Ryer et al: Depth distribution, habitat associations, and differential growth of Chionoecetes bairdi 
263 
200 r 
c 150 
Carapace width (mm) 
Figure 8 
Cumulative size-frequency distribution of southern Tanner 
crabs (Chionoecetes bairdi) from all sites, months, and depths 
surveyed in 2010 and 2011 around Kodiak Island, Alaska. A 
tabulation of median size, size range, and molt increment for 
each molt stage is provided in Table 1. 
df=2, P=0.070). As seen in the scrape data, crab den- 
sity estimated by divers in quadrat surveys increased 
with depth (Wald x 2 =12.8, df=3, P=0.005). This depth 
effect was consistent over months (monthxdepth: Wald 
X 2 =3.5, df=4, P=0.474). Although crab density appeared 
to mirror the observed depth distribution of worm tubes 
(Fig. 7B), worms tubes were not a significant covariate 
in the GLM analysis (Wald x 2 =0.9, df=l, P=0.354), in- 
dicating no association at this spatial scale. 
Table 1 
Medians and ranges of carapace widths (mm) for pro- 
posed molt stages of southern Tanner crabs ( Chionoece- 
tes bairdi) collected during 2010 and 2011 from all 4 
sites that were surveyed around Kodiak Island, Alaska. 
Increment percentages indicate the relative increase in 
carapace width from one molt stage to the next. 
Stage 
Median 
(mm) 
Range 
(mm) 
Increment 
(%) 
Cl 
3.4 
2.6-4. 1 
_ 
C2 
4.8 
4. 2-5. 6 
41 
C3 
6.8 
5.7-8. 2 
42 
C4 
9.7 
8.3-11.3 
43 
C5 
13.4 
11.4-15.2 
38 
C6 
17.8 
15.3-20.1 
32 
C7 
23.4 
20.4-26.6 
31 
C8 
31.8 
28.5-35.6 
36 
C9 
42.0 
37.3-45.7 
32 
Age-0 crab molt intervals and size distributions 
Cumulative size-frequency distributions (all sites 
and months) were examined initially for each 
year (2010 and 2011). However, because patterns 
were identical, we combined both years into a 
single distribution (Fig. 8; Table 1). Break points 
between peaks in the size-frequency distribu- 
tion were taken to be boundaries between suc- 
cessive molt stages. Percent increase in size over 
the prior stage averaged 36.9%, within a range 
of 31-43% (Table 1). There was a trend for molt 
increments to decrease in later molt stages. Using 
this classification, we assigned individual crabs 
to stages, thereby allowing us to examine differ- 
ences in population composition between months 
and study sites. 
A comparison of the 4 study sites surveyed 
in August of 2010 revealed a significant differ- 
ence in molt-stage frequencies (x 2 =349.13, df=12, 
P<0.001). C4 instars were dominant at Womens 
and Kalsin, and C3 instars were dominant at Hol- 
iday and Pillar. Accordingly, the mean carapace 
widths of crabs at Womens (10.1 mm [SE 0.1]) 
and Kalsin (9.1 mm [SE 0.1]) were larger than 
those at Holiday (7.2 mm [SE 0.1]) and Pillar (7.0 
mm [SE 0.1]; Kruskal-Wallis: P<0.001; multiple 
comparisons: P<0.05). 
Examination of data from surveys conducted in 2011 
revealed that differences between study sites in fre- 
quency distribution of molt stages were established 
relatively early in the late summer (Fig. 9). Differences 
in molt-stage composition between sites were evident 
in May (x 2 =41.36, df=6, P<0.001). Cl instars dominated 
populations, although C2, C3, and even C4 instars were 
present at Kalsin and in lessening degrees at Pillar 
and Holiday (we did not include Womens in our analy- 
sis because only several Cl instars were encountered 
there). As a consequence, the mean carapace widths of 
crabs at Kalsin (4.5 mm [SE 0.2]) and Pillar (4.0 mm 
[SE 0.1]) were larger than the mean at Holiday (3.6 
mm [SE 0.1]; Kruskal-Wallis: P<0.001; multiple com- 
parisons: P<0.05). By June, crabs were present at Wo- 
mens and were typically one stage further along than 
those at the other sites (x 2 =184.89, df=9, PcO.OOl). Ac- 
cordingly, the mean carapace width of crabs at Wom- 
ens (5.7 mm [SE 0.2]) was larger than the means for 
crabs at Kalsin (4.2 mm [SE 0.1]) and Pillar (3.8 mm 
[SE 0.1]), where crabs in turn were larger than crabs 
at Holiday (3.5 mm [SE 0.1]; Kruskal-Wallis: P<0.001; 
multiple comparisons: P<0.05). Similarly, in July, C3 
instars were prevalent at Womens, whereas C2 instars 
dominated the other sites (x 2 =436.06, df=12, P<0.001). 
As a result, the order of mean carapace widths of crabs 
at the study sites was Womens > Kalsin > Holiday > 
Pillar, and all sites differed significantly from each 
other (Kruskal-Wallis: P<0.001; multiple comparisons: 
P<0.05). Lastly in August, C4 instars dominated at 
Womens, a mixture of C3 and C4 instars dominated 
at Kalsin, C3 instars were dominant at Holiday, and a 
