STRIPED BASS: WHAT HAVE WE LEARNED IN THE PAST TWENTY YEARS?

 

I was digging around in my archives, and came across a report that was released almost exactly two decades ago.  Titled “A Population Study of Atlantic Striped Bass 2001 Report to Congress,” it was prepared by Gary R. Shepherd of the National Marine Fisheries Service’s Northeast Fisheries Science Center, in cooperation with the U.S. Fish and Wildlife Service, and the Atlantic States Marine Fisheries Commission’s Atlantic Striped Bass Technical Committee.

That report played a big role in the debate preceding adoption of the most recent amendment to the ASMFC’s striped bass management plan, Amendment 6, which was finalized in 2003.  Much like the nascent debate over the proposed Amendment 7, the conversation twenty years ago pitted a group of angler/conservationists who wanted to see a healthy striped bass stock, managed in a sustainable manner, against pro-harvest stakeholders and Atlantic Striped Bass Management Board members, who were trying to push harvest up as high as the science might possibly allow.

Thanks to the Report to Congress, the conservationists didn’t lose.

I can’t say that we won, because we were seeking a target fishing mortality rate somewhere below 0.25, and the final Amendment 6 set it at 0.31.

But I can’t say that we lost, either, because the other side was seeking a rate closer to 0.40, and they didn’t get what they wanted, either.

Instead, the Management Board sort of split the baby down the middle.  As typically happens in such situations, the baby died; or, at least, the Management Board’s actions killed off the likelihood that we’d have a healthy and sustainable striped bass stock over the long term.  But we do have enough bass around to give us a solid base, upon which such a healthy stock can be built, if the Management Board is willing to take appropriate action in Amendment 7.

That, of course, is never a sure thing.

So it’s interesting to see whether the twenty-year-old Report to Congress provides any guidance on the situation that we face today.

In some ways, the report is ‘way out of date.  When it was written, scientists still believed that there was a stock/recruitment relationship that tied spawning success to the size of the spawning stock.  Now, we know that’s not true.

And when it was written, scientists were comfortable with the idea of a target fishing mortality rate around 0.30, with a fishing mortality threshold of 0.40 or thereabouts.  Today, we know that’s not true either.

But there is one bit of wisdom that we ought to think about, because it still applies.  The Report to Congress noted that

“at fishing mortalities higher than 0.2, the proportion of large fish in the population would decline well below the 2000 level.  In this model, large fish are considered all fish greater than age 10 (about equal to or greater than 36 inches in total length).  Since female striped bass are estimated to reach full maturity at age eight, the decline in older fish results in a decreased spawning stock biomass…

“The distribution of age classes in a population has important implications for stock productivity and stability.  Studies on striped bass have shown that larger fish produce larger eggs and larvae, and larger individuals of these life stages have a greater chance of survival…

“…even modest levels of fishing mortality reduce the proportion of older fish in the stock and shift the bulk of egg production towards younger fish…Maintaining a diverse number of ages in the population has the biological advantages of increased spawning potential through inclusion of older more fecund fish, and a reduced risk of poor recruitment associated with dependence on younger fish for egg production…”

Right now, the target fishing mortality rate just happens to be 0.20.  The models used to determine such things have changed in the past 20 years, so we can’t be certain that raising mortality a little bit above 0.2 will necessarily cause the number of larger fish to go into steep decline,  Still, raising the target fishing mortality rate, as some would like to see Amendment 7 do, is certainly ill-advised, absent data indicating that such a decline won’t happen.

Whether 0.20 is still deemed to be the pivotal fishing mortality value or not, retaining a well-structured spawning stock, that contains many older, larger fish, remains important.  Dr. David H. Secor, a biologist who is well-known for his work with striped bass, explained why in his paper “Spawning in the nick of time?  Effect of adult demographics on spawning behavior and recruitment in Chesapeake Bay striped bass.”

