Migratory fish populations, or "diadromous fish," are an important part of Massachusetts' near-shore fauna. Migratory fish includes both anadromous and catadromous fish species. Diadromous fish habitat includes areas that support nurseries, feeding, migration (fish runs), and spawning. Intact waterways (as opposed to waterways fragmented by dams) with sinuous channels and diverse structure are most likely to support fish runs, feeding, and spawning areas. Such waterways might include areas with sandy or gravelly substrate, riffle and pool sequences, slightly undercut banks, fallen logs or boulders, flowing water, wetlands, riparian habitats, vegetative buffers, a diverse benthic community, as well as clean and adequate water flow. These conditions provide food for the aquatic invertebrates preyed upon by migratory fish. These conditions also cool the water and stabilize the riverbanks, preventing soil erosion, high water turbidity and siltation of fish eggs. Note that fish runs occur in waterways and adjacent areas that meet certain water quality standards and drain into coastal ponds and Boston Harbor.
|Figure 1a: Blueback herring (Cornell University)|
|Figure 1b: Alewife (Cornell University)|
|Figure 1c: Rainbow smelt (Cornell University)|
Anadromous fish begin life in freshwater, then migrate to the ocean to spend the majority of their life. Eventually, they return to freshwater rivers or to the brackish upper reaches of estuaries to spawn and lay eggs. Then, in the autumn, most of the juveniles head back downstream to more brackish water. Over 15 species of anadromous fish occur in inland and marine waters along the Commonwealth's coast, including within the Mystic, Back, Charles and Neponset Rivers, for example, and historically within the Weir River. Anadromous fish that currently use the Boston Harbor Watershed include American shad (Charles River contains a principal fish run), blueback herring (Back River) alewives (Charles River), Atlantic salmon, striped bass (Parker River, although no spawning), rainbow smelt (Neponset River estuary), sea-run brook trout (Salvelinus fontinalis), and sea lamprey (Petromyzon marinus). Also present in Massachusetts waters are sea-run brown trout (Salmo trutta) and Pacific salmon species (Oncoryhnchus spp.), both introduced species. Anadromous shortnose sturgeon (Acipenser brevirostrum) and Atlantic sturgeon historically were present in the Charles River, but have been extirpated.
Catadromous fish species begin life at sea and then migrate to freshwater, where they live for most of their adult lives. Eventually, they return to the ocean to spawn and die. The catadromous American eel (Anguilla rostrata) is born at sea, then migrates to freshwater lakes and ponds, eventually returning to the Sargasso Sea to spawn and die.
|Figure 2: American shad (B. Byrnes)|
Unfortunately, these fish and their habitat have diminished over the last several hundred years throughout Boston Harbor Watershed and beyond. Because diadromous fish migrate between fresh- and saltwater, they are affected by conditions in both. Dams, habitat alterations, pollution, and overfishing have led to declines in migratory fish numbers. In the Boston Harbor Watershed, dams have negatively affected migratory fish species since the mid-1600s, when water-powered mills hummed with activity on Massachusetts rivers. Species such as American shad (Alosa sapidissima), Atlantic salmon (Salmo salar), alewife (Alosa pseudoharengus), blueback herring (Alosa aestivalis), striped bass (Morone saxatilis), and rainbow smelt (Osmerus mordax) were all declining in southern New England by 1870, at least in part because of dams that blocked migrations and because of significant water pollution near towns and mills. These species' numbers remain below historic levels. Overexploitation, primarily by commercial fishers, also has reduced populations of migratory fish such as striped bass, Atlantic salmon and Atlantic sturgeon (Acipenser oxyrinchus).
Migratory fish species seek out different habitat characteristics.
- Rainbow smelt. Smelt spend the summer in shallow waters less than a mile from shore and then move into bays and estuaries during the fall and winter.
- River herring (e.g., alewife and blueback herring). Although river herring use both riverine and lacustrine environments as spawning grounds, alewives prefer lacustrine areas, while blueback herring prefer streams or rivers. Once river herring hatch, they swim out to the ocean to spend their adult lives. Eventually, they return to spawn in the freshwater where they were born. River herring occur in schools of thousands near their natal streams, where they feed on plankton (i.e., copepods, amphipods, shrimp, and fish eggs). Many river herring, in turn, are consumed by gulls, terns and other coastal birds, mammals, fishermen, striped bass and bluefish (Pomatomus saltatrix).
