Life history

A characteristic feature of the family Scorpaenidae and the genus Sebastes is longevity. The maximum lifespan in Sebastes is an estimated 20-140 years (Leaman, 1991; Pearson et al., 1991), and many species have extremely low growth rates for up to 50 per cent of their adult lives (Archibald et al., 1983; Leaman and Beamish, 1984). Redfish in the North Atlantic are reported to reach 50 or more years in age and maximum sizes of 45 to 55 centimetres (Kelly and Wolf, 1959; Mayo et al., 1990; Nedreaas, 1990; Saborido-Rey, 1995). Female S. mentella grow slightly faster and reach maturity later than the males. Oceanic S. mentella are reported to reach maturity at a length of 32 (males) and 33 centimetres (females) at an age of 13 and 14 years, respectively, whereas S. mentella from the shelf slopes reach maturity at a length of 42 and 44 centimetres and an age of 16 and 17 years, respectively (Rikhter, 1996). The age determination of redfish remains difficult and international constraints are set up to improve the age reading validation (Anon., 1996). However, extremely low growth rates and the late maturing process in all exploited Sebastes species are main issues in harvesting policies concerning a long-term sustainable yield, comparable to recent constraints according to Patagonian toothfish (Dissostichus elongoides) in Antarctic waters (MacKenzie, 1997).

The reproductive mode in Sebastes has been characterised as ovoviviparity, which is the case for most species of the family Scorpaenidae (Wourms, 1981). After mating and copulation, the eggs are fertilised within the ovary, brooded for about one month and released as relatively undeveloped larvae, 3-9 millimetres long (Boehlert and Yamada, 1991). The contrast with matrotrophic viviparity, wherein the embryos receive additional nutrition during gestation, has been questioned (Boehlert and Yoklavich, 1984; MacFarlane and Bowers, 1995). Nutritional relationships and reproductive styles are important factors in the management of Sebastes stocks, and the reproductive energy costs in this genus may be higher than previously thought (Garrod and Horwood, 1984; Leaman, 1991; Norton and MacFarlane, 1994). The iteroparity in all Sebastes species is assumed to be an adaptive strategy when the probability of successful reproduction is low, or pre-reproductive survival is either lower or more variable that that of adults (Giesel, 1976).

The breeding areas of S. marinus and S. mentella in the North Atlantic reach from the eastern Labrador Sea to the Norwegian coast and the eastern Barents Sea, concentrating in the central Irminger Sea southwestward of Iceland in boreal mixed water of 3 - 8.5 °C (Tåning, 1949; Templeman, 1959; Magnússon et al., 1988; Mukhina et al., 1992; Magnússon and Magnússon, 1995). The size of newly extruded larvae of the oceanic S. mentella ranges from 7.8 to 8.9 millimetres, which is larger than the larvae of the deep-sea S. mentella and S. marinus (Magnússon and Magnússon, 1977). After the extrusion of 15,000 to 63,000 larvae in April and May at depths of 300 to 400 metres and water temperatures of 5.5 to 6.5 °C (Pavlov et al., 1989a), the larvae of the oceanic S. mentella stock drift around Cape Farewell with the Irminger current and then northwestward with the West Greenland current. Larvae remain pelagic for four to five months (Kelly and Barker, 1961), after which they move into deeper waters (oceanic S. mentella) or to the sea floor of the continental slopes (all other Sebastes species). From the nursery grounds off Baffin Island and Labrador, the juveniles move southwards and return to the Irminger Sea as they mature. Once there, they stay in this region and constitute the spawning stock (Magnússon and Magnússon, 1995). The growth rates of juvenile Sebastes species were found to be highly dependent upon temperature (Boehlert, 1981; Boehlert and Yoklavich, 1983), being the slowest in Sebastes species from the North Atlantic (0.109 mm per day; Penney and Evans, 1985) and fastest in S. paucispinis (0.666 mm per day; Moser and Boehlert, 1991).

Since the distribution and growth of larvae, juveniles and adults of redfish is strongly influenced by water temperatures and currents (Dietrich et al., 1961), environmental conditions and life history patterns of these fish have important implications for stock management and commercial fisheries.

Troyanovsky (1992) reported the occurrence of non-maturing deep-water S. mentella in the Northwest Atlantic, first observed in the early 1960s. These fish were probably drifted with the cold East Greenland current from the Irminger Sea into the Labrador region as larvae. Having experienced unfavourable conditions for reproduction, they stay immature, show no sexual dimorphism and don't perform the characteristic migration from shallower waters to greater depths. Similar observations were made for non-maturing roundnose grenadier (Coryphaenoides rupestris). The commercial value of the non-maturing S. mentella 'pseudo-population' should be taken into account, considering that its exploitation does not affect the reproductive capacity of the population.

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