Alewives (Alosa pseudoharengus; Figure 1) are small, silvery forage fish found along the East Coast from Newfoundland to the Carolinas. They are commonly referred to as river herring along with the closely related blueback herring (Alosa aestivalis). Anadromous in their native range, alewives spend most of their lives in the ocean but migrate into freshwater rivers each spring to spawn. Like other anadromous species, dams have blocked migratory routes to upstream spawning habitats, contributing to the decline of alewife populations.
Alewives generally spawn in easily accessible freshwater habitats, but little is known about their spawning behaviors and early life history in freshwater and estuaries. Dr. Ken Able from the Rutgers University Marine Field Station (RUMFS) led a study to examine alewife reproduction and the distribution of larvae and juveniles in the Mullica River-Great Bay Estuary in southern New Jersey. The Mullica River watershed is the primary drainage system of the relatively undeveloped New Jersey Pine Barrens. About 80% of the land in the watershed is preserved from future development. As a result, the Mullica River-Great Bay Estuary is one of the least disturbed estuaries in the Northeastern United States. Previous studies by researchers at RUMFS have found that alewives are the dominant species of river herring that spawn in the Mullica River and its tributaries.
To examine alewife spawning, the research team made visual daytime observations downstream of the dam in the Batsto River from March through May between 2016 and 2018. The Batsto River is a large tributary that was dammed near its confluence with the Mullica River in 1766 to support an iron furnace as well as provide waterpower to the village’s sawmill and gristmill. Today, the earthen dam has a concrete spillway and apron, and is maintained for its historical significance (Figure 2). A subsample of adult alewives was captured using dipnets, seines, fyke nets, and gill nets to confirm the species and determine the sex of the fish. Spawning sites were located by observations and video recordings and confirmed by identifying the presence of alewife eggs.
The researchers found that the adult alewives ascended the river in distinct, short-term pulses throughout the spawning season. Once spawning was completed, the alewives rapidly returned downstream to deeper water. This may benefit the fish as they can use the deep water as a refuge from avian predators such as osprey (Pandion haliaetus) and great blue heron (Ardea herodias).
After analyzing data collected between 1989 and 2018 from several fisheries surveys led by researchers from RUMFS and the nearby Stockton University Marine Field Station, it was found that alewife larvae utilize the freshwater reaches of the Mullica River and its tributaries as nursery habitat (Figure 3). As the larvae grow into juveniles, they expand into brackish areas of the watershed. Specifically, they appeared to concentrate near the freshwater-saltwater interface of the river. As the water temperatures became cooler in the fall, juvenile alewives became less abundant in the rivers and creeks. The researchers hypothesized that the juveniles were likely migrating into the lower reach of the estuary or into the ocean to overwinter.
The researchers also investigated the predation of alewife eggs and larvae by analyzing the gut contents of several predatory fish species caught below the dam. The largest amount of predation was by American eels (Anguilla rostrata; Figure 4). Eels preyed on alewife eggs but there was no evidence that larvae were consumed. American eel, a catadromous species, spawns in the Sargasso Sea and the early life history stages (glass eels, elvers, small yellow eels) live in freshwater until they mature and migrate back to the Sargasso Sea to spawn. In the Batsto River, glass eels, elvers and some younger yellow eels, were found to concentrate at the concrete apron of the dam around the same time the alewives were spawning. Because of this barrier to the migration of both species, predator-prey relationships may be amplified downstream of the dam.
Laboratory experiments were conducted to examine the interactions between the two species by placing alewife eggs and larvae into tanks with eels at various life stages. In contrast to the field gut samples, the glass eels consumed about 96% of the larvae in the trials, preferring larvae over eggs. Elvers and yellow eels readily consumed alewife eggs and larvae. The lab results contradicted what the researchers observed in their field samples. It may be that alewife larvae are weak swimmers and are quickly carried downstream by swift currents, away from the concentration of eels. Alewife eggs may be an important, nutrient-rich food source for eels below dams along the East Coast.
Alewife spawning behaviors may differ between river systems. Though the Mullica River-Great Bay Estuary is among the least impacted estuaries along the East Coast, many of the tributaries are dammed. Further studies will be needed to determine if alewives ascend other dammed rivers in distinct pulses throughout the spawning season as well as how these barriers may influence predator-prey relationships between anadromous alewives and predatory fish species downstream.
Able, K.W., T.M. Grothues, M.J. Shaw, S.M. VanMorter, M.C. Sullivan, and D.D. Ambrose. 2020. Alewife (Alosa pseudoharengus) spawning and nursery areas in a sentinel estuary: Spatial and temporal patterns. Environmental Biology of Fishes 103: 1419-1436.