Background:

The Norton Sound Trawl Survey data is one of a few long time-series data sets available in the northern Bering Sea. The survey has been conducted approximately triennially since 1976 by NMFS (1976-1992) and ADF&G (1996-present) with a similar survey format, in order to monitor abundance of red king crab in this region (Fair 1998). Norton Sound was laid out with evenly-spaced sampling stations approximately 18.5 km (10 nautical miles) apart. One trawl was towed for approximately 2.6 km (1.5 nm; NMFS) or 1.9 km (1.0 nm; ADF&G) at each location, and the catches were enumerated and weighed to the lowest practical taxa. Catch weights for each survey location was standardized by converting into catch-per-unit-of-effort: weight per area swept (kg km-2) calculated as effective width of the net times distance towed (Fair 1998).

A previous study showed that overall biomass of the Norton Sound benthic fauna have increased exponentially by three-fold from 1976 to 2002, with declines in 1991 and 1999, while overall taxonomic composition remained relatively stable (Hamazaki et al. 2005). This is also consistent with findings north of Norton Sound in the southeastern Chukchi Sea and Kotzebue Sound, where biomass of the most dominant species/groups increased two-fold or more between 1976 and 1998, with little change in epifaunal invertebrate compositions (Feder et al. 2005). Those studies suggest that benthic fauna biomass increased significantly without major taxnomic changes across these northern regions since 1976.

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