Historically, Atlantic salmon shared the rivers of Maine with at least 11 other diadromous (sea-run) fishes. Before the construction of dams, their upstream migrations extended well into the headwaters of Maine's largest rivers. Fisheries abounded, with commercial catches reportedly reaching into the millions for some species. Based on historical catch data and the writings of Charles G. Atkins (one of the first Commissioners of Fisheries for Maine), we can infer that diadromous fishes were substantially more abundant historically than they are today.
At historic levels, several of these species likely benefited Atlantic salmon at key life history events (like providing an alternative food source for predators when smolts migrate to sea). Consequently, their diminishment, via habitat fragmentation, pollution and overfishing, may be partly responsible for the long declining and presently low numbers of adult Atlantic salmon returning to Maine to spawn. Unfortunately, little is known about the ecological interactions between the co-evolved members of the diadromous community that once dominated Maine’s rivers. The anecdotal nature of available historical sources makes it difficult to pinpoint the actual degree to which such interactions ultimately affected Atlantic salmon throughout their various life stages. As such, NEST researchers are exploring untested hypotheses related to the ecological importance of an intact diadromous fish community and its potential importance to the successful recovery of Atlantic salmon to the Northeast U.S.
 Atlantic salmon evolved within a fish community dominated by anadromous species (ones that spend their adult life at sea and return to freshwater to spawn) including the alewife, American shad, Atlantic sturgeon, Atlantic tomcod, blueback herring, brook trout, rainbow smelt, sea lamprey, shortnose sturgeon, and striped bass. Like these species, the American eel migrates between freshwater and saltwater as well—however, it spends its adult life in freshwater and returns to saltwater to spawn (it’s catadromous). Together, these species make up the diadromous community.
Ecological roles of co-evolved diadromous fishes |
1. Prey for Salmon. Atlantic salmon are significant predators of various aquatic organisms during most of their life stages. For instance:
- The historical availability of rainbow smelt as a potential food source for post-spawn Atlantic salmon adults (kelts) in lower river zones may have been important in sustaining the viability of this salmon life stage. Conversely, the broad declines in rainbow smelt populations may be partially responsible for the declining occurrence of repeat spawners in Maine’s salmon rivers.
- The larvae of other sea-run fishes probably provided a significant source of food for juvenile salmon. Salmon parr may opportunistically consume juvenile alewives and other small fish to supplement their primary foraging base of aquatic invertebrates.
2. Prey Buffering. Historically, several diadromous species likely provided a robust alternative food source for native predators of Atlantic salmon. Consequently, these fish might have played an important role in minimizing predation on salmon during different life stages by acting as a prey buffer. Likewise, their relatively low abundance today likely limits their capacity to act as a prey buffer. Specifically:
- Pre-spawn adult alewives overlap in time and space with Atlantic salmon smolts. As adult alewives are migrating upstream Maine's rivers to spawn, smolts are migrating to sea. With a similar body size and numbers that exceeded smolts by several orders of magnitude, alewives probably acted as a substantial prey buffer. As such, they would have protected salmon smolts from native predators such as double-crested cormorants, river otters, striped bass and ospreys.
- Adult American shad likely provided a similar prey buffer for Atlantic salmon adults against river otters, harbor seals, and, perhaps, odontocete cetaceans (i.e. toothed whales, dolphins, and porpoises).
- Juvenile American shad and blueback herring may have also acted as a substantial prey buffer for Atlantic salmon fry and parr by native opportunistic predators such as mergansers, great blue herons, mink, and fallfish.
3. Nutrient Cycling. The upstream migration of anadromous species from the sea to spawn provides a conduit for the import and deposition marine-derived nutrients (MDN) into freshwater systems. These are elements sequestered in the marine environment (particularly nitrogen and phosphorous). For example, due to the semelparous nature of sea lampreys (they spawn once and die), they provide an important source of MDN at about the same time that salmon fry emerge from redds. Nutrient cycling (the continuous movement of minerals, compounds, or elements through an ecosystem) via anadromous species likely influenced production in Maine's rivers. For example, aquatic invertebrates (particularly caddisfly larvae) eat the carcasses of post-spawn adult lampreys and then provide a source of food for juvenile fishes, including juvenile Atlantic salmon.
4. Habitat Conditioning. Some sea-run species may have also improved the condition of shared habitat. For example, in constructing their nests, sea lamprey deposit stones in loose piles and scour silt off stones already at the site. The stones and silt-free habitat preferred by lampreys appeal to spawning Atlantic salmon. In addition to creating an attractive spawning area, the lamprey’s silt-cleaning activities during nest construction may improve the “quality” of the surrounding environment with respect to potential diversity and abundance of aquatic invertebrates.
Maine's aquatic ecosystems are drastically different today than they were historically.  Sea-run fish abundance has declined precipitously and with them their roles as prey and prey buffers, sources of nutrients and habitat conditioners. Today, Maine's coastal rivers are resident species rich and diadromous species poor. Non-native residents found in Maine's coastal rivers now inclde predatory species such as smallmouth and large mouth bass, chain pickerel, northern pike (pictured here) and brown trout. These new species now also compete for limited in-stream resources, such as habitat, and nutrients . |
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"The Penobscot fairly swarmed with the finest fish... salmon, shad, and alewives were taken in quantities that now seem almost incredible."
(Ford 1882:"History of Penobscot County, Maine")
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A successful day of Atlantic salmon fishing on the Penobscot River's Bangor Salmon Pool in 1926.
(Courtesy of the Bangor Daily News, used with permission)
 The anadromous alewife (top) and American shad co-evoloved with Atlantic salmon. At historical abundance levels, these two species likely provided a significant prey buffer for the various life stages of Atlantic salmon against predators. |
Striped bass are a co-evolved anadromous predator of smolt-sized fish. Above, a smolt found inside the stomach of a striped bass.
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Rainbow smelt (pictured spawning in the Penobscot River) likely provided a significant food source for Atlantic salmon kelts. Historically, smelt were very abundant and provided a valuable fishery on the Penobscot, Kennebec, Sheepscot, and Pleasant Rivers. Today, the rainbow smelt is listed as a species of concern by NOAA's National Marine Fisheries Service.
River Otter - A sketch by Tim Hiller
(used with permission)
 Paul Music displays a jawless sea lamprey caught in one of NEST’s rotary screw traps on the Narraguagus River. Lampreys may play a particularly important role in both nutrient cycling and habitat conditioning - it brings in MDN when it returns to freshwater to spawn and uses its mouth to move stones during nest construction.
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