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11-08-2010, 02:52 PM #1
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Optimizing sampling effort within a systematic design for estimating abundant escapem
By Juha Lilja, Teri Ridley, George M.W. Cronkite, Hermann J. Enzenhofer, John A. Holmes
Fisheries Research, Volume 90, Issues 1-3, April 2008, Pages 118-127, ISSN 0165-7836, DOI: 10.1016/j.fishres.2007.10.002.
The upstream migration of adult sockeye salmon (Oncorhynchus nerka) in the Horsefly River was monitored by a DIDSON imaging sonar during the dominant stock-cycle year 2005 using a systematic 20-min h-1 sampling scheme. We used a subset of these data collected between 16 and 29 September to investigate whether this sampling protocol was justified based on temporal variation in the salmon migration data. During post-processing, the 20-min sequence was split into two 10-min periods and the number of migrating salmon was counted separately. Cross- and autocorrelation analysis showed that estimates from the first and second 10-min samples were similar (r = 0.65) and variation between them (i.e., within the hour) was random, supporting the conclusion that systematic-hourly sampling is a defensible sampling design for acoustic enumeration when temporal variation in fish migration is unknown a priori. Using a simple benefit-cost model (statistical reliability of point estimates of salmon escapement-sampling effort), we recommend a minimum sampling effort of 10-min h-1 and a maximum effort of 20-min h-1 for projects using a systematic sampling scheme in which the goal is to estimate total upstream salmon escapement. An alternative sampling approach targets high-passage events such as diurnal peaks or periods when total daily upstream escapement exceeds 25 000 fish d-1, for increased sampling effort while reducing sampling effort during low-passage periods. This design will improve the statistical reliability of the resulting point estimates of upstream escapement relative to that achievable with a systematic effort with no overall change in total sampling effort over the course of the migration period.
Keywords: DIDSON; Optimal sampling rate; Sockeye salmon; Temporal sampling; Upstream migration