Reproduction of female spiny dogfish, Squalus acanthias, in the Oslofjord

The spiny dogfish (Squalus acanthias) longest gestation periods of any living is a relatively small shark with a charvertebrate (up to 24 months) (Ketchen, acteristic spine in front of each dorsal 1972; Nammack et al., 1985). fin. Its dorsal side is grayish and has This shark, like other sharks, is very sporadic white spots. Although it may susceptible to overfishing, not only be­ reach a length of 160 cm, most individucause of its long gestation period, but als in the North Sea are in the range also because of slow growth, late ma­ of 80–100 cm (Ford, 1921). It is disturity, and because it bears a small tributed worldwide, absent only from number of offspring (up to 15) (Nam­ tropical and polar regions (Compagno, mack et al., 1985; Fahy, 1989). Exten­ 1984). sive fishing since the early 1960s has The spiny dogfish has been harvestled to a marked decrease in the North ed for more than 100 years mostly for Sea stock. Fishing has also affected the its oil-rich liver (Ketchen, 1986). At population in the Oslofjord where the first, the oil was used for lamp fuel and annual catch declined from 704 tons as a lubricant in machines. The oil was in 1979 to less than 300 tons per year later (during W.W.II) used as a source during the 1990s (Official Statistics of of vitamin A. Today the dogfish is valNorway, 1996). We investigated dogfish ued as food in many countries (Gordon, reproduction in the Oslofjord by com­ 1986). paring the reproduction parameters of The reproduction cycle of the spiny dogfish caught in 1987 and 1997. The dogfish begins with mature females focus of our study was to evaluate the bearing several large (over 40-mm) yelreproductive parameters of spiny dog­ low eggs. As the eggs pass through fish in the Oslofjord and to look for the shell gland they are fertilized and possible changes in these parameters. become enclosed in a protective cap­ sule (candle). The candle passes down the reproductive tract and comes to Materials and methods rest in the uterus. The embryos live off the large yolk sac attached under Sampling the gill region (Gilbert, 1981). As the embryos grow, they slowly absorb the Dogfish were sampled monthly off the yolk sack. Embryos that have comHvaler Islands throughout 1987 and pletely absorbed the yolk sac may still 1997 in gill nets and by longline at remain in the uterus for some time bedepths ranging from 50 to 460 m (Fig. fore being born (Ford, 1921). According 1). The gill nets were composed of mono­ to Jones and Geen (1977), the embryfilament line (0.60 mm) with a mesh os also bear an internal yolk sac which size of 285 mm. This large mesh size nourishes them for up to 2 months afaccounts for catches consisting mainly ter birth. The dogfish reproductive cyof larger dogfish (over 70 cm). The long­ cle takes almost 2 years, one of the 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.