mus). For red knots,it is the final stop before a single direct
1992, Harrington 2001), where, on arrival in early June,
the flight to the Arctic and survival and successful breeding
(Baker et al.2004,Morrison and Hobson 2004,Morrison et
gratory stopover for six shorebird species—red knot,ruddy
turnstone (Arenaria interpres), sanderling (Calidris alba),
semipalmated sandpiper (Calidris pusilla), dunlin (Calidris
with peak counts of more than 400,000 individuals; esti-
mates are that more than 1 million shorebirds used the bay
ew World red knots (Calidris canutus rufa) migrate an-
Western Hemisphere Shorebird Reserve Network (Myers et
spectacular shorebird stopovers, comparable with the Cop-
Yellow Sea in Asia. Delaware Bay had also spawned an eco-
tourism industry with an estimated worth of $34 million
(Eubanks et al.2000).
dance and ease of collection when spawning made them a
prime target as bait for an emerging conch fishery in the
early 1990s.In the five years between 1992 and 1997,the re-
ported harvest of crabs grew 20-fold from about 100,000 to
more than 2 million (figure 2),at an estimated value of $11
the number of hand-collecting permits for Delaware grew
from 10 in 1991 to 132 in 1997, indicating a large increase
Effects of Horseshoe Crab Harvest
in Delaware Bay on Red Knots: Are
Harvest Restrictions Working?
LAWRENCE J. NILES, JONATHAN BART, HUMPHREY P . SITTERS, AMANDA D. DEY, KATHLEEN E. CLARK, PHILLIP W.
ATKINSON, ALLAN J. BAKER, KAREN A. BENNETT, KEVIN S. KALASZ, NIGEL A. CLARK, JACQUIE CLARK, SIMON
GILLINGS, ALBERT S. GATES, PATRICIA M. GONZÁLEZ, DANIEL E. HERNANDEZ, CLIVE D. T. MINTON, R. I. GUY
MORRISON, RONALD R. PORTER, R. KEN ROSS, AND C. RICHARD VEITCH
polyphemus) and refuel for breeding in theArctic.During the 1990s,the Delaware Bay harvest of horseshoe crabs for bait increased 10-fold,leading
to a more than 90% decline in the availability of their eggs for knots. The proportion of knots achieving weights of more than 180 grams by 26–28
May, their main departure period, dropped from 0.6–0.8 to 0.14–0.4 over 1997–2007. During the same period, the red knot population stopping in
Delaware Bay declined by more than 75%, in part because the annual survival rate of adult knots wintering in Tierra del Fuego declined. Despite
restrictions,the 2007 horseshoe crab harvest was still greater than the 1990 harvest,and no recovery of knots was detectable.We propose an adaptive
management strategy with recovery goals and annual monitoring that, if adopted, will both allow red knot and horseshoe crab populations to
recover and permit a sustainable harvest of horseshoe crabs.
Keywords: red knot, Calidris canutus rufa, Delaware Bay, horseshoe crab, Limulus polyphemus
www.biosciencemag.orgFebruary 2009 / Vol. 59 No. 2 • BioScience 153
BioScience 59: 153–164. ISSN 0006-3568, electronic ISSN 1525-3244. Request permission to photocopy or reproduce article content at the University of
California Press’s Rights and Permissions Web site at www.ucpressjournals.com/ reprintinfo.asp. doi:10.1525/bio.2009.59.2.8
dramatic decrease in spawning crabs and thus in the avail-
bird numbers on Delaware Bay were soon falling fast; peak
for 1998–2002 (figure 3,box 1).
(ASMFC) began to implement restrictions,and some states
implement restrictions (ASMFC 1998, 2006a). The notable
Early restrictions, such as stopping the harvest during the
ing the disturbance to feeding birds, but
they did little to reduce the harvest. By
crabs to about 600,000, from a high of
more than 2 million. Although the 2004
harvest was only a quarter of the 1998
peak, it was still well above harvests
increase in the early 1990s (figure 2). In
2006, concern that harvest restrictions
review by the ASMFC stock assessment
vest still exceeded production (ASMFC
2006b).Since May 2008 there has been a
moratorium on the harvest of female
torium on the harvest of all horseshoe
crabs in New Jersey.
