3.7. Spiny dogfish (equilibrium strategy)
The maximum recorded age of spiny dogfish is more than 100 years. Maturation starts at age 14 (male) or 23 (female) in Japanese waters (Muto, 2005). Saunders and McFarlane (1993) reported that spiny dogfish off the west coast of British Columbia had a median age of maturity of 22 years (male) and 35 years (female). This species is ovoviviparous, with litters of 1–20 young, and their gestation period is 18–24 months, which is the longest among the sharks and rays. Equilibrium strategists have a very low intrinsic rate of increase (Smith et al., 1998). As such, equilibrium strategists should exhibit relatively stable population dynamics over time. Conversely, these species are unable to withstand even modest harvest rates without experiencing population depletion and potential stock collapse. The Japanese spiny dogfish catch and CPUE continuously declined since the 1950s with a slight recent recovery (Fig. 11a), although the time series lacks data from the late 1950s and 1960s. In British Columbia, CPUE was rather stable (varying by a factor of 4 only) in contrast to the large interannual variability of catch (Fig. 11b). Owing to the low reproductive rate and high juvenile survival rate arising from ovoviviparity and large size at birth (18–30 cm), responses of equilibrium strategists to climatic regime shifts are expected to be minimal, although data were not available to evaluate this. Any excess catch will deplete stocks, which will require a long time to recover.