617 



Abstract.— Snow crab iChionoecetes 

 opilio^ and Tanner crab (C bairdi) fish- 

 eries in the eastern Bering Sea are 

 managed by using swept-area esti- 

 mates of biomass based on an annual 

 survey conducted with a 83-112 east- 

 ern bottom trawl. Estimates of net ef- 

 ficiency (i.e. the capture probability of 

 crab that occur between the wing-tips 

 of the trawl net) are needed to correct 

 the biomass estimates for any size se- 

 lectivity by the trawl. Data on net effi- 

 ciency were obtained experimentally by 

 attaching an auxiliary net beneath the 

 trawl net to capture crab escaping un- 

 der the trawl footrope. Net efficiency is 

 then the quotient of the trawl catch di- 

 vided by the combined catch of the 

 trawl and auxiliary nets. Mathemati- 

 cal models of the relationship between 

 net efficiency and carapace width were 

 formulated and fitted to the experimen- 

 tal data. Net efficiency for both species 

 first decreased with increasing cara- 

 pace width until a minimum efficiency 

 was reached near 50 mm carapace 

 width. At larger sizes, efficiency in- 

 creased asymptotically with carapace 

 width. Net efficiency for mature female 

 Tanner crab was lower 10.47) than for 

 males and immature females combined 

 (0.72) at the same mean carapace width 

 as mature females (66-107 mm). Net 

 efficiency did not differ between mor- 

 phologically mature and immature 

 male Tanner crabs of the same carapace 

 width. 



Net efficiency of a survey trawl for 

 snow crab, Chionoecetes opilio, and 

 Tanner crab, C bairdi 



David A. Somerton 



Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way NE 



Seattle, Washington 98125 



E-mail address david somertonanoaa gov 



Robert S. Otto 



Kodiak Laboratory 



Alaska Fisheries Science Center 



National Marine Fishenes Service, NOAA 



PO Box 1638 



Kodiak, Alaska 99615 



Manuscript accepted 3 November 1998. 

 Fish. Bull. 97:617-625 ( 1999). 



The commercial fisheries for snow 

 crab (Chionoecetes opilio) and Tan- 

 ner crab (C. bairdi) in the eastern 

 Bering Sea are managed by using 

 swept-area estimates of biomass 

 (Alverson and Pereyra, 1969) pro- 

 vided by an annual Alaska Fisher- 

 ies Science Center (AFSC) bottom 

 trawl survey. In addition to catch 

 and swept-area data, such biomass 

 estimates require a value for the 

 sampling efficiency of the trawP (i.e. 

 the proportion of animals that are 

 captured within the area spanned by 

 the trawl doors; Dickson, 1993a). 

 Trawl efficiency can be considered as 

 a function of sweep efficiency (the 

 proportion of animals within the 

 path of the doors, bridles, and 

 sweeps that are herded into the net 

 path) and net efficiency (the propor- 

 tion of animals that are captured 

 within the path of the trawl net; 

 Dickson, 1993a). For snow and Tan- 

 ner crabs, owing to a lack of any 

 contradictory evidence, sweep effi- 

 ciency was assumed to be zero and 

 net efficiency was assumed to be 

 unit}'. The assumption of zero sweep 

 efficiency has been subsequently 

 supported by experiments on herd- 

 ing,^ but the assumption of com- 

 plete net efficiency has been contra- 



dicted by studies in which low-light 

 video cameras have documented 

 that even large male crabs could 

 often escape under the footrope.'^ 

 Such observations prompted us to 

 experimentally estimate net effi- 

 ciency of the 83-112 eastern trawl, 

 the trawl used for the eastern 

 Bering Sea surveys. 



Previous studies of net efficiency, 

 which were focused at various spe- 

 cies of groundfish, indicated that 

 efficiency generally increases with 

 body size (Engas and God0, 1989; 

 Walsh, 1992). For snow and Tanner 

 crab, however, biological attributes 

 other than size could also influence 

 efficiency. For example, mature 

 males have longer legs and thicker 

 chelae, in relation to their carapace 

 width, than either mature females 

 or immature males. Not only could 

 the difference in body shape result 

 in differing net efficiency, differ- 



' Throughout this paper we will refer to the 

 trawl as an entire fishing gear comprising 

 the net. bridles, and doors. 



- Somerton, D. A., and P. T Munro. 1998. 

 Estimating the sweep efficiency of a bottom 

 trawl. Manuscript in preparation. 



' Munro, P. T. 1998. Alaska Fisheries Sci- 

 ence Center, 7600 Sand Point Way NE Se- 

 attle, WA. Unpubl. data. 



