GLF
01-26-2006, 07:17 PM
Common Name: Alewife (Gaspereau, Sawbelly, Spreau, Kyak, Kiack, River herring, Glut herring)
Scientific Name: Alosa pseudoharengus
Classification:
Phylum or Division: Chordata
Class: Osteichthyes
Order: Clupeiformes
Family: Clupeidae
http://www.greatlakesfisherman.com/gallery/files/1/alewife.jpg
Identification: Adult alewife are typically 10 to 12 inches in length (25 to 30 cm), with a green back and silvery belly; they have a single black spot located behind the eye. The scales that line up in a row along the belly give it one of its common names, the sawbelly. The blue-back herring (A. aestivalis) is a physically similar species, and it is difficult to distinguish between the two. The only definable difference (alewife tend to have larger eyes, and blueback herrings have more "compressed" bodies, but these are difficult to enumerate unless directly comparing the two species in hand) is the color of the peritoneum in the two species. The abdominal cavity in the blueback herring is much darker, almost black, whereas the alewife has a paler abdominal cavity with some black spots. Misidentification between the two species may cause problems in identifying range and abundance.
Original Distribution: The alewife used to be a purely anadromous species, breeding in freshwater rivers but returning to the ocean to complete their life cycle. They were typically found from Newfoundland to the Carolinas, preferring depths of approximately 150 to 350 ft off the coast, and spawning populations were found among the tributaries at a maximum of about 100 miles inland.
http://www.greatlakesfisherman.com/gallery/files/1/alewiferange.gif
Current Distribution: Although the means of introduction are still debated, the alewife seems to have entered the Great Lakes at about the time of canal building in the late 19th century. Perhaps using the Erie as a mode of transportation, the alewife range increased greatly as they entered the Great Lakes and from there became established in all five lakes; cold temperatures in the winter have been known to kill off large populations periodically, but typically the alewife can survive most winter temperatures even in the northern parts of Lake Superior. These introduced populations have forsaken the second part of the anadromous life cycle, and do not return to the sea as adults. Instead, they spend the entirety of their life in fresh water. There are also a number of separate isolated inland populations in Virginia, Kentucky and Tennessee.
Site and Date of Introduction: Alewife were first detected in Lake Ontario in 1873, Erie in 1931, Huron in 1933, Michigan in 1949, and finally Lake Superior in 1954. The Ontario population should be only considered as the first date on record, not necessarily the introduction date.
Modes of Introduction: There are three main introduction theories for the alewife. It was first recorded in Lake Ontario in 1873, and some believe that it was native to the lake, but spread to the others with the decline of Atlantic salmon and lake trout, two natural predators of the alewife. Others have suggested that it was introduced when Ontario was being stocked with American shad in the 1880s. The third theory contends that the alewife used the newly-built Erie canal as an opened introduction point, connecting the Atlantic with the Great Lakes. All the Southern lake populations were introduced as a result of intentional introduction.
If the alewife had been native to Ontario previous to the opening of the Erie canal, it would have had to have traveled up the St. Lawrence river from the Atlantic earlier in its natural history. Genetic evaluation, however, shows that the Great Lake populations and the Atlantic populations are similar enough that the introduction was a recent event, and that the introduced population probably proceeded through the Erie canal. Their introduction into the lakes would not have been possible if not for the over-fishing of the Atlantic salmon and lake trout, or the alewife's ability to survive living only in freshwater, contrary to its natural anadromous life cycle.
Reasons Why it has Become Established: The alewife is mostly a filter feeder, but has been known to be piscivorous, feeding on fry as large as 50mm. Before their introduction, the Great Lakes ecosystem functioned with Atlantic salmon as the main predator, with no dominant filter feeding species. As the Atlantic salmon populations declined, the alewife would have found a suitable ecosystem with no strong competition for food resources. Introduced sea lamprey populations may have contributed to the decline of native species that could have outcompeted the alewife.
The extreme temperatures of the lakes generally support populations off alewife - although Lake Superior can occasionally get too cold and kill of a populatoin. Average temperatures are also suitable for spawning, between 12 and 22.5 degrees Celsius. It is also important to note that Alewife were once an anadromous species, and its vagility was appropriate to this life cycle. A spawning female lays somewhere 100,000 eggs, as an adapatation to the hazards of moving downstream and into the open ocean. Landlocked populations, though, may not face the same perils, and so their survival rate would increase. Current landlocked populations suggest that, on average, a female lays 17,000-38,000 eggs while breeding, but upon first introduction from the Erie canal, the initial populatoin may have had a much larger reproductive rate, allowing for the establishment of the species.
The alewive's ability to adapt from an anadromous life cycle to a landlocked one was a key factor in their establishment. While anadromous populations prefer slowly moving waters, and lay their eggs on sandy or gravelly bottoms, the landlocked populations show no preference for breeding grounds. Similarly, landlocked alewife were able to move from being exclusive filter feeders to also feeding on copepods and larvae. Alewife are generalists, which pre-supposes them to invasion.
