Stiansen et al Efficiency and catch dynamics of square and conical crab pots 
45 
Table 3 
Catch data from square and conical pots used to capture red king crab ( Paralithodes camtschaticus). Median and mean carapace 
length of male and female red king crabs caught by the two types of pot at different soak times are shown. The length distribu- 
tions were compared by means of a nonparametric Mann-Whitney U test for each soak time, n is the number of crabs caught, SE 
is the standard error. 
Carapace length of crabs caught (mm) 
Square pots Conical pots 
Soak time U test 
(days) 
n 
Median 
Mean 
SE 
n 
Median 
Mean 
SE 
P value 
Males 
2 
1797 
161 
154 
0.69 
412 
154 
150 
1.38 
<0.01 
3 
823 
164 
154 
1.04 
369 
151 
148 
1.44 
<0.001 
4-5 
232 
174 
168 
1.46 
95 
166 
159 
2.37 
<0.01 
7-8 
364 
142 
146 
1.42 
250 
142 
143 
1.65 
>0.2 
Females 
2 
1954 
131 
130 
0.39 
755 
132 
131 
0.60 
>0.1 
3 
1030 
130 
130 
0.53 
683 
131 
130 
0.63 
>0.7 
4-5 
312 
139 
137 
0.85 
170 
137 
136 
1.22 
>0.6 
7-8 
548 
129 
128 
0.62 
463 
131 
130 
0.67 
>0.05 
1990; Cyr and Sainte-Marie, 1995), and it is not until 
catches are large, and crabs (by climbing on top of each 
other) can reach to the funnel top, that the entry and 
exit rates reach equilibrium (Zhou and Shirley, 1997a). 
The linear increase in catches, the relative low catches, 
and the stable sex ratio (60% females) until seven to 
eight days of soak time indicated that the exit rate was 
low for the conical pots. 
Unlike the catch in the conical pots, the amount of 
catch taken by the square pots did not increase after 
two days of soak time as was also observed by Zhou and 
Kruse (2000) who used a different square-pot design. 
The initially high rate of entry into pots may lead to the 
bait being eaten within a short time and to the result 
that few additional crabs are attracted into the pot. In 
addition, the likelihood of escape should be higher for 
square pots that have two horizontally orientated fun- 
nels, which are placed lower and made of netting that 
enable crabs to climb up the funnel and escape (Zhou 
and Shirley, 1997c). 
The square pots caught a higher proportion of male 
crabs and male crabs larger than 160 mm CL than did 
the conical pots. If the largest crabs take longer to es- 
cape than small crabs, large crabs will accumulate in 
the square pots over the course of time. Sexual dimor- 
phism, where the two sexes have different probabilities 
of leaving the square pot, would result in more males 
in the catch. Although large crabs accumulated in the 
square pots over time, the proportion of female crabs 
increased from 50% to 60% with longer soak time. This 
increase may also be explained by sex-dependent escape 
probability. Generally, total body length and height 
(critical dimensions in entering and escaping) are great- 
er for females than for males of the same carapace 
length (Zhou and Shirley, 1997b), and the differences 
increase with increasing carapace length. Thus, at a 
carapace length of 100 mm, Wallace et al. (1949) found 
female crabs to be 6 mm longer and 11 mm taller than 
male crabs with the same carapace length. Moreover, in 
the autumn (the season of this study) most female crabs 
larger than 100 mm CL were carrying external eggs, 
which would further increase body length and height. 
Motivational differences related to sex and condition 
may also have influenced the ability of RKC to climb 
into and out of pots (Zhou and Shirley, 1997c). 
The catch characteristics and design (collapsible) of 
the square pots make them highly suitable for com- 
mercial fishing. This pot design may also prove to be 
efficient and practical in other regions, e.g., in the Alas- 
kan king crab and Tanner crab ( Chionoecetes bairdi) 
fisheries. The size selection of the square pots resulted 
in them taking five times as many large male crabs 
(>160 mm CL, approximately 3.5 kg) as the conical pots 
after two days of soak time. The selectivity and catch 
efficiency of the square pots also resulted in a 50% re- 
duction in the catch of females and males smaller than 
137 mm CL (minimum legal landing size). Even though 
discard mortality should be minimal with correct on- 
deck handling, there is nevertheless the likelihood that 
some crabs are injured, e.g., spines, rostrum, and limbs 
can be damaged during handling (Zhou and Shirley, 
1995). RKC without all legs intact are of no commercial 
value and have to be discarded. This problem would 
be mitigated with the use of square pots. A further 
advantage of the square pot over the conical pot is that 
the higher escapement rate afforded by the square pot 
reduces crab mortality at sea when pots that are lost 
at sea continue to fish (ghost fish); the pots used in the 