- American shad. Shad seek out backwater areas, lakes and larger rivers. In Massachusetts, they spawn in larger river systems (Connecticut and Merrimack Rivers), although small populations also occur in smaller waterways (Palmer and Indianhead Rivers). Historically, shad also occurred in rivers such as the Charles. New England shad overwinter in the mid-Atlantic coastal region and then migrate northward in the spring. Postspawn adults and immature fish congregate in the Gulf of Maine and Bay of Fundy during the summer before moving to wintering grounds.
- Sea-run brook trout. Sea-run brook trout prefer cold, fast-flowing streams.
|Figure 3: Sea run brook trout (SRBT Coalition)|
With its many waterways, the Boston Harbor region provides a diversity of habitats resulting in a range of migratory fish species.
- Weir River in Hingham: river herring, smelt, white perch (Morone americana), tomcod (Microgadus tomcod).
- Straits Pond in Cohasset and Hull: river herring.
- Accord Brook in Hingham: river herring.
- Fulling Mill Brook in Hingham: river herring.
- Weymouth Back River in Weymouth: river herring, smelt, white perch.
- Fore River / Monatiquot River in Weymouth and Braintree: smelt.
- Smelt Brook of Weymouth and Braintree: smelt.
- Town River in Quincy: smelt.
- Furnace Brook in Quincy: smelt.
- Neponset River in Quincy, Boston, Milton, Canton, Dedham, Westwood, Norwood, Walpole, and Foxborough: alewife, blueback, American shad, smelt, white perch.
- Charles River in Boston, Cambridge, Watertown, Waltham, Newton, Weston, Wellesley, Needham, Dedham, Westwood, Dover, Natick, Medfield, Sherborn, Millis, Norfolk, etc.: alewife, blueback, American shad, smelt, white perch, gizzard shad (Dorosoma cepedianum). Historically, shortnose sturgeon and brown trout (non-native) occurred in the Charles River.
- Mystic River in Boston, Chelsea, Everett, Somerville, Medford, Arlington, and Winchester: alewife, blueback, American shad, white perch. Historically, shortnose sturgeon and brown trout (non-native) occurred in the Mystic River.
|Figure 4: Smelt eggs at Lower Mills (T. Palmer)|
Unfortunately, the distribution of migratory fish populations and their habitat has waned over the years, due especially to degradation and reduction of habitat. Consider the historic of just a few of the anadromous fish species that occur in the Boston Harbor Watershed:
- Rainbow smelt. Despite natural variation in run size over the long-term, coastal populations of rainbow smelt in southern New England are declining. The Atlantic Coast range also has shrunk in recent decades: although populations once extended from the Delaware River to eastern Labrador and the Gulf of St. Lawrence, spawning populations are now extirpated south of Long Island Sound. Limited spawning occurs between Long Island and Cape Cod. In the Gulf of Maine region, although spawning runs still occur, populations have declined. Factors for the decline in smelt may include spawning habitat conditions, fish health, marine environmental conditions, and fishing. In 2004, the National Marine Fisheries Service (NMFS) of the National Oceanic and Atmospheric Administration listed Rainbow smelt as a Species of Concern, indicating concern about the status and threats to smelt, but insufficient information to list the species under the Endangered Species Act (ESA). The Species of Concern label is meant to encourage smelt conservation efforts, but does not apply protections under the ESA.
- River herring (e.g., alewife and blueback herring). The extent of river herring (key prey for a variety of marine and freshwater fish [e.g., striped bass, cod, and yellow perch], as well as birds) has been reduced by dams that block spawning migrations along rivers, pollution, overfishing, and increased predation. In fact, because several Massachusetts herring runs have declined to historically low levels, in January 2006 the Massachusetts Division of Marine Fisheries (DMF) established a three-year moratorium on the sale and harvest of river herring. In October 2008, that moratorium was extended through 2011 because river herring populations had not sufficiently recovered. The National Marine Fisheries Service also has listed river herring as a Species of Concern, indicating concern about the status and threats to the species, but insufficient information to list under the ESA. A 2001-2002 survey of 215 Massachusetts coastal streams found active herring runs in 100 different coastal waterways (including the Back and Mystic Rivers). The coastal range of blueback herring spans Nova Scotia to Florida. Alewives occur from Labrador to South Carolina.
- American shad. Shad are in severe decline. In Massachusetts, shad have been extirpated or reduced to unsustainable populations in all rivers where they occurred, due to structures blocking spawning migrations, pollution of spawning grounds, changes in land and water use that reduce habitat, nonpoint source pollution, increased water withdrawals from spawning rivers, and overfishing. Climate change, predation, and bycatch in other fisheries also have led to population declines. Shad territory spans the Atlantic coast from southern Labrador to northern Florida.
