2011 S3: Brandon Harris

Comparing two methods for estimating stream fish abundance

Unbiased estimates of stream fish abundance are critical for sound fisheries management.  Most studies investigating bias associated with stream fish population estimates primarily focus on salmonids, yet non-game fishes often comprise a major portion of stream fish assemblages.  We evaluated mark-recapture and removal methods for estimating the abundance of mottled sculpin (Cottus bairdii), a common non-game fish found in streams.  Specific objectives were to: (1) compare abundance estimates using mark-recapture and removal methods, (2) assess potential removal method bias by comparing estimated abundance to known abundance, and (3) evaluate assumption that the population is closed (i.e., no additions or deletions during sampling).  Fish were sampled via backpack electrofishing at eight streams, and each stream was sampled over a 2-day period.  On day one, fish were batch marked in three sections of a 90-m reach in each stream.  On day two, fish were captured and temporarily removed from the stream during four electrofishing passes, and the number and marking status was recorded during each pass.  Thirty individuals were then held in cages overnight to assess survival and loss of marks.  We found removal estimates generated by program CAPTURE were typically lower (38%-44%) compared to mark-recapture abundance estimates.  Removal methods also consistently underestimated (39%-48%) known mottled sculpin abundances.  Minimal movement of marked fish (mean=4.4%; range=0.96%-17.5%) was observed in all but one of our study streams.  Survival of mottled sculpin after capture was 100% (n=405 fish), and no marks were lost among fish held overnight.  Our preliminary results suggest that the closed-population assumption was valid and the removal method yielded negatively biased estimates of mottled sculpin abundances in small streams.  Consequently, we recommend that fisheries managers use mark-recapture methods to estimate abundance of small, non-game fishes.

Faculty Mentor: Carl Ruetz, Annis Water Resources Institute

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