Abundant horseshoe crab eggs are a
particularly valuable food resource for
time-stressed, long-distant migrants, in-
cluding red knot, ruddy turnstone, and
they are easily digested and metabolized
into fat and protein (Castro and Myers
1993,Haramis et al.2007).The digestive
from South America are reduced in size
(Piersma and Gill 1998) and are initially
inadequate to support feeding on knots’
usual prey—hard-shelled bivalves (e.g.,
rich horseshoe crab eggs allows birds to
feed at high rates when they arrive, re-
build their organs and muscles, and
recorded in knots (Atkinson et al. 2007,
Haramis et al. 2007). Consequently, the
stopover duration of Delaware Bay knots is much shorter
(10 to 14 days) than comparable stopovers in other parts
of the world (21 to 28 days) (Piersma et al.2005).However,
is that no similar,easily digested alternative food is available
(particularly the Florida wintering population [Niles et al.
ing to alternative prey does not seem to be an option.Knots
migrating long distances from Tierra del Fuego would have
to arrive earlier and stay longer in Delaware Bay to refuel
adequately and depart on time,as there is only a short time
window for successful breeding in theArctic.
154 BioScience • February 2009 / Vol. 59 No. 2www.biosciencemag.org
Figure 1.Delaware Bay and the main parts of the shore used by red knots (dark lines)
during their spring migration.The inset portrays the entire migration route from
Tierra del Fuego in Chile and Argentina to the Arctic.
the beach surface (Botton et al. 1994), a depth that is in-
individual females unearth existing egg masses when laying
their own eggs, bringing eggs to the surface, where they are
available to shorebirds. Eggs are also brought to the surface
by wave action, which loosens sand and eggs (Botton et al.
Eggs brought to the surface are lost to horseshoe crab re-
quickly desiccate and die.As early as 1997,concern over the
have focused particularly on red knots, ruddy turnstones,
Bay belong to separate populations that breed in the central
Canadian Arctic and winter in Tierra del Fuego, northern
Bay and the populations in all three wintering areas have
suffered a major collapse (Morrison et al. 2004, Niles et al.
Rufa is listed as endangered under the Bonn Convention
and is proposed for endangered status in Brazil (Niles et al.
mined that rufa warranted “threatened” listing under the
Endangered Species Act (50 C.F.R.17),but chose not to list
it because of insufficient staff and fiscal resources,as well as
In April 2007, the Committee on the Status of Endangered
Wildlife in Canada classified the southern wintering popu-
and Florida populations as threatened (COSEWIC 2007).
In this article we review more than a decade of studies of
red knots, horseshoe crabs, and horseshoe crab eggs in
Delaware Bay. We ask whether, after nine years of reduced
horseshoe crab harvest, conditions for knots in Delaware
Bay have improved. We suggest a recovery paradigm—a
series of assumptions about how the recovery of horseshoe
crabs and knots can be accomplished—and propose recov-
ery parameters that should be monitored to ensure that re-
covery proceeds as anticipated. Finally, we describe current
www.biosciencemag.org February 2009 / Vol. 59 No. 2 • BioScience 155
Figure 2.Harvest of horseshoe crabs reported by mid-Atlantic
states.Gray bars represent the estimated harvest,according to
interviews with state marine fish biologists from Delaware and
New Jersey (reliable harvest reports are not available for years
prior to 1997).Black bars represent the sum of the harvest
reported to the Atlantic States Marine Fisheries Commission by
New Jersey,Delaware,Maryland,Virginia,and New York.
Figure 3.Mean peak counts of red knots observed on aerial
surveys of Delaware Bay 1986–2007 in five- to six-year
periods (bars are ±95% confidence intervals).
The conflict between fishermen using horseshoe crabs as bait
and conservationists demanding a healthy shorebird stopover
mirrors other conflicts around the United States over multiple
uses of resources: the spotted owl and forest products industry,
wolves and sport hunters inAlaska,and Pacific salmon and
water resources in the Pacific Northwest.A teachers’curricu-
lum program called“Green Eggs and Sand”takes advantage of
the controversy on the Delaware Bay to provide teachers with a
window into the complexities of resource conflicts and the
methods used to resolve them.Working with biologists,the
program creators designed a curriculum guide to help middle-
school and high-school teachers learn of the complex life his-
tory of each animal and their interrelationships.Then they
focus on how fishery management decisions are made,the
resource-use conflict,and the difficulties of resolution.They
conclude with a discussion of similarly contentious issues
throughout the country.For more on this program,contact
Cindy Etgen at www.dnr.state.md.us/education/ are/ges.html.
