Background

A 2007 Mia J. Tegner Memorial Research Grant in Marine Environmental History and Historical Marine Ecology supported this research, which is focused on the question: Is the recovery of cod (Gadus morhua) along the Maine coast limited by reduced anadromous river herring populations?

Rationale: Many sub-populations of Atlantic Cod (Gadus morhua) along the Maine coast were extirpated 50-70 years ago (1). This is long before the National Marine Fisheries Service (NMFS) fisheries independent trawl survey was initiated in 1963. Very little is known about the population dynamics and factors, either anthropogenic or natural, driving cod populations historically. Consequently, there is a need to use historical ecology to reconstruct the conditions that were once favorable for robust self-sustaining cod populations. River herring Alosa pseudoharengus (Alewife) and Alosa aestivalis (Blueback), considered "forage species" of cod along the Northeast coast of the U.S., are at extremely low coast-wide population abundances. Commercial landings in the New England region have declined by an order of magnitude since a peak in 1950. However, abundance of river herrings were much higher prior to 1900 (Lotze and Milewski 2004), and historical accounts reveal that settlers across the Penobscot region of Maine were concerned about declines in anadromous fish before 1800. Declines are attributed to overfishing and degradation of freshwater habitats that accompanied human expansion across coastal ecosystems of the northeast. This study will test the hypothesis that a relationship is present between declines in river herring populations, due to changes in terrestrial-estuarine systems, and the predator populations that depend on the herring migrations as a major prey item. If there is any chance of rebuilding ecological sustainability along the Maine coast, it must first be determined if the loss of a forage fish base is now a limiting factor for the rejuvenation of coastal cod populations. Understanding how the long term decline of the river herrings affected other populations of fish species is a critical link to new approaches for managing fisheries, such as Ecological Approaches to Fisheries (EAF) or Ecosystem-Based Management (EBM) techniques.


Image from: NOAA Photo Library

The pelagic marine forage fish Blueback Herring (Alosa Aestivalis), Alewife (Alosa Pseudoharengus), Atlantic silversides (Menidia Menidia), Atlantic herring (Clupea harengus) and sandlance (Ammodytes americanus) have all been associated with estuaries in Maine during longer parts of their life history (Personal Observation, 1). Blueback herring, alewives and silversides use estuaries either as spawning areas or pass through to complete anadromous lifecycles. Alewife and Blueback herring, collectively called river herring live in large schools in the ocean and return to freshwater in May to June to spawn in ponds and lakes. Blueback spawning is similar to Alewife spawning, except that the migration into freshwater occurs later and is not as far beyond tidal waters. The later spawning appears to relate to temperature with blueback restricted to 21°C to 24°C water temperatures compared to alewives that appear to prefer 13°C to 16°C (2). Alewives spawn in ponds and in low flow parts of rivers and streams with spawning occuring quickly and spent adults returning to the ocean, resulting in both spent and gravid individuals in any sampling (2). Later in the summer/fall juveniles also migrate to the ocean. Adults are quick to adjust to the changes in salinity, in both directions, but appear less capable of repeated changes between freshwater and saltwater (2). Fisheries target the migrations of herring-like species in embayments along the Maine Coast. American shad (Alosa sapidissima) are less abundant than the alewives and bluebacks, and undergo similar migrations from freshwater to spawn in freshwater. The shad are another schooling fish. Adults enter the streams in spring or early summer at the same water temperatures as alewives, which corresponds to May and June. The historical numbers of shad in the Penobscot appear to be lower than other streams and rivers in Maine (2). Rainbow smelt (Osmerus mordax) are the smallest of Maine's anadromous fish, arriving slightly before the other anadromous species migrations (2).

Without baselines, coastal stakeholders and managers are struggling to devise appropriate restoration and conservation goals. Having historical estimations of prey species and the impact of specific actions on ecological systems will provide managers with a more realistic blueprint. Understanding the significance of the severe reductions of river herring populations on other ecosystem components will be necessary to incorporate shifting baselines into ecosystem-based management and to better understand coastal ecological processes.

  1. Ames, E.P. 2004. Atlantic Cod stock structure in the Gulf of Maine. Fisheries 29:10-28.
  2. Collette B.B. and Klein-MacPhee, G. 2002. Bigelow and Schroeder's fishes of the Gulf of Maine. Collette BB, Klein-MacPhee G, editors. Washington [D.C.]: Smithsonian Institution Press. Online at: http://www.gma.org/fogm/