Opening the Black Box: A Look at the Marine Phase of Anadromous Alewife

Anadromous alewife and blueback herring are together referred to as river herring.  This is misleading, as they spend most of their lives at sea; however, little is known about this phase and it is often treated as a “black box.” To address this, spawning adult alewives were collected at 12 sites distributed on Cape Cod, New Hampshire and Maine, along a north-south gradient to assess differences in geographic distribution of alewives in the ocean and to elucidate overwintering offshore distribution patterns.  Size distributions of spawning alewives were different among geographic regions, in part due to different age distributions between sites. Fish ranged on average between 252–272mm, 4–6 years-old, Fulton's K 0.8–1.3, 43.6–46.1%C, 12.2–13.4%N, 4.0–4.1 C:N. Larger fish were found in the New Hampshire and Maine sites. Stable isotope ratios of δ15N and δ13C ranged between 12.4–13.4‰, and between -19.0 – -18.2‰. Despite similar K and growth rates, there was a distinct trophic position shift between northern and southern fish. Maine and New Hampshire alewives feed more on pelagic offshore zooplankton, whereas Cape Cod fish may be feeding on more nearshore food sources. Furthermore, δ18O data differentiate overwintering distributions of alewives: Androscoggin River alewives feed in colder Gulf of Maine northern ocean waters, whereas Cape Cod alewives overwinter and feed in Southern New England nearshore waters.  Future work could enlarge this study beyond New England.