Box 1. “Green Eggs and Sand”: A Resource for Teachers.
Bay, and show that with minor modification, they can pro-
vides an excellent opportunity to employ an adaptive man-
agement approach (Williams et al. 2001) and stress those
needed to ensure the sustainability of the horseshoe crab
Horseshoe crabs: Trends in population
size and the density of their eggs
Several surveys during the past 20 years (ASMFC 2004) of
adult horseshoe crabs in Delaware Bay have led to various
Smith 2007, Sweka et al. 2007). To illustrate the population
trend, we use standardized data collected since 1990 by
Delaware Division of Fish and Wildlife.A 30-foot trawl net
was towed for 20 minutes (covering about 2 km), once per
month from March to December, on each of nine transects
criticized in anASMFC peer review in 1998 (ASMFC 1998)
in 2000; we report on that survey in this article.
The Delaware 30-foot trawl surveys showed a decline of
88% (r2= 0.76, p < 0.001) in the mean number of crabs
of the late 1990s,but those figures are still much lower than
the levels of the early 1990s. It takes 10 years for horseshoe
crabs to become sexually mature (Schuster and Sekiguchi
the increase in 2006 and 2007 a consequence of harvest re-
strictions that began in 1998.Therefore,a recovery of adult
horseshoe crabs may be under way.
in horseshoe crab numbers started in 2006.Hata (2008) re-
shoe crabs. The mean catch was 35.0 in 2005, 65.1 in 2006,
from 2005 to 2006–2007. Counts of spawning crabs have
been made since 1999 on Delaware Bay beaches (Michels et
per square meter (m2) increased from 3.23 (standard error
in 2007, but the mean number of females per m2showed
little change: 0.82 (SE 0.07) to 0.99 (0.07) to 0.89 (0.07),
sidering the long period of sexual maturation, results from
more years of surveying are needed to measure the strength
and persistence of this trend.
1992 in Delaware Bay, Botton and colleagues (1994) esti-
mated the density of eggs in the upper 5 cm of sediments
age densities per m2ranged from 3125 to 721,354 (mean
gan in 1996 used varying survey methods in its first four
3-meter (m) intervals between the high- and low-tide lines
assuming that in 1990 and 1991,it equaled the mean of the
3406, a decline of about 98% from the estimated density in
the early 1990s. This estimate of the scale of decline should
numbersof crabshavedeclined.Inviewof theevidencethat
of an increase in egg density. However, if horseshoe crab
populations are recovering, as suggested by trawl surveys,
egg densities should begin to improve within the next few
156 BioScience • February 2009 / Vol. 59 No. 2www.biosciencemag.org
Figure 4.Number of adult horseshoe crabs caught on standard-
ized surveys in Delaware Bay conducted by the Delaware
Division of Fish and Wildlife.An exponential curve is fitted to
the years 1990–2005.
Red knots: Trends in population size
wintered in southern South America from Tierra del Fuego
north to Río Colorado in Patagonia (Morrison and Ross
1989, Morrison et al. 2004, Baker et al. 2005a). Estimates of
wintering numbers there were made in 1985 using aerial
surveys (Morrison and Ross 1989), in 1995 using capture-
or their numbers over- or underestimated. Consistency of
method and timing keeps such errors to a minimum.
Tierra del Fuego population fell by almost 50% between
of 85% for 1994–1995 through 1997–1998 to 56% for
1998–1999 through 2000–2001, and recruitment into the
at 25,000 to 30,000 birds between 2002 and 2004,the popu-
lation again plunged to between 17,000 and 18,000 in
2005–2007 and then to 14,800 in 2008 (COSEWIC 2007,
Niles et al.2008).
Smaller numbers of knots winter in northern Brazil at
Maranhão and in the southeastern United States, mainly
Florida. Surveys of Maranhão revealed 8324 birds in 1985
and about 3000 in 2006 (Niles et al.2008).
ularly on the Florida gulf coast,have not been surveyed sys-
tematically. Niles and colleagues (2008) suggested that the
emphasized uncertainty about the true number. In Florida,
1979, between 4000 and 5000 in 2004, and 2142 in 2006
ple winter survey of southeastern United States (Niles et al.
ied from hundreds to nearly 5000,but there are insufficient
during winter from South Carolina and farther north and
Since 1986,fourtosixweekly aerialsurveys of shorebirds
gration in May and early June. The survey covers most bay
sey, where small numbers can be found foraging; therefore,
it does not record total numbers but provides an index of
stopover population size.Peak aerial counts (figure 3) show
peak (12,375) was the lowest ever recorded.
