Non-native American shad seemed to have adapted well to West Coast life over the past 100 years in a life cycle that includes Pacific Ocean sojourns and spawning and rearing in the Columbia River and Sacramento River systems.
But their ideal condition remains the same, and contrary to native salmon.
For shad, warmer is better.
“They don’t like cold temperatures,” said William Pearcy, professor emeritus in biological oceanography at Oregon State University and lead author of a research paper that explores the coming and goings of shad in the Pacific Ocean via catch records of research vessels and commercial and recreation fishers from the past 30 years.
The research paper, “Ocean distribution of the American shad (Alosa sapidissima) along the Pacific coast of North America,” was published in the just-released Fishery Bulletin, Vol. 109, No. 4. The journal can be found at:
Authors are Pearcy and Joseph P. Fisher. Pearcy is affiliated with College of Oceanic and Atmospheric Sciences, Oregon State University, and Fisher with OSU’s College of Oceanic and Atmospheric Sciences.
The quarterly Fishery Bulletin is produced by NMFS’ Scientific Publications Office in Seattle. It is the oldest fisheries journals in the world. It has been an official publication of the U.S. government since 1881, under various titles. The journal features both articles that reflect original research and interpretative articles for all interdisciplinary fields that bear on marine fisheries and marine mammal science.
American shad are native to the east coast of North America, where the fish range from Florida to the Bay of Fundy in Nova Scotia, depending on the season.
“American shad undertake extensive ocean migrations along the east coast of North America to the north in the summer, and south in the fall and winter, before returning to natal rivers in the spring to spawn,” the paper says.
The fish, which live to an average age of about five years, are born and spawn in freshwater, but swim to saltwater between breeding times. They spend 3-6 years in the ocean, according to the research paper.
Shad were first introduced to the Pacific coast in 1871 in the Sacramento River. Mostly as a result of their ocean migrations they had found their way to the Columbia, and the Willamette and Snake rivers by 1885. In rivers feeding the Pacific Ocean they have since been reported as far north as Alaska and as far south as Baja California, and as far west as Kamchatka, Russia.
Despite that known distribution, the primary spawning populations are in the Sacramento and Columbia river basins, according to the research paper.
The shad population increased greatly over the past 70 years in the Columbia as the completion of dams, and resulting large reservoirs, continued. Columbia populations estimated at fewer than 17,000 shad before 1960 blossomed to as many as 5.4 million in 2004, as counted in Bonneville Dam at river mile 146.
Bonneville construction was completed in 1938. Counts at the dam topped one million for the first time in 1987 at 1.4 million. The last sub-million count was 943,875 in 1987, until this year. Since that 2004 peak, the annual shad counts at the lower Columbia’s oldest mainstem hydro project have been steadily declining. This year’s total was 948,070, according to data compiled by the Fish Passage Center, http://www.fpc.org/
Increased incidence of a protozoan parasite, endemic to the Pacific Ocean, has been suspected as a cause of the decline in the Columbia River shad population in recent years, the paper says.
“Other possible explanations for declining numbers include changes in the temperature and river flows that may affect survival (Leggett and Whitney, 1972; Crecco and Savoy, 1986; Petersen et al., 2003), competition for zooplankton with forage fishes in the ocean that increase during cool ocean conditions (Emmett and Brodeur, 2000), dietary overlap with salmonids in the estuary (McCabe et al., 1983), and increased predation by seabirds in the Columbia River estuary,” the paper says.
Otherwise, Columbia River American shad population ebbs and flows seem to follow the whims of Mother Nature in a fashion opposite to that of coho salmon. When ocean conditions show a warm “Pacific Decadal Oscillation,” the shad seem to flourish and the coho do not, and vice versa for a cool PDO.
“During the cool Pacific Decadal Oscillation (PDO) regime between 1970 and 1976 shad counts were comparatively low. At this time coho salmon survival was high (Fig. 9, A and B). During the relatively warm PDO phase from 1977 to 1998 shad counts increased rapidly, whereas coho salmon survival was generally low, especially during the warm ocean conditions of the late 1990s. After 2000 shad counts increased markedly with warm PDOs, whereas coho salmon survival increased to high levels following several earlier years with cool PDOs and then declined.
“From these trends we conclude that ocean survival of shad and coho salmon off Oregon and Washington are inversely related and that warm ocean conditions favor increased shad abundances and cool, more productive ocean periods favor coho salmon survival.”
Pacific Coast shad also have seemed to have zeroed in on a much tighter sphere of influence than their Atlantic kin.
“Migrations to the north off Washington and Vancouver were seen during spring to fall, but we found no evidence for large-scale seasonal migrations to the south during the fall or winter,” the research paper says.
“Data from our research surveys were collected mainly during the spring and summer; however, other data lend little support for shad migrations in the Pacific Ocean far to the south during the winter months. Data from commercial and sport landings of American shad along the Pacific coast indicate limited seasonal migrations along the West Coast.
“These differences in distributional and migration patterns of Pacific and Atlantic American shad are consistent with the phenotypic plasticity that has allowed adaptations to the unique environmental conditions along the Pacific coast over the past 100 years (see also Petersen et al., 2003),” the paper says.
“Rottiers et al. (1992) found that juvenile shad from the Columbia River had higher growth rates than did shad from the Delaware River and that the two stocks differed genetically. Quinn and Adams (1996) concluded that shad in the Columbia River have evolved a migratory pattern that allows greater behavioral response to environmental conditions because they now migrate into the river earlier in the year and at lower temperatures than during the prior 45 years.”