Restoration and migratory species

Species that spend part of their time in one habitat (or region) and part in another (see Section 6.4) can be badly affected by human activities that influence the ability to move between them. The declining populations of river herrings (Alosa pseudoharengus and A. aestivalis) in the northeastern USA provide a case in point. These species are anadromous: adults ascend coastal rivers to spawn in lakes between March and July and the young fish remain in fresh water for 3-7 months before migrating to the ocean. Yako et al. (2002) sampled river herrings three times per week from June to December in the Santuit River downstream of Santuit Pond, which contains the only spawning habitat in the catchment. They identified periods of migration as either 'peak' (>1000 fish week-1)

or 'all' (>30 fish week-1, obviously including the 'peak'). By simultaneously measuring a range of physicochemical and biotic variables, they aimed to identify factors that could predict the timing of juvenile migration (Figure 7.12). They determined that peaks of migration were most likely to occur during the new moon and when the density of important zooplankton prey was low (Bosmina spp.). All migration periods, taken together (30 to 1000+), tended to occur when water visibility was low and during decreased periods of rainfall. It is not unusual for changes in the moon phase to influence animal behavior by acting as cues for life cycle events; in the herrings' case, migration near to the new moon phase, when nights are dark, may reduce the risk of becoming prey to piscivorous fish and birds. The trough in availability of the herrings' preferred food may also play a role in promoting migration, and this could be exacerbated by murky water interfering with the foraging of the visually hunting fish. Predictive models such as the one for river herrings can help managers identify periods when river discharge needs to be maintained to coincide with migration.

Populations of flying squirrels (Pteromys volans) have declined dramatically since the 1950s in Finland, mainly because of habitat loss, habitat fragmentation and reduced habitat connectivity associated with intensive forestry practices. Areas of natural forest are now separated by clear-cut and regenerating areas. The core breeding habitat of the flying squirrels only occupies a few hectares, but individuals, particularly males, move to and from using knowledge of animal movements... ... to restore harvested fish species,...

... to restore habitat for a declining squirrel population ...

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