U.S. Department of the Interior

U.S. Geological Survey

Brown trout population modeling

H.2 RBT recruitment and BNT modelling ($7K)

Using data collected by AZGF and TRGD as part of 

project H

Thanks to Josh Korman, Laura Tennant, Jan Boyer and 

Michael Yard

Resource Goals: Invasive Species

Annual Reporting

January 21, 2021
Outline

 Brown trout continue to increase.

 The competitive interference hypothesis for 

recruitment may have legs.

 Evidence for another immigration event coincident 

with 2018 fall high flow event.

 Influence of data reduction for BNT model.

 Rainbow trout recruitment model update.
Basic modelling framework

• Fit to cpe data 

(2000 – present) 

and markrecapture data 

(2012-present)

• 1 mark-recapture 

site (2012-2016), 

3 sites (2017-

2020), 2 sites 

(FY21)...

• Gap in markrecapture data 

when culling 

required.
Modelling assumptions

 Seasonal time step.

 Size and seasonal variation in growth and 

survival (but not among years).

 Survival informed by a Lorenzen relationship.

 Capture probability allowed to vary by trip 

and size class (random effect).

 Immigration for large adults allowed to vary 

for each interval (random effect).

 Recruitment varies between years (random 

effect).
Somewhat good news: Abundance 

of largest brown trout stable over 

last ~2 years.

(Preliminary, 

do not cite)
But increasing abundance of size 

class 2 (150 – 350 mm) brown trout.

(Preliminary, 

do not cite)
And recruitment keeps trending 

upwards (these are fall estimates).

(Preliminary, 

do not cite)
So, what is driving year to year 

variation in recruitment?

 Allee

 Temperature

 High flow experiments

 Spawner interference

https://youtu.be/sUcpTQcTvMg?t=36

(Runge et al., 2018)
Reproductive rate: Number of brown 

trout in the fall per small adult in 

preceding winter (x4 for large adults)
Quantifying RBT spawner

abundance

 Not necessarily correlated with RBT total 

abundance.

 Not necessarily correlated with RBT 

recruitment.

 Recall showed relationship between 

condition factor and maturity.
Our best guess of spawner N as of right now, courtesy 

of Josh, based on integrating cpe and mark recap data.
Moderate to strong negative correlation 

depending on how you analyze the relationship.
What about immigration?

 We have known for a while that there was a 

large immigration event coincident with 2014 

fall HFE, but other competing hypotheses 

(see Runge et al., 2018).

 Now there is some indication of a second 

immigration event coincident with 2018 fall 

HFE.

A brief history

 TWG / GCMRC leadership proposed to eliminate two 

mark-recapture sites and maintain cpe monitoring 

over other monitoring designs for brown trout.

 TWG requested biologists meet to integrate cpe and 

mark-recapture and find a way to monitor two sites.

 Two meetings, not a ton of progress/concessions. 

Some discussion of need for pseudo-power analysis 

were effects of different designs on brown trout 

model output are considered.
There are some potential issues.

All data Drop 1 MR Drop 2 MR

Drop 2 MR

Drop cpe

Drop 1 MR

Drop cpe Drop cpe

For the most part, brown trout 

piscivory does not appear to be 

affecting RBT recruitment yet.

Questions

cyackulic@usgs.gov