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Fishery Bulletin 99(4) 
stage (from the newly fertilized eggs up to embryos 10 
cm long) that lasted from October to November the fol- 
lowing year. Fertilization occurred from the beginning of 
October to the beginning of February. Free-living embryos 
were observed from October to September of the follow- 
ing year, followed by parturation of fully mature embryos 
from September to December. The duration of the preg- 
nancy for spiny dogfish in the Oslofjord ranged from 18 to 
24 months. This period of time is calculated from the time 
of fertilization to the time of parturation. These estimates 
of the reproduction phases are summarized in Figure 5. 
fish embryos was followed throughout their second year of 
development. Growth was approximately 1 cm per month 
for both year classes in the size range from 8 to 24 cm, 
which corresponds approximately to the embryos in the 
second year of development. However, growth was not uni- 
form; it could be divided into two growth phases (Fig. 6). 
The first phase lasted from October to May with a slow 
growth of 0.6 cm/month. The second phase lasted from 
May until December and was characterized by a more 
rapid growth of 1.2 cm/month. 
Embryonic growth 
Discussion 
The embryos ranged from 4.4 to 24.9 cm total length. The 
growth of the combined 1986 and 1996 year classes of dog- 
Free-living embryos 
Candled embryos 
Fertilization 
Parturation 
i l t i — l i t i i i i i i — i l l t t t- i i i i — i t l i i 
SONDJ FMAMJJ ASONDJ FMAMJ JAS 
Month 
Figure 5 
Schematic time representation of the reproductive cycle of the 
female spiny dogfish by month (n=194). 
Growth 
The lack of an asymptote is probably due to an absence of 
older, larger fish. This is most likely due to over-harvesting 
of dogfish in the Oslofjord. According to local fishermen, 
heavy catches of large individuals (exceeding 110 centime- 
ters) were taken in the late 70s and early 80s. 2 
Age and length at maturity 
Age at 50% maturity was found to be 17.6 years in 
1987 and 17.0 years in 1997. For spiny dogfish captured 
between the East Coast of Scotland and Norway, Aasen 
(1961) estimated age at 50% maturity to be in the range 
12 to 14 years. In an area northwest of Scotland, Holden 
and Meadows (1962) reported age at 50% maturity of 9 
years. However, Aasen (1961) and Holden and Meadows 
(1962) did not take into account worn spines, which com- 
bined with the difficulty of identifying all of the annual 
rings, means that their estimates are most likely biased 
downwards. 
Length at 50% maturity in the Oslofjord should not de- 
viate greatly from sizes observed elsewhere in the north- 
east Atlantic. Length at 50% maturity in the Oslofjord was 
found to be 81 cm in both 1987 and 1997. This is in general 
agreement with other observations in the northeast 
Atlantic where length at 50% maturity varied from 
74 to 83 cm (Hickling, 1930; Holden and Meadows, 
1964; Fahy, 1989; Gauld 1 ). 
Fecundity 
There was no significant difference between the 
number of eggs observed in 1987 and 1997. How- 
ever, the number of eggs in the ovaries was higher 
than the number of embryos in the uterus (average: 
8.1 eggs and 6.6 free-living embryos in 1987 and 8.9 
eggs and 7.5 free-living embryos in 1997). This is 
probably due to the absorption of eggs in the ovaries 
following fertilization that was observed by Hanchet 
(1988), Holden and Meadows (1964), and Nammack 
et al. ( 1985). 
2 Johansen, P. A. 1997. Personal commun. Dogfish fish- 
erman from the Oslofjord. Gudeberg Alle 1, 1600 Fredrik- 
stad, Norway. 
