Biology, Assessment, and Management of North Pacific Rockfishes 411 Alaska Sea Grant College Program • AK-SG-07-01, 2007 Variability in Trawl Survey Catches of Pacific Ocean Perch, Shortraker Rockfish, and Rougheye Rockfish in the Gulf of Alaska David M. Clausen and Jeffrey T. Fujioka National Marine Fisheries Service, Auke Bay Laboratory, Juneau, Alaska Abstract Little information is available on the comparative variability of Pacific ocean perch, shortraker rockfish, and rougheye rockfish catches in Alaska trawl surveys. In this study, data were first examined for these three species from three localized experimental bottom trawl surveys in the Gulf of Alaska in 1993, 1998, and 1999 to compare the variability of catch rates for each species. When catch rates were analyzed within favorable fishing areas for each species, the coefficients of variation for Pacific ocean perch were approximately 2.5 to 3 times greater than those of shortraker or rougheye rockfish, indicating that the latter two species are much more even in their distribution. To provide a broader comparison of variability, catch rates of the three species were also examined from seven large-scale bottom trawl surveys conducted in the Gulf of Alaska from 1984 to 2001. Using geographic information systems software, subsets of the hauls in these surveys were selected in what were determined to be favorable fishing areas for either Pacific ocean perch or for shortraker and rougheye rockfish. Analysis of the species’ catch rates in these hauls indicated results that were very simi- lar to those of the experimental trawl surveys; the variability of Pacific ocean perch was 2.1-2.3 times greater than that for either shortraker or rougheye rockfish. The results of this study indicate that a trawl survey for shortraker and rougheye rockfish based on a random design may be logistically feasible, but that an alternative design would be needed for Pacific ocean perch in order to be cost effective. 412 Clausen and Fujioka—Variability in Trawl Survey Catches Introduction Pacific ocean perch Sebastes( alutus), shortraker rockfish Sebastes( borealis), and rougheye rockfish Sebastes( aleutianus) are ecologically important and commercially valuable species that inhabit the outer continental shelf and upper continental slope of the Gulf of Alaska. Pacific ocean perch (POP) as adults are found on the outer shelf and upper slope at depths of ~100-300 m. In contrast, adult shortraker and rougheye rockfish typically inhabit deeper waters of the slope at depths of ~300-500 m (Clausen et al. 2003). Due to the fact that shortraker and rougheye often co-occur in hauls and are superficially similar in appearance, until 2005 the two species were grouped together as an assemblage in the Gulf of Alaska for fishery management purposes, whereas POP has been managed as a single species for many years. POP is the most abundant rockfish in Alaska, and in recent bottom trawl surveys of the Gulf of Alaska has ranked second in biomass among all groundfish species in this region (Martin 1997, Britt and Martin 2001). Shortraker and rougheye rockfish, while of less overall abundance, are nonetheless two of the most common species in the 300-500 m stratum on the slope. POP supports the largest rockfish trawl fishery in the Gulf of Alaska, with catches averaging about 11,000 t in the years 1999-2003 (Hanselman et al. 2003). Catches of shortraker and rougheye rockfish have been much lower, but still substantial, with an average during these years of about 1,600 t for the assemblage (Clausen et al. 2003). Despite the importance of all three species, there is little informa- tion on their comparative variability in trawl catches. POP are known to have a highly clustered distribution (Lunsford 1999); hence, randomly located hauls targeting this species usually result in many hauls with low or zero catches and infrequent hauls with relatively large catches. There has been little published on the variability of shortraker and rougheye rockfish catches, except that Ito (1999) states that they are “contagiously distributed and highly dispersed,” which implies the two fish may be similar to POP in having a clustered distribution and a rela- tively high variability. Information on the variability in trawl catches for each of these species is particularly needed, as bottom trawl surveys provide much of the data used in assessing their stock condition and in determining appropriate harvest levels. These surveys have been conducted every two or three years by the Alaska Fisheries Science Center (AFSC) since 1984, and they attempt to survey all groundfish species in offshore waters of the Gulf of Alaska out to a depth of 500 m (to 1,000 m in some surveys). However, the surveys have oftentimes been problematic for rockfish due to three major reasons: (1) sampling density in the areas inhabited by rockfish appears to be inadequate for species such as POP with a patchy distribution; (2) the