11-18-2021, 07:01 PM
(This post was last modified: 11-18-2021, 07:18 PM by wiperhunter2.)
Here is a link to the stocking numbers in Oakley in recent years:
https://idfg.idaho.gov/ifwis/fishingPlan...9388421673
Here is some info on gill netting at Oakley is 2016:
The Walleye Sander vitreus population in Oakley Reservoir was assessed using a Standard Fall Walleye Index Netting (FWIN) survey on October 5 and 6, 2016. An effort of seven net-nights yielded a total catch of 244 Walleye. Catch per unit effort was 35 + 15 (95% CI). The PSD of the catch was 7 + 3 (95% CI). Stock density of the catch was 78, 4, 0, 0, and 0 % for RSD-P, RSD-M, and RSD-T, respectively. A subsample of the catch was aged (n = 195). Eleven age classes were present and ranged from 2 to 13 years. Mean relative weights for each Walleye stock size were 81, 89, 100, 0, and 0 % for stock-, quality-, preferred-, memorable-, and trophy-sized Walleye, respectively. Relative weights of female and male Walleye were 81. Visceral fat indices were 2.3 for males and 1.8 for females. Gonadal somatic indices for males and females were 2.9 and 1.87, respectively. Maturity rates were for males and females were 43% and 4%, respectively. Annual mortality of Walleye (sexes combined) based on weighted catch curve analysis was 13%. The overall FWIN ranking was 1.75 on a scale of 1-3, which classifies the population as “unhealthy and unstable” to “stressed and unstable”.
Oakley Reservoir is managed as a mixed-species fishery that includes Rainbow Trout Oncorhynchus mykiss, Yellow Perch Perca flavescens, and Walleye Sander vitreus. Other species present include sculpin Cottus sp., Largescale Sucker Catastomus macrocheilus, and Spottail Shiner Notropis hudsonius. Spottail Shiner was introduced in 1989 to provide additional Walleye forage. Oakley Reservoir is one of only three waters in Idaho managed to provider a Walleye fishery. As such, the Walleye population at Oakley Reservoir is monitored at 3-5 year intervals. The objective of this survey was to resample Oakley Reservoir and compare FWIN results to the established average baseline set by FWIN sampling in 2007-2009.
Standard Fall Walleye Index Netting (FWIN, Morgan 2002) protocol described in the Manual of Instructions – Fall Walleye Index Netting guided sampling efforts on Oakley Reservoir in 2016. Based on a maximum reservoir surface area, target sample size was 16 gill-net nights. A biological threshold of 300 Walleye was set prior to sampling. Sampling was discontinued when either sample size or biological threshold were met. Gill nets were eight panel monofilament nets 1.8-m deep, 61.0-m long, with 7.6-m panels measuring 25-mm, 38-mm, 51- mm, 64-mm, 76-mm, 102-mm, 127-mm, and 152-mm stretched mesh. Net locations were randomly selected and are listed in Appendix A. Net sets were equally split between two depth strata including 2 – 5 m and 5 – 15 m. All nets were placed perpendicular to the shoreline. Netting was conducted when water temperatures were between 10 and 15 °C.
A total of 244 Walleye were sampled using seven net-nights, resulting in a CPUE of 35 + 15 fish/net-night (95% CI). Total length of sampled Walleye ranged from 144 to 541 mm (Figure 12). The PSD of the catch was 7 + 3 (95% CI). Stock density of the catch was 78, 4, 0, 0 and 0 % for RSD-P, RSD-M, and RSD-T, respectively (Figure 13). A subsample of the catch was aged (n = 195). Eleven age classes were present, and ages ranged from 2 to 13 years (Figures 14- 15). Weights ranged from 22 to 1,845 g. Mean relative weights for each Walleye stock size were 81, 89, 100, 0, and 0 % for stock-, quality-, preferred-, memorable-, and trophy-sized Walleye, respectively. Relative weights of female and male Walleye were 81%
ll decreased rating in population health and stability, an increased CPUE, a drastic decline in Walleye > 450 mm, absence of preferred, memorable and trophy sizes, a substantial decrease in the maximum age of walleye, as well as a noticeable decrease in VFI and GSI indices. Based on catch and length-at-age estimates, the majority of the population is comprised of age 2-6 Walleye and is dominated by the age 3-4 cohorts. Walleye were not stocked during 2012-15 suggesting that natural recruitment has been annual recently. In comparison to the number of age-1 Walleye caught in the 2007-2009 surveys, relatively few <200 mm were collected in 2016 indicating relatively poor recruitment in 2015. However, this notion should considered with caution as sample sizes for this portion of the Walleye population are small and Walleye are not fully recruited to the gear at this length. Growth rates peak around age-6, which differed from the past surveys and potentially indicates a forage-limited situation for younger Walleye. However, growth recovers slightly for the older age classes suggesting available forage in Oakley may exceed the gape size of younger Walleye. A substantial decline in GSI and VSI indices provides further evidence of a potential forage limitations. The relatively high CPUE implies there was good recruitment 3-4 years previous to 2016, when no stocking occurred, with those progeny growing at similar rates as seen in the past survey
