Evaluation of hypotheses for explaining temporal trends in Atlantic salmon parr densities in Northeast U.S. Rivers

Atlantic salmon Salmo salar in the USA have declined dramatically and their persistence is heavily dependent on stocking juvenile fish, predominantly fry. The success of stocking hatchery fry is evaluated annually throughout New England by electrofishing surveys targeting age-1 parr. The objective of this study was to examine temporal trends in Atlantic salmon parr densities throughout New England and determine how trends vary among river basins. We fit generalized additive mixed models to investigate potential linear and nonlinear temporal trends in parr density. Akaike's information criterion was used to evaluate competing hypotheses about how temporal trends vary regionally. The top-ranked model suggested two types of trends. The first type (the Penobscot River) showed a nonlinear trend in which parr densities increased until the 1990s and then rapidly decreased through 2008. The second type (all other rivers) showed a linear decrease throughout the time series. Parr density trends reflected trends in spawning escapement for each river group. We conclude that fry stocking has not been able to overcome the decrease in spawning escapement in altered stream ecosystems in New England and that additional management strategies should be considered.