January 16, 2018
Ancient populations of Atlantic Salmon in Lake Ontario were completely landlocked, never venturing to the ocean, according to a recent study from Canadian researchers. Now-extinct strains of Atlantic Salmon (Salmo Salar) in the easternmost of the Great Lakes apparently had a life cycle that was based entirely in fresh water, contradicting decades of accepted scientific consensus in regards to the migratory behavior of the species, which have a native range from the Upper Atlantic Seaboard to northern Europe and Russia. The functional extinction of Atlantics within Lake Ontario was recorded by the year 1900, and is widely considered to be the initial motivation for modern fisheries conservation policy in the Americas.
Lead authors Eric Guiry and Suzanne Leeds-Howard from the University of British Columbia and the Perca Zooarchaeological Center analyzed the carbon, nitrogen and sulphur isotopes found in scale samples from taxidermy mounts dating back to the 1850's, and in skeletal remains from native Iroquoian campsites both on the northwestern shore of Lake Ontario and from the middle section of the St. Lawrence river, much closer to the sea. The St. Lawrence is the massive downstream outlet for the entire Great Lakes system, draining the largest freshwater impoundment on the planet. However, any upstream movements by salmon into the upper lakes system starting with Lake Erie would be prevented by an insurmountable migratory barrier, Niagara Falls.
The researchers found that the isotopic signatures in the samples from the western Lake Ontario data set showed clear differences from those from the St. Lawrence, indicating long term exposure to either fresh water or marine elements during the fish's life cycle. This means that while the St. Lawrence populations were anadromous (running downstream to the ocean to mature), the western fish were entirely potadromous -- living their entire lives in fresh water after being spawned in tributary streams, and using the giant lake itself as a maturing ground. Outside of the incidence of small numbers of relatively stunted "Landlocked Salmon" in New England lakes (probably of introduced origin) this is counter to most perceptions of Atlantic Salmon being uniformly anadromous and requiring access to salt water to support self-sustaining populations.
The cause for the initial demise of Salmo Salar in Lake Ontario is still subject to speculation, despite the application of modern science to the question. However, historical overfishing, overall pollution and degradation or blockage of spawning stream resources have all been implicated as a cause. Despite this, modern aquaculture and sporting introductions of species of Pacific salmon into the Great Lakes system -- notably Chinook and Steelhead -- have been wildly successful, despite those species exhibiting anadromous behaviors in their home range of the Pacific northwest and far eastern Russia. This would indicate that current environmental conditions for reintroduction of Atlantic salmon into their ancestral potadromous home range could be acceptable. The Great Lakes have been subject to decades of regulatory directives that have resulted in improved water quality since a low point in the late 1960's, with the start of the modern environmental movement after the Cuyahoga River near Cleveland caught fire due to hydrocarbon pollution.
It is unknown if the currently established pacific species in the Great Lakes were forcibly adapted to the necessity of landlocked behavior, or if a subset of the introduced population carried a predisposition in the first place. If so, that knowledge would be crucial for fisheries planning. The authors of the study conclude that re-establishing the Lake Ontario Atlantics will be dependent on identifying strains or certain races of the fish that are genetically inclined to potadromy, if the goal is to have similar success to that of the Pacific species.