Myers et al.: Population growth rate of Gadus morhua 
767 
Table 1 
Estimates of rate of population growth (r m ), slope at the origin ( a) estimated from the Ricker model, age of maturity (a), bottom 
temperature, and NAFO/ICES management units for 20 cod populations in the North Atlantic. 
NAFO/ICES 
ID no. 
Stock location 
management unit 
r m 
a 
a 
Temperature 
1 
West Greenland 
1 
0.23 
2.4 
6 
1.75 
2 
Labrador/N.E. Newfoundland 
2J3KL 
0.17 
2.3 
7 
0.00 
3 
S. Grand Bank 
3NO 
0.27 
3.5 
6 
1.75 
4 
N. Gulf of St. Lawrence 
3Pn4RS 
0.20 
3.0 
7 
1.00 
5 
St. Pierre Bank 
3Ps 
0.31 
4.6 
6 
2.50 
6 
S. Gulf of St. Lawrence 
4TVn 
0.15 
1.9 
7 
1.75 
7 
E. Scotian Shelf 
4VsW 
0.36 
9.8 
6 
3.75 
8 
S.W. Scotian Shelf 
4X 
0.36 
2.5 
3.5 
6.75 
9 
Georges Bank 
5Z 
0.60 
2.2 
2 
8.00 
10 
S.E. Baltic 
22-24 
0.74 
8.4 
3 
7.00 
11 
Central Baltic 
25-32 
0.53 
3.1 
3 
5.00 
12 
Celtic Sea 
Vllg.f 
0.62 
5.3 
3 
11.00 
13 
Faroe Plateau 
Vb 
0.44 
4.2 
4 
7.40 
14 
Iceland 
Va 
0.24 
4.3 
7 
5.80 
15 
Irish Sea 
Vila 
1.03 
23.1 
3 
10.00 
16 
Kattegat 
South Ilia 
0.53 
3.8 
3 
6.50 
17 
Barents Sea 
I 
0.26 
6.6 
7.5 
4.00 
18 
North Sea 
IV 
0.56 
9.0 
4 
8.60 
19 
Skagerrak 
North Ilia 
0.82 
11.2 
3 
6.50 
20 
West of Scotland 
Via 
0.80 
6.4 
2.5 
10.00 
pendence of r m on age-at-maturity, but there is no 
corresponding significant relation between a and 
age-at-maturity (Fig. 6). Consistency demands that 
there be a relation between age-at-maturity and tem- 
perature, and, indeed, Table 2 and Figure 7 show 
that there is a significant correlation between these 
two variables. 
We repeated the above analysis with a calculated 
at the median slope of the six observations with the 
lowest spawner abundance. The estimates calculated 
with this robust procedure were generally compa- 
rable with those estimated from the Ricker model, 
although the Ricker values were generally higher 
(Fig. 8). The larger discrepancies in the two meth- 
ods occurred for the populations in which there were 
low estimates of recruitment at the largest popula- 
tion sizes, e.g. Irish Sea cod (Fig. 3). These points, 
although they are farthest from the origin, resulted 
in a higher estimate of the slope at the origin be- 
cause the Ricker model assumes a linear relation 
between egg-to-recruit mortality and SSB. 
We repeated the regression analysis of rate of popu- 
lation growth and slope at the origin ( a ) at mini- 
mum population size versus bottom temperature 
with the robust estimate of the slope, and found simi- 
Table 2 
For each of the three variables r m (population growth rate), 
a (standardized slope of the spawner- recruit curve at the 
origin), and a (age-at-maturity), the estimated slope pa- 
rameter of the regression on temperature ( T ) (e.g. r m = a + 
bT) is presented, labeled b . For r m and a , there are two b 
values based respectively on the Ricker fit to each spawner- 
recruit data set and the median of the first six points of 
each spawner-recruit data set. Also shown are the signifi- 
cance levels of the regressions on temperature and the cor- 
responding r 2 . The results are presented for the Northwest, 
Northeast, and entire Atlantic. 
Variable 
Ricker 
b 
Ricker 
P(b= 0) 
Ricker 
r 2 
Median 
b 
Median 
P(6+0) 
Median 
r 2 
r m 
West 
0.04 
0.001 
0.63 
0.02 
0.03 
0.51 
East 
0.09 
0.003 
0.64 
0.04 
0.06 
0.35 
All 
0.06 
0.00008 
0.59 
0.04 
0.00003 
0.62 
a 
West 
0.32 
0.5 
0.06 
-0.16 
0.3 
0.15 
East 
0.83 
0.2 
0.16 
0.09 
0.7 
0.02 
All 
0.67 
0.04 
0.21 
0.13 
0.2 
0.08 
a 
West 
-0.62 
0.00005 
0.92 



East 
-0.45 
0.06 
0.34 
— 
— 
— 
All 
-0.47 
0.00002 
0.65 
— 
— 
— 
