1. Fish and Zooplankton Interactions Wetzel Chapter 16, pp. 460-468

2. Interactions between Fish and ZP Predation Alteration of trophic structures Nutrient cycling

3. Feeding Relationships Among Fish Feeding defined: acquisition of both energy and nutrients for growth and reproduction Cost/Benefit Analysis: do the costs of food acquisition exceed energy gained from consumption – Maximize return per unit effort

4. Prey Density and Diet Shifting If prey are large and scarce, fish consume them one at a time If prey are small and abundant, fish will filter water column – Mechanical Sieving Obviously, these statements apply to fish capable of doing this and of the correct trophic status – i.e. adult walleye will not do this, but walleye larvae will

5. Ontogenetic Diet Shift in Yellow Perch – Wu and Culver 1992 As zooplankton occur less and less frequently, yellow perch young-of-year switch from zooplankton to benthic macroinvertebrates This was found both in the lab and in nature Function of the development of digestive organs

6. Ontogenetic Diet Shift

8. Predation of ZP by Fish Study from Liu and Herzig –Distribution in lake of possible prey

9. Distribution of Prey Species

10. Predation of ZP by Fish Study from Liu and Herzig –Distribution in lake of possible prey –Selectivity of larger sized individuals Prey sizes present in gut vs. prey sizes in nature

11. Selectivity of Large Individuals

12. Predation of ZP by Fish Study from Liu and Herzig –Distribution in lake of possible prey –Selectivity of larger sized individuals Prey sizes present in gut vs. prey sizes in nature –Selectivity vs. Length

13. Selectivity vs. Length

14. Predation of ZP by Fish Study from Liu and Herzig –Distribution in lake of possible prey –Selectivity of larger sized individuals Prey sizes present in gut vs. prey sizes in nature –Selectivity vs. Length –Selectivity vs. Eye Diameter

15. Selectivity vs. Eye Diameter

16. Predation of ZP by Fish Study from Liu and Herzig –Distribution in lake of possible prey –Selectivity of larger sized individuals Prey sizes present in gut vs. prey sizes in nature –Selectivity vs. Length –Selectivity vs. Eye Diameter Conclusion: Prey visibility and parts that are visible make a difference

17. Predation of ZP by Fish Predation by fish can shape communities of zooplankton Relationship between size selection and foraging efficiency Ex: Bluegill (p. 462 in Wetzel & refs therein) –Selects prey on basis of size –Growth rates increase with increasing food size Increased efficiency (higher return with large prey) –Size selection based on search time and handling time –When there is low density of prey, more likely to take prey of any size –As prey density increased, search time decreased and small classes are consumed less frequently

18. Predation of ZP by Fish Another example of Size Selectivity Crystal Lake, Connecticut (Brooks and Dodson, 1965) No alewife – large zp Alewife introduced – shift to smaller zp But why were larger present in the first place and no coexistence of all sizes?

19. Size Efficiency Hypothesis When size-selective predation not occurring in lakes, small zooplankton not found to occur with larger zooplankton The Hypothesis: (1)Planktonic herbivore zp compete for small food particles in the pelagic zone of a lake (2)Larger zp filter more efficiently and can take larger particles – this allows for reduced metabolic maintenance demands allowing for more devotion of energy to reproduction

20. Size Efficiency Hypothesis (3) Therefore, larger forms come to dominate (4) However, if fish size-selective predation is intense, larger forms are selected to be eaten, allowing smaller forms to dominate (5) If predation pressure is moderate, large forms are reduced and this allows coexistence of forms -Some evidence for this – Threshold food concentrations -Amount of food to satisfy metabolic upkeep decreases with increasing body sizes -In Cladocerans, Gliwicz 1990; however, rotifers may not follow the same patterns

21. Food Limitations in ZP If food is limiting, body sizes and mass are reduced and reproduction is minimal Interaction of food limitation and size selective feeding shapes the community –Which is more important depends on lake characters

22. Conclusions Fish and ZP interact through predation How this actually shapes the communities is dependent of many different forces