Question

Two populations of fish were isolated by the introduction of a dam, one living in the lake above and the other living below the dam in the river. After a long period of time, the fish below the dam are now much smaller on average than the fish above the dam due to the lack of predators below in the river. Predict what changes, if any, might occur in the populations after the introduction of a fish “ladder”, allowing access of the fish below to the lake above. Describe the situation in evolutionary terms and include the biological definition of a species.

Answer 1

he environmental consequences of large dams are numerous and varied, and includes direct impacts to the biological, chemical and physical properties of rivers and riparian (or "stream-side") environments.The dam prevents migration between feeding and breeding zones. The effect can become severe, leading to the extinction of species, where no spawning grounds are present in the river or its tributary downstream of the dam.

The dam wall itself blocks fish migrations, which in some cases and with some species completely separate spawning habitats from rearing habitats. The dam also traps sediments, which are critical for maintaining physical processes and habitats downstream of the dam (include the maintenance of productive deltas, barrier islands, fertile floodplains and coastal wetlands).

Another significant and obvious impact is the transformation upstream of the dam from a free-flowing river ecosystem to an artificial slack-water reservoir habitat. Changes in temperature, chemical composition, dissolved oxygen levels and the physical properties of a reservoir are often not suitable to the aquatic plants and animals that evolved with a given river system. Indeed, reservoirs often host non-native and invasive species (e.g. snails, algae, predatory fish) that further undermine the river's natural communities of plants and animals.

Fish ladder, also known as a fishway, provides a detour route for migrating fish past a particular obstruction on the river. Designs vary depending on the obstruction, river flow, and species of fish affected, but the general principle is the same for all fish ladders: the ladder contains a series of ascending pools that are reached by swimming against a stream of water. Fish leap through the cascade of rushing water, rest in a pool, and then repeat the process until they are out of the ladder.

Fish ladders have a mixed record of effectiveness. They vary in effectiveness for different types of species, with one study showing that only three percent of American Shad make it through all the fish ladders on the way to their spawning ground. Effectiveness depends on the fish species' swimming ability, and how the fish moves up and downstream. A fish passage that is designed to allow fish to pass upstream may not allow passage downstream, for instance.Fish passages do not always work. In practice a challenge is matching swimming performance data to hydrodynamic measurements.Swim tests rarely use the same protocol and the output is either a single-point measurement or a bulk velocity. In contrast, physical and numerical modelling of fluid flow (i.e. hydrodynamics) deliver a detailed flow map, with a fine spatial and temporal resolution. Regulatory agencies face a difficult task to match hydrodynamic measurements and swimming performance data.