Review
of “King Crabs of the World”
Review by Braxton Dew
NMFS, Ret.
The Elephant In the Room
Alaskans who pick up
Brad Stevens’ book, King Crabs of the
World, are likely to browse through its pages looking for answers to the
question: “Whatever happened to the king crab?”. If they make it to page 589,
readers will be disappointed to hear editor Stevens’ say, “…. We will never know what caused the dramatic changes in the
abundance of Bering Sea RKC after 1978”. One of the reasons we will never
know is that this book studiously avoids taking an honest look at the elephant
in the room — the breaching of red king crab no-trawl sanctuaries by commercial
groundfish trawling during the late 1970s and early 1980s.
The central theme
running through the book is that adult king crab are immeasurably more likely to be
killed by natural causes, including a slight rise in sea-surface temperature,
than to be killed by being dragged up in a bottom trawl (in which chances of
survival are said to be good). The stated opinion of editor Stevens (p.369), is
that crab bycatch mortality in groundfish trawl fisheries is low. According to Stevens, critics’ claims
to the contrary verge on the fantastic and are often tinged with prejudice. The
bottom line is that bycatch “probably
represents only a small fraction of overall king crab mortality” (p.xviii).
Natural Mortality Is the Only Mortality
In case there is any
confusion as to the uber-dominant role of natural mortality, authors Zheng and
Kruse in their management model (p.530) categorize all trawl-killed crab as
having succumbed to natural mortality. Using this slight-of-hand, any
fishing mortality spikes caused by
commercial trawling in the newly breached sanctuaries were interpreted as
natural deaths. Zheng and Kruse offer the usual laundry list of
natural-mortality elements — predation, cannibalism, senescence, and disease —
but are not forthcoming as to the model’s inclusion of trawl bycatch as a
source of “natural” mortality. The Zheng-Kruse Bering Sea red king crab model
may be the only fisheries model in the world that so profoundly conflates man’s
fishing (F) with natural mortality (M). Go to the link below to see an example
of “natural” mortality according to the Zheng-Kruse model. No wonder the authors
had to inflate M (by nearly 10X) during 1980-1984 (p.530).
Where
Did They Go?
What, then, could
explain the spatially explicit disappearance of red king crab from the
southwestern sector of their Bristol Bay range, as noted by Loher and
Armstrong? Below is a Bering Sea trawling-distribution map that may help
interested readers form their own hypothesis as to what might have happened to
crab in the southwestern sector of their range.
 |
| The trawl-density distribution of observed tows and observed joint-venture deliveries within 25-km2 grid squares during 1973–2001. The white boundary line shows the old (1959-1977) Japanese Broodstock Sanctuary (Dew 2010b). Trawling intensity is highest in the Unimak area and relatively low in the more offshore area to the northeast, where the present-day broodstock remains (Fig.3, Dew and McConnaughey, 2005) |
A glance at the Bering
Sea trawl distribution will enable many readers to come up with a theory more
credible than that advanced by editor Stevens (p.588) that the crab, whose
population had grown in an unrestrained manner to unsustainable heights,
essentially did it to themselves. According to Stevens, the height from which
RKC fell was an abundance level that was “anomalous”,
resulting from a supportive environment and lower fishing levels, which have
never reoccurred. “What we should wonder
about is not so much that the abundance …. declined but that they ever reached
such a high abundance level in the first place. Even without fishing it was
ecologically unsustainable and would have declined due to natural mortality ….”.
In sum, Stevens goes on, the decline of adult red king crab should not have
been a surprise to anyone (p.587).
Improbable Population Trend Is
Suspect
Editor Stevens is right about one
thing, though: we should have wondered — not about the crab but about the NMFS
population estimates, which were increasingly biased during 1975-1980 by an
inexplicable number of extra, non-design-based tows made at high-abundance,
male-dominant stations (Dew 2010a). Too often the NMFS estimates are published
and accepted as if they are free of bias or sampling error. But because of the
patchy spatial distribution of podding crab, each year’s estimate of red king
crab abundance is associated with a large measure of uncertainty. Figure 17.5
of Chap.17 (Zheng and Kruse) shows population estimates surrounded by
confidence intervals, many of which are at least ±30-60% of the mean, with some
approaching or exceeding ±100% (e.g., 1984 and 1991). The ±80-90% uncertainty
surrounding the 1991 estimate, for example, was caused by a single survey tow
that hit a large pod of adult legal male crab on the “bachelor grounds” (Dew
2008).
