Bering Sea/Aleutian Islands King and Tanner Crab stocks
Contributed by Robert Foy, Kodiak Laboratory, Resource Assessment and Conservation Engineering Division, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA
Last updated: October 2010
Description of the Indices: Eastern Bering Sea crab abundance indices are based on the annual National Marine Fisheries Service bottom trawl survey area swept estimates, Alaska Department of Fish and Game (ADF\&G) trawl surveys, ADF\&G pot surveys, and from commercial catch data. There are ten crab stocks in the current Fishery Management Plan for Bering Sea/Aleutian Islands King and Tanner Crabs: four red king crab Paralithodes camtschaticus (Bristol Bay, Pribilof Islands, Norton Sound, and Adak), two blue king crab Paralithodes platypus (Pribilof District and St Matthew Island), two golden king crab Lithodes aequispinus (Aleutian Islands and Pribilof Islands), and two Tanner crab stocks (southern Tanner crab Chionoecetes bairdi and snow crab C. opilio).
Overfishing and overfished status of crab stocks are based on a five tier system where mature male biomass is currently used as a measure of the productive capacity of the stock (B). Snow crab and Bristol Bay red king crab are managed as Tier 3 stocks with length based models where proxy limit reference points are estimated based on life history information. Snow crab was declared overfished in 1999 and is under a rebuilding plan. Tanner crab, Pribilof Islands red and blue king crab, and St. Matthew blue king crab are Tier 4 stocks where data on life history and a spawner-recruit relationship are lacking. The Pribilof Islands blue king crab was declared overfished in 2002 and remains at a low overfished biomass. St. Matthew blue king crab was declared overfished in 1999, was officially considered rebuilt in 2009, and supported a commercial fishery in 2009. The southern Tanner crab stock is now overfished based on the 2009 survey estimates and 2009/2010 catch data. The remaining stocks are Tier 5 stocks with no reliable estimates of biomass or natural mortality and are managed on average catch data.
Status and trends: Red king crab Bristol Bay red king crab estimates of total survey biomass of adult males increased to 150,193 metric tons (t) in 1977 decreased sharply to a low of 9,582 t in 1983, and then remained steady between 20,000 and 50,000 t through 2010 (Figure 56). Recent above-average year classes have recruited into the fished population and there is no evidence any strong year classes recruiting. As a result both immature and mature biomass has declined in the last 3 years.
Pribilof Islands red king crab were not prevalent in the Pribilof Islands until the early 1990s. The large male biomass peaked in the 1990s at 9,687 t and then declined to biomasses between 2,000 to 3,000 t between 1998 and 2010. Recruitment indices are not well understood for Pribilof red king crab largely due to the difficulty in catching the smaller crab in the nearshore habitat and due to their small numbers. There was a substantial decrease in abundance for all crab size groups in this stock in 2009 that leveled in 2010.
Norton Sound red king crab adult male abundance was highest during the 1970s at around 5,611 t, declined into the 1980s and 1990s to a low of 805 t, and since 1996 has gradually increased to 2,468 t in 2010. Juvenile male abundance has fluctuated over the time series but has increased gradually in recent years to over 400 t in 2009.
Adak red king crab estimates of biomass are not available for this stock. Fishery catches decreased from a 9,613 t peak in the 1960s to less than 3 t in the late 1990s. Since the end of the 2003/2004 fishing season the fishery has been closed due to poor recruitment indices from periodic pot surveys.
Blue king crab Pribilof Island blue king crab adult male biomass peaked in the late 1970s between 15,798 and 38,756 t before a precipitous decline to less than 752 t in 1989. Biomass estimates have remained low in this designated overfished stock with average estimates less than 50 t. Juvenile male blue king crab biomass in the Pribilof Islands fluctuated between 0 and 232 t in the last decade. Survey results from 2009 and 2010 showed a slight increase in adult males with no apparent incoming year classes.
St. Matthew blue king crab adult male biomass fluctuated between low and high biomass over three periods: 1978 to 1985, 1986 to 1999, and 2000 to current. Historical peaks in adult male biomass were 13,947 t in 1982 and 9,137 t in 1997. Currently the stock has increased from a low of 1,126 t in 2003 to the current 2010 biomass estimate of 8,141 t.
Tanner crab Tanner crab adult male biomass in the survey peaked in the 1970s around 149,948 t and early 1990s at 99,991 t (Figure 56). From 1990s through 2007, adult male biomass increased to 50,149 t. However, in 2008 and 2009 there was a substantial decrease to 27,949 t in 2010. Juvenile male crab have fluctuated similarly with peaks of 85,825, 58,938, and 62,537 t in 1980, 1989, and 2007. Recent juvenile crab abundance estimates have declines to less than 150 million crab in 2009.
Snow crab Snow crab adult male survey biomass peaked in the mid to late 1970s and again at 356,511 t of adult male crab in 1990 and 144,022 t in 1996 (Figure 56). After a decline to 53,757 t in 2000 the adult male portion of the snow crab stock has gradually increased to the current adult male biomass of 107,131 t. Snow crab recruitment has varied substantially with peaks of 263,362 t and 359,485 t of juvenile males in 1987 and 1995 respectively. Recent juvenile male snow crab survey biomass estimates were between 97,000 t and 146,000 t.
Golden king crab Stock abundance estimates are not available for golden king crab stocks in the eastern Bering Sea/Aleutian Islands. Fluctuations in Aleutian Islands golden king crab and Pribilof Islands golden king crab fishery catch per unit effort have led to speculation about changes in recruitment. In the Pribilof Islands, commercial catches ranged from 16 to 155 t in the late 1990s and early 2000s and have dropped to 0 in recent years. In the Aleutian Islands, catches of golden king crab remain steady with average catches of 4,536 t in the 1980s and 2,722 t in the 1990s and 2000s.
Factors Influencing Trends: Fluctuations in crab stocks have coincided with variable fishing pressure and changes in environmental conditions affecting benthic organisms in the eastern Bering Sea although no one cause has been identified to explain the wild fluctuations in some stocks and the precipitous decline from the 1970s and 1980s in other stocks.