Atlantic Salmon Growth and Survival As an Indicator of Habitat Quality in Small Tributaries of the Machias River Watershed

The Machias River harbors one of the few remaining wild populations of Atlantic salmon (Salmon salar) in the U.S. and provides an ideal system for investigating the productive capacity of currently inaccessible first and second order stream reaches to juvenile salmon due to impassable culverts.  In spring 2010, salmon fry were scatter-stocked in twenty study reaches to quantify growth and survival across multiple environmental gradients.  In late summer, fry abundance per 100 m reach averaged 43 individuals and ranged from 0 to 200 fry. Apparent survival averaged between 0 and 45.1%, with a mean of 10.9%.  Mean length at time of capture was 53.9 mm and ranged from 35.0 to 78.0 mm, and mean mass was 1.4 g and ranged from 0.3 to 5.2 g.  Mean size of fry varied significantly among stream reaches for both length (p < 0.0001) and mean mass (p < 0.0001). Mean density was 0.26 fry/m² and ranged from 0.00 to 1.09 fry/m², whereas mean biomass was 0.37 g of salmon tissue/m² and ranged from 0.00 to 1.34 per reach.  Brook trout (Salvelinus fontinalis), the dominant fish throughout the study area, were collected in 85% of study reaches.  Of the 20 habitat variables measured, temperature, water depth, interstitial space availability, and LWD DBH were correlated most strongly with growth and survival.  We anticipate results that will empirically validate the importance of headwater streams as critical nursery and rearing habitat for juvenile salmon, thus providing the impetus toward culvert removal and the reestablishment of watershed connectivity.