500 
Fishery Bulletin 95(3), 1997 
35 -| 
34 - 
33 - 
32 - 
31 - 
30 - 
29 - 
28 - 
27 - 
26 - 
1992 
SE area 
25 
i i -i — i — i — i — i — i — i — i 
0 10 20 30 40 50 60 70 80 90 100 
.I' 
35 
JJ 
03 
34 - 
t 
33 - 
& 
O 
32 - 
IT) 
31 - 
03 
30 - 
E 
29 - 
O^ 
_c 
28 - 
o> 
27 - 
c 
0 
26 - 
_J 
25 - 
0 
1993 
“i i — i — i — i — i — i — i — i — i 
10 20 30 40 50 60 70 80 90 100 
35 
34 - 
33 - 
32 - 
31 - 
30 - 
29 - 
28 - 
27 - 
26 - 
25 -- 
0 
1994 
NW area 
SE area 
“T I I I I 1 1 1 1 1 
10 20 30 40 50 60 70 80 90100 
Bottom depth (m) 
Figure 4 
Female yellowfin sole length at 50% 
maturity, with respect to bottom depth 
(D) and geographic area (NW and SE) 
for years 1992-94. Curves illustrate 
predicted values of fish length (L) from 
the logistic equation: MAT = 1/(1 + 
e ~(n+P L+a area+8 D)^ t where the pro- 
portion mature (MAT) is 0.5. 
deeper depths than can slower-growing individuals. 
Length-at-age estimates for immature yellowfin sole 
(<8 years of age) from shallower waters, therefore, are 
biased low in comparison with those from deeper wa- 
ters (Fig. 3). The cessation of this depth effect with in- 
creasing bottom depth after 8 years of age (Fig. 3) may 
indicate the approximate age at which immature yel- 
lowfin sole leave the size-depth continuum and become 
migratory (i.e. to shelf-slope waters in winter and back 
to nearshore waters in spring-summer). The timing of 
first maturity may very well coincide with the initia- 
tion of a “spawning” migration. 
' ~ ] 1 992-94 
14 - 
13 - 
12 - 
1 1 - 
10 - 
9 - 
8 H — i — i — i — i — i — i — i — i — i — i 
0 10 20 30 40 50 60 70 80 90 100 
Bottom depth (m) 
Figure 5 
Female yellowfin sole age at 50% matu- 
rity, with respect to bottom depth (D), 
pooled among years 1992-94. Curves il- 
lustrate predicted values of fish age (A) 
from the logistic equation: MAT = 1/ 
(l +e ~(H+P A+8 D)-. j where the propor- 
tion mature (MAT) is 0.5. 
o 
in 
<D 
o> 
< 
The increase in female length and age at maturity 
with increasing bottom depth was largely due to the 
variation of immature female length distributions as 
depth increased. In spring-summer, when yellowfin 
sole have migrated nearshore for spawning, distri- 
butions of mature migratory individuals and imma- 
ture “ontogenetically driven” individuals overlap. The 
differences between these two migration patterns act 
to separate immature and mature fish of similar 
lengths (and ages) along a bottom depth gradient. 
Immature females were distributed unevenly by size 
across depth; larger sizes (25-32 cm TL) were more 
common at deeper depths (Fig. 7). In contrast, ma- 
ture females were distributed similarly by length 
between shallow (<30 m) and deeper (>30 m) bottom 
depths (Fig. 7). 
The absence of these larger immature females in 
shallow water resulted in lower values of length at 
maturity for shallow waters and higher values for 
deeper waters (Fig. 7). Trippel and Harvey (1991) 
demonstrated how age at maturity of white suckers 
( Catostomus commersoni) could be affected if year 
classes falling within the progression from immatu- 
rity to maturity are missing. Missing length classes, 
similarly, affect estimates of length at maturity. Sam- 
pling of female yellowfin sole in shallow waters (<30 
m) misses critical length and age classes of imma- 
ture individuals on the verge of maturity. 
Length or age at maturity, as with growth rela- 
tions, are often measured simultaneously across 
multiple cohorts and are therefore approximations 
