Graduate Thesis Or Dissertation
 

The dynamics of an isolated population of Acartia tonsa Dana (Copepoda) in Yaquina Bay, Oregon

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  • In the upper estuary of Yaquina Bay, Oregon, there is an annual population explosion of Acartia tonsa, (Dana) a calanoid copepod, during the months of July, August and September, followed by a rapid decline to virtual extinction in November. The restricted estuarine distribution affords an excellent opportunity to study the factors governing the population dynamics of A. tonsa without the disadvantage of potential mixing with other populations on different schedules of development. Field densities of A. tonsa during the 1972 summer were determined by twice weekly sampling with Clarke-Bumpus plankton samplers. In addition, explanation of the production of A. tonsa in the field was attempted by measuring the rates of egg production and development in the laboratory under temperature (21°C) and salinity (25‰) conditions comparable to those in the upper estuary. Daily egg production was found to be 30.5 eggs*day⁻¹*female⁻¹. The median rate of development from egg to adult required 11.5-11.6 days at 21°C in conditions of either laboratory cultured or wild food organisms. Mortality rates were substantial during the experiments as only 20-30% of the original populations survived to adulthood. The cause of mortality is not known but may relate to the molting process. In the field, six successive generations of A. tonsa were observed during the population explosion. The mean generation time ranged from 16 days in August to 19 days in October. A maximum density of 16,800 adults*m⁻³ in late August was followed by a crash to 2,100 adults*m⁻³ 10 days later. Predation and over exploitation of food resources are two possible causes of the crash. However temperature appears to be the major factor for the subsequent decline and disappearance of A. tonsa in the fall. This thesis presents the first results of ongoing research into the population dynamics of A. tonsa.
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