U.S. Department of the Interior 

U.S. Geological Survey 

Population effect of incentivized harvest on Brown Trout 

Charles B. Yackulic1 , Josh Korman2 , David Rogowski3 , Jeff Arnold4 , and Brian Healy1 

1 US Geologic Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center 

2 Ecometric Research 

3 Arizona Game and Fish Department 

4 National Park Service 

Resource Goals: Invasive Species 

Annual Reporting 

January 23, 2024 This information is preliminary and is subject to revision. It 

is being provided to meet the need for timely best science. 

The information is provided on the condition that neither 

the U.S. Geological Survey nor the U.S. Government shall 

be held liable for any damages resulting from the 

authorized or unauthorized use of the information.
 

 

Basic modelling framework 

 

 

Size Class 1 

< 150 mm 

• 

Size Class 2 

150-350 mm 

• 

Fit to catch per 

unit effort data 

(2000 – present) 

and markrecapture data 

(2012-present) 

1 mark-recapture 

site (2012-2016), 

3 sites (2017-

2020), 2 sites 

(2021 – present). 
Modelling assumptions 

 Seasonal time step. 

 Size and seasonal variation in growth. 

 Temporal variation in survival (random effect) 

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). 
 

 

 

 

Incentivized harvest has 

been ongoing for over three 

years and increased 

substantially in the last year. 

Preliminary data, do not cite. 
 

 

 

We can see changes in adult brown trout 

abundance estimates over last year 

Preliminary data, do not cite. 
Harvest 101 

 Is mortality additive or compensatory? 

 Is recruitment weakly or strongly density 

dependent? 
Survival

Compensatory 

0 0 

But are declines entirely because of harvest? 

Harvest rate 
 

 

 

Survival 

And use this to estimate what survival would have 

been without harvest (assuming additive model) 

and then compare to prior years without harvest. * 

* 

* We can estimate survival and harvest 

0 Harvest rate 

0 

But are declines entirely because of harvest? 
We see lowered survival of large adult 

Brown Trout over last year 

Preliminary data, do not cite. 
 

Which appears to be mostly additive 

Observed 

Back calculated (harvest removed) 

Preliminary data, do not cite. 
 

 

 

However, for small adults, there appears 

to have been fairly strong compensation 

Observed 

Back calculated (harvest removed) 

Preliminary data, do not cite. 
 

Little evidence of density dependent recruitment 

(at moderate spawner abundances) 

2023 

Preliminary data, do not cite. 
 

 

 

What does this mean for asymptotic 

Brown Trout population growth (lambda)? 

Observed 

Back calculated (large 

adult harvest removed) 

Preliminary data, do not cite. 
 

 

 

Take home messages 

 Adult Brown Trout population declined 

significantly in 2023, a significant portion of 

which can be attributed to incentivized harvest. 

 Rigorous analysis of incentivized harvest (and 

other removal efforts) benefits from accurate 

estimates of vital rates (recruitment, survival, 

etc.) 
Questions 

cyackulic@usgs.gov