Based on these comparisons, the Walleye population in Oakley Reservoir is highly abundant which has led to reduced prey populations, poor Walleye condition, and slow growth rates. Management options to improve habitat for Walleye and their forage in Oakley Reservoir are limited. The reservoir primary purpose is to supply water for crop irrigation. Changes in snow pack and subsequent runoff directly affect the quantity and quality of habitat for Walleye and their forage. Drastic vertical changes in pool elevation can restrict access to critical cover and preferred spawning habitat such as submerged willow Salix sp. for Yellow Perch. Habitat quality is dependent upon annual precipitation and water management. Recovery of forage population are unlikely due to highly-abundant Walleye and inconsistent availability of flood vegetation for perch spawning especially during drought cycles. Forage supplementation would likely be ineffective until Walleye populations are reduced and habitat is improved. In the recent past, IDFG has requested 750,000 Walleye fry to supplement suspected limited natural recruitment in Oakley Reservoir. Ryan et al. (2007) concluded that post-stock fry survival may be variable ranging from 0 to 22% (Ryan et al. 2007). Our analysis seems to contradict this notion. Natural recruitment appears to be consistent as several age classes were present from non-stocking years, including several that appear to be relatively strong based on catch curve residuals. Future stocking numbers and frequency need to be reduced especially until Walleye population abundance declines and until forage populations increase. Collectively, the changing variables documented in this survey describe a high-density Walleye population potentially limited by forage, exhibiting suppressed growth, and with absence of larger-sized fish once found in the fishery. This description is in agreement with the overall ranking (1.75) that classifies the status of the fishery as “unhealthy and collapsed”, but approaching “stressed and unstable”. The fishery
Recommendations
1. Preserve the five-year trend sampling rotation. Resample Oakley Reservoir in 2021. 2. Estimate catch rates and angler exploitation to determine if additive mortality is impacting population size structure and abundance. 3. Reduce stocking numbers and frequency until forage populations increase.
https://idfg.idaho.gov/ifwis/fishingPlan...9388421673
Here is some info on gill netting at Oakley is 2016:
The Walleye Sander vitreus population in Oakley Reservoir was assessed using a Standard Fall Walleye Index Netting (FWIN) survey on October 5 and 6, 2016. An effort of seven net-nights yielded a total catch of 244 Walleye. Catch per unit effort was 35 + 15 (95% CI). The PSD of the catch was 7 + 3 (95% CI). Stock density of the catch was 78, 4, 0, 0, and 0 % for RSD-P, RSD-M, and RSD-T, respectively. A subsample of the catch was aged (n = 195). Eleven age classes were present and ranged from 2 to 13 years. Mean relative weights for each Walleye stock size were 81, 89, 100, 0, and 0 % for stock-, quality-, preferred-, memorable-, and trophy-sized Walleye, respectively. Relative weights of female and male Walleye were 81. Visceral fat indices were 2.3 for males and 1.8 for females. Gonadal somatic indices for males and females were 2.9 and 1.87, respectively. Maturity rates were for males and females were 43% and 4%, respectively. Annual mortality of Walleye (sexes combined) based on weighted catch curve analysis was 13%. The overall FWIN ranking was 1.75 on a scale of 1-3, which classifies the population as “unhealthy and unstable” to “stressed and unstable”.
Oakley Reservoir is managed as a mixed-species fishery that includes Rainbow Trout Oncorhynchus mykiss, Yellow Perch Perca flavescens, and Walleye Sander vitreus. Other species present include sculpin Cottus sp., Largescale Sucker Catastomus macrocheilus, and Spottail Shiner Notropis hudsonius. Spottail Shiner was introduced in 1989 to provide additional Walleye forage. Oakley Reservoir is one of only three waters in Idaho managed to provider a Walleye fishery. As such, the Walleye population at Oakley Reservoir is monitored at 3-5 year intervals. The objective of this survey was to resample Oakley Reservoir and compare FWIN results to the established average baseline set by FWIN sampling in 2007-2009.
