REBUILDING STRIPED BASS

 

In April 2019, the Atlantic States Marine Fisheries Commission’s (ASMFC) Atlantic Striped Bass Management Board (Management Board) accepted a benchmark stock assessment that found the striped bass stock to be both overfished and subject to overfishing. The Management Board acted quickly to reduce fishing mortality and end overfishing, but was much slower to initiate the 10-year rebuilding plan required by Amendment 6 to the Interstate Fishery Management Plan for Atlantic Striped Bass.

 

Instead, the Management Board elected to first develop a new Amendment 7 to the Interstate Fishery Management Plan for Atlantic Striped Bass (Amendment 7), a process that took it a full two years. Amendment 7 directly addresses the rebuilding issue, containing a provision that reads,

 

If the 2022 stock assessment results indicate the Amendment 7 measures have less than a 50% probability of rebuilding the stock by 2029 (as calculated using the low recruitment assumption) and if the stock assessment indicates that at least a 5% reduction in removals is needed to achieve [the fishing mortality rate needed to rebuild the stock], the Board may adjust measures to achieve [such fishing mortality rate] via Board action (change management measures by voting to pass a motion at a Board meeting).

Such provision allows the Management Board to quickly impose restrictions needed to rebuild the stock, without the need for public hearings or analysis of hearing comments, and gives it a better chance to rebuild the stock by the 10-year deadline.

At its August 2022 meeting, the Management Board discussed various ways to achieve any needed fishing mortality reduction; at the time, it appeared that the majority of its members expected such reduction to be needed. However, the magnitude of any such reduction wouldn’t be known until the ASMFC’s Striped Bass Stock Assessment Sub-Committee and Atlantic Striped Bass Technical Committee released the 2022 stock assessment update (Assessment Update) prior to the Management Board’s November 2022 meeting.

The draft Assessment Update was made public on October 25, 2022, and it contained the unexpected news that,

Under current fully-recruited fishing mortality (F=0.14), female [spawning stock biomass] is expected to reach or exceed the [spawning stock biomass] threshold by 2023 with a probability of 70.2%, and exceed or reach the [spawning stock biomass] target by 2025 with a probability of 56.1%. By the rebuilding deadline of 2029 there is a 78.6% chance the stock will be at or above the [spawning stock biomass] target and a 96.7% chance the stock will be at or above the [spawning stock biomass] threshold. Under [the fishing mortality target rate] (F=0.17), female [spawning stock biomass] is expected to reach or exceed the [spawning stock biomass] threshold by 2023 with a probability of 61.9%, and exceed or reach the [spawning stock biomass] target by 2028 with a probability of 52.0%. Under [the fishing mortality threshold rate] (F=0.20), female [spawning stock biomass] is expected to reach or exceed the [spawning stock biomass] threshold by 2023 with a probability of 53.2%, but it has less than a 50% probability of reaching the [spawning stock biomass] target in any year. [internal references omitted]

Both striped bass fishermen and fishery managers were surprised to learn that, provided that overfishing isn’t allowed to recur, it’s more likely than not that, at some point within the next year, striped bass will no longer be considered overfished. And so long as fishing mortality is kept at or below its target level, the stock is likely to rebuild before the 2029 deadline.

Still, nothing is certain, and there remains a chance that the stock will not be rebuilt by 2029.

Successful rebuilding depends upon keeping fishing mortality at or below the target rate, and that might not be easy to do. Recreational fishing effort tends to move in concert with striped bass abundance, so as the stock rebuilds, more anglers are likely to enter the fishery, and those already participating in the fishery are likely to fish more often. Such increased effort is likely to lead to increased fishing mortality, due to both an increase in the number of fish retained by anglers and, because more fish are being caught and released, an increase in release mortality as well.

If angling effort, and the resultant fishing mortality, increases at a faster rate than the striped bass population, the stock’s recovery could be threatened. Fortunately, Amendment 7 requires the Management Board to reduce fishing mortality if it exceeds its target level for two consecutive years (provided that spawning stock biomass remains below target during at least one of those years), so such threat to the stock’s recovery should be promptly abated should it arise.

Rebuilding is also contingent upon the precision of the estimates of fishing mortality and spawning stock biomass. All such estimates contain some level of uncertainty; although the Assessment Update deems its estimate of fishing mortality to be “precise” and its estimate of spawning stock biomass “very precise,” such estimates are nonetheless thought to somewhat underestimate fishing mortality and overestimate biomass. While, according to accepted scientific protocols followed by the Assessment Update, the uncertainty in the estimates was not great enough to justify any adjustment in their calculation, such uncertainty still could have some bearing on the speed of the stock’s recovery.

The one thing that shouldn’t have a significant impact on the stock’s ability to rebuild by the 2029 deadline is the number of young fish recruiting into the stock. The relatively poor spawns that occurred in the Maryland portion of the Chesapeake Bay, the single most important striped bass spawning ground, during 2019, 2020, and 2021 have already been incorporated into the Assessment Update; most striped bass mature when they are somewhere between six and eight years of age, so any fish spawned after 2023 are unlikely to affect rebuilding.

 

While the small 2022 year class could, in theory, have some effect on the rebuilding projection, such effect will be minimal, as the Assessment Update assumes that striped bass will be in a low recruitment regime throughout the recovery period. 2029 rebuilding remains on track.

Yet rebuilding the spawning stock biomass is only one milestone on the road to a sustainable striped bass fishery. Once the stock is rebuilt, spawning stock biomass must remain at or reasonably near its target level for the stock to remain healthy in the long term. Such long-term abundance is far from guaranteed.

The Maryland juvenile abundance index (JAI), arguably the best predictor of the future health of the striped bass stock, has maintained a long-term average of 11.3. The JAI for 2022, at a mere 3.6, was less than a third of the average JAI, yet was still the highest reported over the past four seasons; it marked the first time in the 65-year history of the JAI that the index remained below 4.0 for four consecutive years. The average JAI for the past four years, 3.16, matched a time series low that was first established during the period 1983-1986, when the stock was just beginning to recover from its collapse during the late 1970s.

While the Assessment Update was calculated using a low recruitment assumption, the average JAI for the striped bass year classes that define such “low recruitment,” 2007 through 2016, was still substantially higher than the average JAI for the most recent four, six, or even eight year periods. If such low recruitment continues, current regulations will probably not be restrictive enough to maintain the spawning stock biomass at its target level.

Unfortunately, there is little that managers can do to improve recruitment. Spawning success isn’t dependent on the size of the spawning stock; a very small spawning stock may produce a very large year class, while a large spawning stock biomass is no guarantee against spawning failure. Instead, spawning success is largely dependent upon environmental conditions in the spawning rivers, with cold, wet winters followed by cool, late springs leading to strong year classes, while warm winters and dry, early springs typically lead to a disappointing spawn.

 

The best that managers can hope to do is manage the stock conservatively, and maintain a high enough population of older, larger fish that, even after years of poor recruitment, there will be enough spawning stock available to take advantage of favorable spawning conditions when they again occur.

Right now, it looks like managers will fully rebuild the striped bass stock. Whether they will then be able to maintain that stock at a healthy level remains an open question.

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This essay first appeared in “From the Waterfront,” the blog of the Marine Fish Conservation Network, which can be found at http://conservefish.org/blog/