2015 Assessment of the Deepwater Flatfish Stock Complex in the Gulf of Alaska
February 21, 2015
The "flatfish" species complex previous to 1990 was managed as a unit in the Gulf of Alaska (GOA). It included the major flatfish species inhabiting the region, with the exception of Pacific halibut. The North Pacific Fishery Management Council divided the flatfish assemblage into four categories for management in 1990; "shallow flatfish" and "deep flatfish", flathead sole and arrowtooth flounder. This classification was made because of significant differences in halibut bycatch rates in directed fisheries targeting the shallow-water and deepwater flatfish species. Arrowtooth flounder, because of high abundance and low commercial value, was separated from the group and managed under a separate acceptable biological catch (ABC). Flathead sole were likewise assigned a separate ABC since their distribution over depths overlaps with that of the shallow-water and deepwater groups. In 1993, rex sole was split out of the deepwater management category because of concerns regarding the bycatch of Pacific ocean perch in the
rex sole target fishery.
The deepwater complex, the subject of this chapter, is composed of three species: Dover sole (Microstomus pacificus), Greenland turbot (Reinhardtius hippoglossoides) and deepsea sole (Embassichthys bathybius). Dover sole dominates the biomass of the deepwater complex in research trawl surveys and fishery catch (typically over 98%). Little biological information exists for Greenland turbot or deepsea sole in the GOA. More information exists for Dover sole, which allowed the construction of an age-structured assessment model in 2003 (Turnock et al., 2003).
Greenland turbot have a circumpolar distribution and occur in both the Atlantic and Pacific Oceans. In the eastern Pacific, Greenland turbot are found from the Chukchi Sea through the Eastern Bering Sea and Aleutian Islands, in the GOA and south to northern Baja California. Greenland turbot are typically distributed from 200-1600 m in water temperatures from 1-4 °C, but have been taken at depths up to 2200 m.
Dover sole occur from Northern Baja California to the Bering Sea and the western Aleutian Islands; they exhibit a widespread distribution throughout the GOA (Hart, 1973; Miller & Lea, 1972). Adults are demersal and are mostly found at depths from 300 m to 1500 m.
Dover sole are batch spawners; spawning in the GOA has been observed from January through August, peaking in May (Hirschberger & Smith, 1983). The average 1 kg female may spawn 83,000 advanced yolked oocytes in about 9 batches (Hunter, Macewicz, Lo, & Kimbrell, 1992). Although the duration of the incubation period is unknown, eggs have been collected in plankton nets east of Kodiak Island in the summer (Kendall & Dunn, 1985). Larvae are large and have an extended pelagic phase that averages about 21 months (Markle, Harris, & Toole, 1992). They have been collected in bongo nets only in summer over mid-shelf and slope areas in the GOA. The age or size at metamorphosis is unknown, but pelagic postlarvae as large as 48 mm have been reported and juveniles may still be pelagic at 10 cm (Hart, 1973). Juveniles less than 25 cm are rarely caught with the adult population in bottom trawl surveys (Martin & Clausen, 1995).