Who's knocking on the door? Using DIDSON to assess the presence of American shad in the Penobscot River

American shad in the Penobscot River are presumed to be few; shad currently access only a small portion (approximately 23%) of their historic range in the Penobscot watershed. Restoration projects scheduled in the Penobscot system include the removal of the two lowest mainstem dams (Veazie Dam at rkm 48, and Great Works Dam at rkm 60) and improvements to fish passage at the third dam (Milford Dam at rkm 62). These improvements are expected to restore shad access to approximately 93% of their historic range. In anticipation of this restoration, our goal was to better characterize the population of adult shad available for recolonization post-dam removal. Beginning in 2009, we conducted continuous Dual-Frequency Identification Sonar (DIDSON) imaging surveys during two spawning seasons (2009, 2010) at Veazie Dam. Sonar data were used to measure the sizes of fish at the base of the fishway (which is impassable to shad), to develop size distributions of migrating shad, and to investigate how these distributions changed through time. Our results indicate that size distribution is an effective method for making species determinations while minimizing impacts to endangered species, and for providing a semi-quantitative index of fish approaching Veazie Dam.