The condition of anadromous fish habitat has declined significantly, due in part to water pollution, blocked fish passage along waterways key to spawning migrations, alteration of riparian habitat, and low stream flow, among other factors. Because migratory fish use both fresh and salt water over their lifetimes, they are vulnerable to changes in both areas. In the words of John Moring in the "Recent Trends in Anadromous Fishes" chapter of The Decline of Fisheries Resources in New England: Evaluating the Impact of Overfishing, Contamination, and Habitat Degradation, edited by Robert Buchsbaum, Judith Pederson and William E. Robinson, 2005:
- "Because these fishes are dependent on diverse environments during different portions of their life cycle, they can be especially vulnerable to a variety of environmental changes. During their early life stages, these fishes are sensitive to deleterious alterations in freshwater. Later, when they pass through estuaries and into the marine environment, coastal pollution can affect survival. At maturity, habitat alterations, pollution, and commercial harvest can have profound impacts on spawning grounds. Therefore, not only are anadromous fishes subject to environmental and harvest pressures at sea, they encounter dams, pollution, urbanization impacts, and habitat changes in freshwater."
The reduction and degradation of migratory fish habitat can be described within four main categories. Each of these stressors are prevalent in the Boston Harbor region.
- Barrier to migration. Low water levels, dams, aging or improperly maintained or sited culverts, large debris, inefficient, degraded or outdated fish ladders, and other blockages, can obstruct fish spawning migrations along waterways. Even some fish passage structures can act as barriers; some fish species will not use fish ladders. Another barrier to migration occurs in the form of sand accumulation in the outlets of spawning area ponds, due to sandy soils, shoreline development, and/or beach nourishment. Sand accumulation can trap juvenile herring in the ponds and delay or prevent downstream migration during years of low water.
- Instream habitat quality. During migrations, most anadromous fish stay in coastal waters, fairly close to land. The pollutants in this water can damage the health of migratory fish (especially juveniles). Such pollutants may include heavy metals, pesticides, hydrocarbons and effluents. Other pollutants may affect the fish in a less direct manner, such as when pollution containing high nutrient levels (such as untreated sewage) sparks dense aquatic plant growth that endangers fish. Pollutants of both recent and historical origin – for example, PCBs (manufacture banned since 1979) and DDT (use banned since 1972), occur in Boston Harbor Watershed. In addition to toxins, instream habitat can be degraded by the introduction of excessive silt, via removal of riparian buffers, riverbank erosion, street runoff, and construction. Siltation can block fish migration and reduce habitat, prevent successful spawning, kill embryonic and juvenile fish, and smother eggs, especially at spawning grounds. Fish spawning in the upper portion of estuaries especially are vulnerable to siltation, via smothering of eggs – blueback herring, rainbow smelt, white perch, and tomcod. Spawning migration of alewives and American shad heading toward the headwaters of rivers that drain into estuaries can be blocked by siltation. Removal of riparian habitats and buffers reduces the ability of a waterway to maintain cooled water, to provide food and wildlife habitat, and to avoid pollution from surface water and erosion. Channelization and hardening of river channels (with cement, riprap, seawalls, etc.) simplify river structure and reduce instream habitat.
- Spawning habitat quality. Pollution degrades migratory fish spawning grounds, as do lack of water, lack of appropriate substrate and damaged habitat. Spawning habitat degradation also results from removal of riparian habitats, erosion, sedimentation, hardening of river channels, decreases in submerged aquatic vegetation and increases in algae, as well as acid precipitation, which results in water acidification. Finally, environmental changes due to climate change compound these issues.
- Hydrology. As water withdrawals continue to increase – pulling from groundwater and surface water sources, instream water levels drop, yielding "low flow" conditions that damage fish habitat and fish populations. Agricultural practices also divert streamflow from waterways, causing low water levels and potentially blocking fish passage and entrapping and stranding juvenile fish. In addition, increased periods of drought associated with climate change yield more low instream water levels.
