Nasby-Lucas et al : Use of submersible transect data and multibeam sonar imagery for habitat assessment 



741 



invertebrates. These characteristics have made Heceta 

 Bank one of the largest and most important of the heavily 

 fished rocky banks in the Pacific Northwest. Along with its 

 commercial importance, the bank has been the subject of 

 substantial scientific research, which has made it an ideal 

 site for developing these methods. 



Direct observations of Heceta Bank were first made in 

 1987 in a series of 16 submersible dives used to character- 

 ize fish populations and habitats on the bank (Pearcy et 

 al.. 1989). These initial dives were used to select represen- 

 tative transects which were repeated in 1988, 1989, and 

 1990 using the manned research submersible Delta (Hix- 

 on et al.'*; Stein et al., 1992). The objective of these surveys 

 was to investigate relationships between the abundance 

 of groundfish and macroinvertebrates and the topography 

 and texture of the seafioor, as well as to document inter- 

 annual variation in these relationships. Fish observed 

 during these sui-veys included 69 taxa, representing 24 

 families, dominated by 24 species of rockfish. Multivariate 

 analysis detected statistical relationships between habi- 

 tat characteristics and fish distribution and abundance by 

 species and provided comprehensive data on fish-habitat 

 associations, as well as a baseline for future comparisons. 



Although invaluable, data from this set of submersible 

 dives provided detailed "snapshots" of very limited areas 

 of the Bank. To complement this study and to provide a 

 broad view of the bank, a survey was performed in 1988 

 with hull-mounted, Simrad EM300 multibeam sonar, 

 which provided high-resolution bathymetry and backscat- 

 ter imagery of most of Heceta Bank (Merle et al.*). 



Materials and methods 



Submersible dives 



Submersible observation data were collected by using the 

 manned submersible Delta at six predetermined stations 

 along Heceta Bank. Detailed descriptions of submersible 

 operations used in our study can be found in Hixon et al.^ 

 and Stein et al. (1992), and a brief description follows. 

 Surveys were conducted in the month of September and 

 consisted of 18 dives in 1988, 12 in 1989, and 12 in 1990, 

 for a total of 42 dives and 84 transects (Fig. 2). Each dive 

 consisted of two 30-minute timed transects and a 10-15 

 minute "quiet period" between transects to assess the 

 effects of the submersible's lights and motors on local 

 fish distribution. The average length of each 30-minute 

 transect was approximately 1015 m. Data on fish species, 

 size, and abundance were collected by direct observations 

 through the forward view port from approximately 2 m 

 above the bottom, providing a bracketed transect width 

 of about 2.3 m. During the transects, observations were 

 verbally tape-recorded and visually recorded with a VHS 



44"15'N_ 



44'10'N. 



44 05'N 



44 OON. 



43'55'N_ 



Merle, S., R. W. Embley, J. Reynolds, D. Clague, C. Goldfinger, 

 and R. Yeats. 1998. A high-resolution image over the Heceta 

 Bank off Central Oregon. Trans. Am. Geophys. Union 78, Fall 

 Meeting Suppl., F818. 



Figure 2 



Simrad EM300 multibeam bathymetry illuminated from the 

 northwest, which creates shadows to the southeast of the 

 relief The 1988, 1989, and 1990 Delta submersible transects 

 (six clusters of white lines) have been overlaid on top of the 

 bathymetric data. The isobathymetric contours are at 50-m 

 intervals, beginning with 100 m on the east side of the bank. 



