282 



Abstract. -A bottom-trawl catch- 

 ability coefficient was estimated for 

 rockfish by comparing trawl catch rates 

 to densities estimated from a manned 

 submersible. A total of 16 comparisons 

 were completed in 1989 and 1991 at 

 depths of 180-283 m. Pacific ocean 

 perch, Sebastes alutus, was the target 

 species and accounted for 12 c r of the 

 rockfish caught in the trawl. The sea- 

 floor area sampled by the trawl was cal- 

 culated from the distance between the 

 wingtips of the net and the distance the 

 net traveled across the seafloor. The 

 seafloor area sampled from the sub- 

 mersible was calculated from the dis- 

 tance the submersible traveled and the 

 lateral range of visibility. A larger vol- 

 ume of water was sampled from the 

 submersible because counts of rockfish 

 included fish to 7 m above the seafloor. 

 whereas the vertical opening of the 

 trawl was 1.8 m; approximately 50% of 

 the rockfish observed from the sub- 

 mersible were >1.8 m above the sea- 

 floor. Even though a larger volume of 

 water was sampled from the submers- 

 ible, catchability coefficients of 0.97 and 

 1.27 were estimated for minimum and 

 maximum detection probabilities of 

 rockfish from the submersible. Appar- 

 ently, rockfish dove or were herded hori- 

 zontally into the opening of the net. 

 Density estimates of rockfish and other 

 groundfish species are feasible from a 

 submersible, and these estimates can 

 be used to determine bottom-trawl 

 catchability coefficients. 



Catchability coefficient for rockfish 

 estimated from trawl and 

 submersible surveys 



Kenneth J. Krieger 

 Michael F. Sigler 



Auke Bay Laboratory. Alaska Fisheries Science Center 



National Marine Fisheries Service, NOAA 



I 1305 Glacier Hwy. Juneau, Alaska 99801-8626 



Bottom-trawl surveys are used to es- 

 timate absolute abundance of most 

 groundfish species in Alaska. The 

 method is founded on the basic as- 

 sumption that catch per unit of effort 

 ( CPUE ) is a function of stock density. 

 CPUE is converted to biomass from 

 the area swept by the trawl and the 

 sampling efficiency of the gear ( catch- 

 ability coefficient) (Alverson and 

 Pereyra, 1969). The basic equation 

 relating population size to CPUE is 



Py=Ci j /qfy > 



where i 



Manuscript accepted 7 December 1995. 

 Fishery Bulletin 94:282-288 ( 1996). 



time period; 

 location or area; 

 exploitable population 

 size in weight; 

 catch in weight; 

 catchability coefficient; 

 and 

 / = fishing effort (Alverson 

 and PereyTa, 1969). 



Because there is no simple method 

 of measuring catch efficiency, a 

 catchability coefficient of 1.0 is used 

 to convert CPUE data to biomass in 

 bottom-trawl surveys (assuming a 

 I00 r /t catch efficiency across the 

 horizontal spread of the net [wing- 

 tip to wingtip]). However, if fish are 

 distributed above the headrope, are 

 buried in the substrate, swim out 

 of the path of the trawl, or escape 

 through the net, the catchability 

 coefficient is <1.0 and abundance is 

 underestimated. Conversely, if fish 

 are herded into the trawl by the 



bridles and trawl doors and do not 

 escape above or under the net, the 

 catchability coefficient is >1.0 and 

 abundance is overestimated. Catch- 

 ability coefficients depend on bio- 

 logical factors such as sensory ca- 

 pabilities as well as behavioral re- 

 sponses of the target species, and 

 environmental factors such as light, 

 temperature, and bottom type (see 

 Gunderson, 1993, for an overview). 

 A wide range of catchability coeffi- 

 cients were estimated in studies 

 that monitored efficiency of bottom 

 trawls: Harden-Jones et al. (1977) 

 attached acoustic tags to flatfish 

 and monitored their behavior in re- 

 sponse to trawl gear; 61 f /r of the fish 

 in the path of the net were captured. 

 Main and Sangster ( 1981 ) observed 

 the catch process of several ground- 

 fish species from a towed underwa- 

 ter vehicle; each species responded 

 differently with regard to net avoid- 

 ance, escapement, and herding by 

 the bridles and doors. Korotkov 1 ob- 

 served groundfish behavior in front 

 of a bottom trawl from an underwa- 

 ter towed apparatus; catchability 

 coefficients of 0.1 to 0.4 were esti- 

 mated, depending on trawl rigging 

 and trawl orientation to the bottom. 

 These studies relied on species that 

 could be tagged or on species located 

 at shallow depths where ambient 



Korotkov, V. K. 1984. Fish behaviour in 

 a catching zone and influence of bottom 

 trawl rig elements on selectivity. Int. 

 Counc. Explor. Sea. Council Meeting 1984, 

 paper/B:15, 14 p. 