Red knots: Trends in weight gain
Since 1997, red knots have been captured, banded, and
later. Peak numbers usually occur during 14–28 May, after
their weights have shown considerable year-to-year varia-
trend (figure 7a). However, during 21–27 May, and 28 May
to 3 June, when most birds depart, weights have shown a
www.biosciencemag.orgFebruary 2009 / Vol. 59 No. 2 • BioScience 157
Figure 5.Estimated density of horseshoe crab eggs (eggs per
square meter in the top 5 centimeters of sand) on Delaware Bay
beaches.Data for 1990–1991 are from Botton and colleagues
(1994).The y-axis is log scale,and the bars are ±1 standard
error.Data on variation are not available for 1990–1991 or
1996–1999,so error bars cannot be shown.
Figure 6.Number of red knots counted during surveys of their
wintering grounds in southern South America,1985 and
2000–2008,and estimated using capture-recapture methods
in 1995 (González et al.2004).
quadratic relationship with year, declining strongly in the
the main departure period, dropped significantly from be-
tween 0.6 and 0.8 g in 1997–1998 to between 0.14 and 0.4 g
in 2006–2007 (figure 8). The decline in weight late in the
stopover period could result from a trend for birds to arrive
plies.Although some birds arrive late every year,there is no
evidence (e.g., from aerial counts) of a systematic trend to-
in Delaware Bay (1998–2005) by Atkinson and colleagues
(2007) showed that early arrivals increase mass at approxi-
gains two to three times higher, thereby making up for lost
time. However, this relationship broke down in 2003 and
of massgainbecauseof inadequatefoodsupplies.Thisstudy
indicates that birds arriving later in the stopover period re-
quire a superabundant supply of horseshoe crab eggs be-
weight to fly to Arctic breeding grounds, survive adverse
Therefore,it is likely that the main reason for the decline of
the red knot population is reduced availability of horseshoe
crab eggs,their primary food resource on Delaware Bay.
Botton and colleagues (2003) reviewed the synchrony of
tion. They pointed out that variations in the peak period of
horseshoe crab spawning, caused by yearly variations in
water temperature, can further contribute to inadequate
mass gains in shorebirds if peak spawning occurs before
birds arrive or after they depart. They also noted that peak
spawning periods have narrowed as the horseshoe crab
population has decreased.
Trends in other shorebird species
Several other shorebird species forage on horseshoe crab
turnstone were large and highly significant (table 1). Ex-
of shorebird species other than red knot declined by a com-
bined total of more than 14,000 birds per year; ruddy turn-
stones alone declined by 8145 per year.
158 BioScience • February 2009 / Vol. 59 No. 2www.biosciencemag.org
Figure 7.Mass (in grams) of red knots in Delaware Bay during
three weeks of their spring stopover: (a) 14–20 May,(b) 21–27
May,and (c) 28 May–3 June,plotted against year.Trend lines
are those predicted by the equations using the dataset means for
the Julian date.All predictors in each equation are significant at
p ≤ 0.001 except for year in the equation for 14–20 May,which is
nonsignificant (p = 0.53).The equation for 21–27 May does not
include the data for 2003 (marked by the arrow),which was an
atypical year when large numbers of red knots arrived late,
leading to very low weights in the latter part of the normal
stopover period.Bars are ±95% confidence intervals.
Figure 8.Proportions of red knots with 95% confidence inter-
vals in the more than 180 gram (g) body-mass category in
Delaware Bay near the departure time each year (26–28 May)
over 1997–2007.Numbers are total birds sampled.The trend
was fitted using binary logistic regression of body mass greater
than 180 g (1 = yes,0 = no) on year (continuous independent
variable),with sigma-restricted parameterization where the
year coefficient = –0.194,standard error = 0.015,and p < 0.001.
have been reported in flyways where food resources have
the flight to the Arctic.Among the causes of food depletion
Asia (Barter 2002,Moores 2006).