An assumption of assessment
surveys is that the observed trend in annual survey estimates is reasonably
well-correlated with a real abundance trend in the underlying population.
However, if confidence intervals around the annual estimates are wide enough
(i.e., if the uncertainty of the survey estimates is great enough), the
survey-derived trend may bear little or no resemblance to the actual abundance
changes in the population. The high level of imprecision of NMFS king crab
survey estimates, combined with bias from ad hoc extra sampling at high
abundance stations, raises the question whether the 1975-1980 data can be used
to define a trend, or whether such a trend tells us much, if anything, about
actual abundance changes in the underlying population. My own statistical
examination reveals that there were no significant (P < 0.05)
differences among the consecutive six years, 1975 through 1980, nor were there
significant differences among any of the 15 year-pairs (e.g., 1975 vs. 1978, a
period during which the population was thought to increase by >100%). Thus,
it is likely that the “anomalous” and
“ecologically unsustainable”
abundance spike reportedly achieved by Bristol Bay red king crab was nothing
but a mirage — the product of an imprecise survey biased by repeated, nonrandom
sampling in areas of high legal-male crab abundance. For more details, click on
the link below to Podding Behavior of
Adult King Crab and Its Effect on Abundance-Estimate Precision:
NMFS Warned Us
In an effort to show that the most
spectacular crash in the history of U.S fisheries management did not blindside
NMFS officials, Stevens (p.587) claims that, based on “warnings issued by the NMFS” (no documentation as to what warnings), the Bering Sea collapse “should not have been a surprise to anyone”.
Stevens then erroneously uses Orensanz et al. (1998) as support, stating
(p.588) that exploitation of the legal male stock rose to >70%, sex ratios
became highly skewed in favor of females (M:F = 1:6), and the occurrence of
barren females increased dramatically. Stevens goes on to say, ”Clearly, this was an unsupportable level of
effort, and under such pressure it is no surprise that the [Bering Sea] crash occurred so quickly”.
The problem with all of this is that
Orensanz et al. (1998) did not write about Bering Sea crab. The Orensanz
warning signs reiterated by editor Stevens were for Kodiak, not Bering Sea king crab. As for Bering Sea warnings, instead
of Orensanz we had Otto (1985) declaring that Bering Sea imbalances in sex
ratio have been negligible and have placed no constraints on reproductive
success; and Otto (1986) specifying that the “Bristol Bay catch never exceeded 60% of the estimated legal stock”.
Both of these pieces of information later turned out to be incorrect (Dew and
McConnaughey 2005). Moreover, because the NMFS survey is conducted during king
crab spawning and spawning occurs well inshore of the survey-sampling boundary,
investigators have been unable to reliably monitor mating success (barren
females) or king crab sex ratios in the Bering Sea (Dew 2008). It is simply not
credible to imply that NMFS or any other organization was savvy enough to warn
us of the impending dismantling of Alaska’s most valuable fishery.
Ex-Managers In Denial
On p.395 Stevens says the question of
whether or not trawling contributed to the decline of Bering Sea king crab
stocks in the early 1980s is “debatable”.
However, Stevens’ book presents little or no debate. Instead, there are denials
(p.128) that the work of others is valid. For example, on p.931 of their paper,
Dew and McConnaughey (2005) stated that the rate of decline of broodstock
density with trawling in the Unimak area was 1.9 times that of a more lightly
trawled offshore area, and that these rates of decline were statistically
different (P < 0.05). This is not a startling finding — more trawling, less
crab remain on the grounds. No one fabricated data — the crab densities and the
number of trawl drags came from NMFS. Yet author Otto, without presenting any
data or analysis of his own, flatly denies that there was any statistically
discernible effect of trawling on the rates of decline in crab abundance within
the two areas.
Quibbles
About Survival
Why does
author Otto bother trying to discredit an unsurprising revelation that trawling
captures crab and more trawling captures more crab? Consider the following from
Dew and McConnaughey (2005, pp.931-932):
“Given the rate and magnitude of these
declines, and considering that two different levels of trawling activity in the
Unimak and offshore areas each moved the local population quickly toward
extinction, it is apparent that the survival of discarded, trawl-caught crab is
negligible. That is, the long-term survival of red king crab affected by
commercial trawling is insufficient to offset the mortality and social
disruption caused by past and present levels of trawling”.
