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Fishery Bulletin 107(2) 
and immature females) of horseshoe crabs using the 
form 
log e (VFf) = log e (fW)-a + log e (6), (1) 
where Wt = weight of a horseshoe crab (kg); 
PW = prosomal width (mm); 
a = slope; and 
b = y-intercept (PROC REG, SAS). 
We could not develop equations for each group of 
horseshoe crabs by state because of the small sample 
size collected from some states. 
Testing current and developing 
alternative conversion factors 
The predictive accuracy of the NOAA Fisheries conver- 
sion factor was tested using data collected from four 
data sets: the three spawning surveys and the Delaware 
commercial fishery. We calculated total biomass for each 
sample using the NOAA Fisheries conversion factor and 
then compared it to the total observed biomass for each 
sample. 
We used various data sets (i.e., three spawning sur- 
veys, the Delaware commercial fishery, the HCRC trawl 
survey, unpublished data) and previously published 
studies to generate the average PW and weight for male 
and female horseshoe crabs from each state (Table 1). 
For some states, the average weight of horseshoe crabs 
was not available, and therefore we used the PW-weight 
equations that were derived from this study to estimate 
the average weight of horseshoe crabs based on an aver- 
age measured prosomal width. For states where average 
weight data were available, we compared the observed 
weight to the estimated weight (i.e., using PW-weight 
equation) to determine the accuracy of the PW-weight 
equations. 
Results 
Prosomal-width-to-weight relationship 
The average weight differs between male and female 
horseshoe crabs. Mature female horseshoe crabs 
were significantly larger (i.e., prosomal width; df=l, 
346; F= 1488.03; P< 0.0001) and heavier (df=l, 346; 
F=2245.72; PcO.0001) than mature male horseshoe 
crabs. The weight of horseshoe crabs was significantly 
different among sex and maturity stages (df=7, 924; 
F=6.86; P=0.0090; Table 2). Significant differences did 
not occur in the PW-weight relationship of mature male 
and mature female horseshoe crabs (df=3, 577; F=2.19; 
P=0.1396; Table 2); however, when comparing only 
horseshoe crabs of overlapping size ranges (PW=181-292 
mm; weight= 0.88-3.14 kg), the PW-weight relationship 
of mature female horseshoe crabs was significantly dif- 
ferent than that of mature males (df=3, 626; F=8.21; 
P=0.0043). 
Separate PW-weight equations were developed for 
all females, mature females, immature females, all 
males, mature males, and immature males (Table 3). 
The derived PW-weight equations were used to esti- 
mate an average weight of horseshoe crabs from each 
state based on the observed prosomal width (Table 1). 
However, we used only the PW-weight equation derived 
for mature horseshoe crabs (i.e., one for mature males 
and one for mature females) to estimate weight because 
the PW-weight relationship was significantly different 
between sexes of mature horseshoe crabs, and the com- 
mercial fishery is directed only at mature horseshoe 
crabs. The estimated average weight of both male and 
female horseshoe crabs with the derived PW-weight 
equations was relatively accurate compared to the ob- 
served average weight for each state (Table 1). 
Testing current and developing 
alternative conversion factors 
The conversion factor used by NOAA Fisheries (i.e., 
1.21 kg/horseshoe crab) consistently overestimated the 
total weight of horseshoe crabs collected during spawn- 
ing surveys and from the Delaware commercial fishery 
(Table 4). For female horseshoe crabs from Mid-Atlantic 
populations (i.e., Delaware Bay and Raritan Bay), this 
conversion factor provided a relatively close estimate of 
total weight. However, when estimating the total weight 
of male horseshoe crabs, the NOAA Fisheries conversion 
factor overestimated total weight. The weight of horse- 
shoe crabs from the New England population (i.e., Great 
Bay) was overestimated to the greatest degree, by more 
than 70% for both males and females. 
The average weight of a horseshoe crab also varies 
by location. Horseshoe crabs between Rhode Island and 
South Carolina are larger and heavier than horseshoe 
crabs from Maine, New Hampshire, Massachusetts, and 
Florida; and the conversion factors that have been used 
by most states reflect the differences in size and weight 
among states (Table 1). For those states where a single 
conversion factor has been used in the past to estimate 
the weight for both male and female horseshoe crabs 
(i.e., Maine, Rhode Island, Virginia, North Carolina, 
South Carolina, and Florida), the weight of at least 
one sex, in most cases the weight of female horseshoe 
crabs (Table 4) has been predicted inaccurately. Most 
states in the Mid-Atlantic have derived two conversion 
factors (i.e., one for each sex) that are relatively close to 
the average weight of mature horseshoe crabs collected 
within that area (Table 1). 
Discussion 
Female horseshoe crabs are much larger than male 
horseshoe crabs; therefore separate conversion factors 
should be used for each sex. Our results indicate that 
horseshoe crabs exhibit considerable sexual size dimor- 
phism with mature female horseshoe crabs being sig- 
nificantly larger and heavier than males. Males in any 
