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Kodiak Laboratory: Shellfish Assessment Program

Eastern Bering Sea Annual Bottom Trawl Survey: Crab Resources

The 2007 eastern Bering Sea trawl survey took place from 4 June to 2 August conducted jointly by the Shellfish and Groundfish Assessment Programs aboard the chartered fishing vessels Aldebaran and Arcturus. Three hundred seventy-six standard stations were sampled from Bristol Bay to west of St. Matthew Island on the Bering Sea shelf (Fig. 14). In addition, four extra stations were sampled after encountering a Tanner crab (Chionoecetes bairdi) “hot spot”, defined as 100 or more legal-sized males.

When a hot spot is encountered, four additional tows are made 5 nmi miles in each direction (north, south, east, and west). Over 50,000 commercially important crab species including red king crab (Paralithodes camtschaticus), blue king crab (P. platypus), snow crab (C. opilio), Tanner crab, and hair crab (Erimacrus isenbeckii) were measured, and other biometric information, including shell and egg condition, weight, chela height and presence of parasitism were recorded.

Due to colder than normal bottom water temperatures at the beginning of the survey for the second year in a row, only 51% of the female red king crab caught in Bristol Bay had extruded new clutches of eggs. Spawning stock assessment models rely upon future recruitment and mating success so an accurate assessment of female reproductive condition is necessary.

Figure 14 map, see caption
Figure 14.  Click image to enlarge.

Figure 15 map, see caption
Figure 15.  Click image to enlarge.

Figure 16 map, see caption
Figure 16.  Click image to enlarge.

Figure 17 map, see caption
Figure 17.  Click image to enlarge.


After most of the standard stations were completed at the end of July, the Aldebaran returned to Bristol Bay and resampled 32 stations (Fig. 14); over 98% of the females had completed the mating and molting cycle and extruded new clutches by this time.

Station locations and numbers of legal (i.e., 6.5-in width and greater) male red king crab, Tanner crab and snow crab caught per square nautical mile are shown in Figures 15, 16, and 17 respectively. Due to low stock abundance, the fishery for blue king crab in the Pribilof area has been closed since 1999 and in St. Matthew since 1998. The red king crab fishery in the Pribilofs has also remained closed due to blue king crab bycatch concerns. Historically there has been a fishery for hair crab, although there has not been one since 2000.

The following are the 2007 abundance estimates for the five species of crab with the percentage of change from 2006. These estimates are derived for the annual Report to Industry.

Red king crab, Bristol Bay

Legal males: 13.3 million crabs; 6% increase.
Pre-recruits: 10.2 million crabs; 37% increase.
Large females: 35.4 million crabs; 19% increase.

Status: The abundance of legal males increased slightly in 2007. The 2007 index of pre-recruit males showed a notable increase, while that for small males decreased by 23%. The abundance of mature females increased relative to 2006, although that for pre-recruit females declined by 72%. While the stock is not considered to be overfished, it remains well below the peak population levels of abundance of the 1970s.

Red king crab, Pribilof District

Legal males: 1.6 million crabs; 25% increase.
Pre-recruits: 0.2 million crabs; 8% decrease.
Large females: 1.7 million crabs; 85% increase.

Status: Crabs are highly concentrated, and indices of abundance of all categories are characterized by very poor precision. Male abundance estimates in this district are highly influenced by the results of a limited number of tows with positive crab catches. The overall male plus female population abundance in 2007 increased by 50%; total males increased by 28% and total females increased by 86%.

Blue king crab, Pribilof District

Legal males: 0.1 million crabs; 46% increase.
Pre-recruits: 0.1 million crabs; 160% increase.
Large females: 0.2 million crabs; 49% decrease.

Status: The population is extremely low overall and trends in abundance are not easily detectable. Indices of male and female abundance are characterized by very poor precision. All male size categories increased in abundance relative to 2006, although the abundance in all female size categories declined. The overall male plus female population abundance in 2007 declined by 6%. Irrespective of the percent change in abundance relative to 2006, the 2007 assessment reveals indices among the lowest on record.

Blue king crab, Northern District (St. Matthew)

Legal males: 1.4 million crabs; 1% decrease.
Pre-recruits: 2.3 million crabs; 212% increase.
Large females: 0.2 million crabs; 27% decrease

Status: Indices of abundance in this district are affected by the portion of the stock occupying inshore rocky untrawlable grounds. They are also characterized by low precision. The overall male plus female population abundance in 2007 increased by 113%. The 2007 assessment showed encouraging signs of a wider distribution of crabs around St. Matthew Island than encountered in recent past. Assessment of this stock is clouded by large uncertainty in estimated female abundance. The current assessment for small and pre-recruit males and small females are among the highest population estimates on record which may indicate future recruitment to the stock.

Tanner crab, Eastern District

Legal males: 12.1 million crabs; 17% decrease.
Pre-recruits: 92.5 million crabs; 26% increase.
Large females: 40.8 million crabs; 6% decrease.

Status: Since 2004, this stock demonstrated encouraging signs of recovery and increasing abundance in both 2005 and 2006. In 2007, with the exception of the pre-recruit male category, all sex-specific size categories decreased relative to 2006. The overall male plus female population abundance in 2007 decreased by 12%. The legal male abundance index is characterized by low precision, and legal-sized males continue to represent only a small portion of mature male stock abundance. The current estimates of small and pre-recruit male abundance are among the highest population estimates on record, which suggest future recruitment to the stock.

Snow crab, all districts combined

Large males: 150.9 million crabs; 5% increase.
Pre-recruits: 344.3 million crabs; 19% increase.
Large females: 1244.4 million crabs; 19% increase.

Status: The abundance indices of all sex-specific size categories increased slightly in 2007 relative to 2006 with the exception of the small female category. The overall male plus female population abundance increased by 2%. The female reproductive stock is evidenced by high frequencies of old and very old shell crab which is of concern in terms of expected reproductive output. The mode of apparent male recruitment in 2006 was not replaced by new recruitment in 2007. With the exception of the pre-recruit and large male size categories which are average, the current assessment of small male abundance, and small and large female abundances are among the lowest estimates on record. There is apparent continued recruitment failure in the small male and female size categories; the recruitment trend since 1994 is dramatically low and future outlook for the stock is uncertain.

Hair crab, all districts combined.

Legal males: 2.0 million crabs; 91% increase.
Total females: 1.3 million crabs; 65% decrease.

Status: Since the early 1990s, this population has shown persistently declining trends in abundance. In 2007, the abundance indices of all male size categories increased relative to 2006, while female abundance declined substantially. Recruitment trends in this stock are unclear due to poor representation of small crabs in the survey and to the extremely poor precision of the abundance estimates. Current stock status is not well estimated.

Additional Research

Also, in 2007 the Bering Sea Fisheries Research Foundation (BSFRF), receiving funds from the fishing industry, NPRB, and NMFS, conducted a separate red king crab survey in Bristol Bay beginning in May. Comparative tows with the NMFS Bering Sea trawl survey vessels were carried out on 9 and 10 June. The BSFRF vessel, the F/V American Eagle, made 5-minute tows alongside the NMFS vessels, which made 19 standard 30-minute tows. Catch data from this effort is currently being analyzed.

By Jan Haaga, Lou Rugolo, and Claire Armistead

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