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Report: Determination of Gerrard Rainbow Trout Stock Productivity at Low Abundance Final Report
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The recent unprecedented collapse of the lakes' Kokanee population has provided an ideal opportunity to obtain information on the population dynamics at low stock abundance, a key data piece required for determining stock status. This report summarizes spring 2016 survey work conducted on the Lardeau and Duncan rivers to assess juvenile recruitment at low stock abundance.
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Author: G. Andrusak
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Old Reference Number: COL-F17-F-1200
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Old Reference System: FWCP Fish Wildlife Compensation Program Columbia
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Date Published: Jun 2016
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Report ID: 52339
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Audience: Government and Public
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Annual stock assessment of the juvenile Gerrard Rainbow Trout (Oncorhynchus mykiss) population in the Lardeau and Duncan rivers is considered a management priority within the Kootenay Region. Assessing and determining the Gerrard rainbow trout stock's reproductive capacity and productivity is essential for managing this highly exploited population. Assessment of the in-river juvenile abundance in unison with adult escapement data provides important information for defining a stock-recruitment relationship used in determining useable biological reference points for management of this unique population. The recent unprecedented collapse of the lakes' Kokanee population has provided an ideal opportunity to obtain information on the population dynamics at low stock abundance, a key data piece required for determining stock status. This report summarizes spring 2016 survey work conducted on the Lardeau and Duncan rivers to assess juvenile recruitment at low stock abundance.
Snorkel surveys provide a proven method in obtaining juvenile abundance estimates on a medium to large size river in comparison to more labour intensive and more expensive methods such as electrofishing. This project has utilized night time snorkel methods since 2006 to assess Gerrard juvenile abundance on the Lardeau and Duncan rivers. Starting in 2011, increased sampling effort has improved precision thus reducing uncertainty in abundance estimates. Surveys conducted in the spring of 2016 covered approximately 10.4% Lardeau River and 6.6% of the Duncan River. A total of 8.7% of the mainstem shoreline and 17.7% of the side channel habitat were surveyed in the Lardeau River. Similarly, a total of 7.6% of the mainstem shoreline and 0.7% of the side channel habitat were surveyed in the Duncan River in 2016.
In 2016, age 1 abundance for the Lardeau and Duncan rivers was estimated to be 43,570 (95% CRI 30,990-59,110) which is substantially lower than previous years. Spawner escapements at Gerrard using area under the curve (AUC) from daily counts have provided an index of abundance since 1961. The 2016 juvenile recruitment estimates are the progeny from the 2015 spawn of Gerrard Rainbow Trout which was estimated to be 301 AUC, well off the record high observed in 2012 of over 1,500 AUC.
Data analysis fitted a Beverton Holt stock-recruit (SR) curve for Gerrard Rainbow Trout based on river recruit and spawner data. Preliminary results of the stock-recruit relationship suggest no appreciable increase in recruitment in juvenile abundance as spawner abundance increased above 500 AUC. This relationship also suggests that most of the density dependent mortality in the early life stages occurs prior to age 1. Average annual recruitment is estimated to be approximately 85,000 age 1 juveniles. Based on the 2016 data, the SR also indicates that the maximum reproductive performance (recruits per spawner) has declined, indicating the population did not compensate in survival at low abundance. It is uncertain whether the decline is a function of decreased egg production or an actual decline in overwinter mortality associated with limited or no Kokanee production in the rivers. Therefore, results should be used with caution until more information at low stock abundance can be obtained.
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Report Type
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Fish and Aquatic Habitat Information |
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Subject
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Fish Species - Rainbow Trout - Oncorhynchus mykiss |
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Region - Kootenay |
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Fish and Fish Habitat - Stock Assessment |
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Watershed Groups - 340 - Lower Kootenay |
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Program Specific Area - Kootenay Compensation Program |
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