Background
Forage fish are small fish that occupy low and middle levels of marine and aquatic food chains. Species found in Massachusetts include:

• River herring (alewife and blueback herring)
• American shad
• Atlantic menhaden
• Scup
• Sand lances
• Butterfish
• Atlantic silversides
• Rainbow smelt
• Killifish and mummichogs
• Sticklebacks
• Sheepshead minnow

Forage fish provide direct economic benefits through commercial harvest; the annual global commercial catch value of forage fish in 2014 was estimated to be 5.6 billion dollars ²⁰. Forage fish also provide indirect economic benefits through important supportive ecosystem services by transferring energy to higher levels of food webs. This occurs as forage fish typically consume lower trophic level species such as invertebrates and plankton, and are then consumed by a number of commercially and recreationally important higher level marine predators such as sportfishes, seabirds, marine mammals, and squid. In an attempt to highlight the intrinsic value of forage fish, a recent study estimated the yearly value of global fisheries supported by forage fish at around 11.3 billion dollars ¹⁹.

Many forage fish populations have declined in recent decades due to commercial harvest and incidental catches in other fisheries, habitat obstructions, and pollution. These threats impede the recovery of forage fish which reduces the overall availability of food resources for predators ¹⁵. Many forage fish have been targeted by fisheries due to the loss of other overfished species, and as a result, there have been declines of larger more valued fishes such as cod and tuna ^18,29.

Environmental changes in marine ecosystems from climate change and increased fishing pressure have reduced the body size and age at reproduction of some species ⁸. This, along with other biological adaptations, may impact the ability of forage fish to adapt to and withstand future stressors brought about by climate change.

Climate Impacts
Climate change impacts are broad, and events in one location can cascade and impact ecosystems far away. Climate-induced shifts in the Arctic Ocean have triggered ecological changes in North Atlantic ecosystems that impact forage fish in New England in various ways. For example, since the 1990s, continental shelf waters have become fresher (lower salinity levels) and more stratified (divided into distinct layers). This has altered the availability and distribution of types and sizes of primary (phytoplankton) and secondary (zooplankton) producers, which juvenile and other small fish depend on for food ^1,3,10,19. We are just beginning to understand how changes in ocean temperature affect the flow and stability of energy in marine systems.

Increased temperatures, changes in hydrology, increased periods of drought, and sea level rise puts stress on our local forage fish communities. Vulnerability of forage fish to climate change will vary widely depending on the complexity of their life cycles, habitat and food preferences, movement patterns, and individual species’ biology. 
Climate change vulnerability and exposure of forage fish is dependent on whether they are year-round “residents” within certain coastal habitats, or “transient” species that display seasonal habitat use and movement behaviors. Increasing temperatures can alter the proportion of migrant and resident fish within populations, thus contributing to their relative exposure to different stressors ¹⁶.

Resident Forage Fish
Resident forage fish exhibit relatively localized movements that are typically limited to estuaries, embayments and coastal rivers ²⁷. In Massachusetts, resident forage fish include:

• Killifish
• Mummichogs
• Sheepshead minnows
• Sand lances
• Sticklebacks

Killifish, mummichogs, and sheepshead minnows are typically restricted to brackish-freshwater estuarine habitats, while sand lances and sticklebacks can tolerate a wide range of salinities ^4,9,23.

Because resident forage fish primarily occur in nearshore environments, they are vulnerable to stressors that impact both terrestrial and aquatic systems. Intensification of precipitation and storm surge events associated with climate change are likely to increase terrestrial runoff, and affect delivery and distribution patterns of sediment and pollutants. Projected increases in sea level and storm surge will also alter coastal sediments, which many forage fish rely on for incubating eggs, foraging, and predator avoidance ²³.

In response to warming temperatures, many predatory fish species are shifting their distribution and the amount of time they spend seasonally in northern habitats ^2,17,22. In some cases, this is leading to a novel suite of predators and changes in the composition and structure of existing food webs ^21,28. Therefore, the vulnerability of resident forage fish to future climate change will depend on their ability to disperse, adapt to new environmental conditions, and withstand ecosystem-wide changes in predators and competitors ^11,13.

Transient Forage Fish
Transient forage fish are species that undergo large movements throughout their life cycle that can span tidally-driven streams and rivers, estuarine, and open ocean habitats. In Massachusetts, transient forage fish include:

• Silversides
• White perch
• Scup
• Butterfish
• Rainbow smelt
• Atlantic menhaden
• River herring (alewives and blueback herring)
• American shad

Many transient species inhabit near-shore waters during the summer and then migrate to deeper offshore waters during winter. Some species, such as the scup and butterfish, make movements between offshore and estuarine waters many times throughout their lives ^7,25. Scup and butterfish appear to be benefiting from increased water temperature, as seen by their increasing populations in New England ⁵.

Alewife, blueback herring, American shad, and rainbow smelt are all anadromous species, spending most of their lives in salt water but migrating to freshwater to reproduce. This life history exposes anadromous species to stressors associated with both offshore marine and inland waters such as habitat connectivity and obstruction (e.g. dams and culverts), pollution, and fishing pressure ^6,12,15. In addition, smaller species such as rainbow smelt are sensitive to changes in salinity and temperature, particularly in spawning streams ¹³.