A field energy budget for northern pike, an aquatic piscivore

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. 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 Energy budget

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

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

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

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

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

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

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

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