Integration of submersible transect data and high-resolution multibeam sonar imagery for a habitat-based groundfish assessment of Heceta Bank, Oregon

Fishery Bulletin, Oct, 2002 by Nicole M. Nasby-Lucas, Bob W. Embley, Mark A. Hixon, Susan G. Merle, Brian N. Tissot, Dawn J. Wright

One of the limitations of this habitat-based approach to stock assessments has to do with strong reliance upon the fish-substratum association. The distribution and abundance of groundfish has been shown to be strongly correlated with substrate type (see introduction), but fish distributions and densities may vary with other factors as well, such as depth, currents, nutrients, and food availability. In this study there was an attempt to address this potential problem of over emphasizing the fish substratum relationship through the grouping of habitats into patches. The designation of habitat patches allowed the grouping of areas of potentially similar biotic and physical characteristics. Thus, the use of patches as areas for fish density estimates allowed for increased accuracy in forming abundance estimates from the habitat-groundfish association information. For example, this advantage was apparent by high variance in fish density estimates among patches of similar bottom type, such as the high density of juveniles in one of the three rock ridge patches. Variations in density in similar patch types of our study were also observed for Dover sole, rex sole, and pygmy rockfish. These patterns may be due to differences in depth, food availability or variations in percent composition of substrate type in separate areas of the bank. The other benefit of using habitat patches was that it allowed the testing of new groundfish assessment methods without making predictions for areas of high uncertainty where submersible transects were not performed.

Hixon et al.'s study (4) is one of only a few comprehensive habitat-ground fish studies available and has provided a foundation for testing this new approach. Hixon et al's dataset provided invaluable habitat information, but had several shortcomings. One problem was that of the inconsistency in positional data because of the use of Loran-C (GPS was not yet available). Another problem, characteristic of all submersible studies, was the overall limited spatial sampling provided by the survey. The stations for the study were chosen as representative habitats for Heceta Bank from exploratory submersible dives conducted by Pearcy et al. (1989) in 1987. However, not all of the representative habitat areas were sampled because maps of high-resolution bathymetry and backscatter were not available at that time. Lack of complete habitat data made it difficult to extrapolate bottom-type and fish-density data to the entire bank.

The use of observational data from submersibles for determination of fish density, and extrapolation from these data to total abundance within regional habitat patches, were based on several assumptions. First, we assumed that the areas sampled were representative of the entire regional patch to which each transect belonged. Whatever error was associated with this assumption, our approach is certainly more accurate that any method that ignores habitat variation. Second, we assumed that all fish within each submersible transect were accurately identified and counted. Identification of similar species of rockfish can be problematic, and counts clearly become estimates when dense schools are encountered. Third, we assumed that avoidance or attraction of fish in response to the submersible was minimal, and thus did not affect counts substantially. This assumption was tested by Hixon et al. (4) by observing the local distribution and abundance of fish around the submersible just before and after the 10-15 minute "quiet periods" on the bottom, during which all motors and lights were turned-off. Quiet periods were conducted for all transect dives, and there was no indication that fish behavior was altered by the presence of the submersible. However, midwater schools of yellowtail rockfish sometimes circled the submersible during transects, which could have affected counts if fish were counted more than once. The final assumption was that transect width, which varied as a function of the altitude of the submersible above the seafloor, was constant. Certainly, there was some variation in altitude, yet the error introduced was presumably insubstantial except perhaps in areas of extremely heterogeneous vertical relief.