UNDERSTANDING STRIPED BASS RELEASE MORTALITY, AND ITS IMPACT ON THE STOCK

Ever since the latest benchmark assessment of the striped bass stock, released in 2019, revealed that nearly half of all fishing mortality resulted from fish that didn’t survive after being returned to the water by anglers, striped bass release mortality has been a hot topic for East Coast fishery managers.

A Work Group assembled by the Atlantic States Marine Fisheries Commission’s Atlantic Striped Bass Management Board, in order to provide some direction on issues that should be addressed in the proposed Amendment 7 to the ASMFC’s striped bass management plan, noted last August that

“Multiple members of the [Work Group] indicted that recreational dead discards may be the single most important issue at this time, and addressing (or reducing discards) is the most important action that can be taken going forward.  Many [Work Group] members pointed to the fact that recreational discards accounted for just under 50% of the fishing mortality as basis for the critical need to address this issue…”

A desire to reduce striped bass fishing mortality was also the impetus behind the provision in Addendum VI to Amendment 6 to the Atlantic Striped Bass Interstate Fishery Management Plan that requires all states to adopt regulations requiring striped bass fishermen to use non-offset fishing hooks when fishing with bait.

Given the impact of recreational release mortality on the striped bass stock, the Management Board asked the ASMFC’s Atlantic Striped Bass Technical Committee to run the striped bass stock assessment model again, using different assumptions for the release mortality rate, to determine whether such mortality rates would materially impact the conclusions reached in the benchmark assessment.

The short answer is no.  

Whether the release mortality rate was the best-case 3%, the worst-case 26%, or the 9% used in the original benchmark assessment (all of which come from a Massachusetts study and a subsequent paper authored by Paul J. Deodati and R. Anne Richards in 1996), or a blended mortality rate that takes differing seasonal or regional mortality rates into consideration, the conclusions would have been the same:  the striped bass stock is overfished, and experiencing overfishing.

Since the results of the Technical Committee’s research were reported at last week’s Management Board meeting, it seems that some individuals in the angling community might have misinterpreted exactly what the report itself says about the importance of release mortality.  Some appear to believe that the Techincial Committee concluded that release mortality isn't very important.

That's not true.

What the Technical Committee did, and later reported on, was to perform a “sensitivity analysis” to determine how sensitive the population model used for the benchmark assessment would be to differing rates of release mortality.  In performing such analysis, the Technical Committee looked backwards, to see how using different fishing mortality rates would have affected the outcome of the benchmark stock assessment.

They concluded that the impacts would be minimal.

That’s not surprising.   The benchmark assessment was based, in part, on a number of fishery-dependent and fishery-independent surveys that, when run through a population model, allow biologists to estimate the size of the current striped bass population, along with the size of each individual year class of fish as they make their way through the stock.  

To come up with that estimate, scientists must know, among other things, how many fish are removed from the stock each year, whether such removals are a result of natural mortality, harvest, or fish that die after being released.  Knowing how the stock responds to such removals, in light of the current population size, estimated recruitment of new fish into the stock, and similar factors, allows biologists to calculate the target fishing mortality rate, the fishing mortality rate where overfishing begins to occur, the target biomass or spawning stock biomass, and the point at which biomass drops so low that a stock becomes overfished.

All of the parameters are interconnected.  Thus, if fishing mortality was higher than the scientists originally believed—perhaps because the release mortality rate was 26% instead of 9%--the initial population had to be larger than scientists originally believed, too.  Otherwise, it would be impossible for there to be as many striped bass around as there are today. 

Similarly, if the release mortality rate was lower than believed—for example, 3% instead of 9%--then the stock was probably a little smaller than believed, as well, because even at the lower mortality rate, the stock is no bigger than it is at this time. 

Unfortunately, some anglers are looking at the fact that differing estimates of past release mortality didn't materially change the current status of the stock, and coming to the conclusion that, looking forward, the rate of release mortality will not materially affect the fishing mortality rate in the future, nor the size of the spawning stock biomass.

That conclusion is wrong.

In 2017, the terminal year of the benchmark stock assessment, recreational release mortality constituted 48% of all striped bass fishing mortality.  Every fish that dies after being released by an angler contributes to overall fishing mortality, as does every fish that is retained by an angler, harvested by a commercial fisherman, or is shoveled over the side and dies after ending up in a trawl.

It doesn't matter how a fish dies; all are removed from the spawning stock biomass.

Thus, if recreational fishing mortality could be cut in half, we could see an approximately 24% reduction in overall striped bass fishing mortality—one-third more than the reduction that Addendum VI was hoped to achieve—without making any reduction at all in the number of fish actually landed by recreational or commercial fishermen.

Thus, reducing release mortality, to the extent reasonably possible, is a worthwhile goal for fisheries managers, but only when viewed in the context of reducing overall fishing mortality.  For it’s overall mortality that matters.

Striped bass won’t be rebuilt unless the overall fishing mortality rate is low enough to allow that to happen.  So managers should try to eliminate unnecessary release mortality; requiring circle hooks to be used when fishing for bait is one important step toward that goal.

But managers would be wrong to focus merely on release mortality.  If they were able to reduce overall mortality enough to allow the stock to rebuild, even if that meant that release mortality increased to some degree, the bass would be in a better place than they are today.