Protection and Restoration Potential
To restore and rehabilitate anadromous fish populations, we must protect and improve the condition of the freshwater and estuarine resources on which they rely, by implementing projects to reduce aquatic pollution, restore fish passage, reduce water withdrawals, and rehabilitate or protect nursery and spawning habitats. When planning such conservation efforts, it is beneficial to consider the interplay of directly affected and other species. In the words of John Moring in the "Recent Trends in Anadromous Fishes" chapter of The Decline of Fisheries Resources in New England: Evaluating the Impact of Overfishing, Contamination, and Habitat Degradation, edited by Robert Buchsbaum, Judith Pederson and William E. Robinson, 2005:
- "…[A]s striped bass numbers increase, more of these predators will be entering the lower portions of New England rivers. The success of their feeding in these habitats will depend, among other things, on the success of programs to increase runs of Atlantic salmon and American shad. Survival of Atlantic salmon smolts, rainbow smelt, and alewives, in turn will be influenced by the breeding success of federally-protected aquatic birds, such as double-crested cormorants (Phalacrocorax auritius), the popularity of new sea-run brown trout sportfisheries (potential predators of smolts and prey for striped bass), and the success of striped bass restoration programs, as well as fishing and nonfishing human activities. Thus, rehabilitation programs must be managed from a broader perspective."
A range of agencies has worked to maintain, restore and boost diadromous fish populations and habitat in the Boston Harbor Watershed, and their efforts occur within a network of national, regional and state-level guidelines concerning species management. Such efforts to restore diadromous fish populations and habitat in the Boston Harbor Watershed targeted species key to recreational fisheries, like rainbow smelt, American shad and river herring that also are important members of the food web, feeding predators like striped bass and bluefish. These efforts have experienced some success – fish populations have been established in areas where they previously had disappeared due to human activities, fish passage for some species has been restored along some waterways, and populations of migratory fish have increased in some areas. However, it should be noted that although a migratory fish species may be observed to increase in number here in Massachusetts that increase may be due to restoration efforts elsewhere in their migratory path (e.g., striped bass, which no longer spawn in Massachusetts).
A sampling of agencies involved in migratory fish and/or aquatic resources conservation in Massachusetts include:
|Figure 5: Neponset River, Quincy (T. Palmer)|
Anadromous fish habitat restoration efforts may be divided into these categories:
- Removing barriers to migration. Blockages along waterways key to spawning migrations can be removed to restore fish passage. The recent restoration focus has shifted from managing for specific fish species to enabling them to bypass an obstruction and re-establishing annual migrations of groups of species. Of the various methods available for restoring fish passage, dam removal provides the most ecological benefits – restoring river hydrology, connectivity, water quality, recreation, ecology, and fish passage for a variety of fish species, unlike the limited species whose passage can be restored via fish ladders. However, dam removal projects in the Boston Harbor region can be complicated by pollutants in river-bottom sediments, producing added expense and time. Some contaminated sediments require removal, treatment and proper disposal. Note that when a dam cannot be removed, fish passage may be restored by installing fishways: upstream and downstream fish ladders, fish lifts (elevators), or nature-like fishways. The nature-like fishways pass more types of fish than fish ladders. Nature-like fishways include rock ramps and by-pass channels, which
mimic natural river morphology. For migratory fish, the Denil-type, Alaskan steep pass fishways are the most effective.
Figure 6: Public Health Advisory sign on Neponset River (T. Palmer)
- Improving and restoring instream habitat quality. Often altered in urban areas, river contours and riparian habitats and buffers can be restored to a more natural state to improve water quality. Water pollution can be reduced. Waterway blockages can be removed or altered to restore waterway connectivity and water flow.
- Improving spawning habitat quality. Spawning habitat substrate can be restored and protected. Water quality can be improved. Riparian buffers can be restored. Water withdrawals can be reduced and groundwater recharge can be increased in the watershed.
- Improving hydrology. Water withdrawals can be reduced and groundwater recharge can be increased in the watershed, to improve water flow in the waterway.
|Figure 7: Walter Baker Dam, Neponset River (T. Palmer)|
A migratory fish restoration technique that is less directly tied to habitat is to propagate fish to re-stock depleted waterways. Fish, their larvae or eggs can be harvested from one river and moved to another. The juvenile fish imprint on the new site and eventually return there to spawn, thereby boosting local fish populations. River herring and American shad have benefited from propagation.
Protection and restoration opportunities for diadromous fish habitat in the Boston Harbor region include the following:
- Support existing restoration efforts. DMF has identified and begun to implement many fish passage projects in the Boston Harbor region. There is a need for help with montioring and data gathering for some of these projects. A protection/restoration priority is to build local capacity to aid in the implementation of these DMF projects.
Develop a herring warden/local steward network. Develop a network of stewards for the diadromous fish habitat in the Boston Harbor region by connecting existing herring wardens or identifying a local steward for each fish run or watershed. Create a network for sharing information and experiences.
Gather and incorporate local information. After creating a network of fish habitat stewards, use the resources to identify restoration priorties not identified by DMF. Incorporate fish warden and watershed association perspectives into the protection and restoration opportunities.
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