Date Awarded

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

John E. Olney

Abstract

In 1988 laboratory presentations, bay anchovy, and Atlantic menhaden tested positive as potential predators. Consumption of striped bass larvae by bay anchovy increased at higher prey densities to a maximum of 42 larvae/h at prey densities of 1,650/m&\sp3&. Examination of 229 stomachs of bay anchovy collected during peak spawning in 1988 and 1989 provided direct evidence of predation on striped bass eggs and larvae. The relationship between patterns of survival inferred from the back-calculation of juvenile hatchdates were compared with data sets describing predator and prey fields, egg production, food abundance, and changing environmental conditions during the 1988 and 1989 spawning seasons. The physical factors pH, rainfall, dissolved oxygen, and water temperature were measured at 30-minute intervals and observations of predator, prey, egg and larval densities were recorded weekly or semi-weekly. In 1988, a year of average recruitment, only 11% of the juveniles aged (n = 78) were born during the week of peak spawning when over 48% of the eggs were produced. In 1989, a year of high recruitment, the temporal distribution of juvenile hatchdates more closely followed the distribution of eggs with 30% of the juveniles (n = 96) born during the week of peak spawning when 40% of the eggs were spawned. Potential fish predators were collected in greater numbers during peak spawning in 1988 (61/100m&\sp3&) than during the same period of time of 1989 (12/100m&\sp3&). Water temperatures were lower during peak spawning in 1988 (16&\sp\circ&C) than during 1989 (19&\sp\circ&C). Also, prey items for first-feeding larvae decreased the week following peak spawning in 1988 from an average number of 194 to 74 individuals/l. Lower water temperatures and reduced food densities in 1988 may have produced a combined effect of prolonging developmental stages of striped bass eggs and larvae, thereby making them more susceptible to elevated predator densities. Dissolved oxygen values were inversely related to water temperature, reaching low values of about 6 mg/l after water temperature reached about 25&\sp\circ&C and spawning had ceased. Although pH values were usually around 7.0 throughout the 1988 and 1989 spawning seasons, pH depressions associated with rainfall were noted during both years.

DOI

https://dx.doi.org/doi:10.25773/v5-va57-0m29

Rights

© The Author

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