Butler et al Multibeam sonar mapping techniques for estimates of Haliotis sorenseni 



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Hobday and Tegner, 2000; Hobday et al., 2001). Using 

 this assumption, the total amount of white abalone habi- 

 tat in southern California was estimated to be only 752 

 ha (Davis et al., 1998; Hobday et al., 2001). The results 

 of surveys from the present study at only three sites 

 (Tanner and Cortes Banks and San Clemente Island) 

 revealed 3646 ha of rocky substrate between 30 and 60 

 m, which is much higher than estimates based on the 

 39c rocky habitat assumption. Thus, the total available 

 habitat area for white abalone and other benthic dwell- 

 ing organisms in southern California is likely far greater 

 than previously reported. 



The definition of available white abalone habitat, 

 however, may need further refinement if white aba- 

 lone exhibit a preference for a certain type of rocky 

 substrate. Cochrane et al. (2005) suggested that white 

 abalone prefer the edges of reefs at the sand-rock in- 

 terface rather than the areas in the middle of the reef. 

 Although there is some qualitative evidence support- 

 ing this idea, there have been no conclusive studies 

 on specific habitat preferences. The only conclusion 

 that can be drawn in relation to habitat preference 

 from the present study is that white abalone are not 

 typically found in areas with no rock (i.e., sand only). 

 Even if white abalone were found only at the sand-rock 

 interface, our estimates of available habitat would still 

 far exceed past estimates. Although the time and effort 

 involved in mapping habitat may lead to the tempta- 

 tion to make broad generalizations to estimate habitat 

 types and amounts, the results of this study have shown 

 that specific habitat mapping is necessary to accurately 

 identify the amounts of each type of habitat existing in 

 a particular area. 



The depth distribution of white abalone at these three 

 sites was grouped into three depth bins (30-40 m, 

 40-50 m, and 50-60 m) and was less stratified than 



predicted according to preliminary surveys at Tanner 

 Bank. The number of abalone was highest in the mid- 

 dle depth bin (40-50 m) at Tanner Bank during 2002 

 and 2004, but only significantly so in 2002. The high- 

 est number of abalone observed at Cortes Bank was in 

 the shallowest depth bin (30-40 m). Too few abalone 

 were sighted at San Clemente Island and they were not 

 included in the analysis. These results indicate that 

 there are not clear, broad trends in depth distribution 

 that can be applied to all sites, although the deepest 

 depth bin did yield the fewest abalone sightings at all 

 sites. It is also possible that the only realistic trends 

 observed were those at Tanner Bank in 2002 because 

 of the relatively large number of animals observed at 

 this site. 



The white abalone population in California has been 

 estimated to number less than 3000 (Davis et al., 1998; 

 Hobday and Tegner, 2000; Hobday et al., 2001). In this 

 study a total of between 15,187 and 22,123 white aba- 

 lone was estimated at two offshore banks and one is- 

 land location. Our population estimate is greater for 

 two reasons: 1) the multibeam sonar maps revealed 

 more habitat than was previously known to exist, and 

 2) search effort, and therefore the calculation of density, 

 was dependent on suitable habitat, i.e., densities were 

 not artificially deflated by including portions of surveys 

 conducted in inappropriate habitat (e.g., sand). 



The highest densities (19.8 abalone/ha between 40 

 and 50 m) were observed at Tanner Bank in 2002. 

 Previous surveys at Tanner Bank underestimated habi- 

 tat area and included the entire search area without 

 regard to depth or habitat to determine density. A 

 re-examination of data from surveys conducted by the 

 Delta submersible in 1999 at Tanner Bank (Haaker et 

 al.-) yielded an overall density of 15.7 abalone/ha. This 

 value varies only slightly from our overall density esti- 



