A few issues back we examined some factors affecting natural feeding in predator fish, specifically smallmouth bass. Some common factors affecting feeding are turbidity (water clarity), cover (plants and wood, etc.), temperature, weather, water quality, and other stressors.
On a larger, more basic scale, we did not examine what affects the prey fish that smallmouth bass consume, such as shads and other prey fish. The whole system collapses if the prey fish don't have enough food to eat (and thrive). The scientific term is called a trophic cascade.
That brings us to the base of the food chain - planktons of all types and what happens in their sphere.
In Complex Interactions Between Fish and Zooplankton: Quantifying the Role of an Open-water Planktivore by D. R., DeVries, and R. A. Stein in Canadian Journal of Fisheries and Aquatic Sciences. 49:1216-1227.1992. an examination of plankton predation showed extreme results. In this study, an open-water planktivore, Gizzard shad, were found to cause complex interactions among fish and zooplankton. In this case, crustacean zooplankton density declined to near zero immediately after larval gizzard shad abundance peaked. This decline can be attributed to increased death rates due to predation and to a reduced number of eggs per cladoceran.
The authors noted that in an enclosure experiment, young-of-year gizzard shad at typical densities significantly reduced the density of crustacean zooplankton and rotifers within two weeks. In addition, phytoplankton that were edible by zooplankton were reduced likely due to a combination of direct grazing by gizzard shad and reduced nutrient availability due to uptake by the growing gizzard shad. Gizzard shad not only directly influenced zooplankton via predation, they also indirectly affected zooplankton by reducing phytoplankton abundance.
The study also sought to evaluate the potential for competition between larval Bluegill and larval gizzard shad that occurred in the open water zone (larval Bluegill spend several weeks in the open water zone before returning to the nearshore zone in larger waters). Because larval Bluegills also rely on these planktons for food, their ultimate recruitment was reduced by gizzard shad. Through direct (i.e., predation) and indirect (i.e., influencing plankton abundance), gizzard shad can cause zooplankton to decrease to very low levels, thereby reducing the survival of other fishes and upending community composition.
The whole-system effect of predators has been well described for planktivore-plankton interactions. In general, plankton-eating fishes remove large zooplankters, and invertebrates remove small forms of plankton.
The standard model states that when plankton-eating fish occur in a lake, large zooplankters are selectively removed and smaller plankton forms dominate. Furthermore, because fish, as predators, select larger prey, they also remove invertebrate predators, which tend to be large, thereby reducing their predation on tiny zooplankton and reinforcing the dominance of small plankton forms. Without fish, competitive interactions among various plankton and invertebrates are complex and influenced by body size and environmental conditions. When grazing fish are abundant, whole-system results may be extreme if unmanaged.
While these specific results are fascinating, they further show the reliance of all fishes on the plankton base and the numerous types of plankton and their interactions. With this, one might ask how does supplemental feeding impact the situation? One analysis is found in The Effect of Feed Composition on the Structure of Zooplankton Communities in Fishponds, by Toth, F.; Zsuga, K.; Kerepeczki, E.; Berzi-Nagy, L.; Jakabne Sandor, Z.; Kormoczi, L. in Water 2020,12,1338.
The authors' main goal of this paper was to assess the effect of different supplementary feed types (fish food) on zooplankton communities. Three different feeds were utilized in common carp ponds:
- Fishmeal- and fish oil-based feed, a conventional and commercial feed
- Plant meal- and plant oil-containing feed
- Cereals, a common supplementary feed, as a control
The authors theorized that the impact of experimental feed on community composition would not differ from that of conventional and traditional feeds. Thus, experimental feed does not pose a greater environmental burden and can be recommended as a sustainable alternative to conventional fishmeal-based feeds. Remember that while being omnivorous (plant and animal eating), common carp lean strongly in favor of being vegetarian (herbivore). Carp can survive and thrive on plant-based supplemental feed, whereas most pond fish can't.
Viewing the typical environmental background factors and the feed components, the different feed treatments had little effect on the plankton community composition, as expected. The community structure was determined by season rather than feed treatment. In general, plankton communities were not impacted by the different diets. All in all, the different treatments did not represent special conditions for the zooplankton communities, which would cause a change in their composition in both positive and negative directions. Similarly, to fishmeal-based commercial feed, the typical zooplankton communities for carp monoculture were formed, corresponding to the fish's natural diet, and developed using experimental feed. Based on these results, the plant-based experimental feed had no adverse effect on planktonic assemblages, which makes it suitable as a sustainable fish feed in carp pond aquaculture. More importantly, the fish meal-based diet commonly used in ponds did not negatively affect the plankton community.
Reprinted with permission from Pond Boss Magazine