McDowall’s emphasizes the role of diadromous fish – species that migrate between fresh water and the sea, like salmon – in structuring freshwater fish communities. Because they disperse through the sea, diadromous fish are a “source” for natural restoration of existing rivers and colonization of new rivers (e.g. melting glaciers in Alaska). Freshwater fish communities should be viewed as more open to colonization and invasion that previously thought. McDowall documents high frequency of diadromous fish species in river systems with high disturbance and extirpation rates, e.g. volcanism in New Zealand and glaciation in North American. Human modification of drainage systems has altered fish communities, by allowing new access points to diadromous fish (Great Lakes canals) and by preventing access (Pacific Coast dams building). The implication for fisheries management and restoration is that diadromy provides a means of natural restoration which is rapid, cost-free, and results in well-adapted stocks.
I am fascinated by the dispersal abilities of fish and implications for freshwater fish community assemblies, as well as the idea of “natural” restoration. The paper’s weakness is that it underplays the variation among diadromous species’ ability to disperse and colonize new river systems, as well as the extent of human modification to rivers.
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