Alvarez and Alemany: Birthdate analysis and its application to the study of recruitment of Sardina pilchardus 
189 
ments (Methot, 1981). The length ranges by sample 
of the analyzed specimens (n= 220) are given in Table 
1. Fish were measured (standard length and total 
length by 1-mm size classes), weighed (0.1 g), and 
their otoliths were removed and mounted on micro- 
scope slides with Eukitt mounting medium. It is pos- 
sible to determine the ages of juveniles and their daily 
growth rates because the daily deposition of growth 
increments has been validated for sardine (Re, 1984), 
the time of formation of the first daily growth ring is 
known (Alemany and Alvarez, 1994), and because it 
is possible to distinguish false or subdaily increments 
from true daily growth rings. It should be pointed 
out that no gaps were observed in the daily growth 
rings in the sagittal otoliths of the analyzed speci- 
mens. Each daily ring was counted and its width was 
measured, along a transect located at ± 5° of the long- 
est radius from the focus to the posterior margin of 
the otolith, with a video coordinated digitizer con- 
nected to a microcomputer (Methot, 1981). To reveal 
increments, the otoliths were progressively polished 
between readings by using 30-, 9-, and 0.3-micron 
lapping film. Magnifications of x60, x640, and xl,000 
were used. Data from several replicate transects per 
otolith, at different magnifications, were combined 
to estimate age. Occasionally, daily increments were 
difficult to resolve within short (<60 microns) seg- 
ments of the otoliths. In these cases, widths of rings, 
and hence their number, were interpolated by using 
linear approximation based on the widths of previ- 
ous and later clearly visible daily rings. Data from 
an otolith reading were rejected as unreliable if the 
interpolation process affected more than 5% of the 
readings. 
Environmental conditions 
During the main sardine spawning season off the 
northern and northwestern coasts of the Iberian pen- 
insula, 5 cruises were conducted between March and 
July 1992 (Lopez-Jamar et al., in press). The proto- 
cols established during a pilot cruise carried out in 
April 1991 (Lopez-Jamar et al. 7 ) were followed. The 
7 Lopez-Jamar, E., S. Coombs, F. Alemany, J. Alonso, F. Alvarez, 
C. Barrett, J. M. Cabanas, B. Casas, G. Diaz del Rio, M. L. 
Fernandez de Puelles, C. Franco, A. Garcia, N. C. Halliday, A. 
Lago de Lanzos, A. Lavin, A. Miranda, D. Robins, L. Valdes, 
and L. M. Varela. 1991. A SARP pilot study for sardine 
Sardina pilchardus off north and northwest Spain in April/May 
1991. ICES Council Meeting 1991/L:69, 36 p. (mimeo). 
Table 1 
Summary statistics for the Atlanto-Iberian sardine Sardina pilchardus by port and sampling date in 1992. TL=total length (mm), 
SD=standard deviation, BD=birthdate, Age=days. 
Port and 
sampling date 
Mean TL 
SD 
Min. TL 
Max. TL 
Mean BD 
Mean age 
SD 
Min. age 
Max. age 
La Coruna 
7 Jul 
90 
6 
82 
103 
18 Jan 
173 
11 
156 
193 
24 Jul 
113 
7 
105 
127 
13 Jan 
194 
13 
172 
215 
9 Sep 
116 
3 
111 
122 
23 Jan 
231 
8 
216 
246 
17 Sep 
130 
3 
125 
133 
21 Jan 
243 
13 
225 
265 
1 Oct 
128 
9 
121 
155 
20 Jan 
256 
19 
231 
294 
6 Oct 
126 
6 
118 
135 
1 Feb 
249 
19 
228 
295 
13 Oct 
118 
11 
105 
139 
1 Feb 
254 
22 
215 
298 
28 Oct 
133 
6 
121 
140 
30 Jan 
273 
13 
241 
291 
4 Nov 
115 
2 
112 
120 
14 Feb 
265 
22 
217 
293 
13 Nov 
109 
3 
105 
113 
13 Feb 
275 
21 
256 
290 
Vigo 
30 Jun 
83 
3 
77 
87 
26 Jan 
157 
5 
151 
169 
8 Jul 
89 
5 
80 
97 
25 Jan 
165 
5 
154 
173 
16 Jul 
97 
3 
94 
105 
22 Jan 
177 
13 
162 
207 
8 Sep 
104 
3 
92 
102 
9 Feb 
212 
11 
193 
224 
22 Sep 
112 
6 
107 
127 
3 Feb 
235 
13 
218 
254 
1 Oct 
104 
5 
93 
111 
11 Feb 
234 
10 
220 
247 
9 Oct 
107 
6 
99 
122 
10 Feb 
243 
7 
238 
260 
14 Oct 
113 
5 
103 
120 
30 Jan 
259 
11 
240 
279 
22 Oct 
131 
6 
121 
141 
8 Feb 
259 
9 
249 
273 
27 Oct 
138 
13 
101 
145 
1 Feb 
270 
14 
254 
292 
11 Nov 
122 
4 
117 
132 
1 Mar 
258 
17 
231 
280 
19 Nov 
121 
8 
112 
137 
23 Feb 
271 
11 
247 
287 
