Determination of Gerrard Rainbow Trout Stock Productivity at Low Abundance-2018 (COL-F19-F-2703). Prepared for the Fish and Wildlife Compensation Program, the Habitat Conservation Trust Foundation
Author: G.F. Andrusak, Thorley J.L
This study focused on obtaining information on the recruitment of Gerrard Rainbow Trout under low stock abundance. Stock abundance (spawner numbers) is expected to be at or near record lows over the next 3-5 years, due to the near collapse in the Kokanee population on the lake (MFLNRO 2016a). Obtaining estimates of juvenile production from the low abundance of spawners is expected to be highly informative in understanding the population dynamics of these unique trout population. Such information will provide necessary data in developing a SR relationship for this ecotype and assist fisheries managers in future using an abundance based management framework, similar to that for Steelhead in BC (MFLNRO 2016b). Project objectives: - Estimate the spring abundance of age-1 fish by year. - Estimate the egg deposition. - Estimate the stock-recruitment relationship between the egg deposition and the abundance of age-1 recruits the following spring. - Estimate a Limit Reference Point (LRP) for the egg deposition (and number of spawners). - Estimate the maximum reproductive rate (spawners per spawner at low density).
Surveys conducted in the spring of 2018 covered approximately 8% Lardeau River and 2% of the Duncan River, much lower than in previous years.
In 2018, age 1 abundance for the Lardeau and Duncan rivers was estimated to be 97,967(95% CRI 67,939-142,639), a substantial increase from the revised estimates of 58,000 in 2017. Spawner escapements at Gerrard using area under the curve (AUC) from daily counts have provided an index of abundance since 1961. The 2018 juvenile recruitment estimates are the progeny from the 2017 spawn of Gerrard Rainbow Trout which was estimated to be 252 AUC, well off the record high observed in 2012 of over 1,500 AUC and the lowest since records began in 1961.
Data analysis fitted a Beverton Holt stock-recruit (SR) curve for Gerrard Rainbow Trout based on river recruit and spawner data converted to egg production. Preliminary results suggest no appreciable increase in recruitment in the age 1 juvenile abundance at egg deposition above 1,000,000. Information on the maximal reproductive rate (the number of spawners per spawner at low abundance without fishing indicates the Gerrard stock has a high degree of resilience. The maximum reproductive rate (not accounting for fishing mortality) increased from around 12.5 adults per recruit (log-transformed ~2.5 adults per recruit) in the early to mid-2000s to over 30 adults per recruit (log-transformed ~3.5 adults per recruit) in 2007 before dropping to under 5 adults per recruit (logtransformed ~1.6 adults per recruit) in 2010. The Gerrard stock appears to indicate time variation in in-lake survival that alters the productivity of the stock over time.
Based on the degree of resiliency from the stock, the limit reference point (LRP) for the Gerrard stock is defined as a spawner abundance of 108 AUC (๐0.5Rmax) and/or egg deposition of 326,000 (E0.5Rmax) and defined as the spawner abundance or egg deposition that produces 50% of the maximum recruitment, known as the half saturation constant. Based on the defined reference point, it appears that egg deposition (E0.5Rmax) has fallen below the LRP since 2015.
Surveys conducted in the spring of 2018 covered approximately 8% Lardeau River and 2% of the Duncan River, much lower than in previous years.
In 2018, age 1 abundance for the Lardeau and Duncan rivers was estimated to be 97,967(95% CRI 67,939-142,639), a substantial increase from the revised estimates of 58,000 in 2017. Spawner escapements at Gerrard using area under the curve (AUC) from daily counts have provided an index of abundance since 1961. The 2018 juvenile recruitment estimates are the progeny from the 2017 spawn of Gerrard Rainbow Trout which was estimated to be 252 AUC, well off the record high observed in 2012 of over 1,500 AUC and the lowest since records began in 1961.
Data analysis fitted a Beverton Holt stock-recruit (SR) curve for Gerrard Rainbow Trout based on river recruit and spawner data converted to egg production. Preliminary results suggest no appreciable increase in recruitment in the age 1 juvenile abundance at egg deposition above 1,000,000. Information on the maximal reproductive rate (the number of spawners per spawner at low abundance without fishing indicates the Gerrard stock has a high degree of resilience. The maximum reproductive rate (not accounting for fishing mortality) increased from around 12.5 adults per recruit (log-transformed ~2.5 adults per recruit) in the early to mid-2000s to over 30 adults per recruit (log-transformed ~3.5 adults per recruit) in 2007 before dropping to under 5 adults per recruit (logtransformed ~1.6 adults per recruit) in 2010. The Gerrard stock appears to indicate time variation in in-lake survival that alters the productivity of the stock over time.
