmental conditions in the Chesapeake’s tributaries will be when it’s time for the striped bass to spawn.  If the conditions are good for an extended period of years, striped bass abundance can remain high for a while.  But if the conditions are poor for a number of years in a row, as they were for most of the years between 2004 and 2010, striped bass abundance can plummet; the fact that striped bass spawning stock biomass peaked in 2003 didn’t prevent it from declining sharply in later years.  There is always uncertainty about when the next strong year class will be produced.

Thus, there is only one way to maintain a healthy striped bass stock.  As Dr. Armstrong also noted,

“We have to husband the big year classes along the best we can.  The only way to do that is to keep [fishing mortality] low.”

That being the case, it only makes sense to maintain the current biological reference points.  Lowering the biomass threshold to 1993 levels, as suggested in the PID, is contrary to that goal, and the fishing mortality reference point associated with such a 1993 threshold and target would be higher than the fishing mortality reference point being used today.

There is also another reason for maintaining the current reference points.  A higher fishing mortality target that would be associated with a lower spawning stock biomass target would tend to truncate the age and size structure of the spawning stock, and that, in turn, increases the risk to the striped bass. 

That’s something people often don’t think about; it’s somewhat intuitive to assume that if fishing mortality is increased, that increase will have an equal impact on every year class in the population, but that’s not how things actually work.  Higher fishing mortality rates have their greatest impact on the older age classes; when such rates increase, the population tends to lose its fish, and reduce the number of year classes in the spawning stock.

That’s not a good thing.

Twenty years ago, biologist David H. Secor, who is well-known for his work with striped bass, published a paper in the ICES Journal of Marine Science that addressed the issue.  In that paper, Dr. Secor noted that

“reduction in year-class diversity renders a population more vulnerable to recruitment failures.”

He also observed that, with respect to striped bass spawned in the Chesapeake Bay,

“Lowest year-class strengths were observed during periods when age structure [of the spawning stock] was severely truncated.”

That’s apparently due to the fact that female striped bass of different ages spawn at different times, with the older, larger fish generally being the first to spawn.  When there are many different year classes of bass represented in the spawning stock, it makes it more likely that, even in years with generally unfavorable spawning conditions, at least some striped bass will time their spawn to coincide with a period when spawning conditions are somewhat better, and thus prevent spawning failure.

When the age structure of the spawning stock is truncated, the stock loses much of its spawning time diversity; the fish are all about the same age, spawn at about the same time, and fail to produce many juveniles if, at that time, they encounter hostile spawning conditions.

Finally, having older fish in the population provides a buffer against long periods of below-average spawns.  Dr. Secor points out that, even after the striped bass stock collapsed in the late 1970s, it managed to produce a fairly successful spawn in 1982; that 1982 year class later become the foundation for the  stock’s recovery. He notes that

“most egg production in 1982 was attributable to striped bass >10 years in age.  Old remnant females produced during the 1960s were a hedge against a long period of recruitment overfishing that occurred during the 1970s.  Striped bass epitomize periodic strategists, spreading risk of failed replacement through variability in spawning behavior over many spawning seasons.  This life history tactic indicates that a truncated age distribution would result in stock abundance being more closely linked to annual changes in year-class strength.  [emphasis added]”

To put that in a PID context, if the reference points are changed, and a lower spawning stock target and higher fishing mortality target are adopted, and the age structure of the spawning stock becomes more truncated as a result, the stock will become more vulnerable to extended periods of below-average recruitment such as we saw throughout most of the 1970s and 1980s, and saw again, just a few years ago, in the period 2004-2010.

Thus, maintaining the current biological reference points is the best way to avoid truncating the age and size structure of the spawning stock, and by doing so, ensure the continued resiliency of the striped bass stock, and minimize its vulnerability to periods of low recruitment.

In response to Issue 2 of the PID, the current biological reference points should not be changed.

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In the next edition of One Angler’s Voyage, we’ll look at the issues of management triggers, intended to compel the Management Board to act when spawning stock biomass falls too low or fishing mortality rises too high, and how to rebuild the spawning stock.