Age estimation, growth, maturity, and distribution of the bluemouth rockfish Helicolenus d. dactylopterus (Delaroche 1809) from the Rockall Trough

C. J. Kelly 0 P. L. Connolly 0 J. J. Bracken 0 0 C. J. Kelly, and P. L. Connolly: Marine Institute, Fisheries Research Centre , Abbotstown, Dublin 15, Ireland. J. J. Bracken: Department of Zoology, University This paper presents data on the age, growth, maturity, and distribution of Helicolenus d. dactylopterus (Delaroche 1809) from the Rockall Trough area. The results are based on a series of deep-water trawl surveys carried out on the continental slope west of Ireland and Scotland between 1992 and 1993. Ages ranged from 1 to 43 years for males and 1 to 37 years for females with most fish aged between 3-14 years. The species was found to be sexually dimorphic with males attaining a greater length and weight (L=37.2 cm; W=747 g) than females (L=31.0 cm; W=700 g). Age and growth estimates were determined for both sexes and the Brody growth coeYcient for both sexes indicate that this species is relatively slow growing (male K=0.06; female K=0.09). Ripe female fish were only found in April and May in a specific area. Using total lengths 50% of male fish were mature at 26 cm (ages 15-16) while 50% of female fish were mature at 23 cm (age 13). Catch data were sporadic but the largest catches were obtained on the southern slope of the Rockall Trough in April at a depth of 650-950 m. The fish from this area had a much greater range of sizes than fish from the eastern slopes of the Rockall Trough. - Introduction Helicolenus dactylopterus (Delaroche 1809) is a benthopelagic deep-water fish, found at depths of 2001000 m in both the North and South Atlantic (Barsukov, 1979; Whitehead et al., 1986). Throughout this range there are five subspecies which may be diVerentiated by morphometric characteristics (Eschmeyer, 1969; Barsukov, 1979). The subspecies Helicolenus d. dactylopterus is found in the North-eastern Atlantic and in the Mediterranean. Commonly called bluemouth rockfish or blackbelly rosefish in English, in French it is known as rascasse du fond, or simply rascasse and is sold in France as sebaste chevre (H. Dupouy, pers. comm.). In the early part of the century, bluemouth rockfish was sold on the London market as red bream (Holt and Byrne, 1906). These authors further reported that it was commonly found in the North Sea especially in the 200 m hole oV Troup Head in Aberdeenshire. The depth distribution for this species was recorded as ranging from 3 m (in the Humber estuary) to 1134 m (oV the Canary islands). Wheeler (1969) and Whitehead et al. (1986) describe its contemporary distribution in the North-eastern Atlantic to be from Norway to Morocco, the Azores, Madeira, the Canaries, and the Mediterranean in depths of 2001000 m. Gordon and Duncan (1985) give the depth range in the Rockall Trough as 5001000 m, and Merrett et al. (1991) give its depth distribution in the Porcupine Seabight at 230848 m. Recent records from North Sea ground fish surveys have reconfirmed the presence of H. d. dactylopterus in the North Sea since 1991. The stock is dominated by a single year class (1990) that has been spreading south Discarded in Long-line fishery for ling tusk Not caught Discarded in Long-line fishery for ling tusk By-catch or discarded in Long-line fishery for ling tusk By-catch in trawl fishery for roundnose grenadier and discarded in Long-line fishery for ling tusk By-catch in trawl fishery for roundnose grenadier and discarded in Long-line fishery for ling tusk By-catch in irregular fishery on other deepwater species By-catch in the crustacean and artisinal fisheries Caught in the directed Long-line fishery Discarded in Long-line fishery for ling tusk and east of Shetland since 1991 (Heessen, 1994). Its present distribution is given as the entire North Sea and parts of the Wadden Sea to depths of less than 100 m (Heessen, 1994). Since 1993 juvenile bluemouth rockfish have also been taken by the Irish groundfish surveys in depths as shallow as 60 m (Fisheries Research Centre, FRC, unpubl. data). McPherson (1985) studied this species oV the northwest coast of Africa finding that bluemouth rockfish feed mainly during the day remaining otherwise inactive. Merrett and Marshall (1981) described the diet of this species in the same area as highly selective, comprising over 90% ophiuroids, but including small quantities of crustaceans and fish. This does not seem to be the case for bluemouth rockfish in the Rockall Trough where Mauchline and Gordon (1985) describe the diet as relatively diverse containing small crustaceans and polychaetes. Whitehead et al. (1986) and McPherson (1985) found that cephalopods, echinoderms, and pelagic and demersal fish species are eaten by large individuals. Bluemouth rockfish is taken as a by-catch in ICES Subareas VI and VII by the trawl fishery for roundnose grenadier (Coryphaenoides rupestris Gunnerus 1765). It is also taken by the artesinal fisheries in ICES Division IX, and by a directed longline fishery in ICES Division X (Anon, 1996a). In ICES Subareas with longline fisheries directed at ling (Molva molva L.) and tusk (Brosme brosme Ascanius 1772) (ICES Subareas II, IV, V, VI, VII & XIV) bluemouth rockfish is generally discarded (Anon, 1996a, Table 1). Fertilization and development of the larvae have been described by Sanches and Acha (1988). These authors state that fertilized eggs are retained in a gelatinous material within the ovary, that they may be extruded onto the seabed at various stages of development, and that as the gelatinous material dissolves the larvae are released into the water column and take up a pelagic existence. Spawning seems to occur later in the year with increasing longitude (Pierano and Tunesi, 1986; Whitehead et al., 1986). Very little has been published on age estimation and growth of bluemouth rockfish from the North-east Atlantic. However, several institutes throughout Europe archive unpublished age readings (Connolly et al., 1995). Data from more southern waters give maximum age estimates and values for the Brody growth coeYcient (K), that vary considerably (Pierano and Tunesi, 1986; Isidro, 1987; Ragonesse, 1989; Monteiro et al., 1991; Anon, 1996b). This paper examines the catch rates and distribution of H. d. dactylopterus taken with a commercial bottomtrawl net on the continental slope to the west of Ireland and Scotland. The first age and growth estimates for this species from Rockall Trough are presented. Predicted maximum ages and size ranges are compared to those recorded from the Mediterranean and Azores areas. The first estimates for length and age at first maturity are calculated for male and female fish from the Rockall Trough. Seven research exploratory and commercial cruises to the slopes of the Rockall Trough and Porcupine Bank between May 1992 and September 1993 provided the data used in this study. The bulk of the samples were collected during the 1993 deepwater demersal sampling programme conducted by the FRC using funding provided by the EU STRIDE programme for Ireland. The samp (...truncated)