Based on the degree of resiliency from the stock, the limit reference point (LRP) for the Gerrard stock is defined as a spawner abundance of 108 AUC (๐0.5Rmax) and/or egg deposition of 326,000 (E0.5Rmax) and defined as the spawner abundance or egg deposition that produces 50% of the maximum recruitment, known as the half saturation constant. Based on the defined reference point, it appears that egg deposition (E0.5Rmax) has fallen below the LRP since 2015.
Resources Data:
Name: COL-F19-F-2703_FINAL_REPORT-MFLNRORD-GERARD_RAINB_1568747924976_8745244559
Format: PDF
URL: http://a100.gov.bc.ca/appsdata/acat/documents/r57434/COL-F19-F-2703_Final_Report-MFLNRORD-Gerard_Rainb_1568747924976_8745244559.pdf
Additional Info
Study Years: 2017
Published: 2018
Determination of Gerrard Rainbow Trout Stock Productivity at Low Abundance-2018 (COL-F19-F-2703). Prepared for the Fish and Wildlife Compensation Program, the Habitat Conservation Trust Foundation
Author: G.F. Andrusak, Thorley J.L
Tags: Abundance, Duncan River, FWCP, Gerrard Rainbow Trout, HCTF, Juvenile Recruitment, Lardeau River, Oncorhynchus Mykiss, Population, Stock Assessment
This study focused on obtaining information on the recruitment of Gerrard Rainbow Trout under low stock abundance. Stock abundance (spawner numbers) is expected to be at or near record lows over the next 3-5 years, due to the near collapse in the Kokanee population on the lake (MFLNRO 2016a). Obtaining estimates of juvenile production from the low abundance of spawners is expected to be highly informative in understanding the population dynamics of these unique trout population. Such information will provide necessary data in developing a SR relationship for this ecotype and assist fisheries managers in future using an abundance based management framework, similar to that for Steelhead in BC (MFLNRO 2016b).
Project objectives:
- Estimate the spring abundance of age-1 fish by year.
- Estimate the egg deposition.
- Estimate the stock-recruitment relationship between the egg deposition and the abundance of age-1 recruits the following spring.
- Estimate a Limit Reference Point (LRP) for the egg deposition (and number of spawners).
- Estimate the maximum reproductive rate (spawners per spawner at low density).
Summary
Surveys conducted in the spring of 2018 covered approximately 8% Lardeau River and 2% of the Duncan River, much lower than in previous years.
In 2018, age 1 abundance for the Lardeau and Duncan rivers was estimated to be 97,967(95% CRI 67,939-142,639), a substantial increase from the revised estimates of 58,000 in 2017. Spawner escapements at Gerrard using area under the curve (AUC) from daily counts have provided an index of abundance since 1961. The 2018 juvenile recruitment estimates are the progeny from the 2017 spawn of Gerrard Rainbow Trout which was estimated to be 252 AUC, well off the record high observed in 2012 of over 1,500 AUC and the lowest since records began in 1961.
Data analysis fitted a Beverton Holt stock-recruit (SR) curve for Gerrard Rainbow Trout based on river recruit and spawner data converted to egg production. Preliminary results suggest no appreciable increase in recruitment in the age 1 juvenile abundance at egg deposition above 1,000,000. Information on the maximal reproductive rate (the number of spawners per spawner at low abundance without fishing indicates the Gerrard stock has a high degree of resilience. The maximum reproductive rate (not accounting for fishing mortality) increased from around 12.5 adults per recruit (log-transformed ~2.5 adults per recruit) in the early to mid-2000s to over 30 adults per recruit (log-transformed ~3.5 adults per recruit) in 2007 before dropping to under 5 adults per recruit (logtransformed ~1.6 adults per recruit) in 2010. The Gerrard stock appears to indicate time variation in in-lake survival that alters the productivity of the stock over time.
Based on the degree of resiliency from the stock, the limit reference point (LRP) for the Gerrard stock is defined as a spawner abundance of 108 AUC (๐0.5Rmax) and/or egg deposition of 326,000 (E0.5Rmax) and defined as the spawner abundance or egg deposition that produces 50% of the maximum recruitment, known as the half saturation constant. Based on the defined reference point, it appears that egg deposition (E0.5Rmax) has fallen below the LRP since 2015.