Diagram of trawl gear set up for fish. Image: IEP
Fish monitoring surveys in the San Francisco Estuary net different numbers of different fish species depending upon when and how they sample.
According to a new study
published in San Francisco Estuary and Watershed Science, even surveys that target the same part of the water column can come up with significantly different catches. The study’s authors analyzed decades worth of data from the California Department of Fish and Wildlife (CDFW) Fall Midwater Trawl, which spans from San Pablo Bay to the upper Delta; CDFW’s San Francisco Bay Study midwater and otter trawls, covering the South Bay to the central Delta; and the UC Davis Suisun Marsh Fish Study, also an otter trawl. The two midwater trawls, which sample the middle of the water column, were found to be generally more successful at catching pelagic (open-water) fish like Delta smelt and American shad than the two otter trawls, which sample the bottom of the water column. The latter were more effective at catching demersal (bottom-dwelling) species like prickly sculpin and Sacramento sucker. The research also turned up unexpected distinctions between the two types of trawls that may have more to do with how and where equipment is deployed than the depth at which they operate, says lead author Brock Huntsman of the U.S. Geological Survey. “There was a decent amount of variability to which species were captured more efficiently with one gear type or one survey than the other, even when they were technically sampling the same habitat.” The new findings will help researchers studying fish populations Estuary-wide to more accurately combine and compare data from four key long-term surveys, Huntsman says.