1) The study examined changes over time in the body size of walleye in over 350 lakes in Wisconsin using data on over 100,000 fish collected between 1987-2012. 2) They found that while the body size of juvenile walleye had increased on average due to declining population densities and warmer temperatures extending the growing season, the body size of adult walleye had decreased slightly over the same time period. 3) Selective harvest played a minimal role in influencing body size variation, and the decline in adult size was possibly due to an energetic trade-off of allocating more resources to increased juvenile growth and earlier reproduction.

1. Selective harvest effects on body size of
exploited walleye stocks in north temperate lakes
1Daisuke Goto, 2Andrew Rypel, 1Jereme Gaeta, 2Gretchen
Hansen, 2Greg Sass, and 1Jake Vander Zanden.
1Center for Limnology, University of Wisconsin-Madison
2Wisconsin Department of Natural Resources (WDNR)
Photo credit: Alex Latka

2. Patterns and consequences of selective harvest
Patterns
• Size-selective harvesting is widespread in
(commercial and recreational) fisheries.
how we fish (e.g., fishing gear selectivity)
how we manage (e.g., minimum fish size limits)
 Skewed size distribution in harvested stocks.
Photo credits: Alex Latka and greatlakesmapping.org
Consequences
Change in life history traits;
e.g., body size-at-age
age-at-maturity
fecundity
recruitment
 Change in population dynamics and productivity.
?

3. Selective harvest in Wisconsin (WI) walleye stocks
100 300 500 700 800
Numberoffish
0
100
200
300 Pre-harvest
Post-spearing
Post-angling
Mixed fisheries of walleye
(Annual total “safe harvest” level
– <35%)
1. Tribal fishery (spearing)
• No min. size limit.
• low harvest and low size-
selectivity
2. Recreational fishery (angling)
• min. size limit for some stocks.
• high harvest rate and high size-
selectivity.
WI walleye size distributions before
& after harvest
Wisconsin, USA
Heavily harvested stock
(total fishing mortality, F = 0.33)

4. Research objectives
1. How did body size of exploited walleye vary among
stocks over time?
2. How did changing population size, climate, and harvest,
influence body size variation of walleye stocks?

5. Data
Fishery-independent surveys:
• Annual surveys in spring by WNDR
• Fyke-netting in spring (after ice out)
at ~15-30 per year
• N of lakes = >350
• N of years = 26 (1987 – 2012)
• N of fish = >100,000
Individual-level (intrinsic) data:
• Body size (total length)
• Sex
• Age (scales for <300 mm
and spines for >300 mm)
• Stock type (stocked or
naturally reproduced)
Wisconsin, USA
Approach: multilevel linear mixed effects modeling

6. Approach: multilevel linear mixed effects modeling
ln(body
length)
ln(age)
sex
stock type
(natural or stocked)
growth year
e.g., ln(age) x sex
[lake]
~
[year class]
+ +
Fixed effects 2-way interactions Random effects
intercept:
slope: [ln(age)]
• Model selection – 1) restricted maximum likelihood (REML) for random
effects; then 2) maximum likelihood (ML) for fixed effects.
• Deviance information criterion (DIC) >4.
Model
A baseline model with individual-level (intrinsic) covariates – Full model:

7. Results: multilevel linear mixed effects modeling
ln(body
length)
ln(age)
sex
growth year
ln(age) x sex
[lake/
~
year class]
+ +
2-way interactions Random effectsFixed effects
intercept:
slope: [ln(age)]
ln(age) x growth year
• Body length-at-age changed over time, which varies among age classes.
• No stocking effect on body length-at-age.
stock type
(natural or stocked)
Model
A baseline model with individual-level (intrinsic) covariates – Best model:

8. Results: observed and projected temporal shifts in
lengths-at-age of walleye stocks
1990
2000
2010
Female Male
1990
2000
2010
356 lakes; 100,563 individuals; 26 years
• Body size of juvenile increased over time.
• Adult male and female body size decreased over time.
Adult
(–)
Adult
(–)
Juvenile
(+)
Juvenile
(+)

9. Lake-level (extrinsic) drivers of walleye body size
variation
Walleye density
1. Density dependence
• Population estimate per lake area
 Declining density over time

10. Results: projected body lengths – temporal shift in
density effect
Age 4 (juvenile) Age 8 (adult) Age 12 (adult)
• Stronger density dependence in recent years for all fish (e.g., declining
prey availability).
Female
Male
Growth year: – 1990 – 2000 – 2010

11. Lake-level (extrinsic) drivers of walleye body size
variation
Walleye density
Growing degree days
1. Density dependence
• Population estimate per lake area
 Declining density over time
2. Climate (growing degree days, GDD)
• Base temperature = 5 ºC
 Increasing growth season over time

12. Results: projected body lengths – temporal shift in
growth season effect
• Increasing growth season in recent years larger juvenile and adult body
size.
Female
Male
Growth year: – 1990 – 2000 – 2010
Age 4 (juvenile) Age 8 (adult) Age 12 (adult)

13. Lake-level (extrinsic) drivers of walleye body size
variation
Walleye density
Growing degree days
Harvest rate
1. Density dependence
• Population estimate per lake area
 Declining density over time
2. Climate (growing degree days, GDD)
• Base temperature = 5 ºC
 Increasing growth season over time
3. Fishing (annual creel surveys)
A. Tribal spearing
B. Total angling
C. Angling for large fish (20” or 508 mm) only
 little change over time
F = ~0.14

14. Results: projected body lengths – temporal shift in
harvest effect
• No spearing or total angling effect on body length of all age classes.
• Very weak effect of angling of large (>508 mm) fish on body length variation.
Female
Male
Growth year: – 1990 – 2000 – 2010
Age 4 (juvenile) Age 8 (adult) Age 12 (adult)

15. Results: projected body lengths – density x growth
season effects
• Change in growth season does not affect density dependence on body size
variation of all age classes.
GDD: – Min. (1097 degree-days) – Median (1655 degree-days) – Max. (2189 degree-days)
Female
Male
Age 4 (juvenile) Age 8 (adult) Age 12 (adult)

16. Results: projected body lengths – density x harvest
effects
• Increasing harvest rate  larger juvenile body size at low density (
indirect harvest effect).
• Weak harvest effect on density dependence for adults.
Harvest (>20” angling) rate: – Min. (F = 0.001) – Median (F = 0.087) – Max. (F = 0.82)
Female
Male
Age 4 (juvenile) Age 8 (adult) Age 12 (adult)

17. Conclusions
Juvenile walleye body size
• Mean body size of juvenile walleye in Wisconsin lakes has been
increasing.
• Declining density and warmer climate are correlated with this trend.
• Size-selective harvest may have indirectly influenced body size variation.
Juvenile
1990 2000 2010
Declining density Warmer climate
Size-selective harvest
(+) (+)
(–)

18. Conclusions
Adult
Declining density Warmer climate
Size-selective harvest
1990 2000 2010
(+) (+)
(–)
?
?
Adult walleye body size
• Mean body size of adult walleye in Wisconsin lakes has been declining.
• Little effect of selective harvest on body size variation.
• Potential mechanism – an energetic trade-off (?)
Increasing juvenile body size (observed)
 earlier maturation + more reproductive investment (?).
?

19. Acknowledgements
 Tom Cichosz (WDNR)
 Alex Latzka (CFL, UW-Madison)
Funding
Wisconsin Department of Natural Resources (WDNR)
U.S. Geological Survey (USGS)