454 



Fishery Bulletin 93(3). 1995 



low population and threshold levels of overfishing is 

 actually less than that which was realized, a differ- 

 ence that increases the risk of overfishing. 



ROVs have the ability to identify the degree of 

 species' attraction to or avoidance of the sampling 

 gear. Three species showed a strong response to the 

 ROV: Pacific whiting, sablefish, and catsharks. Pa- 

 cific whiting was the only species that was attracted 

 to the ROV (Table 3). These three species are usu- 

 ally observed off-bottom and in motion, rather than 

 resting in contact with the bottom. Fishes that are 

 commonly in motion should be in a better position to 

 respond quickly to stimuli (motion, light, etc.) around 

 them. Understanding and accounting for fish behav- 

 ior could improve the accuracy of an estimate. For 

 trawl gear, the level of attraction or avoidance is 

 unknown. Anecdotal evidence for attraction (crowd- 

 ing offish between doors suggested by Krieger [1993]) 

 and for avoidance (net sounder readings of rockfish 

 rising up out of the path of the net [Adams 3 ]), have 

 been reported, but there is no simple way of evaluat- 

 ing these phenomena for trawl gear. 



While the presence of large seasonal or interannual 

 variation in fish numbers could have introduced bias 

 into the estimates, there is no reason to believe this 

 occurred. None of the 20 most common species cap- 

 tured by the three trawl surveys, which covered al- 

 most the entire study period, exhibited a consistent 

 seasonal or overall trend in fish abundance. Also, the 

 consistency of trends in abundance derived from 

 ROVs and trawls from all depths, even though they 

 were sampled at different times, suggests that large 

 varations in abundance were not an important source 

 of bias. However, if this bias had occurred, it would 

 have been a larger problem for the ROV estimates 

 than for the trawl estimates. The ROV samples at 

 depth were taken during one time period, with the 

 exception of the 400-m depth, whereas the trawl 

 samples were taken at all depths during three cruises 

 over nearly the entire time period. 



Both ROV and trawl surveys contain additional 

 information that may be critical to the goals of an 

 abundance study. ROV surveys provide much biologi- 

 cal information, particularly on habitat and species 

 association. In two instances, we observed feeding 

 behavior. Trawl surveys deliver the fish on deck; for 

 common types of biological studies, such as ageing 

 or food habits, these specimens are critical. In addi- 

 tion, these specimens enable accurate species iden- 

 tification. Identifcation, however, is expected to be- 

 come less of a problem as video technology improves. 



3 Adams, P. National Marine Fisheries Service, Tiburon Labora- 

 tory Southwest Fisheries Science Center, 3150 Paradise Dr., 

 Tiburon, CA 94920. Personal commun., 1986. 



Finally, there is the chronic problem of the low sta- 

 tistical power of tests of these abundance estimates 

 owing to large, associated variances. Since increas- 

 ing the sample size is often not practical, the only 

 alternative is to stratify the sampling more effectively 

 rather than simply on the basis of depth. Better 

 stratification can come only from a greater understand- 

 ing of the biological factors responsible for fish distri- 

 butions. An adequate understanding would include the 

 association offish with microhabitat and the biological 

 behavior of fishes that leads to patchiness. Surveys 

 could then be stratified on the basis of areas where fish 

 are occurring at background levels and in large patches. 

 Trawl surveys have been unsuccessful in achieving such 

 separation. Information gained from the ROV could 

 lead to the biological understanding necessary to 

 achieve more efficient stratification designs. 



Acknowledgments 



This work would not have been possible except for 

 the effort of the scientific parties and crews of the 

 RV Point Lobos and of the RV David Starr Jordan. 

 Data for the photogrammetric calibration were kindly 

 provided by Christopher Herald at the Monterey Bay 

 Aquarium. This manuscript gained much from criti- 

 cal reviews by Kenneth Krieger, Richard Methot, 

 David Somerton, and Mark Wilkins. The ideas de- 

 veloped here benefitted greatly from many long con- 

 versations with Mark Wilkins. The ROV portion of this 

 research was supported by a grant from the West Coast 

 Center of the National Underwater Research Program. 



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