Date of Award

1961

Degree Type

Thesis

Degree Name

Master of Science in Biological Oceanography

Department

Oceanography

Abstract

This report presents the results of a study from March, 1959, to June, 1960, of the spawning run of the alewife, Alosa pseudoharengus (Wilson), and the seaward migration of juveniles. It is based on examination of migrating adults and juveniles in a small, fresh water pond and outlet stream in southern Rhode Island.

In the spring adults enter fresh water in very distinct and separate groups, termed “waves” for the purposes of this study. Water temperature appears to play a major role in the formation of these waves. Early migrants are predominantly older male fish. As the spawning migration progresses, female increase in abundance and the average adult size decreases. It is apparent that males become sexually mature at an earlier date than females.

Adults spend from several days to two weeks on the spawning grounds. Available data indicated that considerable mortality occurs while adults are in the fresh water. Males outnumber females on the spawning grounds about two to one. The postulate that females undergo one more year of mortality at sea before spawning grounds. Spent fish return to the sea so soon after spawning that many pass others coming up. Spawning occurred over a twenty-five day period, with May 1st as a mean spawning date. A significant loss in the condition of adults of both sexes occurred during the breeding season. Average grows of juveniles in fresh water was estimated.

Observations indicate that rate of current flow out of the pond, primarily determined by amount of rainfall, explains much of the variation in dates of departure from the nursery grounds. The success of spawning, in terms of numbers of juveniles migrating to the sea, was exceptionally low during the 1959 season. Less than one juvenile entered the sea for every adult that reached the spawning grounds.

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