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Comparing Two Common Methods of Estimating Stream Fish Abundance

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Abstract

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 salmonines, 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. Individuals from each stream were then held in cages overnight to assess survival and loss of marks. We found removal estimates generated with program CAPTURE were significantly lower (22%-58%) compared to mark-recapture abundance estimates. Removal methods also consistently underestimated (38%-59%) known (i.e., number of marked fish in the study reach) mottled sculpin abundances. Minimal movement of marked fish (mean=11.5%; range=0.0%- 42.0%) was observed in all but one of our study streams. Survival of mottled sculpin 1 day after capture (i.e., individuals held in cages) 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 for all but one stream 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.