stratified random survey design is a based on a stratification scheme that may not be optimal for POP in the Biology, Assessment, and Management of North Pacific Rockfishes 413 Gulf of Alaska (Lunsford 1999); and (3) the trawl gear used in the survey is not able to sample many of the rough bottom locations where species such as shortraker and rougheye rockfish are often found. As a conse- quence of these problems, the present trawl surveys have sometimes yielded wide confidence intervals around biomass estimates (often the case for POP; Hanselman et al. 2003) or uncertain trends in abundance (especially for shortraker and rougheye rockfish; Clausen et al. 2003). Therefore, to improve trawl survey results for rockfish, it appears that new surveys, designed specifically for rockfish, may be necessary. We undertook the present study to investigate the variability of trawl survey catches of POP, shortraker rockfish, and rougheye rockfish. The main objectives are to determine how the variability of shortraker and rougheye catches compares with that of POP and to provide infor- mation that will be needed to design improved trawl surveys for rock- fish. In the first part of our study, we examine results from three small, experimental trawl surveys for rockfish conducted by the chartered F/V Unimak. We were particularly interested in these surveys because we believe they may represent the best trawl data for these species in Alaska. In the second part of the study, we analyze results from seven of the large-scale AFSC trawl surveys of the Gulf of Alaska. Finally, we compare our results for the Unimak and the large-scale gulf-wide sur- veys and discuss our study’s implications for future surveys. Experimental surveys by F/V Unimak Methods and analysis of Unimak surveys The AFSC chartered a commercial factory trawler, the 56 m F/V Unimak, to conduct specialized experimental bottom trawl surveys of rockfish in 1993, 1998, and 1999. (In 1993 and 1998, the vessel was named the Unimak Enterprise). These surveys had some unique advantages when compared with the standard AFSC trawl surveys, especially the fol- lowing two. (1) In each experimental survey, the vessel was equipped with a rockfish net fitted with “rockhopper” gear along the footrope. Rockhopper gear consists of a series of split automobile tires mounted on the center section of the footrope so the gear can bounce over boul- ders and obstructions (for a detailed description of the net see Clausen et al. 1999). This allows the net to be fished over much rougher bottom than can the standard AFSC survey net. Rockhopper gear is now rou- tinely used by most commercial vessels in Alaska that bottom trawl for any species of rockfish. (2) The same skipper was used in each of the Unimak surveys—a very experienced Alaska rockfish fisherman. This is in contrast to the AFSC surveys, which used many different skippers over the years, most of whom had limited fishing experience for rock- fish in Alaska. 414 Clausen and Fujioka—Variability in Trawl Survey Catches Alaska x xxxxxxxxxxx xxxxxxx xxxxx 1999 survey n=38 hauls xxx ×xxxx×xx Gulf of Alaska xxx×xx×x xx 1998 survey n=57 hauls # eg. N 55 d 200 m contour 150 deg. W 140 deg. W Figure 1. Location of random hauls for Pacific ocean perch (POP) that were sampled in the F/V Unimak surveys. Hauls were considered to be in favorable fishing localities for POP. The actual purpose of the 1993 survey was to use rockhopper gear to conduct a pilot survey of shortraker and rougheye rockfish only (Ito 1999), whereas the purpose of the 1998 and 1999 surveys was to con- duct adaptive sampling experiments for these two species as well as for POP (Clausen et al. 1999; Hanselman et al. 2001, 2003). In each survey, a number of hauls were located at random positions, and for the present study we restricted our analysis to just these random hauls. The loca- tion and total numbers of hauls used in the analysis are shown in Figs. 1 and 2, which indicate the hauls in each survey covered relatively small geographic areas of the central and eastern Gulf of Alaska. It should also be noted that the number of random hauls fished was relatively small and ranged from 19 for shortraker and rougheye in the 1999 survey to 57 for POP in the 1998 survey. The locations of these random hauls were in what was believed to be “favorable fishing localities” for each species, which for the purpose of this paper we define to be positions where one would reasonably expect to sometimes encounter a large catch. For shortraker and rougheye rockfish in the Unimak surveys, favorable fishing localities were considered to be depths of 300-500 m on the slope and, for POP, depths of 150-300 m on the outer shelf and Biology, Assessment, and Management of North Pacific Rockfishes 415 Alaska °$r°r$r #°°°$rr$°° r°$r °#°°° r$rr°$rr$r °#°° r°r°r° °# rr °# rr rr ° 1993 survey, n =27 hauls r 1998 survey, n=23 hauls r 1999 survey, n=19 hauls Gulf of Alaska # g.