Identification of Alewife Stock Structure in the Gulf of Maine Using Otolith Microstructure

Alewives are an integral part of and link between freshwater and marine ecosystems in the Gulf of Maine. Little is known about the life history and distribution of alewives during their time at sea, however, by-catch and unfavorable ocean conditions during the marine phase has been suggested as contributing factors to the decline and lack of recovery of some stocks. While some rivers in Maine support productive runs of alewives, others only support a few thousand returning adults annually. Depending on mixing of fish at sea, individual populations may be disproportionately vulnerable to by-catch events or unfavorable ocean conditions. This analysis used otolith microstructure as a natural marker to differentiate alewife collected within and among Maine watersheds.  Measurements of otolith increment widths during the first 30 days of life of adult fish revealed significant differences across watersheds. Fish from rivers east of Penobscot Bay tended to exhibit slower growth (tighter growth increments) compared to fish from west of Penobscot Bay (wider growth increments). Otolith microstructure shows promise in discriminating among discrete stocks of alewives. An understanding of population structure and a means to discriminate the source population of ocean-caught fish are needed to address mechanisms behind alewife population declines.