Part 3: The Population Dynamics/Regulation of Rainbow Trout

Population Dynamics/Regulation

To determine what factors could have an effect on rainbow populations, we can see what factors a rainbow population would effect. To do this, we have to remove rainbow trout from an area they currently inhabit. The organisms that increase in number would be ones that the rainbow trout impact negatively either directly, or indirectly. The organisms that decrease in number would be those that the rainbow trout enhances either directly, or indirectly. We can then determine how the trout impact these organisms, and in turn their effect on, or importance to the trout. In Castle Lake, CA, stocking of rainbow trout was discontinued and the effects studied for the next three years (Elser et al. 1995). When the rainbows were removed, the following events occurred in the lake:

1) Brook trout and golden shiners increased
2) Increase in Diacyclops thomasi
3) Decrease in zooplankton herbivores
4) Decline in water transparency
5) Increase in primary productivity

All these effects can be explained by the concept of “Trophic Cascade”. The rainbow trout directly affect one level in the food web, which in turn affects another, and so on. Essentially every part of an ecosystem could be affected by the removal, or introduction of, a single species.

In the case of Castle Lake, the series of events following the removal of rainbow trout were determined as such: Rainbow trout fed primarily on brook trout, golden shiners, and Diacyclops thomasi. When the trout were removed, predation was let up, and the numbers of these increased (Elser et al. 1995). Brook trout, golden shiners, and Diacyclops thomasi in turn consumed more zooplankton herbivores, as there were more of them to eat these (Elser et al. 1995). The phytoplankon that these herbivores normally ate was able to increase, thus making the water less transparent, but at the same time increasing primary productivity (Elser et al. 1995).

This study shows us factors that could serve to regulate rainbow trout populations (at least in accordance with the Castle Lake system, other systems might have a different set of factors). In Castle Lake, phytoplankton needs to be present. This feeds the zooplankton, which the rainbow’s main foods need to survive.

In other systems, with different factors, different interactions will occur. Without study of each separate system, there is no way to determine what will effect the trout population. For example, in some areas brown trout operate as a regulation, as predators on young rainbows (Courtenay 1984). Also, as already covered, water temperatures, dissolved oxygen, or other environmental factors could regulate the rainbow population. When it comes to population dynamics and regulation, the particular population in mind should be studied, as each will differ in this area.

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