Diverse, well-connected habitat promotes healthy, resilient aquatic communities by allowing fish and other wildlife to move to the best areas to meet particular needs in their life cycles (such as spawning and rearing) and by providing refuges during extreme conditions. Reduced habitat connectivity is a clear problem for migratory species, but barriers can also strongly affect populations of less mobile organisms by harming habitat conditions or blocking the movement of animals they rely on.
Structures such as dams, weirs, and culverts can break the important connections between habitats by blocking or slowing upstream and downstream migrations. Channelization, diking and other changes to the river banks can limit natural connections to adjacent wetlands and floodplains. Restrictions to the flow of tide water (such as undersized culverts, tide gates) can substantially alter the duration and frequency of tidal flooding in coastal areas.
In addition to the physical blockage of movement described above, other factors such as water temperature, water chemistry and dissolved oxygen can act as barriers to many organisms even when passage appears possible. This is often the result of behavioral responses to unnatural or inhospitable habitat conditions.
The rivers of the PIE-Rivers region are fairly low gradient systems and, in their natural conditions, generally lacked permanent barriers to migration (like waterfalls). The construction of numerous dams and river crossings (bridges and culverts) has greatly limited habitat access for many species. Additionally, connections to river-side (or riparian) wetlands have been reduced and salt marsh characteristics have been greatly altered (through practices such as mosquito ditching).
Consequences of Migration Barriers
Habitat fragmentation can substantially reduce a river system’s capacity to support populations of many aquatic species. In the case of diadromous (or sea run) fish species, migration barriers can essentially remove entire populations from the system. Migration barriers are listed as a key factor in the region-wide decline of diadromous species including river herring and “salter” brook trout. Additionally, many freshwater “resident” species migrate within a watershed either to complete specific portions of their life cycles (like spawning) or for more general purposes (such as following food sources, seeking shelter). The populations of many freshwater species including white sucker, Eastern brook trout and fallfish are reduced in the PIE-Rivers region due, in part, to reduced habitat connectivity in the watersheds.