In the past eight years, horseshoe crabs and their eggs in
sources for migrant red knots and other shorebirds are not
sufficient to provide the nutrition needed to enable birds to
continue migration and reach the Arctic in good enough
al. 2004, Niles et al. 2008). Since 2000, despite a reduced
horseshoe crab harvest, neither the horseshoe crab popula-
take 10 years to reach sexual maturity, so recovery rates are
and red knot numbers recover to 1990 levels.
Bringing the bay back:
A recovery strategy for
horseshoe crabs and
The evidence outlined
above leads to a strong
inference about the pri-
mary reason rufa knots
have declined: greater
harvest of crabs led to a
The decline in egg densities coincided with a decline in de-
parture weights of knots, and resulted in lower annual sur-
tentially be secondary factors exacerbating the decline of
312 adults (and possibly 1000 more) migrating through
following years when feeding conditions in the bay were
poor. Thus there can be no doubt that the Delaware Bay
food supply has played a critical role in rufa’s decline.
The following paradigm provides a rationale for rufa
recovery. To restore the population, adult survival and re-
cruitment of juvenile birds need to improve.To achieve this
goal, the number of birds reaching threshold departure
weights of 180 g in Delaware Bay needs to increase; this
which requires low harvests until they rebound to former
targets for egg densities, horseshoe crab populations, and
objectives,andstrategies.All areintended toachieve our re-
decisionmaking process that reduces uncertainty of man-
management (ARM), can be characterized as “learning by
doing,”and has been used throughout the field of resource
ment of waterfowl (Nichols et al. 2007). In 2007, ASMFC
authorized both their horseshoe crab and shorebird techni-
shorebirds. Here we propose specific actions and identify
www.biosciencemag.orgFebruary 2009 / Vol. 59 No. 2 • BioScience 159
Table 1. Trends in the number of shorebirds recorded from aerial surveys of Delaware Bay
Table 2. Summary of changes in horseshoe crab and red
knot populations, 1980–2007.
1980–1989 Horseshoe crab harvest probably at low to moderate
levels; high horseshoe crab and red knot populations.
1990–1999High horseshoe crab harvest; horseshoe crab popula-
tions and horseshoe crab eggs decline by at least
80%; red knots and red knot weights begin to decline
by the late 1990s.
2000–2007Horseshoe crab harvest is reduced by the ASMFC
through four addendums to the original management
plan. Despite reductions and additional cuts made by
New Jersey and Delaware, including a moratorium on
harvest in New Jersey, the total harvest remained high-
er than estimated in the 1980s; horseshoe crab popu-
lation stable at a low level; red knot population size
and departure weights decline sharply; rufa red knots
are recognized as threatened under the Bonn Conven
tion and in Brazil, Canada, and the United States.
As these actions are taken, monitoring will reveal whether
anticipated effects occur and will provide evidence to revise
the recovery model as needed.
Recovery goals. The goal is a “restored” rufa population.
Niles and colleagues (2008) proposed that rebuilding the
rufa population to the 1980 level, estimated at 100,000 to
150,000 birds by Morrison and Harrington (1992), was a
targets (conditions that would warrant delisting) that are
smaller than the former or ideal population level,especially
whenthesizeof thehistoricalpopulationisuncertain.If we
servative estimate of the historical population, a recovery
gest two conditions for considering that rufa knots are re-
no factors are known that would be likely to cause a reduc-
tion to below 80,000.
vival and Delaware Bay departure weights.As noted above,
Baker and colleagues (2004) showed that annual survival
rates of adult rufa were about 85% in the late 1990s, which
is very similar to the 85.8% reported by Boyd and Piersma
(2001) for islandica knots (which breed in northeastern
tial target for adult survival of 80%.
Baker and colleagues (2004) showed
that annual survival was related to
parture condition was also shown to be
knots departing from their final spring
2007). Survival appeared to become
weighing at least 180 g at the end of the
ure 8). We therefore propose that the
the birds when they leave Delaware Bay
for theirArctic breeding grounds.
Recovery strategies. Strategies include
sity targets requires specification of an
the area. Niles and colleagues (2008) identified bay beaches
ditions for knots,categories that apply to about half the bay
beaches.We suggest using these areas in defining the target
for horseshoe crab eggs. Work by Botton and colleagues
declined in the 1990s, sometimes (in two of seven beaches
knots and other species that feed on the eggs. For example,
per m2in the top 5 cm of sand. We therefore propose that
tic, however, because suitability for spawning has been
determined only remotely (Niles et al. 2008).Achieving the
sonable as an initial goal until the relationships between
have to be adjusted in the light of future studies, including
those evaluating the impact of gulls, which also consume
numerous eggs (Burger et al.2004).
