NEAFWA 2018

AUTHORS: Kevin Lachapelle; Gayle Zydlewski - University of Maine

ABSTRACT. Non-invasive techniques are attractive when studying rare and endangered species. We have applied various non-invasive hydroacoustic techniques to describe behavior and estimate the population size of Shortnose Sturgeon (Acipenser brevirostrum), an endangered species, in the Gulf of Maine. This species exhibits a peculiar behavior that makes them particularly available for direct visualization. All individuals within a river system aggregate into one or a few dense and distinct groups for the entire winter season. In the Penobscot River this occurs from December through April. Over 5 years we used a DIDSON (Dual IDentification SONar) to observe and enumerate aggregations of Shortnose Sturgeon in their chosen winter sites. Since sonar imaging uses acoustic waves instead of light, the area visualized at each sample point was standardized, and much larger (up to 10 m) than what is possible with underwater cameras (10s – 100s cm). The DIDSON produces near-video quality real-time images, allowing easy differentiation of fish from stationary debris (e.g., logs). This contrasts with side-scan sonar (SSS) imaging which creates a picture where stationary debris can be confused with fish targets. SSS was attempted on various occasions without as much success, likely due to the shallow imaging area, as using a DIDSON. Comparisons of these hydroacoustic techniques will be presented. Resulting DIDSON footage allowed us to visualize aggregation behavior in real time, and to calculate average densities throughout wintering habitat based on counts of fish and area imaged. Spatial statistics were applied to the density data to calculate Shortnose Sturgeon aggregation estimates (539 to 1186 individuals). These estimates were comparable to other seasonal estimates created using mark/recapture methods (641 to 1306 individuals). The DIDSON technique required much less sampling time than mark/recapture and did not require direct handling of fish, both features being beneficial for estimating population sizes for managing an endangered species.