A critical assessment of the ecological assumptions underpinning compensatory mitigation of salmon derived nutrients

Document Type

Conference Proceeding

Publication Date



Stream and river restoration efforts have grown dramatically over recent decades, with estimates of >$1billion spent on the protection, preservation, and restoration of lotic environments. Annually, millions of dollars are appropriated to Pacific salmon (Oncorhynchus spp.) recovery efforts within the Columbia River and its tributaries, often through compensatory mitigation programs with the intent of offsetting lost ecological services provided by salmon. The concept of increasing freshwater productivity through nutrient additions is not novel, yet this approach is being increasingly applied to mitigate for reductions or loss of returns of salmon by replacing the delivery of marine-derived nutrients to streams. Here we critically evaluate some of the key ecological assumptions underpinning the use of such mitigation tools. These assumptions include: (1) mitigation is needed to replace nutrients lost with diminished salmon return and to stimulate primary and invertebrate production in streams, (2) food resources in rearing habitats limit populations of salmon and some resident fishes, and (3) nutrient mitigation mimics the functional roles of salmon. First, though seemingly straight forward, we show that assumption 1 may require qualification based on a more complete understanding of nutrient cycling and the concept of nutrient limitation in streams. Second, we evaluate the empirical evidence supporting assumption 2, and find it has been only weakly tested. Third, assumption 3 is called into question by an array of evidence which points to the multi‐faceted role played by spawning salmon, including red-building disturbance, nutrient recycling by live fish, and consumption by terrestrial predators—none of which are mimicked by standard nutrient mitigation approaches. Additional evidence suggests that the role of salmon may vary with environmental context. On the basis of our assessment, we offer caution in the application of nutrient mitigation as a management tool and recommendations regarding research priorities aimed at further evaluating its scientific foundation.

Publisher's Statement

Publisher’s version of record: http://www.idahoafs.org/documents/2012AnnualMeeting.pdf

Publication Title

Idaho Chapter of the American Fisheries Society Annual Meeting