Alosa pseudoharengus has a high environmental impact in the Great Lakes. Realized:
Alewife populations grew rapidly in the 1950s and 1960s in Lake Huron, Lake Ontario, and Lake Michigan, until they largely dominated fish communities as populations of top predators declined due to increased commercial fishing pressure (Bogue 2000) and Sea Lamprey (Petromyzon marinus) predation (Grady 2007). Bottom trawls of Lake Michigan revealed that Alewife abundance increased from <500 lbs/hour of trawling in 1963 to as high as 1500 lbs/hour of trawling in 1966 (Brown 1968). As the abundance of Alewife continued to increase in the absence of predators, massive annual die-offs of Alewife began in Lake Ontario, Lake Huron, and Lake Michigan. Beaches and nearshore regions were littered with “huge windrows” of fish (Brown 1968), reportedly removed by bulldozer (Alewife explosion 1967). After the introduction of salmonids in the late 1960s to both control Alewife abundance and create a sport-fishing industry, Alewife populations have decreased steadily over time, with intermittent periods of growth and decline which could have been due to predation pressure, climate, or limited zooplankton availability (Eck and Wells 1987; Rand et al. 1995; Mills et al. 2005; Madenjian et al. 2008).
It was estimated that Alewife populations were responsible for 28% of the total consumption (by wet weight) in Lake Michigan in 1987, and 96% of the total predation on invertebrates in Lake Ontario in 1990 (Rand et al. 1995). The abundance of Alewife combined with a diet preference of zooplankton and larval fishes has been shown to affect both the zooplankton community and certain native fish populations over time (Crowder 1980). Preference for macrozooplankton and microcrustaceans has shifted the zooplankton community structure towards a prevalence of small species. Following an Alewife decline in Lake Michigan in the mid 1970s, Evans (1990) noted a significant increase in abundance of Limnocalanus macrurus and Diaptomus sicilis, two of the largest copepods. Similarly, a 1987-1995 study of Lake Ontario found that abundances of cyclopoids and other larger species of zooplankton increased during this period of Alewife decline (Johannsson et al. 1998). Changes in zooplankton abundance and structure caused by Alewife can lead to changes in the phytoplankton community (Shapiro et al. 1975).
Disappearance of native planktivorous salmonids, such as Lake Whitefish (Coregonus clupeaformis), in the Great Lakes has been attributed in part to the introduction of Alewife because of reduced zooplankton populations (Crowder and Binkowski 1983; Todd 1986; Page and Laird 1993). Crowder (1984) speculated that a Cisco native to Lake Michigan, the Bloater (C. hoyi) evolved fewer and shorter gill rakers, and shifted to benthic habitat and diet as a result of competition with Alewife. Smith (1970) attributed the extermination of the Cisco and decline of chub species in the Great Lakes to the Alewife. Smith (1970) also discussed the various interrelated changes that took place in each of the Great Lakes as Alewife abundance increased. Christie (1972), on the other hand, argued that the Alewife was not responsible for these changes.
In a review of the adverse effects of Alewife on Great Lake fish communities, Madenjian et al. (2008) presented evidence that agreed with Eck and Wells (1987), who stated that Alewife likely has a larger effect on native fish populations through predation of larvae than competition for food resources. Using time-series data for various fish populations along with change point regression analysis, they concluded that predation of larvae by Alewife likely contributed to the decline of Yellow Perch (Perca flavescens), Deepwater Sculpin (Myoxocephalus thompsonii), Burbot (Lota lota), Atlantic Salmon (Salmo salar), Lake Trout (Salvelinus namaycush), and Emerald Shiner (Notropis atherinoides) (Madenjian et al. 2008).
Furthermore, Alewife has an elevated level of thiaminase, an enzyme that can degrade thiamine in those species that prey on Alewife (Tillitt et al. 2005). Alewife has thus been shown to cause thiamine deficiency and, consequently, early mortality syndrome (EMS) in populations of Alewife predators. EMS and its adverse effects on recruitment and fish populations is well-documented for Coho Salmon (Oncorhynchus kisutch), Lake Trout, and Atlantic Salmon (in which it is also referred to as Cayuga syndrome), among other fishes (Fitzsimons et al. 1999; Ketola et al. 2000; Madenjian et al. 2008; Ladago et al. 2020). In a spawning reef in Lake Ontario, 50 to 75% of newly hatched Lake Trout fry were estimated to suffer from EMS from 1992–1999 (Mills et al. 2005).
Potential:
In high abundances, Alewife could restructure a lake's food web, leaving less food for native species (USEPA 2008). For example, Alewife and Rainbow Smelt predation in Lake Champlain may prevent Mysis diluviana (formerly Mysis relicta) from recovering from pre-1995 (zebra mussel invasion) densities (Ball et al. 2015). In inland lakes, young-of-year Largemouth Bass (Micropterus salmoides) grow slower and have lower trophic position due to the strong effects Alewife has on the zooplankton community (Boel et al. 2018). EMS also has the potential to cause a genetic bottleneck in populations of heavy Alewife predators by increasing fry mortality and inhibiting recruitment (Mills et al. 2005).
Alosa pseudoharengus has a high socio-economic impact in the Great Lakes.
Realized:
Alewife is a very important species in the history of biological invasions in the Great Lakes. Periodic large-scale die-offs littered the beaches of the Great Lakes with rotting fish in the 1960s. These mortality events happened with such frequency that they became known as “the annual spring and summer die-off” (Brown 1968). Such die-offs cause widespread beach closures and can pose both a nuisance and a health hazard (Becker 1983).
Alosa pseudoharengus has a high beneficial effect in the Great Lakes.
Realized:
Prompted by calls for Alewife management, Pacific salmonids were introduced to both control Alewife populations and utilize Alewife as a food source for sport fisheries. Non-native salmonids in the Great Lakes now support a multimillion dollar sport fishing economy and have caused Alewife populations to decline to the extent that salmonid stocking has been reduced to bolster Alewife abundance and sustain the sport fisheries (Dettmers et al. 2012). Chinook (Oncorhynchus tshawytscha), Coho (Oncorhynchus kisutch), and Atlantic Salmon (Salmo salar) all rely on Alewife as forage in Lake Ontario (Mumby et al. 2018). In late summer 2016, Alewife dominated Lake Trout (Salvelinus namaycush) diets in northeastern Lake Michigan (Luo et al. 2019).
Indeed, the threat from Alewives now in some cases is their declining abundance. Since there are large predators (e.g., Chinook salmon) that focus on Alewife as prey, these fisheries are heavily reliant on Alewife as a food source, and managers are attempting to balance predator-prey ratios to sustain these populations/fisheries (personal communication, Jesse Lepak, May 21, 2021).
In the Great Lakes, Alewife consumes the invasive cladocerans Bythotrephes and Cercopagis (Keilty 1990; Mills et al. 1992; Bushnoe et al. 2003), with the highest consumption rates nearshore (Keeler et al. 2015). Alewife also heavily preys upon the invasive bloody red shrimp (Hemimysis anomala) (Boscarino et al. 2020).
Alewife helps sustain populations of the native double crested cormorant (Phalacrocorax auritus), however, at higher than historic abundances cormorants can have significant negative impacts on sport and commercial fisheries, other waterfowl (Madura and Jones 2016), and island plant communities (Boutin et al. 2011).