1. A field energy budget for northern pike, an aquatic piscivore
James S. Diana
School of Natural Resources and Environment
University of Michigan

2. Philosophical debate
A man has only enough time to do what he truly thinks is necessary (Goethe)
An animal only has sufficient energy to do what is important to improve its fitness
Evolutionary fitness = maximize production of successful offspring
Measures of fitness = number of eggs produced, number of spawnings, growth rate
Basic theoretical constraint behind energy budgets, which are believed to be highly evolved

3. Energy budget Really defines how an animal makes a living
Can parallel it to a bank account
Paycheck = amount of food eaten
Uses = body maintenance, activity, growth, reproduction
Can borrow on the short term from energy reserves in lipids, body protein, etc.
On long term – has to balance, no loans

4. Bioenergetic models
Take known physiological information, along with growth rate, prey types, and temperature of an ecosystem/species to predict food consumption by prey type
Unless ration is also measured in field, there is no way to corroborate predictions
Usually assumes something regarding fish activity, for example, no cost of activity or activity doubles metabolic rate
Used widely in fishery management

5. Energy budget for pike in Lac Ste. Anne
We set out to determine all components of pike energy budgets in order to evaluate growth dynamics of pike and growth-reproduction tradeoffs
Measured growth, activity, and ration in field, metabolism, feeding efficiency, and digestion costs in lab at field temperatures
Then applied to test fit of model to real data, and evaluate reasons for errors

6. Growth methodology
Collect and sacrifice fish over regular periods of summer (monthly) and winter (every 2-3 months)
Gillnets as collection method
Only feasible method for winter collection
Not very size selective for pike because they mainly catch by their teeth
Evaluated seasonal dynamics for 3-year-old fish, annual values for ages 0-4

7. Pattern of pike growth

8. Pike pattern Males and females grow in body over summer
Females grow in gonads over winter, males in body
Ovary growth much higher than testicular growth
Overall females grow faster than males, must eat more

9. Age
Body kcal
Gonad kcal
Males
554 4 1
372 19 2
284 18 3
177 22
541? 34
Females
376
141
268
373
436
278
454

10. Ration methods Determine stomach contents and number of empty stomachs
Pattern = asynchronous feeding with no diel pattern
At any time, meal frequency is percent empty related to digestion time, fish with food estimate meal size
Coupled with lab data at each temperature on digestion rate
Ration = meal size divided by meal frequency

11. Feeding pattern

12. Diet and ration contribution
Species
Number eaten
Calories eaten
Perch
970 (69%)
6286 (54%)
Spottail shiner
322 (24%)
829 (7%)
Burbot
71 (5%)
1313 (11%)
Sucker
29 (2%)
2592 (22%)
Whitefish
3 (0%)
140 (1%)
Walleye
2 (0%)
366 (3%)
Pike
1 (0%)
23 (0%)

13. Size of food important Shiners and perch numerous but small
Suckers and burbot rare but large
Contribute over 1/3 of annual consumption

14. Daily rations Month Sex Meal Size (kcal/kg) Time between meals (days)
Daily ration (kcal/kg/d)
May
Male
30.4
3.1
9.6
Female
32.4
2.3
14.0
June
35.0
1.9
18.1
66.5
2.2
30.9
July
36.5
2.1
11.5
54.1
2.8
19.2
August
23.1
3.8
6.0
25.4
2.6
9.8
September
22.5
3.5
6.4
31.4
4.2
7.5
October
17.4
7.9
16.5
8.6
January 34
0.3
22.0 23
1.0
March
10.9 22
0.5
21.6 26
0.8
April
14.8 59

15. Ration results Females eat more than males (17.4 vs. 11.4)
Highest consumption in spring (30-18)
Spawning fast in April
Low but significant consumption all winter

16. Telemetry Surgically implanted transmitters
Followed fish using boats and hydrophones
Had to use shore landmarks and compasses for location

17. Northern pike movements
Moved largely over nearshore zone
Returned to similar locations at time
Home range? – if so very large
Did use specific habitats

18. Distances moved

19. Pike habitat

20. Pike activity methods Measure regularly from multiple points
Determine locations over short time intervals
Can evaluate activity pattern and swimming speeds
Could also use buoy array or other new methods

21. Pike diel activity

22. Activity summary Fish were commonly inactive, sit-and-wait predators
No displacement over 80% of the intervals observed
When moved, generally moved rather slowly but constantly
Most likely the cost of activity is negligible in an energy budget

23. Overall energy budget balance
Calculate ration from observations, compare to ration predicted from Wisconsin bioenergetics model
Evaluate errors and determine fit
Evaluate reason for errors

24. Budget balance

25. Budget balance Lots of variation in summer, but correct overall trend
Error most likely due to errors in ration estimate
For next part, accept that models of metabolism and measured growth are accurate

26. Pike age-related costs
Growth
Reproduction
Maintenance
Males 0
558 (42%)
776 (58) 1
137 (8)
77 (4)
1606 (88) 2
238 (10)
108 (5)
1992 (85) 3
192 (7)
94 (4)
2391 (89)
Females 1
102 (5)
279 (14)
1662 (81)
190 (7)
286 (11)
2081 (81)
287 (8)
549 (16)
2609 (76)

27. Other poor fits - esocids

28. Applying bioenergetics and energetic models
Growth and reproductive tradeoffs
Larger size = more energy for protecting nest, also more capable
Larger size = more fecundity
Older age = less likely to survive to breed
Maturation is a shift of energy away from future growth into current reproduction
Natural selection acts strongly on this

29. Latitude and pike energetics
Growth of pike in Michigan
Variation in winter 3 to 5 months
Similar levels of maximum temperature
Compared growth and maturation across 3 lakes
Found no major differences in growth for fish from each lake

30. Latitude and pike maturation
Murray
Houghton
Vieux Desert
Males
71%
100%
80% 2
94% 3
Females 1
17%
31%
67%

31. Pike maturation Not a clear latitudinal cline
Was related to intensity of fishing
Fishing adds mortality, size selective for older fish, that may reduce frequency of late maturing fish in gene pool

32. Stunting in pike Common pattern in inland lakes
Mature early, grow slowly, all adults reach a terminal size

33. Stunting in pike Common ideas for mechanisms
High density and competition
Warm water and lack of thermal refuge
Lack of large prey?
Perfect system for energetic modeling

34. Temperature profiles

35. Stunting simulations

36. Problems with such simulations
No limits on fish growth, unlike nature
Produces potential growth but not necessarily possible growth

37. Conclusions
Energy budgets can describe major decisions and allocations that have evolved in animals
They require much site specific work to produce a corroborated budget
They can lead to good understanding of the limits to fitness
They can be useful in understanding how animals adapt to environmental challenges