Dover sole are also known as “slime” or “slippery” sole because they excrete mucous that makes them difficult to hold. Dover sole is sold fresh and frozen whole, headed and gutted as well as dressed, and in fillets, however, it is nearly always filleted due to its slimy skin. Dover sole from the Pacific has a mild taste and delicate texture, although it is not as mild as European Dover sole. Since flatfish quality can vary immensely, buyers recommend looking for Dover sole that has uniform color and lacks bruises. They also recommend against purchasing these fish whole since soft-fleshed fish may not be detected until after they’ve been filleted. Availability of fresh Dover sole varies throughout the year while frozen or thawed Dover sole primarily from Alaska is available year-round.
Key sustainability notes for Dover sole from landings averaged from 2011-2013 based on 2016 Seafood Watch ratings:
~80% of Pacific landings get a "Best Choice (green)" rating (U.S. West Coast)
~20% of Pacific landings get a "Good Alternative (yellow)" rating (British Columbia)
Within the "Best Choice" ratings, ~60% of Dover sole is landed in Oregon, ~30% in California, and ~10% in Washington. In recent years, ~35% of the total allowable catch (TAC) for Dover sole was harvested in the U.S. West Coast and ~60% in British Columbia.
Dover sole are flatfish with both eyes placed on one side of their head allowing them to easily hide and camouflage themselves under the sand. They also excrete mucus onto their scales, which makes them extremely slippery and difficult to pick up, giving them the name “slippery sole.”
Males live up to 58 years, and females up to 53 years. However, female Dover sole grow faster and larger than males, growing up to 30 inches (76 cm) in length. Males grow to about 27 inches (69 cm) in length.
Spawning season varies by location for Dover sole, but is always done near the ocean floor in water deeper than 1440 feet (439 meters). In the Gulf of Alaska it is from January to August, off the coast of Oregon it is from November to April, and in Puget Sound it is from January to March. Females produce fairly large eggs (0.1 inches), which are fertilized externally in the upper part of the water column. The hatched larvae settle in deeper water farther inshore in suitable nursery areas. Some populations can remain as larvae for as long as two years due to unfavorable environmental conditions. Scientists call these populations “holdover” larvae. All flatfish are born upright with their eyes on either side of their head. As they mature, their body tilts and they begin to swim flat with both eyes on one side.
Dover sole feed in daylight using sight and smell. Their small mouths are adapted to eating the small crustaceans that live in the sediment. Larvae feed on small or larval crustaceans, eggs, and plankton. Juveniles and adults feed on worms, bivalves, brittle stars, and small crustaceans. Larval Dover sole is preyed upon by seabirds and surface-dwelling fish. Sharks, bottom-feeding marine mammals, cod, and flounder prey upon adult Dover sole.
Dover sole are found along the Pacific Coast of North America from the Navarin Canyon in the Bering Sea and western Aleutian Islands to San Cristobal Bay, Baja California, Mexico. Dover sole live near the ocean floor – with adults and juveniles preferring soft mud or sand bottoms. They inhabit waters from 7 feet (2 meters) to 4,500 feet (1371 meters) deep, with the highest abundance generally occurring between 656 feet (200 meters) to 984 feet (300 meters) deep. Juveniles are found in deep near shore waters and travel to deeper waters as they age. In the summer and fall, adults and juveniles can be found in shallow water feeding grounds. In late fall and early winter, Dover sole migrate offshore into deeper waters to spawn. While Dover sole make an inshore-offshore seasonal migration, little north-south coastal migration occurs.
Science & Management
A 2015 paper from the Fisheries Oceanography journal shows that Dover sole is largely unaffected by the depletion of oxygen in water due to algal blooms and ‘dead zones.’ Their large gill size and ability to reduce their oxygen consumption as needed allow them to comfortably live in such conditions.
Coastal states and treaty tribes conduct portside monitoring programs, providing biological data used in stock assessment science, and aid in management decisions. A vessel monitoring system is also in place to collect data on fishing activity.
Dover sole can adjust to a thermal habitat ranging from 5°C to 27°C (41°F to 80°F); however, their thermal optima for growth lies within 20°C to 25°C (68°F to 77°F). More information is needed to further research for Dover sole aquaculture farming such as determining the optimal growth temperature for juvenile populations as well as their nutritional requirements.
NOAA Fisheries and Pacific Fishery Management Council (PFMC) manage the Dover sole fishery in California, Oregon, and Washington under the Pacific Coast Groundfish Fishery Management Plan (FMP). Implemented in 1982, the Pacific Coast Groundfish FMP covers over 90 species including rockfish, skates, and other flatfish caught off the US West Coast. The US commercial groundfish fishery is comprised of three components: Limited Entry (LE), Open Access (OA), and Nearshore (NS). The LE and OA sectors are managed by the PFMC while the NS sector is jointly managed by the PFMC and the states of Oregon and California respectfully.
