Fish Habitat - What Fish Need

Brook Trout

Flowing water or ponds?
All fish need water to live, but some fish need flowing water for all or part of their lives, and some don't. We tend to think of rivers and streams as flowing water, but dams change portions of rivers and streams into de-facto ponds called impoundments. In general, natural, free-flowing rivers and streams are dominated by fish species that need flowing water for at least part of their life cycle. The fish communities in the Assabet and Sudbury Rivers today is dominated by pond-fish (macrohabitat generalists) because the rivers are broken up into a series of impoundments by the many old mill dams and water supply dams along their length. Read more about our work on river restoration and dams.

Enough water and safe passage?
Fish also need a range of streamflows throughout the year to access breeding areas and to move safely up and down the rivers finding food and refuge. Changes in the watershed's hydrological cycle caused by stressors like increased impervious surfaces (paved areas), increased pumping of water, either directly out of a stream or from a nearby well, or changes in the climate. These stressors can reduce flows or even dry up a river or streambed in a dry year. In addition, poorly designed or maintained bridges and culverts can cut off a fish's access to parts of the stream by creating large drops, constricted flows, or very shallow areas. Read more about our work on river continuity.

Three classes of habitat requirements
To look at fish populations in the river, fish can be divided into general categories or classes depending on their habitat requirements: fish that need flowing water for all or a portion of their life cycle, and fish that don't.

Fluvial specialists
Fluvial specialists, such are brook trout, are almost always found only in streams and rivers or are described as needing flowing water habitats throughout their life cycle. Occasionally a fish may be found in a reservoir or lake but the vast majority of the fish live in flowing streams. Assabet watershed fluvial specialists include:

Blacknose dace
Brown trout
Brook trout
Creek chubsucker
Longnose dace (present in 1954)
Rainbow trout

Fluvial dependents
Fluvial dependents, such as white suckers, are found in a variety of habitats but require access or use of stream habitats or flowing waters at some point in their life cycle. For example, white suckers can swim far up into the tributary streams to breed in the spring. These fish species may have significant lake or reservoir populations that use tributary streams for some life requirement. Assabet watershed fluvial dependents include:

White suckers

Macrohabitat generalists
Macrohabitat generalists, like bluegill and large mouth bass, are commonly found in lakes, reservoirs, and streams and are capable of completing their life cycle in these systems. These fish live in highly diverse habitat types over wide geographic areas. In other words, these fish are very adaptable and can live almost anywhere. Assabet watershed macrohabitat generalist include:

American eel
Brown bullhead
Yellow bullhead
Black crappie
Banded sunfish
Redbreast sunfish
Largemouth bass
Smallmouth bass (present in 1954)
Common carp
Chain pickerel
Redfin pickerel
Golden shiner
Spottail shiner
White perch
Yellow perch