First-Summer Survival and Growth of Juvenile Atlantic Salmon in Headwater Streams: Implications for Restoring Connectivity at Road Culverts

The Machias River, located in downeast Maine, harbors one of the few remaining wild populations of Atlantic salmon Salmon salar in the U.S. and provides a model system for investigating the productive capacity of headwater streams currently inaccessible to wild salmon because of impassable culverts. Historically, headwater streams provided high-quality nursery habitat to juvenile salmon as they encompass > 70% of the total stream area in a watershed, and have more favorable and consistent temperature regimes, more available food, and fewer predators than larger rivers. In spring 2010 and 2011, we stocked salmon fry in twenty study reaches and quantified survival and growth across multiple environmental gradients. Fry migration was quantified at representative sites with directional traps, where movement was invariably in a downstream direction, negligible in distance, away from suboptimal habitat, and predominantly within days of stocking. Despite near drought conditions in 2010, late summer electrofishing resulted in fry abundance, apparent survival, growth, biomass, density, and PBI (potential biomass index) estimates that were comparable to 2011 values. Due to the low water event in 2010, the physical habitat variables prevailed, as warmer early season water temperatures, greater water depths, more abundant large wood, reduced detritus substrate, faster late season velocity, and larger drainage areas contributed most to fry metrics. In 2011, a more benign environment persisted, and biotic habitat variables were also important, as brook trout Salvelinus fontinalis, early season abundance and mass of drifting invertebrates, and late season drifting Diptera, Ephemeroptera, Plecopetera, Trichoptera (EPT), and terrestrial inputs contributed most to fry metrics. Our results indicate that headwater streams, although variable in their productive capacity, are essential rearing habitat for juvenile Atlantic salmon. We suggest that based on a select suite of habitat variables, problematic culverts can be targeted for removal, thus reconnecting the highest-quality streams for restoration purposes.