Alvarez and Alemany: Birthdate analysis and its application to the study of recruitment of Sardina pilchardus 
193 
An alternative area of origin for the 1992 recruits 
is the coastal waters off northern Portugal, where 
eggs are presumably spawned during the winter. 
Relatively large larvae were found in March-April 
in southern and western Galicia. These larvae may 
not have been locally spawned, because spawning is 
low in this area (Garcia et al., 1992; Lopez-Jamar et 
al., in press), but rather spawned during the winter 
off the coasts of north Portugal where high winter 
spawning has been observed (Re et al., 1990). Sup- 
porting evidence for such an area of spawning was 
deduced by Lopez-Jamar et al. (in press) from the 
results of a 1992 sampling, where the progressive 
northwards and westwards displacement (around 
Galicia) of a group of larger larvae (mean length >13 
mm) was documented. Using a larval growth-rate 
estimate of 0.59 mm/day in March off Galicia 
(Alemany and Alvarez 5 ), we found that the length 
range of this group of larvae is in accordance with a 
February birthdate. Moreover, such a spawning area 
is consistent with the poleward flow of winter circu- 
lation in the coastal ocean off southwest Europe 
(Frouin et al., 1990). 
The significant differences between the mean 
birthdates of recruits sampled at Vigo and those 
sampled at La Coruna are also consistent with the 
hypothesis of a larval drift from the south. The lar- 
vae that were produced at the beginning of the pe- 
riod, when the displacement northwards took place, 
would reach areas farthest from the spawning zone. 
Thus, the mean birthdate of specimens at La Coruna 
would be earlier than that of specimens at Vigo, as 
was observed. It is suggested from the evidence of 
the overall distribution pattern of sardine larvae in 
the Cantabrian Sea that, during the 1992 spawning 
season, most larvae drifted westwards and were dis- 
persed offshore in northern Galicia (Lopez-Jamar et 
al., in press). This pattern could be explained by the 
hydrographically dynamic area observed off north- 
western Galicia in spring (Lopez-Jamar et al., in 
press; Chesney and Alonso-Noval 11 ). On the other 
hand, several studies have also suggested that re- 
cruitment of the Atlanto-Iberian sardine stock de- 
pends mainly on spring spawning in the Cantabrian 
Sea and on upwelling features along the west coast 
of the Iberian peninsula. These studies have been 
based on empirical relationships (Dickson et al., 1988; 
Cabanas et al. 8 ) and a qualitative approach (Robles 
et al., 1992; Roy et al. 9 ). Possible mechanisms asso- 
ciated with physical factors that could influence early 
11 Chesney, E. J., and M. Alonso-Noval. 1989. Coastal up- 
welling and the early life history of sardine Sardina pilchardus 
along the Galician coast of Spain. ICES Council Meeting 1988/ 
H:61, 54 p. (mimeo). 
life-stage larvae from spring spawning in the 
Cantabrian Sea are suggested in these works. How- 
ever, the present study is a process-oriented ap- 
proach, which accounts for intraseasonal effects of 
the biotic and physical environment on the survival 
of a fish cohort. 
There is a possibility that spring-spawned recruits 
could exist off Galicia, but owing to their later incor- 
poration into the juvenile fishery, they may not have 
been present before the sampling of recruits was fin- 
ished in November 1992, when the March-June lar- 
vae may not have yet recruited to the fishery. How- 
ever, on the basis of the age range given in Table 1, 
these spring-spawned recruits would be caught by 
the fishery from August and should be distinguish- 
able in birthdate distribution. Moreover, the routine 
sampling of sardine length-frequency distributions 
carried out for stock assessments at the same area 
from January 1993 onwards has not revealed the 
presence of smaller sizes, 12 which would be an indi- 
cation of recruitment from spring-spawned larvae in 
the Cantabrian Sea. 
In summary, our results reinforce the suggestion 
of an alternative origin for the Atlanto-Iberian sar- 
dine recruits of the Galician area, at least in some 
years. The particular hydrological conditions along 
the northern coast of Portugal would favor either 
spring- or winter-spawned recruits, as outlined by 
Lopez-Jamar et al., (in press). If upwelling in spring 
is weak, larvae spawned at this time in the 
Cantabrian Sea could be transported to the Galician 
area. On the other hand, if upwelling is intense, they 
may drift offshore at Cape Ortegal, as was postu- 
lated by Lopez-Jamar et al. (in press) for the 1992 
spawning in the Cantabrian Sea. In this latter case, 
the recruits in the Galician area would come from 
winter-spawned larvae in northern Portugal, which 
could reach the Galician area by northward-flowing 
surface currents during the winter. The influence of 
these mechanisms on year-class abundance remains 
to be investigated. 
Acknowledgments 
We express our gratitude to P. Cubero for the pa- 
tient work of sampling juveniles, and to the person- 
nel of the laboratories participating in this Project 
(Centro Oceanografico de La Coruna and Centro 
Oceanografico de Malaga (IEO), Plymouth Marine 
Laboratory, Alfred Wegener Institut fur Polar und 
Meeresforschung, and Institut fur Hydrobiologie und 
12 Porteiro, C. 1994. Institute Espanol de Oceanografia, P.O. 
Box 1552, 36280 Vigo, Spain. Personal commun. 