Ecological Role: alewife are important zooplanktivores. They feed extensively on zooplankton, as well as small insect and fish larvae. They have three different feeding methods: gulping, individual particulate feeding, and filtering. Gulping involves opening the mouth wider for larger objects, as opposed to particulate feeding, during which the alewife open their mouth a small ways. When filtering, the alewife leaves its mouth open and captures any zooplankton and other small organisms present in its feeding area. Alewife may be good competitors for this particular niche, based on their success at transforming the zooplankton community in the Great Lakes. They also seem to have out-competed any native zooplanktivore species.
Alewife serve as food for larger organisms, including Atlantic salmon and lake trout. Herons and other pescivorous birds, as well as otter, mink and other aquatic mammals are all alewife predators. In addition, humans have been known to consume A. pseudoharengus. There are no known large species, however, that depend on the alewife for food - its removal from the Great Lakes, in other words, would probably not be particularly detrimental to larger species.
There are also a number of parasites that have been found in alewife, including Acanthocephala, cestodes, trematodes and copepods.
Benefits: Alewives feed on zookplankton so extensively that they increase water clarity in the Great Lakes; this may, most of the time, serve as an attraction to tourists who want "purity" in the lakes, but this can cause large algal blooms from time to time. Alewives also serve as a food source for many predators, including the diminishing Atlantic salmon. As conservationists attempt to re-stock the Great Lakes with the once-native salmon, alewife may become an important resource. Humans also consume alewife, and states along the Eastern seaboard have taken measures to support dwindling populations of anadromous alewife. Maine, in particular, has seen a dramatic decrease in population sizes, and has made efforts to restore historical spawning runs. Fishing licenses for alewife, as well as the potential tourists they invite, can be beneficial for the local economy.
Threats: Alewives have fundamentally altered the Great Lakes ecosystem. Since their invasion, all trophic levels have been effected by their extensive predation of zooplankton. This ecosystem now, in some places, significantly revolves around the alewife. Native zooplanktonivores have been out-competed. Zooplankton are fed upon extensively, clarifying the water but also allowing for algal blooms. Any potential damage from the alewife has already been done, as integration into the ecosystem seems to have come to completion, at least if temperatures in the lake remain stable. Warming in Lake Superior may cause an increase in alewife populations, leading to greater changes in the ecosystem.
Scientific Name: Alosa pseudoharengus
Classification:
Phylum or Division: Chordata
Class: Osteichthyes
Order: Clupeiformes
Family: Clupeidae
http://www.greatlakesfisherman.com/gallery/files/1/alewife.jpg
Identification: Adult alewife are typically 10 to 12 inches in length (25 to 30 cm), with a green back and silvery belly; they have a single black spot located behind the eye. The scales that line up in a row along the belly give it one of its common names, the sawbelly. The blue-back herring (A. aestivalis) is a physically similar species, and it is difficult to distinguish between the two. The only definable difference (alewife tend to have larger eyes, and blueback herrings have more "compressed" bodies, but these are difficult to enumerate unless directly comparing the two species in hand) is the color of the peritoneum in the two species. The abdominal cavity in the blueback herring is much darker, almost black, whereas the alewife has a paler abdominal cavity with some black spots. Misidentification between the two species may cause problems in identifying range and abundance.
Original Distribution: The alewife used to be a purely anadromous species, breeding in freshwater rivers but returning to the ocean to complete their life cycle. They were typically found from Newfoundland to the Carolinas, preferring depths of approximately 150 to 350 ft off the coast, and spawning populations were found among the tributaries at a maximum of about 100 miles inland.
http://www.greatlakesfisherman.com/gallery/files/1/alewiferange.gif
Current Distribution: Although the means of introduction are still debated, the alewife seems to have entered the Great Lakes at about the time of canal building in the late 19th century. Perhaps using the Erie as a mode of transportation, the alewife range increased greatly as they entered the Great Lakes and from there became established in all five lakes; cold temperatures in the winter have been known to kill off large populations periodically, but typically the alewife can survive most winter temperatures even in the northern parts of Lake Superior. These introduced populations have forsaken the second part of the anadromous life cycle, and do not return to the sea as adults. Instead, they spend the entirety of their life in fresh water. There are also a number of separate isolated inland populations in Virginia, Kentucky and Tennessee.
Site and Date of Introduction: Alewife were first detected in Lake Ontario in 1873, Erie in 1931, Huron in 1933, Michigan in 1949, and finally Lake Superior in 1954. The Ontario population should be only considered as the first date on record, not necessarily the introduction date.
