502 



Fishery Bulletin 101(3) 



result. For our study, all samples were called "tows" because 

 our study was a trawl survey. 



When little information is available to preset a fixed 

 criterion value, order statistics are often used to choose 

 a criterion value (Thompson and Seber, 1996). The basic 

 idea is that an initial random sample is conducted. Next, 

 the values of the random tows are ordered, and ACS is 

 conducted around the top r stations. The variable r is de- 

 cided by the experimenter and depends on the amount of 

 resources available and the suspected aggregation of the 

 population. The criterion value is then set at the value of 

 the next highest tow ( ;+ 1 ). This was the design used in the 

 1998 adaptive cluster sampling survey for rockfish (Quinn 

 et al., 1999, Hanselman et al., 2001). The use of order 

 statistics has several limitations, however. First, initial 

 random samples must be taken before the adaptive phase 

 can begin. This procedure can be inefficient, because the 

 experiment may have to move a large distance back to the 

 previous tows that exceeded the criterion, by which time 

 the aggregation may have moved or dispersed. In some 

 cases, this procedure may result in a very small criterion 

 value that leads to an overwhelming amount of adaptive 

 sampling around some tows. Second, the process of achiev- 

 ing simple unbiased estimates of abundance is more com- 



plicated with order statistics because the criterion value is 

 dependent on the sampling. 



In our study, we address methods to avoid these limita- 

 tions and illustrate these methods with a 1999 ACS survey 

 for Gulf of Alaska rockfish. The primary target of the sur- 

 vey was Pacific ocean perch iSebastes alutus [POP] ). These 

 fish have extremely uncertain biomass estimates in the 

 Gulf of Alaska (Heifetz et al.'). The estimates are based in 

 part on a standardized stratified random survey conducted 

 by the National Marine Fisheries Service every three years 

 (every two since 2000). This uncertainty is likely due to 

 their highly clustered distribution (Lunsford, 1999) and 

 has led to two independent surveys ( 1998, 1999) to test the 

 benefits of ACS in sampling POP. Shortraker (S. borealis) 

 and rougheye (S. aleutianus) rockfish combined (SR-RE) 

 are also tested to compare the results of a population that 

 is considered highly clustered (POP) versus one that is 

 considered more uniformly distributed (SR-RE). SR-RE 

 are combined because they co-occur in identical habitat 

 and are managed as a complex. 



Materials and methods 



In June 1999, ACS was carried out between 140° and 144° 

 west longitude near Yakutat in the Gulf of Alaska (Fig. 2). 

 Approximately 75% of sampling was directed toward the 

 POP depth stratum ( 180-300 m) and 25% directed toward 

 SR-RE depths (300^50 m). A 182-ft. factory trawler, the 

 Unimak, was chartered to conduct trawl samples. Fish- 

 ing and field operations are described in Clausen et al.^ 

 Duration of all trawl hauls was 15 (POP) and 30 (SR-RE) 

 minutes on the bottom. SR-RE tows were made parallel to 

 the depth contours in a linear pattern (Fig. 1) because the 

 slope that SR-RE inhabit is too steep for perpendicular 

 tows. Travel time between all tows was recorded to exam- 

 ine time efficiency. 



Initially, a set of systematic random tows was conducted 

 from west to east across the entire study area to determine 

 the criterion value. Samples were chosen systematically by 

 longitude and distributed randomly by depth within each 

 longitudinal strip. This procedure was a necessary proxy for 

 simple random sampling because of poorly known bathym- 

 etry in the area. The use of simple random latitudes and 

 longitudes often results in the selection of sites that are well 

 out of the sampling depth interval. After random sampling 

 was completed, we compiled and examined the data to set 

 the criterion value. Criterion values were chosen based on 

 a hierarchy of three alternatives described below. Next, we 

 conducted a new set of random tows from east to west across 

 the area, in which any tows exceeding the criterion value 

 were adaptively sampled. A distance of 0.19 km (0.1 nmi) 

 was used between all adaptive tows and the initial random 

 tow to avoid depletion effects on the catches. 



' Heifetz, J., D. L. Courtney, D. M. Clausen, J. T. Fujioka, and J. N. 

 lanclli. 2001. Slope rockfish. In Stock as.scssment and fish- 

 ery evaluation for the groundfish resources of the Gulf of Alaska, 

 72 p. North Pacific Fishery Management Council. 605 W. 4"' 

 Ave. Suite 306, Anchorage, AK 99501. 



2 Clausen, D.M.,D.H. Hanselman, C. Lunsford, T. Quinn II, and J. 

 Heifetz. 1999. fZ/i/niaA' enterprise cruise 98-01 rockfish adap- 

 tive sampling experiment in the central Gulf of Alaska 1998, 

 49 p. Auke Bav Lab, NMFS, NOAA, 1 1305 Glacier Hwy, Auke 

 Bay, Alaska, 99801. 



