Date Awarded

2008

Document Type

Thesis

Degree Name

Master of Science (M.Sc.)

Department

Virginia Institute of Marine Science

Abstract

Stable isotope techniques were applied to summer flounder, Paralichthys dentatus, in Chesapeake Bay to elucidate the relative importance of different prey groups on the growth and productivity of this species. Prior to field application, a laboratory diet-shift study was conducted to evaluate methodological assumptions and obtain necessary isotopic parameters. Specifically, the goals of the laboratory study were to 1) determine isotopic turnover rates and fractionations of δ13C and δ15N in liver, whole blood, and white muscle and 2) estimate the relative importance of growth and metabolic processes on isotopic turnover. Groups of captive juvenile summer flounder (130-255mm total length) were monitored for up to 180 days after switching their food to a new diet with different stable isotope values. Although differences existed between carbon (C) and nitrogen (N), the rate of isotopic change was consistently ranked liver>blood>muscle for the three tissues due to increased metabolic activities of liver and blood. Half lives ranged from 9-21, 20-44, 49-73 days for liver, blood, and muscle respectively. Fractionation estimates for δ15N in muscle (range: 2.4-4.2‰) corresponded with previous research, but estimates for δ13C (range: 0.1-4.8‰) tended to be greater than the traditionally assumed values of 0-1‰. Liver and blood fractionation estimates were similar to those of muscle, differing by usually <1‰. A generalized model for predicting the time scale of isotopic turnover from growth-based turnover parameters was also developed to help evaluate assumptions of isotopic equilibrium in the field. Information obtained from the laboratory study facilitated the use of stable isotopes as dietary tracers for wild summer flounder (138-624mm total length) in Chesapeake Bay. Summer flounder tissues (liver, blood, and muscle) and commonly consumed prey species were sampled seasonally during late spring / early summer (May-July) and fall (November) in 2006 and 2007. To account for similarity in isotopic measurements and to apply mixing models, prey species were aggregated into two trophic guilds: crustaceans (mysid shrimp, sand shrimp, mantis shrimp) and fishes (bay anchovy, juvenile sciaenids, spotted hake). Lack of δ13C differentiation among trophic guilds and summer flounder prevented the use of δ13C as a useful dietary indicator. Analysis of δ15N revealed that crustaceans comprised the majority of summer flounder diet, accounting for ~85-100% of flounder diets on average, except in spring of 2006 when fishes and crustaceans were equally represented in the diet. Summer flounder tended to occupy the same trophic level as the other fishes, suggesting more of a competitive relationship than a predatory one. However, a positive trend in δ15N with length in all tissues indicated that larger summer flounder fed at ~1 trophic level above smaller flounder. Differences in isotopic values between slow and fast turnover tissues did not reveal this ontogenetic dietary pattern at the level of the individual, because the changes in feeding were of small isotopic magnitude and occurred too gradually for reliable detection. Based on stable isotopic analysis, growth and production of summer flounder in Chesapeake Bay are highly dependent on assimilation of mysid, sand, and mantis shrimps, more so than previously expected based on stomach content research.

DOI

https://dx.doi.org/doi:10.25773/v5-rckk-1a79

Rights

© The Author

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