A network of abundant and healthy habitat is vital to the survival
of fish species. Food, a safe place to raise young, shelter, and
clean water are as important to fish as they are to people.
An important aspect of habitat is that the individual pieces of habitat are interdependent. Prey availability is affected by the amount of dissolved
oxygen, which is affected by water temperature, which is affected by the presence of healthy riparian vegetation. This is a simple example of how the different pieces of the habitat puzzle are linked in a network of ecological interactions.
Food: Fish obtain a variety of foods from a variety of locations, and food requirements change as the young fish matures into an adult. As a newly hatched fry, diadromous fish need aquatic insects, copepods, and detritus as food sources. As the fish grow, they will become increasingly dependent on larger prey such as small fish, crustaceans, and squid. Food sources include wetlands, streambed, open water, and intertidal areas.
An example of an important food source for many fish and shellfish species is a wetland. Wetlands offer a great food supply for fish because wetland plants grow rapidly and support a variety of animals. Some fish eat the wetland plants themselves and other fish eat small insects and crustaceans that live on wetland plants.
Spawning and Nursery Areas: Fish need specific environmental conditions for adequate spawning areas and juvenile habitat. Spawning fish need good water quality, protection from predators during this vulnerable life stage, and an adequate place to deposit eggs (the specific requirements vary from species to species). Juvenile fish need food, good water quality, and protection from predators.
For example, some diadromous species such as coho salmon lay their eggs in the stream channel. Defenseless and immobile eggs need to be in a location that will protect them from predators, provide sufficient water and oxygen flow, and not smother them with silt. In addition, when the fish hatch, they need places to hide, feed, and grow. These habitat requirements differ for each species.
Refuge: All diadromous fish must seek refuge from a variety of threats, including predators, floods, and other adverse environmental conditions. Adult and juvenile fish utilizing freshwater streams seek protection from floods, low flow conditions, and predators. They depend on instream vegetation, off-channel habitat, large woody debris, and riparian cover for protection. Juvenile fish are especially susceptible to predation and adverse environmental conditions.
Similarly, fish in estuarine and marine environments must seek refuge primarily from predators, but occasionally from storm conditions as well. They depend on eelgrass beds, rocky shores, and offshore reefs for protection. Again, juvenile fish are especially susceptible to predation and adverse conditions.
An example of refuge from adverse environmental conditions is the movement of the rainbow smelt. Rainbow smelt are sensitive to water temperature and will move into deeper cooler waters during the hot summer season. Cooler water holds more oxygen, and is better for the rainbow smelt's physiology.
Water Quality: Clean water is important to diadromous fish. Changes in salinity, dissolved oxygen, temperature, and other water quality conditions can have adverse impacts on a fish's ability to survive. Human activities such as urban development, industry and some farming and landscaping practices cause abrupt changes in salinity, oxygen levels, nutrients, temperature, and water chemistry.
Wetlands and riparian vegetation are also important in maintaining good water quality. Wetlands act as filters by removing harmful pollutants and excess nutrients. Riparian vegetation helps filter out pollutants, slows down runoff, reduces streambank erosion, and provides shade to streams. Shade is essential to keep water temperature down, which in turn keeps dissolved oxygen levels up.
In marine and estuarine areas, water quality is equally important. Excess nutrient input can cause harmful algal blooms (HAB), which can harm or kill fish. Pollution problems are often magnified in estuaries, where runoff from the land is focused and concentrated before dispersing into the open ocean. Therefore, estuaries are often the focal point for harmful water quality conditions and their effects.
As you can tell from the discussion above, diadromous fish need a variety of high quality interconnected habitat throughout their lives. The protection of all of these habitats is essential for the survival of diadromous fish.