The majority of Dover sole landings occur in California, Oregon, and Washington. Based on 2014 catch data, the US West Coast fishery is not overfished nor subject to overfishing. The fishery is considered well-managed due to measures such as:
A limited entry program – limiting the number of commercial fishing permits available
Minimum size and total catch limits
Seasonal and closed areas to protect sensitive habitats
A vessel monitoring system to ensure vessels are complying with closed areas
Beginning in 2011, LE trawl permit holders were allowed to participate in a catch share program. Participants in the program receive an Individual Fishing Quota (IFQ) of the total catch of the 29 commercial species/species complexes along the US West Coast. Fishers participating in the program can fish their quota at anytime during the season and can use non-trawl gear to catch their quota shares. Whereas non-IFQ fisheries have varying levels of at-sea observer coverage, the catch share program requires 100 percent at-sea and dockside monitoring. A subset of the IFQ, the California Groundfish Collective, comprises 11 fishing operations that have entered into an agreement to pool member’s IFQs.
NOAA Fisheries and the North Pacific Fishery Management Council manage the Dover sole fishery in Alaska under the Fishery Management Plan (FMP) for Groundfish in the Gulf of Alaska and the Fishery Management Plan for Groundfish of the Bering Sea and Aleutian Islands.
Dover sole are considered abundant in the Gulf of Alaska and are managed as part of the “Deep Water Flatfish Complex.” Under the FMP for Groundfish in the Gulf of Alaska fishers must adhere to:
Total allowable catch limits (TAC). TACs are allocated in the Gulf of Alaska by gear type and by processing sector
A limited entry program – limiting the number of commercial fishing permits available
Closed areas to protect sensitive habitats
Strict reporting and record keeping requirements
The Deep Water Flatfish Complex is managed well and is not overfished nor subject to overfishing according to a 2015 stock assessment.
Dover sole are managed under the Bering Sea/Aleutian Islands Groundfish Fishery Plan (FMP) in the Bering Sea and Aleutian Islands as part of the “Other Flatfish” complex. Under the FMP fishers must adhere to measures applying to all groundfish such as: TACs, a limited entry program, and gear modifications. A percentage of the TAC is allocated to a community development quota program to benefit fishery dependent communities in western Alaska. Fishery managers also set a limit on the amount of halibut and crab that may be caught incidentally in the groundfish fishery. Once those limits are exceeded, the groundfish fishery will close for the rest of the season.
The Other Flatfish Complex in the Bering Sea and Aleutian Islands was last assessed in 2015; however, the data was insufficient in determining whether the complex is overfished. The complex is not subject to overfishing according to NOAA.
Dover sole are managed by the Department of Fisheries and Oceans Canada (DFO) under the Integrated Fishery Management Plan (IFMP) for Groundfish in Canada. There are two stocks of English sole found in British Columbia waters – one in the Hecate Strait, Queen Charlotte Sound, and the west coast of Haida Gwaii and the other off the west coast of Vancouver Island. Among management measures the DFO establishes under the IFMP for Groundfish include:
A TAC for the fishery (100 percent of which is allocated to the trawl fishery)
An Individual Vessel Quota (IVQ) system in which IVQs can be reallocated between vessels and fisheries as necessary
100 percent at-sea and dockside monitoring
Habitat conservation measures such as spatial closures to avoid bycatch of sensitive corals and sponges
Pacific Dover sole is not actually sole but a flatfish more closely related to flounder. Larger in size than European Dover sole, Dover sole found in the Pacific is long-lived and has medium fecundity. Dover sole, found in the waters off California, Oregon, Washington, British Columbia, and Alaska, have variable abundance that is considered healthy overall. Dover sole levels in the Gulf of Alaska have been increasing in the past 15 years and the biomass there is now more than double the target population level.
Habitat impacts (Wild)
Dover sole are primarily caught using trawls. Other gear used includes hand lines and traps. While trawls can negatively impact rocky seafloors and reefs, the trawls in this fishery target the flatfish in the soft muddy areas where they live. These kinds of areas in the Gulf of Alaska and off the West Coast tend to be more resilient to trawling.
There is little bycatch in the Pacific Dover sole fishery, helped by a relatively new catch-share management plan on the West Coast and gear improvements that help trawler avoid bycatch hotspots, according to the Environmental Defense Fund.
Pacific Dover sole are managed by NOAA Fisheries, the Pacific Fishery Management Council on the U.S. West Coast, the North Pacific Fishery Management Council in Alaska, and Fisheries and Oceans Canada in British Columbia. On the West Coast they are managed with other groundfish species, and in Alaska under two flatfish complexes. In Alaska, management measures include a conservative annual catch limit, and limitations on the number, size and operation of vessels targeting the fish. In the Washington, Oregon, and California groundfish fisheries, only about 400 federal limited entry permits are available. Other measures there include gear restrictions and a trawl rationalization catch share program implemented in 2011. Seafood Watch reports from 2014 characterize Dover sole management as strong.