Date of Award

2004

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

Barbara K. Sullivan

Abstract

I investigated the importance of predation on fish eggs and larvae by the lo bate ctenophore, Mnemiopsis leidyi, in Narragansett Bay, RI, USA, by measuring the abundance and distribution of M. leidyi and its ichthyoplankton prey at five stations in the East and West Passages of the bay from May to August, 2002. During early-June, the M. leidyi population reached an abundance of 682 m-3 in the mid-bay, while fish egg densities were 3.2 m-3 and fish larvae were absent. In late June, a maximum larval fish density of 34 100m-3 was observed, and ctenophore abundance wasbay, but there is not a substantial amount of temporal overlap. Coincident with the ctenophore abundance measurements, I conducted in situ gut content analyses of 1,031 M. leidyi during the period of highest ichthyoplankton abundance. This revealed that 6.9% of the ctenophore guts examined contained at least one fish egg and one fish larva was observed from May-August. During June, 14.6% contained at least one fish egg. These data provide evidence that fish eggs are consumed by ctenophores in the bay. There was no evidence for frequent predation on fish larvae. Individual feeding rates ranged from 0.04 to 0.6 fish eggs ingested per ctenophore h-1. Predation rates on fish eggs were calculated from the numbers of ichthyoplankton prey found in M. leidyi, temperature-specific digestion times determined in the laboratory, and the field densities of predator and prey. Accordingly, estimates of predation on fish eggs in Narragansett Bay ranged from% to 330% of the standing stock of the fish eggs ingested h-1 during periods of low and high ctenophore abundance, respectively. Predation on fish eggs was not detected in samples taken after June 26. An electivity analysis was performed to compare the proportions of fish eggs versus other prey in the diet of M. leidyi and that found in the environment. Electivity of M. leidyi was examined at the Fox Island station and was found to be positive 23% of the time with respect to fish eggs and negative 8% of the time. All positive and negative values were significantly different from 0 (p<0.01). These results support that M. leidyi preys upon fish eggs in Narragansett Bay.

M. leidyi, zooplankton, fish eggs, and fish larvae abundances were sampled through the summer of2002 in Narragansett Bay. These data were combined with literature values of microzooplankton abundance and physiological processes and a bioenergetics model was developed to simulate seasonal ctenophore biomass from June to July at Dutch and Fox Island. The goal of the model was to examine which prey groups were most important in supporting the observed M. leidyi biomass during the initiation or rapid population growth. The magnitude, rate of biomass increase, and timing output by the model was compared with field estimates of M. leidyi biomass. The model simulations clearly show that the carbon present in copepod biomass alone could account for the magnitude and high rate of increase of the ctenophores in early to mid-June. Later in the season after the copepods have been depleted, it appears that other sources of carbon become increasingly important to M. leidyi.

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