There, he noted that striped bass spawning success is not dependent upon the number of spawning females, but rather on temperature, water flow, and other conditions in the rivers where the fish spawn.  Such changes vary not only from year to year, but also from time to time during the same year; even during years when conditions are generally unfavorable, there may be short windows of time when more favorable conditions exist.  Since older, larger striped bass are believed to spawn earlier in the season, while younger, smaller fish spawn later, having a broad range of age and size classes in the population makes spawning failure less likely.

In Dr. Secor’s words,

“Increased age diversity in the spawning stock may increase the temporal and spatial frequency of spawning (spawning dispersion) and thereby increase the probability that some offspring will encounter favorable conditions.

”…In periodic strategists like striped bass, moderately long reproductive lifespans and low adult mortality rates result in accumulation of spawning stock biomass.  When environmental conditions favor recruitment, this large reproductive reservoir generates dominant year classes which drive population growth.  Diversity in age structure and spawning behavior contribute to this storage effect.  If minority spawning behaviors—for instance spawning during early or late parts of a season—in some years resulted in successful offspring, a diverse age structure should contribute to sustained annual recruitment and accumulation of spawning stock biomass.

“Striped bass epitomize periodic strategists, spreading risk of failed replacement through variability of spawning behavior over many spawning seasons.  This life history tactic suggests that a truncated age distribution would result in stock abundance being more closely linked to annual changes in year-class strength.  [emphasis added]”

Not only does a broad age structure better assure that at least some females will meet with spawning success each season, but a truncated age structure would make striped bass more vulnerable to years when conditions in the spawning rivers are unfavorable.  It’s not hard to see how consecutive years of below-average spawning conditions, and so below-average spawning success, suchas most recently occurred between 2005 and 2010, could significantly increase the risk to the spawning stock, particularly if there were fewer larger, older fish to provide what Dr. Secor referred to as “a hedge against a long period of recruitment overfishing,” or a long period of poor recruitment due to any other cause.

Which brings us back to today, and the Public Information Document For Amendment 7 to the Interstate Fishery Management Plan for Atlantic Striped Bass which, among other things, asks for comment on two closely related questions:  Are the current goals and objectives of the management plan still germane to striped bass management, and are the biological reference points used to gauge the health of the stock appropriate, or should they, too, be changed.

The current goal of the management plan is

“To perpetuate, through cooperative interstate fishery management, migratory stocks of striped bass; to allow commercial and recreational fisheries consistent with the long-term maintenance of a broad age structure, a self-sustaining spawning stock; and also to provide for the restoration and management of their essential habitat.  [emphasis added]”

There are a number of different objectives intended to help to achieve that goal, but none is more relevant to this discussion than the one directing the Management Board to

“Manage fishing mortality to maintain an age structure that provides adequate spawning potential to sustain long-term abundance of striped bass populations.  [emphasis added]”

So even 20 years ago, we knew that raising fishing mortality will reduce the number of older, larger striped bass in the spawning stock, make it more dependent upon younger, smaller fish, and increase the risk to the striped bass population.  And Dr. Secor informs us that maintaining a broad age structure helps to maintain the stability of the striped bass stock.

Thus, it’s difficult to understand why the Public Information Document would ask

“Are the existing goals and objectives of Amendment 6 still in line with fishery needs and priorities?”

Because it’s hard to imagine anything more important--and more "in line with fishery needs and priorities"--than a heathy, stable, and sustainable striped bass stock (and if you are one of those who honestly believes that management stability and flexibility matter more than the health of the stock, you aren’t fit to be making decisions that impact a public fishery resource).

And it’s difficult to understand why the Public Information Document would be seeking comments on whether to change the biological reference points to something that would allow a higher fishing mortality rate, and reduce the number of older, larger fish in the population, knowing that by doing so, it can only increase the risk to the stock.

The title of this edition of One Angler's Voyage asks “What have we learned in the past twenty years” about managing striped bass.

What have we learned?

Sadly, when I see such questioins being asked by the Management Board, it seems that the answer is “Nothing.”