The number of horseshoe crabs needed to produce the
is probably to express the target in terms of the catch on
standardized surveys. Although the Delaware 30-foot trawl
was designed for finfish, it is the only survey of horseshoe
160 BioScience • February 2009 / Vol. 59 No. 2www.biosciencemag.org
Figure 9.Vision,goal,objectives,and strategies for recovering horseshoe crabs and rufa red
knots.The arrows represent the progression of recovery and numeric targets that character-
ize it: for example,increased horseshoe crabs and egg densities (strategies) lead to increases
in red knot departure weight and survival (objectives),leading to recovery of the red knot
population,and thus it is the most informative benchmark.
bers obtained in 1990 and 1991 (figure 4). However, the
the US Fish and Wildlife Service; and the states of New
Jersey, Delaware, Maryland, and Virginia. This joint effort,
be responsible, then, for estimating suitable horseshoe crab
of theASMFC and its recommendations implemented.
particularly the red knot population,depends on the recov-
ery of the horseshoe crab population.Therefore,a conserv-
approach until it is clear that horseshoe crab populations
are recovering and likely to reach the target.
Secondary recovery parameters.Althoughourrecoverypar-
gration route, or Delaware Bay may become unsuitable for
rameters identified above, certain secondary recovery para-
meters need to be monitored. The following are three
parameters that seem particularly important:
1. The proportion of the red knot population that uses
Delaware Bay. A minority of knots has always stopped over
bypassing Delaware Bay than did in the past.This could re-
flect improved conditions at other sites as well as poorer
conditions in the bay. Nevertheless, the proportion of the
2. Productivity. Annual production of juvenile knots might
ber of young passing through each location. This approach
erally migrate at different times and may winter in different
adults and second-year birds that winter together.
3. Recruitment into the breeding population.Anothermeans
of measuring recruitment is to monitor the birds migrating
north for the first time through Delaware Bay. This would
assess the number of new adults by comparing estimates of
the size and survival of the previous year’s adult population
with the total number stopping over in Delaware Bay in the
Here we briefly describe ways to monitor primary and sec-
working on knots and crabs to coordinate their efforts to
achieve these accuracy targets and monitor progress.
Monitoring rufa population size. Monitoring rufa popula-
tion size is best done by counting birds on their wintering
grounds. Aerial surveys already cover Tierra del Fuego, the
most important area. The only sources of error are missing
veys are required every year to monitor ongoing status and
and whether conservation measures are effective.
Much better information is needed from Brazil and the
rufa could be counted in Florida and Georgia (compared
with other wintering areas),and their status as a threatened
given particularly high priority.
Monitoring rufa survival rates.A major research and moni-
toring program focused on migrant shorebirds has been
700 knots have been captured annually, along with similar
numbers of ruddy turnstones and sanderlings. Each year,
several thousand resightings are recorded of each species,
making this program one of the most intensive for any
migratory wildlife species.
Annual adult survival rates may be estimated from
between-year recaptures and resightings of marked birds.
Not all rufa knots pass through Delaware Bay,and at least a
few visit the bay in some years but not in others.Therefore,
survival rates for the rufa population as a whole can be esti-
mated reliably only if they are based on data from through-
out the flyway.Additionally, it is also important to monitor
the annual survival of each wintering population separately
because each faces differing conditions.
Monitoring rufa weights. This study has demonstrated the
value of determining the status and condition of individual
birds by monitoring their weights.It also affords one means
of assessingthegeneralstateof environmentalconditionsin
www.biosciencemag.orgFebruary 2009 / Vol. 59 No. 2 • BioScience 161
Delaware Bay. Therefore, it is important to continue moni-
to 3 June,as done in the past.
arrival and departure times for samples of birds.These data
can be used along with weight at capture and the model to
estimate the departure weight for each bird. This analysis
would permit estimation of the proportion of birds weigh-
this estimate and to provide backup in case within-year re-
captures are insufficient for estimation, the proportion of
birds weighing more than 180 g during 26–28 May should
be monitored routinely (see figure 8), though this may not
reflect the success of the stopover if migration phenology
of global warming.
