Across North America, thousands of dams installed during the last two centuries have dramatically reduced or even stopped the yearly migrations of anadromous fish-species that spawn in inland freshwater systems but travel to coastal waters for at least some part of their lives. During the last several decades, the ecological and economic impacts of stopping these migrations have become increasingly apparent. Inland food webs that were traditionally supplied with a yearly influx of fish from the ocean have been altered, and many commercial fisheries for anadromous species have collapsed. In response, the dams have begun to be removed and fish ladders are being installed, allowing fish species access to inland waters that have been isolated in many cases for a century or more.

Restoring fish migration patterns is an experiment of unprecedented scope and scale, with implications for ecological processes ranging from species conservation to water quality management. While species such as the Atlantic salmon have garnered the preponderance of attention in the coastal ecosystems of New England, ecological interactions and water quality may be most strongly influenced by less glamorous, but historically more abundant, fishes such as the river herring (a name used to collectively refer to alewife, Alosa pseudoharengus and blueback herring, Alosa aestivalis).

The complex life histories and long-distance migrations of anadromous fishes has inspired generations of research in organismal biology, yet we still know little about the role of many of these species in regulating ecological processes in the ecosystems where they reside. This is true for alewife. Over the next several years, the Post Lab will start to address questions related to the ecology and evolution of alewife and the spatial linkages they produce between coastal marine and freshwater ecosystems.

Alewives were historically abundant along the Atlantic coast from Labrador to the Carolinas, and the alewife fishery is one of the oldest documented commercial fisheries in North America. Anadromous alewives spawn in freshwater systems and spend the first summer of their life in lakes and ponds. Most alewives migrate to the ocean in the fall of their first year of life, grow to maturity in three to four years, and return to freshwater each spring thereafter to spawn. There are, however, a considerable number of landlocked alewife populations that have lost the anadromous lifestyle and now spend their entire lives in freshwater systems. It is suspected that alewife populations can become landlocked naturally, as can populations of many other anadromous fish species. However, the evolutionary origin of landlocked alewives is not known, and is a subject of ongoing research. Landlocked alewife populations have also been spread to many inland lakes by intentional human introductions to augment forage fish densities and by natural dispersal. In many of these lakes, alewives have had large negative effects on water quality.

Alewives play important roles in aquatic communities and ecosystems. Alewives are importantprey for top predators, such as bluefish, in the coastal ocean and important prey and predators in freshwater lakes and ponds. In lakes, predation by landlocked alewife can greatly reduce the abundance of large zooplankton such as Daphnia, that are among the most important grazers in lakes and ponds. Lakes that lack Daphnia are more susceptible to algal blooms and degraded water quality. Anadromous alewife can also act as nutrient vectors by transporting large quantities of nitrogen and phosphorous from marine to freshwater ecosystems which could promote algal blooms in lakes and ponds. Thus, alewife may play a dual role in regulating inland water quality

The importance of alewife to both the coastal ocean and local freshwater food webs means that management decisions in one ecosystem can have important consequences for ecological dynamics in other, often seemingly disparate, ecosystems. For example, management decisions for striped bass and bluefish fisheries in Long Island Sound that impact alewife populations could end up indirectly affecting water quality in lakes 10s to 100s of miles inland.

As restoration efforts once again provide access for anadromous fishes to lakes and ponds along the Atlantic coast, there are growing concerns among local lake associations and land owners that the recovery of anadromous herring, in particular alewife, will cause water quality problems in their lakes. At the same time, EPA restrictions on total daily loads of nutrient pollutants are increasing pressure to limit non-point source nutrient pollution. The addition of anadromous herring as a potential nutrient vector troubles lake managers, and lake residents become resistant to restoration efforts when they see images of algal blooms and fish die-offs that occasionally occur in lakes containing landlocked populations of alewife.

In the watershed of the St Croix River, alewives have been excluded from fish ladders over fears that their recovery could be detrimental to popular sports fishes such as smallmouth bass. Yet, river herring were a natural part of these ecosystems for thousands of years. In watersheds that still support strong anadromous populations, alewives are important prey for fish, birds and mammals. Furthermore, it is not clear that anadromous herring have the same impacts upon water quality as landlocked populations. Young-of-the-year anadromous alewives reside in lakes for just a few months, and adults on spawning runs probably do not feed (although this is not clearly documented). This could reduce the impacts of anadromous alewife on food web structure as compared to landlocked alewife populations. Furthermore, the recovery of anadromous alewives, in some cases, will occur in ecosystems that currently contain landlocked populations. This secondary contact (when two species or populations that have had time to evolve separately come back into contact) has important implications for the reestablishment of anadromous populations. There are both genetic and ecological processes (e.g., strong competition) that could seriously limit the ability of anadromous alewives to become established within ecosystems already dominated by landlocked alewives.

Of particular current interest to the Post Lab are questions about the dual role of alewife in determining inland water quality, the evolutionary origins of the landlocked life history, the potential outcome of complex competition/predation interactions among freshwater predators and alewife, and the outcome of secondary contact between landlocked and anadromous alewife. Future questions will broaden the research to include interactions both within the coastal ocean and between the coastal ocean and freshwater ecosystems, and to explore the social/economic impacts of alewife, particularly those related to their effects on water quality and local fisheries. The removal of dams and addition of fish ladders provides a unique opportunity to study the ecological and evolutionary effects of alewife restoration and to explore links between coastal ocean and inland food webs provided by the migration of these important anadromous fish.