By any conventional measure, the Atlantic Herring stock is
in good shape. Not only is the stock
neither overfished nor experiencing overfishing, but fishing mortality is well
below the threshold and biomass is more than double the target.
If we looked at the stock from a pure biomass standpoint, it
is exceedingly healthy, and even considering all of the factors that go into a
single-species model, the herring are doing well.
But the problem, of course, is that we’re using a
single-species model, and when you’re dealing with a forage fish such as
herring, which is preyed upon by a host of other species, single-species models
don’t tell the whole story.
That’s why a lot of people might have been surprised
recently, when they read that bluefin
tuna in the Gulf of Maine, which feed mostly on herring, are not eating enough
to get into prime condition needed for the long migrations south to spawning grounds
in the Gulf of Mexico.
That seeming contradiction puzzled scientists, too,
when they noticed it some time ago. They
set out to understand why, and in a recent paper, entitled “The paradox of the
pelagics: why bluefin tuna can go hungry
in a sea of plenty,” provided their explanation.
It turns out that in order to obtain proper nutrition,
bluefin tuna need to feed on older, larger herring; an abundance of small herring don’t provide enough nutrients to allow the fish to build up the fat
reserves needed for their southern migration.
That lack of fat reserves has real implications for the
health of the bluefin population. The
paper’s authors note that
“A reduction in [fat] stores can have profound effects on
fish life history…Faced with reduced energy, spawning age fish can reduce
growth and reallocate more reserves to gonadal investment. Such a strategy can maintain fecundity in
times of reduced condition, but may also contribute to higher rates of
post-spawning mortality. Alternatively,
fish may skip spawning in times of reduced condition or change reproductive
output (fewer eggs, lower quality) for females.
Given the effect of total [fat] content on egg production, the influence
of prey quality on predator life history parameters and the relationship
between bluefin tuna weight and fecundity, a reduction in bluefin tuna
condition on the foraging grounds could have affected reproductive output…”
That, in turn, may help to resolve one of the big debates in
bluefin tuna management—why the harvest restrictions imposed to date have not
allowed the stock to rebuild to 1970s levels and, whether
environmental conditions are such that the western stock of bluefin is not as
productive—that is, cannot produce as many young fish—as it was a half-century
or so ago.
Although no firm conclusions can be drawn from the data currently
available, the paper notes that
“…total allowable catches have been in line with scientific
advice for over 2 decades. If fishing
mortality were the sole factor limiting population recovery, levels of fishing
mortality set by management should have allowed this stock to rebuild much
faster than currently observed, even for a species with long generation times
like bluefin tuna. A slow recovery may
be related to fishing mortality set too high, despite the scientific advice, or
could be the results of environmental factors affecting life history (growth,
reproduction, migration) or synergistic effects between the two…”
But if the lack of large herring in the population is
causing the bluefin tuna stock to be less productive, and thus preventing
the stock from rebuilding to historical levels, there is no clear biological
path to remedy the problem.
First, the reason why there aren’t enough large herring isn’t
at all clear.
As the paper’s authors note,
“Studies…on George’s Bank suggest the shift to a smaller herring
community (as indicated in the negative relationship between herring size and
abundance) is a direct result of density dependent growth.”
In other words, when herring are abundant, they may grow
more slowly and perhaps never grow as large as they would if there were fewer
fish.
“Increased fishing pressure can reduce density dependence and
potentially lead to more positive weight anomalies for these small pelagics. However, intense fishing also reduces the
abundance of older, larger fish, and management strategies that produce a high
abundance of faster growing but smaller individuals would shift the size
spectrum towards smaller fish.”
In that case, anything gained by reducing the abundance of
herring so that fish might grow larger could be lost by a fishing regimen that
harvests most individuals before they could attain such large size.
Second, while bluefin appear to benefit when large herring are
available, even if herring abundance is down, other predators need an abundance
of smaller individuals to thrive.
Humpback whales,
for example, feed by rising through schools of herring and engulfing many
fish at one time. They get greater benefit from abundance than from the size of the
individual fish.
And that, in the end, is what makes managing forage fish so
challenging, and so different from managing species at higher trophic
levels. Single-species management, which
emphasizes sustainable harvest, does not and cannot adequately account for
the needs of all of the various predators that also rely on the targeted
resource.
As the authors of “The paradox of the pelagics” note,
“indicators of size structure and body condition should be
considered when managing prey species to ensure higher predator fitness. The challenge will be implementing such a strategy
within the context of current assessments and management models and determining
the relative importance of prey abundance and size for a range of predators
with different foraging strategies and metabolic needs.”
That is true not only for Atlantic herring, but also for such
species as menhaden, which serve as forage for a very wide array of predators.
And it illustrates why a very simplistic management
approach, which merely looks at abundance and fishing mortality to determine whether
there will be enough fish around to catch in the future, is not nearly good
enough to manage forage species.
When we talk about managing forage fish, biomass is
only one bit of data that’s needed to make decisions on harvest. How that biomass is structured, and how it is
used by a myriad of predators, must also be known before decisions are made.
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