126 
Abstract .—This study compared 
the hatching season and the actual 
spawning season of spring- and au- 
tumn-spawning herring in the north- 
ern Gulf of St. Lawrence as determined 
by otolith characteristics and maturity 
stages, respectively, to measure the 
crossover between the two spawning 
populations. The growth characteristics 
of two cohorts that showed significant 
crossover were contrasted with those of 
a cohort that did not. It was concluded 
that variable juvenile growth does in- 
fluence the adoption of the season of 
first spawning in these populations, 
and therefore the progeny of a given 
seasonal-spawning population may re- 
cruit to a local population that has a 
different reproductive season. It was 
also shown that the twinning of year- 
class strength can be explained by the 
crossover of a large number of individu- 
als from one seasonal spawning popu- 
lation to another. The data presented 
indicate that the spawning season that 
is established at the time of first matu- 
ration is maintained for the remainder 
of adult life. The present study there- 
fore does not support the concept of dis- 
crete sympatric seasonal-spawning 
populations in Atlantic herring. 
Manuscript accepted 7 August 1996. 
Fishery Bulletin 95:126-136 ( 1997). 
Year-class twinning in sympatric 
seasonal spawning populations of 
Atlantic herring, Clupea harengus 
Ian H. McQuinn 
Division Poissons et Mammiferes marins, Ministere des Peches et des Oceans 
Institut Maurice Lamontagne, C.P. 1000, 850 Route de la Mer 
Mont-Joli (Quebec), Canada G5H 3Z4 
E-mail address: l_McQuinn@qc.dfo.ca 
The current theory on Atlantic her- 
ring population structure, as it re- 
lates to sympatric spring- and au- 
tumn-spawning herring, considers 
them to be discrete populations with 
independent life histories (lies and 
Sinclair, 1982). This concept has 
largely been based on evidence of 
significant homing precision to 
spawning grounds as revealed by 
tag returns (Hourston, 1982; Wheeler 
and Winters 1984, a and b; Steven- 
son et al. 1 ; Hart et al. 2 ; Stobo 3 ) and 
on studies that have noted signifi- 
cant differences in meristic and 
morphometric measurements, such 
as fin-ray counts and otolith char- 
acteristics (Messieh, 1972; Parsons, 
1973; Postuma, 1974), as well as in 
life history parameters, such as 
mean fecundities (Baxter, 1959; 
Messieh, 1976). 
However, several observations 
are difficult to explain within the 
discrete population concept: 1) typi- 
cal spring-type otoliths are often 
found in autumn-spawning herring 
and vice versa (Messieh, 1972; 
Aneer, 1985); 2) “twinning” of re- 
cruitment strength between spring- 
and autumn-spawning year classes 
(see below); and 3) the lack of ge- 
netic divergence between seasonal 
spawning populations as demon- 
strated from numerous electro- 
phoretic and mtDNA studies (Grant, 
1984; Kornfield and Bogdanowicz, 
1987; Safford and Booke, 1992). In 
light of the conflicting evidence for 
stock discreteness, an alternative 
concept has been proposed whereby 
seasonal spawning populations are 
seen as subunits of a larger popula- 
tion, within which there exists a 
“dynamic balance” characterized by 
extensive gene flow (Smith and 
Jamieson, 1986). 
Otolith characteristics and matu- 
rity stages have been used for many 
decades to determine the spawning 
affinity of individual herring from 
sympatric seasonal-spawning popu- 
lations. Maturity stages are the pre- 
ferred method for determining the 
actual spawning season of mature 
herring because the state of matu- 
ration can be used reliably to ascer- 
tain the spawning season through- 
out the year (McQuinn, 1989). On 
the other hand, otolith characteris- 
tics, being related mainly to envi- 
ronmental conditions at birth, are 
used to determine the hatching sea- 
1 Stevenson, J. C., A. S Hourston, K. J. Jack- 
son, and D. N. Outram. 1952. Results 
of the West Coast of Vancouver Island her- 
ring investigation, 1951-52. Report of 
the British Columbia Provincial Fisheries 
Department, Victoria, British Columbia, 
Canada, p. 57-87. 
2 Hart, J. L., A. L. Tester, and J. L. McHugh. 
1941. The tagging of herring ( Clupea 
pallasii) in British Columbia: insertions 
and recoveries during 1940-41. Report of 
the British Columbia Provincial Fisheries 
Department, Victoria, British Columbia, 
Canada, p. 47-74. 
3 Stobo, W. T. 1982. Tagging studies on 
Scotian shelf herring. Northwest Atlan- 
tic Fisheries Organization (NAFO) Re- 
search SCR Document 82/IX/108. Ser. No. 
N617, 16 p. P.O. Box 638, Dartmouth, 
Nova Scotia, Canada B2Y 3Y9. 
