December 26, 2020

THE MIDDLE BLUE RIVER STOCKING HISTORY

Ever wondered how/why the Middle Blue River is stocked?  Gold Metal status in Silverthorne is maintained via brood cull stocking which means stocking smaller numbers of large fish raised in a hatchery, generally 14 inches or longer.  Colorado Parks & Wildlife drops 700-800 big fish in twice a year and this is a point of contention amongst a lot of anglers.  Although, there happens to be a lot more to this story that we must consider before jumping to conclusions. 

Wade electrofishing CPW Report Page 1

Jon Ewert, CPW fisheries biologist, completed a Blue River Fishery Management Report focused on the stretch below Dillon Reservoir based on annual electrofishing surveys conducted from 2010 to 2017.  I did my best to summarize this report below and highly encourage everyone to take some time and read the whole thing.  

How to identify if a rainbow trout was hatchery raised?

"The white tips and excellent condition of the dorsal and anal fins on this 2017 rainbow suggest that it was not raised in a hatchery," (CPW Report, pg 10).

"The tail fin of this larger rainbow captured in 2017 is in excellent condition, with straight fin rays coming to clean points. Fish raised to adult sizes in hatcheries rarely have fins this clean, suggesting that the origin of this fish is likely a past fingerling plant," ((CPW Report, pg 10).

What types of fish are stocked in The Blue River?

  Mainly fingerlings (3-4”) and brood  (>14”) rainbow trout.  The brown trout are a wild/self sustaining fish population. 

Data from CPW Report Page 2

Fingerling Trout Timeline 

Fingerlings stocked in 2010 - 2012.

CPW stopped stocking 10” rainbows in 2011 to better monitor fingerlings.

In 2012, CPW found minimal growth in the 2010/2011 fingerlings and a void in the 9-12” size range.  They also observed a very large number of the 2010/2011 fingerlings stacked up on each other.

CPW decides to omit fingerlings in 2013 to prevent further competition all these smaller fish to grow 

CPW begins stocking fingerlings again in 2018 after discovering a higher number of high-quality rainbow trout captured in 2017.  CPW believes the origin of these fish was not a hatchery suggesting the fingerlings that disappeared in 2012 may have survived. 

The largest rainbow from campground reach in 2017, 23.3" (CPW Report, Page 9)

Brood Trout Timeline

All brood fish stocked between 2011 and 2015 have clipped adipose fins.

December 2015 and April 2016 brood calls have no clips or marks.

2017 brood fish are marked with a clipped left pelvic fin.

Based on 2017 surveys, an overwhelming number of rainbow trout originated from brood stocking

Fish stocked since 2018 have not been marked because CPW can now identify individual plants without these markings

Brood Trout are the only type of fish stocked here in town.

"The largest fish captured in 2014. 23”, 4.2 pounds. Interestingly, this fish appeared to be of hatchery origin but was not marked," (CPW Report, Page 5).

Special Events Worth Noting

1.  2012 saw an unusual number of larger brown trout, attributed to fingerling stocking.

2.  All water released from Dillon Reservoir in 2014 was bottom release.  2014 was the only year when several fish were captured and exceeded their hatchery weight suggesting a larger food supply via mysis shrimp.  All fish captured in previous years were lighter than their hatchery weight.  

3.  Higher number of high-quality rainbow trout were captured in 2017.  CPW believes the origin of these fish was not a hatchery suggesting the fingerlings that disappeared in 2012 may have survived. 

Closing Thoughts

So what does this all mean?  It is still unclear whether the fingerling stocking is effective but it is promising that CPW started stocking them again in 2018.  It is clear that brood fish are not maintaining their weight, nor growing. These brood fish are the main reason we have maintained Gold Medal Status in town. 

In summary, fish stocking in this stretch is necessary for the fishery under current conditions.  However, developing a self-sustaining rainbow trout population in the process is a huge uphill battle.  One of, if not the biggest, issues is there is not enough food to sustain these fish.  Why is that you may ask?  Keep an eye out for our next Blue River Watershed blog focused on CFS and water temperature. 

Best, 

Ben McCormick