The effects of environmental and biological factors on the length of Atlantic Salmon age-1+parr in three Maine drainages

Objective: Atlantic Salmon Salmo salar in the United States have been the focus of recovery efforts for over 150 years, but long-term analyses of juvenile demographics are limited. We examined how parr size (fork length [FL]) varied across three Maine drainages (East Machias, Narraguagus, and Sheepscot rivers) during 1980-2014 and was affected by habitat and biological variables using a long-term electrofishing data set. Methods: We fitted generalized additive mixed models (GAMMs) to determine how parr FL varied with explanatory variables, including mean summer air temperature, elevation, mean channel width, juvenile salmon density, age-0 parr stocked, a metric for the number of effective fry stocked, and year. When examining model-averaged GAMM results, we found that parr size varied throughout the 34-year time period across the three drainages. Result: Between 1980 and 2014, parr FL in the Sheepscot River drainage (mean = 143.9 mm) steadily increased (similar to 5-mm increase). Parr FL in the Narraguagus River drainage (mean = 124.4 mm) declined from 1980 to about 2005 and then increased from 2006 to 2014. Parr FL in the East Machias River drainage (mean = 127.7 mm) experienced a decrease of approximately 10 mm during the study period. Density dependence was evident across drainages, suggesting that habitats were at capacity or that parr were experiencing competition. Given that the production of parr in Maine is highly reliant on stock enhancement, localized high stocking densities may limit parr growth without further restoration or habitat improvements. Stocking intensities generally decreased in all three drainages after 2000, possibly relaxing density dependence and partially explaining the plateau or increase in FL during recent years in the Sheepscot and Narraguagus rivers. Relationships between FL and summer air temperature were drainage specific, while wide channels and lower elevations were consistently associated with larger parr. Conclusion: Given our results, management should continue to prioritize habitat improvements to improve local carrying capacity and potentially reduce density-dependent growth so as to increase stock enhancement efficacy within an adaptive management framework at the southern edge of the Atlantic Salmon's range.