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.
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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