Mark-recapture methods are commonly used in ecology to estimate population size. Assumptions, such as random sampling, must be filled in order to obtain an unbiased population estimate and this can often prove difficult in the field, furthermore it can prove even more difficult to get a verification of such a population estimate and thereby gauge the value of your work.
In 2009, Kintama participated in the yearly mark-recapture study done in the Bulkley River at the Moricetown Canyon Falls to estimate the adult steelhead population. Every year tagging (beach seine) is done immediately below the falls and recapture (dipnet) is 300m upstream at the top of the falls; in 2009, a sub-sample of the mark-recapture fish was equipped with sonic tags in addition to the standard anchor tags. Movements after release were monitored by acoustic receivers located above and below the sampling sites.
This study allowed for an investigation of the number of fish that dropped down-river upon release, the amount of time fish spent in the area below the falls (potentially exposing themselves to recapture in the gillnet below the falls), the extent of down- and up-stream travel following release and most importantly – how many fish actually passed the recapture site following tagging. Our 2009 study found that only between 56-59% of tagged steelhead were available for recapture, and therefore that the mark-recapture abundance estimates would be over-estimated.
The addition of acoustics methods like these can provide valuable supplemental information into mark-recapture studies, providing an indication of their effectiveness and indicating areas where improvement of estimates is needed.
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