Monitoring horseshoe crab parameters.Studies cited above
measure horseshoe crab abundance, egg densities, and har-
iment are readily entrained by small waves as they wash
across a beach; then,as the tide recedes,eggs may be left ex-
eggs quickly desiccate. Thus, samples of surface-sediment
accurate estimation requires frequent sampling—possibly
more frequent than currently practiced—and perhaps a
model-based estimation approach. The best sampling plan
Monitoring secondary recovery parameters.Givenestimates
of rufa population size from the winter surveys, the pro-
portionof thebirdsusingDelaware Baycanbeestimated as
period (though some may arrive before or leave later). We
assume that the aerial surveys are a systematic sample from
birds present at the time of the flight. Under these assump-
tions, it may be shown that Lx/t is an essentially unbiased
estimate of the number of knots visiting the bay, where x is
the mean number of knots recorded per survey and t is the
ing the survey period,estimated from resighting surveys.
Monitoring productivity by reference to the number or
proportion of juveniles during southward migration is very
that method does not allow us to estimate the productivity
breeding distributions). Juveniles and adults of the Tierra
and second-year birds that winter together.
Recruitment can be measured by partitioning change in
population size into recruitment and adult survival rates.
population size between two years as
next May, Ai + 1is the number of birds first appearing in
Delaware Bay (i.e., “additions”) in year i + 1, and Riis re-
cruitment in year i. Simay be estimated from between-year
captures and resightings.Rimay be estimated as
where siis the estimator of Si.The estimates of productivity
can also be used to improve the estimate of recruitment.
migrant is daunting, and there are few other widespread,
threatened species for which such data are available. But as
shown above,programs have been running for several years
and placed on a sound institutional basis.
Our analysis shows that the best chance for halting and
reversing the decline of the red knot would be through
restoration of horseshoe crabs and their eggs to levels pre-
horseshoe crab population to a level that produces egg den-
migrant shorebirds,as well as a sustainable harvest of crabs.
almost exactly the size of their population and can monitor
In contrast, there is great uncertainty about the size of the
horseshoe crab population, the numbers that can be har-
According to the latest (2003) estimate, the Delaware Bay
162 BioScience • February 2009 / Vol. 59 No. 2www.biosciencemag.org
horseshoe crab population numbers 20 million; however,
this estimate is based on only 48 recaptures out of 17,543
(13millionto28million) (Smith etal.2006).Moreover,the
ulation can stand. Conservationists have argued for a com-
approach; regulatory authorities have acted to reduce the
be. On the basis of data up to 2005, the Horseshoe Crab
Stock Assessment Subcommittee of the ASMFC concluded
that“the current harvest appears to be in excess of the max-
imum sustainable yield”(ASMFC 2006b).
Worldwide, and particularly in America, fisheries have
sustainability; for example, the Pew Oceans Commission
had been studied were already overfished or being fished at
unsustainable rates,” concluding that “we catch too many
age the resources of the oceans.
years of support for shorebird research and conservation.
Conserve Wildlife Foundation of New Jersey, with funding
Dodge Foundation,supported Delaware Bay work and sus-
thank the New Jersey and Delaware Divisions of Fish and
Wildlife for long-term support. Stewart Michels, of the
Delaware Division of Fish andWildlife,provided horseshoe
and Annette Scherer,with the US Fish and Wildlife Service,
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Bart is a research wildlife biologist with the US Geological Survey,Forest and
Rangeland Ecosystem Science Center, Snake River Field Station, in Boise,
Idaho. Humphrey P. Sitters is an editor with the International Wader Study
Group Bulletin in Exeter, United Kingdom. Amanda D. Dey is principal
zoologist, and Kathleen E. Clark is supervising zoologist, at the New Jersey
Trenton. Phillip W. Atkinson is research manager, Nigel A. Clark is head of
ogist, all with the British Trust for Ornithology in Norfolk, United Kingdom.
Allan J. Baker is head of the Department of Natural History of the Royal
is with the Victoria Wader Studies Group in Melbourne, Australia. R. I. Guy
Morrison is a research scientist (NationalWildlife Research Centre,Carleton
University), and R. Ken Ross is head of the population management unit
164 BioScience • February 2009 / Vol. 59 No. 2 www.biosciencemag.org