Title:

Timing the Spring Run: Using Acoustic Telemetry to Characterize Spawning Movements of Rainbow Smelt (Osmerus mordax ) in the Penobscot River, Maine

Anadromous rainbow smelt (Osmerus mordax) migrate in large numbers into tidal rivers and estuaries across the Gulf of Maine to spawn in the early spring and early summer. Assessing their spawning movements in relation to areas of contamination may help explain spatial patterns of exposure, as these localized movements dictate when, where, and how long rainbow smelt are exposed to environmental conditions and contaminants. However, the specific timing of rainbow smelt spawning migration is not well understood in most systems. In this study, we used acoustic telemetry to characterize movements of adult rainbow smelt during and after spawning in the Lower Penobscot River, Maine, a large system with a history of mercury contamination. Fifty adult rainbow smelt (18 females, 32 males) were tagged with Innovasea V6 69-kHz transmitters and an array of 69 kHz receivers were deployed to detect fish movements from late March through the end of July 2025. We observed two major pulses of rainbow smelt entering spawning grounds, one at the end of March and another during the second week of May. Telemetry data revealed that rainbow smelt tended to move into tributaries during evening high tides, often leaving and returning to the same tributary or moving between tributaries over 1-5 consecutive nights. These findings provide insights into the timing and spatial dynamics of rainbow smelt spawning behavior and may help address knowledge gaps related to contaminant exposure in the Penobscot River, Maine. These data can support tribal leaders in the development of health advisories and restoration strategies to help reconnect communities to culturally important resources.

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