Sunday, November 14, 2021

ARE WESTERN ATLANTIC BLUEFIN TUNA FINALLY ON THE RIGHT TRACK?

 I haven’t killed a bluefin tuna since 2006, and that one was an accident.  We were trolling south of Fire Island Inlet at the time, when a small bluefin, probably from the big 2003 year class, came up right behind the spreader bar splashing on the third wave, and gulped the trailing Green Machine down into its gills. 

Fortunately, that tuna, though small, was a couple of inches over the minimum size, so I bled it out, tossed it on ice, and headed home.  With the one bluefin bag limit that was in force back then, I didn’t want to risk deep-hooking a second fish that, although mortally harmed, would nevertheless have to be returned to the sea.

I haven’t often targeted bluefin tuna since, because the Western Stock—that is, the bluefin that spawn in the Gulf of Mexico, as opposed to the Eastern Stock, which spawns in the Mediterranean Sea—has been badly delpted, and it didn’t feel right to kill them when the population was in bad shape.  For most of that time, I haven’t even engaged in catch-and-release, as I’ve had little desire to have to keep another exhaused or seriously injured fish.

A lot of people believe that there are plenty of bluefin around, and to them I will say two things:  First, I'm primarily concerned about the health of the Western Stock, and from what I understand, a lot of the school, large school, and medium bluefin that we’ve seen in recent years (including the 200-plus-pound stuff that set up east of New York Harbor last summer) are probably Eastern Stock fish, which frequently cross the Atlantic to feed off our shores, with a lesser number of Western Stock tuna mixed among them. 

Take a look at where the stock was in the 1950s, 1960s, and even early 1970s, compared where it is today, if you want to get an idea of what a truly healthy Western Stock looks like.

The shifting baseline syndrome is very real.

But there is some good news being reported by the biologists at the International Commission for the Conservation of Atlantic Tunas.

The latest ICCAT stock assessment indicates that the biomass of Western Stock bluefin tuna has been slowly but steadily increasing for the past 15 years, and that the recruitment of young fish into the population has also increased over the last few seasons.

Having said that, there are still a lot of unknowns clouding the recent assessment, which

“cautions that conclusions from the latest assessment, using data through to 2020, do not capture the full degree of uncertainty in the assessments and projections and an independent review recommended against using it for management advice.  The various major contributing factors to uncertainties include mixing between the stocks, recruitment, age composition, age at maturity, the possibility of regime shifts, assumptions regarding selectivity, and indices of abundance…  [internal reference omitted]”

The very fact that the results of the new stock assessment, which show a slowly rebuilding stock and increased recruitment, contrast sharply with those of the 2017 stock assessment and 2020 update, which predicted a decline in abundance and low recent recruitment, provide reasons to proceed with caution.

The single issue of stock mixing, of how many of the fish caught in the western Atlantic were spawned there, and how many derive from the Eastern Stock, could easily distort estimates of Western Stock health if biologists get it wrong.  As noted in a 2017 paper, “Modeling the implications of stock mixing and life history uncertainty of Atlantic bluefin tuna,” published in the Canadian Journal of Fisheries and Aquatic Sciences,

“The definition of a fish stock and the spatial delineation of the management unit boundaries establish the spatial framework through which we assess the status of fishery resources and implement management strategies.  Mismatch in the geographic scale of a fish’s life cycle and its management units (i.e., “stocks”) can have profound implications for the accuracy of stock assessment and the effectiveness of fishery management…Stock mixing violates the ‘unit stock’ assumption that underlies many management approaches…mixed stock data can confound indices of abundance, catch estimates, characterization of life history parameters (e.g., growth, maturity-at-age), and stock-recruit relationships.”

If managers overestimate the proportion of Western Stock fish in the population, then quotas are likely to be set too high, and the stock is likely to decline as regulations suited to a more abundant Eastern Stock cause too many Western Stock fish to be killed.  

Biologists have devised ways to measure oxygen isotopes in the otoliths, or “ear bones” of bluefin tuna, and use those measurements to assign such fish to one of the two stocks; population models that make various assumptions about stock mix have also been developed.  Yet ICCAT biologists are not completely comfortable that their estimates accurately reflect the true mix.

The estimates made with respect to stock mixing will also affect the estimates of things such as the age at maturity, since Eastern Stock bluefin are estimated to be fully mature when between just 3 and 5 years old, while Western Stock fish are thought not to mature until between 8 and 12 years old, although some scientists argue for earlier maturities, and the latest assessment of the Western Stock considers both early and late maturity scenarios. 

Estimates of the age composition of the stocks, in turn, are made by sampling the proportion of fish caught in various fisheries.  The recent stock assessment assumes that such fisheries tend to catch a larger proportion of one size of tuna compared to others, something that biologists refer to as “domed selectivity.”  In such situations, older, larger fish manage to evade the fishing gear more successfully than do smaller, younger individuals, and biologists would be wrong to assume that the lack of older fish appearing in landings translates to a lack of such older fish in the bluefin population.

Previous Western Stock bluefin assessments assumed that all sizes and ages of tuna were proportionately represented in the fisheries’ landings, and thus estimated that the biomass was smaller than did the most recent stock assessment, with its dome-shaped selectivity curve, which found more older, larger bluefin in the population.  As a result of assuming domed selectivity, the new assessment set its proxy for maximum sustainable yield about 35 percent higher than it would have otherwise.

The problem with assuming a dome-shaped selectivity curve is that by doing so, biologists assume the presence of “cryptic fish,” also referred to as “ghost fish,” that have been evidenced through a mathematical model, and not through more tangible evidence.  There is always the possibility that the biologists’ assumption is incorrect, that the fisheries do not really select for a particular age and size of fish, and that the reason that older, larger fish are not being caught is because they do not exist in appreciable numbers.  There is also a possibility, as the biologists’ latest report to ICCAT acknowledges, that some fisheries exhibit domed selectivity, while others do not, a possibility that, due to time constraints, was not explored.

Since most of the increase in the estimate of the maximum sustainable yield proxy is attributable to the assumption of domed selectivity, if such assumption is wrong, overfishing could easily occur.

Thus, it is difficult to know what to make of the latest bit of seeming good news about Western Stock bluefin.  The news seems to be encouraging, yet there is a lot of uncertainty inherent in the models which, after all, an outside reviewer found to be not “suitable for management advice.”

For my part, I see enough reason for hope in the ICCAT report that I’ll probably be actively targeting bluefin again next season.

At the same time, I’ll probably end up releasing just about all of the bluefin that I catch, provided that they appear healthy enough to survive the experience.  Caution must still be the watchword.

For I still firmly believe that when fishery data is, for any reason and to any degree, ambiguous, such ambiguity should be resolved in favor of the fish, not the fisherman, just in case the managers got something wrong.

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NOTE:  I’d like to take the opportunity to thank the American Saltwater Guides Association for publishing their recent bluefin blog, which provided the inspiration for this essay

 

 

 

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