This
conclusion renders irrelevant the results from myriad, ever-ongoing survival
studies reported in this book and elsewhere. Moreover, it negates unwarranted optimism
as to the survival of trawl-caught crab (e.g., p.128). After all, when the crab
are gone, how relevant are quibbles about survival?
Male
Reproductive Potential — Chronic Overestimation?
Chap.10, Reproductive
Ecology. Whenever the management of a
major fishery such as Alaska’s king crab fishery is described by reputable
scientists as a “failure” (Wooster 1992) or a “debacle” (Orensanz et al. 1998),
it is probably time to reassess key management assumptions. A present
assumption needing further scrutiny is that mature male king crab, in the wild,
participate in mating shortly after molting. According to Powell et al.(1973)
and Powell et al. (2002), caged males can engage in mating after a post-molt
recovery period of as little as ten days. Compare this with Paul et al. (1995),
who determined that the molt-recuperation period for male Tanner crab was at
least 100 days; and Sainte-Marie et al. (1999, 2002), who reported that male
snow crab did not mate in the same year they molted. Going further, the Powell
et al. assertion rests on the shaky bedrock of yet another assumption — that
investigators can accurately determine the time since last molt, in months, by
visual inspection of a wild crab’s shell. Countering all this is a substantial
body of evidence (link below) that wild king crab do not molt and mate in the
same year
Author
Webb’s chapter on reproduction could have benefitted from a more detailed
discussion of this issue, given that his chapter includes sections on “Molting
and Mating”, “Mating Ability” and “Male Reproductive Potential” (MRP). Although
Takeshita et al. (1990) are cited for their tagging work on king crab
migration, Webb omits the fact that this same tagging data led Takeshita et al.
to conclude that only 50% of Bristol Bay’s mature male population, mostly
old-shell skip-molts, participate in mating in any given year. The other 50%
remains offshore on “bachelor grounds”, far from known spawning grounds,
recuperating from the winter molt (e.g., Dew 2008).
The
answer to the question of whether or not all mature males participate each year
in mating has a profound effect on the management of a male-only fishery. For
example, if Takeshita et al. (1990) are correct, then MRP values calculated by
the model during the past 25 years are likely to have been overestimated by a
factor of 2X (only half of the mature males mate each year, while the other
half is molting). Furthermore, if males do not mate in the same year they molt,
assurances (p.300) that males will be able to mate once or twice before their
harvest are little more than wishful thinking. It is not good science to ignore
reasonable, well-published, data-based hypotheses, either in the management of
a stock or the writing of a book chapter. A better idea might be to provide
evidence, if available, that disproves such hypotheses. Also, with regard to
the presumption that there is some precise correspondence between shell
condition and shell age, it would be prudent to consider the possibility that,
although all new shells are clean, not all clean shells are new.
What Is
Depensation?
In 1966, Kodiak’s Gulf of
Alaska (GOA) fishery provided the bulk of the U.S. red king crab catch (70,000
tons vs. 450 tons from the Bering Sea). We
might expect the dismaying and complete loss of the GOA fishery (commercially
extinct now for more than 30 years) to have stimulated critical examination of
a core management assumption — that calling a halt to fishing will permit an
overfished stock to recover from low abundance levels. This assumption relies
on the idea of compensatory density-dependence, which is the mechanism
underlying all logistic-growth models, including the Ricker stock-recruitment
curve — a mainstay of Bristol Bay king crab management. In addition to positing
reduced per capita production at high stock sizes, the Ricker compensatory
model incorporates the notion that at extremely low population levels — indeed,
when the population is on the verge of extinction — the per capita production
of recruits is highest.
The compensatory model makes no
allowance for populations whose reproductive success, over some range of
population size, increases with the
size of its aggregations. Nor is this model relevant for populations whose per
capita growth rate goes to zero well before the population size goes to zero.
Editor Stevens asks (p.588) why did king crab not rebound when fishing was
halted? The answer is that managers are using a model, developed for salmon,
that does not work for red king crab. That is, they are using a model that does
not allow for depensation, does not make provision for an Allee effect, and
does not account for declining individual fitness and per capita production as
the population nears extinction — all of which are earmarks of Alaska’s highly gregarious
red king crab populations. Both the Bristol Bay red king crab and the North
American passenger pigeon have commonalities that conform to the checklist
below. For more on depensation and environmental
conditioning, click on the following link to Historical Perspective On Habitat Essential To Bristol Bay Red King
Crab:
Dew, C.B. 2008. Red king crab mating success, sex ratio, spatial
distribution, and abundance estimates as artifacts of survey timing in Bristol
Bay, Alaska. North American Journal of Fisheries Management 25:1618-1637.
