A bit of both
Some species are both native and invasive in New York State. They may be native to some watersheds but invasive in others.
Alewives (Alosa pseudoharengus)
Alewives are small herring, usually less than 9 inches long, that live in both deep and shallow waters. Alewives are an anadromous species, which means that the young are born in fresh water but migrate to the sea during their first year and spend most of their adult lives in the ocean, only returning to fresh water coastal streams and rivers to spawn. Ocean migrating alewives are native to several coastal areas of New York, including coastal streams on Long Island, the Lower and Upper Hudson, and the Susquehanna, Chemung and Delaware watersheds.
Alewives also are found in landlocked lakes, where they have been purposely introduced at various times in history as a forage fish (small schooling fish that feed on microscopic plankton and are in turn eaten by seabirds, aquatic mammals, and larger fish, including economically important game fish). Alewives have also been illegally introduced into other waterways in New York. As a result, alewives now occur in all but 4 watersheds throughout the state.
Alewives play a major role in coastal and marine ecosystems. The impact of alewives varies depending on location. Through much of their native range, Alewife populations have declined significantly, due in part to loss of habitat, decreased access to spawning areas as a result of dam construction and other impediments to migration, degradation of native habitats, fishing, and increased predation. As a result, several coastal states now prohibit catching or possessing alewives. The US National Marine Fisheries Service considers the marine alewife a “Species of Concern”, which means there are concerns about status ongoing threats to the species, but that there is not enough information available to support listing the species under the US Endangered Species Act.
In inland waterways alewives are often considered an invasive species. Alewife introduction has led to reduced zooplankton populations and significant alterations of local food webs. Alewives also prey on larvae of native fishes, depressing populations and contributing to the decline and loss of native fish species such as whitefish, lake herring, and chub in the Great Lakes. Native fish that feed primarily on introduced alewives can also experience a thiamine deficiency since alewives contain high concentrations of thiaminase, an enzyme that breaks down thiamine. Thiamine deficient eggs from females that feed heavily on alewives often show Early Mortality Syndrome (EMS), a condition characterized by abnormal behavior, physical abnormalities, and death. EMS has resulted in lowered reproductive success of ecologically and economically important species like landlocked Atlantic Salmon and native lake trout.
Monitoring alewife populations is important from two perspectives, depending on location. In coastal areas it is important to support efforts to restore and maintain dwindling alewife populations, while in landlocked locations inhabited by invasive alewives, to protect native species it is vital to understand the depth and spread of this often environmentally destructive fish.