Modes of Introduction: There are three main introduction theories for the alewife. It was first recorded in Lake Ontario in 1873, and some believe that it was native to the lake, but spread to the others with the decline of Atlantic salmon and lake trout, two natural predators of the alewife. Others have suggested that it was introduced when Ontario was being stocked with American shad in the 1880s. The third theory contends that the alewife used the newly-built Erie canal as an opened introduction point, connecting the Atlantic with the Great Lakes. All the Southern lake populations were introduced as a result of intentional introduction.
If the alewife had been native to Ontario previous to the opening of the Erie canal, it would have had to have traveled up the St. Lawrence river from the Atlantic earlier in its natural history. Genetic evaluation, however, shows that the Great Lake populations and the Atlantic populations are similar enough that the introduction was a recent event, and that the introduced population probably proceeded through the Erie canal. Their introduction into the lakes would not have been possible if not for the over-fishing of the Atlantic salmon and lake trout, or the alewife's ability to survive living only in freshwater, contrary to its natural anadromous life cycle.
Reasons Why it has Become Established: The alewife is mostly a filter feeder, but has been known to be piscivorous, feeding on fry as large as 50mm. Before their introduction, the Great Lakes ecosystem functioned with Atlantic salmon as the main predator, with no dominant filter feeding species. As the Atlantic salmon populations declined, the alewife would have found a suitable ecosystem with no strong competition for food resources. Introduced sea lamprey populations may have contributed to the decline of native species that could have outcompeted the alewife.
The extreme temperatures of the lakes generally support populations off alewife - although Lake Superior can occasionally get too cold and kill of a populatoin. Average temperatures are also suitable for spawning, between 12 and 22.5 degrees Celsius. It is also important to note that Alewife were once an anadromous species, and its vagility was appropriate to this life cycle. A spawning female lays somewhere 100,000 eggs, as an adapatation to the hazards of moving downstream and into the open ocean. Landlocked populations, though, may not face the same perils, and so their survival rate would increase. Current landlocked populations suggest that, on average, a female lays 17,000-38,000 eggs while breeding, but upon first introduction from the Erie canal, the initial populatoin may have had a much larger reproductive rate, allowing for the establishment of the species.
The alewive's ability to adapt from an anadromous life cycle to a landlocked one was a key factor in their establishment. While anadromous populations prefer slowly moving waters, and lay their eggs on sandy or gravelly bottoms, the landlocked populations show no preference for breeding grounds. Similarly, landlocked alewife were able to move from being exclusive filter feeders to also feeding on copepods and larvae. Alewife are generalists, which pre-supposes them to invasion.
Ecological Role: alewife are important zooplanktivores. They feed extensively on zooplankton, as well as small insect and fish larvae. They have three different feeding methods: gulping, individual particulate feeding, and filtering. Gulping involves opening the mouth wider for larger objects, as opposed to particulate feeding, during which the alewife open their mouth a small ways. When filtering, the alewife leaves its mouth open and captures any zooplankton and other small organisms present in its feeding area. Alewife may be good competitors for this particular niche, based on their success at transforming the zooplankton community in the Great Lakes. They also seem to have out-competed any native zooplanktivore species.
Alewife serve as food for larger organisms, including Atlantic salmon and lake trout. Herons and other pescivorous birds, as well as otter, mink and other aquatic mammals are all alewife predators. In addition, humans have been known to consume A. pseudoharengus. There are no known large species, however, that depend on the alewife for food - its removal from the Great Lakes, in other words, would probably not be particularly detrimental to larger species.
There are also a number of parasites that have been found in alewife, including Acanthocephala, cestodes, trematodes and copepods.
Benefits: Alewives feed on zookplankton so extensively that they increase water clarity in the Great Lakes; this may, most of the time, serve as an attraction to tourists who want "purity" in the lakes, but this can cause large algal blooms from time to time. Alewives also serve as a food source for many predators, including the diminishing Atlantic salmon. As conservationists attempt to re-stock the Great Lakes with the once-native salmon, alewife may become an important resource. Humans also consume alewife, and states along the Eastern seaboard have taken measures to support dwindling populations of anadromous alewife. Maine, in particular, has seen a dramatic decrease in population sizes, and has made efforts to restore historical spawning runs. Fishing licenses for alewife, as well as the potential tourists they invite, can be beneficial for the local economy.
Threats: Alewives have fundamentally altered the Great Lakes ecosystem. Since their invasion, all trophic levels have been effected by their extensive predation of zooplankton. This ecosystem now, in some places, significantly revolves around the alewife. Native zooplanktonivores have been out-competed. Zooplankton are fed upon extensively, clarifying the water but also allowing for algal blooms. Any potential damage from the alewife has already been done, as integration into the ecosystem seems to have come to completion, at least if temperatures in the lake remain stable. Warming in Lake Superior may cause an increase in alewife populations, leading to greater changes in the ecosystem.