Dew, C.B. 2010a.
Podding behavior of adult king crab and its effect on
abundance-estimate precision. Pages 129-151 in
Biology and Management of Exploited Crab Populations under Climate Change.
Alaska Sea Grant, AK-SG-10-01, Fairbanks. 562 pp.
Dew, C.B. 2010b.
Historical perspective on habitat essential to Bristol Bay red king crab. Pages
377-402 in Biology and Management of
Exploited Crab Populations under Climate Change. Alaska Sea Grant, AK-SG-10-01,
Fairbanks. 562 pp.
Dew, C. B., and R. A. McConnaughey. 2005. Did trawling on the
broodstock contribute to the collapse of Alaska’s king crab? Ecological
Applications 15:919–941.
Liermann, M., and R. Hilborn.
2001. Depensation: evidence, models and implications. Fish. 2:33-58.
Loher, T., and D.A. Armstrong. 2005. Historical changes in the
abundance and distribution of ovigerous red king crabs (Paralithodes
camtschaticus) in Bristol Bay (Alaska), and potential relationship with
bottom temperature. Fisheries Oceanography 14:292-306.
Orensanz, J.M., J. Armstrong, D. Armstrong, and R. Hilborn. 1998.
Crustacean resources are vulnerable to serial depletion: the multifaceted
decline of crab and shrimp fisheries in the greater Gulf of Alaska. Rev. Fish
Biol. Fish. 8:117-176.
Otto,
R.S. 1985. Management of Alaskan king crab stocks in relation to the possible
effects of past policies. Pages 447–481 in B. R. Melteff, coordinator.
Proceedings of the
international king crab
symposium. Report 85-12. University of Alaska Sea Grant Program, Anchorage,
Alaska, USA.
Otto, R.S. 1986. Management and assessment of eastern Bering Sea
king crab stocks. Pages 83-106 in
G.S. Jamieson and N. Bourne (eds.), North Pacific workshop on stock assessment
and management of invertebrates. Canadian Special Publication of Fisheries and
Aquatic Science 92.
Paul,
A.J., J.M. Paul, and W.E. Donaldson. 1995. Shell condition and breeding success
in Tanner crabs. Journal of Crustacean Biology15(3):476-480.
Powell,
G. C., B. Shafford, and M. Jones. 1973. Reproductive biology of young adult
king crabs, Paralithodes camtschatica (Tilesius), at Kodiak, Alaska. Proceedings of the National
Shellfisheries Association 63:77–87.
Powell,
G.C., D. Pengilly, and S.F. Blau. 2002. Mating pairs of red king crabs (Paralithodes camtschatica) in the Kodiak
Archipelago, Alaska, 1960-1984. Pages 225-245 in Crabs in cold water regions: biology, management, and economics.
University of Alaska Sea Grant, AK-SG-02-01, Fairbanks. 876 pp.
Sainte-Marie,
B., J.-M. Sevigny, and M. Carpentier. 2002. Interannual variability of sperm
reserves and fecundity of primiparous females of the snow crab (Chionoecetes
opilio) in relation to sex ratio. Canadian Journal of Fisheries and Aquatic
Sciences 59:1932–1940.
Sainte-Marie,
B., N. Urbani, J.-M. Sevigny, F. Hazel, and U. Kuhnlein. 1999. Multiple choice
criteria and the dynamics of assortive mating during the first breeding season
of female snow crab, Chionoecetes opilio (Brachyura, Majidae). Marine
Ecology Progress Series 181:141–153.
Takeshita, K., H. Fujita, and S. Matsuura. 1990. A note on
population structure in the eastern Bering Sea adult red king crab, Paralithodes camtschatica. Pages 427–433
in Proceedings of the International
Symposium on King and Tanner Crabs. University of Alaska Sea Grant,
AK-SG-90-04, Fairbanks.
Wilson,
E.O. 1975. Sociobiology: the new synthesis. Belknap
Press, Cambridge. 697 pp.
Wooster,
W.S. 1992. King crab dethroned. Pages 14-30 in
M.H. Glantz, editor. Climate, variability, climate change, and fisheries.
Cambridge University Press, New York.
