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Reproduction of female spiny dogfish, 
Squalus acanthias, in the Oslofjord 
Thomas S. Jones 
Hans Oeverlandsvei 10 
1363 Hoevik, Norway 
E-mail address: tomtrussel@hotmail com 
Karl I. Ugland 
Department of Biology 
University of Oslo, Pb 1064 
0316 Oslo, Norway 
The spiny dogfish (Squalus acanthias) 
is a relatively small shark with a char- 
acteristic spine in front of each dorsal 
fin. Its dorsal side is grayish and has 
sporadic white spots. Although it may 
reach a length of 160 cm, most individu- 
als in the North Sea are in the range 
of 80-100 cm (Ford, 1921). It is dis- 
tributed worldwide, absent only from 
tropical and polar regions (Compagno, 
1984). 
The spiny dogfish has been harvest- 
ed for more than 100 years mostly for 
its oil-rich liver (Ketchen, 1986). At 
first, the oil was used for lamp fuel and 
as a lubricant in machines. The oil was 
later (during W.W.II) used as a source 
of vitamin A. Today the dogfish is val- 
ued as food in many countries (Gordon, 
1986). 
The reproduction cycle of the spiny 
dogfish begins with mature females 
bearing several large (over 40-mm) yel- 
low eggs. As the eggs pass through 
the shell gland they are fertilized and 
become enclosed in a protective cap- 
sule (candle). The candle passes down 
the reproductive tract and comes to 
rest in the uterus. The embryos live 
off the large yolk sac attached under 
the gill region (Gilbert, 1981). As the 
embryos grow, they slowly absorb the 
yolk sack. Embryos that have com- 
pletely absorbed the yolk sac may still 
remain in the uterus for some time be- 
fore being born (Ford, 1921). According 
to Jones and Geen (1977), the embry- 
os also bear an internal yolk sac which 
nourishes them for up to 2 months af- 
ter birth. The dogfish reproductive cy- 
cle takes almost 2 years, one of the 
longest gestation periods of any living 
vertebrate (up to 24 months) (Ketchen, 
1972; Nammack et al., 1985). 
This shark, like other sharks, is very 
susceptible to overfishing, not only be- 
cause of its long gestation period, but 
also because of slow growth, late ma- 
turity, and because it bears a small 
number of offspring (up to 15) (Nam- 
mack et al., 1985; Fahy, 1989). Exten- 
sive fishing since the early 1960s has 
led to a marked decrease in the North 
Sea stock. Fishing has also affected the 
population in the Oslofjord where the 
annual catch declined from 704 tons 
in 1979 to less than 300 tons per year 
during the 1990s (Official Statistics of 
Norway, 1996). We investigated dogfish 
reproduction in the Oslofjord by com- 
paring the reproduction parameters of 
dogfish caught in 1987 and 1997. The 
focus of our study was to evaluate the 
reproductive parameters of spiny dog- 
fish in the Oslofjord and to look for 
possible changes in these parameters. 
Materials and methods 
Sampling 
Dogfish were sampled monthly off the 
Hvaler Islands throughout 1987 and 
1997 in gill nets and by longline at 
depths ranging from 50 to 460 m (Fig. 
1 ). The gill nets were composed of mono- 
filament line (0.60 mm) with a mesh 
size of 285 mm. This large mesh size 
accounts for catches consisting mainly 
of larger dogfish (over 70 cm). The long- 
line was composed of a 5-mm line con- 
nected to a 7/0 dogfish hook. The nets 
were usually checked every 24 hours 
while the longline was taken up after 
a few hours. The samples consisted of 
132 females in 1987 and 101 females 
in 1997. Total length and weight were 
measured to the nearest 0.5 cm and 5 
g, respectively, as described by Saun- 
ders and McFarlane (1993). 
The same fisherman, fishing grounds 
and fishing gear were used in both 
sampling years, so that sampling bias 
was avoided. The fishing gear account- 
ed for catches of dogfish that were 
mainly over 70 cm in length. There 
was a relatively small amount of un- 
marketable-size fish caught and the 
discard rates were approximately the 
same in both years. 
Age determination 
The first and second dorsal spines 
were removed from 217 dogfish. The 
remaining 16 dogfish had spines that 
were either missing, or broken to such 
an extent that age determination was 
not possible for these individuals. The 
spines were air-dried at least 1 week 
before being cooked for approximately 
3 minutes each in tap water. The flesh 
around the base of the spine was then 
removed with a scalpel and tweezers. 
The cleaned spines were subsequently 
dipped in ethyl alcohol and then dried 
with a soft cloth. Next, the spines 
were viewed under an Olympus SZH 
10 zoom stereo microscope and aged 
according to the method described by 
Ketchen (1975). For large individuals 
with worn spines, age was modified by 
Ketchen’s (1975) correction curve for 
age 
Y= 0.50 9 7X 2 55 , 
where X = the diameter of the spine 
base in millimeters; and 
Y = the additional age of the 
spine due to it being worn. 
Ketchen’s correction curve is based 
on fish caught in the Strait of Geor- 
gia, British Colombia. It is quite likely 
that there are differences in juvenile 
growth in the respective dogfish popu- 
lations. The result may therefore be bi- 
Manuscript accepted 15 March 2001. 
Fish. Bull. 99:685-690 (2001) 