Standard Fall Walleye Index Netting (FWIN, Morgan 2002) protocol described in the Manual of Instructions – Fall Walleye Index Netting guided sampling efforts on Oakley Reservoir in 2016. Based on a maximum reservoir surface area, target sample size was 16 gill-net nights. A biological threshold of 300 Walleye was set prior to sampling. Sampling was discontinued when either sample size or biological threshold were met. Gill nets were eight panel monofilament nets 1.8-m deep, 61.0-m long, with 7.6-m panels measuring 25-mm, 38-mm, 51- mm, 64-mm, 76-mm, 102-mm, 127-mm, and 152-mm stretched mesh. Net locations were randomly selected and are listed in Appendix A. Net sets were equally split between two depth strata including 2 – 5 m and 5 – 15 m. All nets were placed perpendicular to the shoreline. Netting was conducted when water temperatures were between 10 and 15 °C.
A total of 244 Walleye were sampled using seven net-nights, resulting in a CPUE of 35 + 15 fish/net-night (95% CI). Total length of sampled Walleye ranged from 144 to 541 mm (Figure 12). The PSD of the catch was 7 + 3 (95% CI). Stock density of the catch was 78, 4, 0, 0 and 0 % for RSD-P, RSD-M, and RSD-T, respectively (Figure 13). A subsample of the catch was aged (n = 195). Eleven age classes were present, and ages ranged from 2 to 13 years (Figures 14- 15). Weights ranged from 22 to 1,845 g. Mean relative weights for each Walleye stock size were 81, 89, 100, 0, and 0 % for stock-, quality-, preferred-, memorable-, and trophy-sized Walleye, respectively. Relative weights of female and male Walleye were 81%
ll decreased rating in population health and stability, an increased CPUE, a drastic decline in Walleye > 450 mm, absence of preferred, memorable and trophy sizes, a substantial decrease in the maximum age of walleye, as well as a noticeable decrease in VFI and GSI indices. Based on catch and length-at-age estimates, the majority of the population is comprised of age 2-6 Walleye and is dominated by the age 3-4 cohorts. Walleye were not stocked during 2012-15 suggesting that natural recruitment has been annual recently. In comparison to the number of age-1 Walleye caught in the 2007-2009 surveys, relatively few <200 mm were collected in 2016 indicating relatively poor recruitment in 2015. However, this notion should considered with caution as sample sizes for this portion of the Walleye population are small and Walleye are not fully recruited to the gear at this length. Growth rates peak around age-6, which differed from the past surveys and potentially indicates a forage-limited situation for younger Walleye. However, growth recovers slightly for the older age classes suggesting available forage in Oakley may exceed the gape size of younger Walleye. A substantial decline in GSI and VSI indices provides further evidence of a potential forage limitations. The relatively high CPUE implies there was good recruitment 3-4 years previous to 2016, when no stocking occurred, with those progeny growing at similar rates as seen in the past survey
Based on these comparisons, the Walleye population in Oakley Reservoir is highly abundant which has led to reduced prey populations, poor Walleye condition, and slow growth rates. Management options to improve habitat for Walleye and their forage in Oakley Reservoir are limited. The reservoir primary purpose is to supply water for crop irrigation. Changes in snow pack and subsequent runoff directly affect the quantity and quality of habitat for Walleye and their forage. Drastic vertical changes in pool elevation can restrict access to critical cover and preferred spawning habitat such as submerged willow Salix sp. for Yellow Perch. Habitat quality is dependent upon annual precipitation and water management. Recovery of forage population are unlikely due to highly-abundant Walleye and inconsistent availability of flood vegetation for perch spawning especially during drought cycles. Forage supplementation would likely be ineffective until Walleye populations are reduced and habitat is improved. In the recent past, IDFG has requested 750,000 Walleye fry to supplement suspected limited natural recruitment in Oakley Reservoir. Ryan et al. (2007) concluded that post-stock fry survival may be variable ranging from 0 to 22% (Ryan et al. 2007). Our analysis seems to contradict this notion. Natural recruitment appears to be consistent as several age classes were present from non-stocking years, including several that appear to be relatively strong based on catch curve residuals. Future stocking numbers and frequency need to be reduced especially until Walleye population abundance declines and until forage populations increase. Collectively, the changing variables documented in this survey describe a high-density Walleye population potentially limited by forage, exhibiting suppressed growth, and with absence of larger-sized fish once found in the fishery. This description is in agreement with the overall ranking (1.75) that classifies the status of the fishery as “unhealthy and collapsed”, but approaching “stressed and unstable”. The fishery
Recommendations
1. Preserve the five-year trend sampling rotation. Resample Oakley Reservoir in 2021. 2. Estimate catch rates and angler exploitation to determine if additive mortality is impacting population size structure and abundance. 3. Reduce stocking numbers and frequency until forage populations increase.