Anthropogenic and natural changes can reduce ecosystem goods and services that coastal watersheds provide to society.  To maintain resilient ecosystems in the face of change, we need to understand structure and function. Fragmentation caused by anthropogenic impacts can disrupt connectivity of stream ecosystems.  However, other species can also produce discontinuities.  Beavers (Castor canadensis), extirpated from Massachusetts in the 18th century, recolonized the state beginning with their reintroduction in the early 1900s.  Beaver populations have greatly increased since 1996 (24,000 to 70,000 over 5 years) when their harvest was reduced due to a ballot measure that outlawed body-gripping traps.  Beavers are ecosystem engineers that can alter stream ecosystems by changing stream velocity, nutrient retention, mesohabitat type (e.g., changing riffle to pool habitat), and creating barriers. Here we examined the extent to which beaver dams have fragmented the Ipswich River watershed, altered fish habitats, and changed fish assemblages. To examine the role of beaver dams in structuring fish assemblages, in the summer of 2009, beaver dams were mapped in Fish Brook (a tributary of the Ipswich River), habitats adjacent to the dams were assessed, and the degree of blockage was measured at three levels of discharge.  Fish were sampled above and below 15 beaver dams and 9 control plots. To examine the impacts of beaver dams on resident and migratory fish communities, beaver dams were divided into two categories, those that changed habitat above the dam (n = 11) and those that did not change habitat but were potential stream barriers (n = 4).  All sites were sampled for fish within 14 days during similar flow conditions with a standardized set of gear (backpack electrofishing, hoop nets, and minnow traps). Potential impact of beaver dams on fish assemblages was quantified as: species richness, diversity, abundance, presence-absence, and proportions of guilds. 

Results/Conclusions

Select fish species, such as the fluvially dependent white sucker (Catostomus commersoni), were present more often below beaver dams compared to control areas without dams.  Total fish abundance, numbers of fluvial species, and species richness were greater below beaver dams that changed habitat. Species richness was higher above dams that were barriers. Taken together, these results suggest that beaver dams altered fish assemblages. However, their impacts are complex and acted both directly by blocking movement and indirectly by altering habitats.  Furthermore, fish responses were multifaceted.  Information concerning these multiple pathways and complex responses need to be incorporated into future stream restoration efforts.