Abstract
Recovery of fish with adipose fin clips (adc) and coded-wire tags (cwt) in escapement surveys allows calculation of expansion factors used in estimation of the total number of fish from each adc,cwt release group, allowing escapement to be resolved by age and stock of origin. Expanded recoveries are used to derive important estimates such as the total number and proportion of hatchery-origin fish present. The standard estimation scheme assumes accurate visual classification of adc status, which can be problematic for decomposing carcasses. Failure to account for this potential misclassification can lead to significant estimation bias. We reviewed sample expansion factors used for the California Central Valley Chinook salmon 2010 carcass surveys in this context. For upper Sacramento River fall-run and late fall-run carcass surveys, the estimated proportions of adc,cwt fish for fresh and non-fresh carcasses differed substantially, likely from the under-recognition of adc fish in non-fresh carcasses. The resulting estimated proportions of hatchery-origin fish in the upper Sacramento River fall-run and late fall-run carcass surveys were 2.33 to 2.89 times higher if only fresh carcasses are considered. Similar biases can be avoided by consideration of only fresh carcasses for which determination of adc status is relatively straightforward; however, restricting the analysis entirely to fresh carcasses may limit precision because of reduced sample size, and is only possible if protocols for sampling and recording data ensure that the sample data and results for fresh carcasses can be extracted. Thus we recommend sampling protocols that are clearly documented and separately track fresh versus non-fresh carcasses, either collecting only definitively adc fish or that carefully track non-fresh carcasses that are definitively adc versus those that are possibly adc. This would allow judicious use of non-fresh carcass data when sample sizes are otherwise inadequate.
Highlights
Quality estimates of Chinook salmon (Oncorhynchus tshawytscha) escapement to California’s Central Valley are needed for several purposes
The mark used is an adipose fin clip, and the tag used is a coded-wire tag, which is a short length of metal wire inserted into the nasal cartilage of fish, bearing an inscribed tag code that is unique to each release group (Johnson 1990; Nandor et al 2010)
In the sections that follow we present general formulas for the sample and production expansion factors and explain how they influence estimates of release group-specific escapement (N ^ i ), total escapement of hatchery-origin fish (N ^ H ), and the proportion of hatchery-origin fish (p ^H )
Summary
Quality estimates of Chinook salmon (Oncorhynchus tshawytscha) escapement to California’s Central Valley are needed for several purposes. Escapement metrics underlie several viability criteria (Lindley et al 2007) applied to Central Valley salmonid populations, which include the threatened spring run and the endangered winter run. A variety of techniques can provide small-scale information on some of these metrics (e.g. scales for aging, otolith microchemistity for natal origin), the largest potential data source relevant to age and stock of origin comes from the marking and tagging program used by all Chinook salmon hatcheries in California. In practice some fish may receive adc but not cwt or vice versa because of the inherent challenges in processing thousands or millions of fish, but typically this occurs at rates below 1%
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