We will use stable isotopes of carbon, nitrogen, and sulfur to examine the diet of commercially import sportfish to determine what prey are important in different environments and in different seasons. We will examine the importance of pelagic and benthic fauna in supporting the diets commercially important sportfish. We will focus the study on Spotted Sea trout, Red Drum and Southern Flounder as they are often caught in the study locations by GCRL researchers and they are target species for many fishermen.
Specimens for the proposed project will be collected during existing fish surveys conducted by CFRD and MDMR. These collaborations greatly reduce the cost of the proposed research and will also minimize impacts of collection on local fish populations. In addition, our results will be integrated with their ongoing diet analysis studies (based on gut content) to better develop local food web models. In addition to stable isotope analysis, estimations of dry weight and caloric density will also be made for each sample. Prey sampling will include pelagic and benthic organisms and will be conducted at the same sites and times as the sport fish samples. These samples will be identified taxonomically to the genus level and will also be analyzed for their stable isotopes compositions.
Delineating food sources for these fish species will allow better decision making for local governances to avoid disturbing the natural food webs that support these and other sport fish in Mississippi Sound. We hypothesize that the relative importance of benthic vs. pelagic trophic pathways to sport fish will differ seasonally based on the movement patterns and seasonal production patterns of different prey (relative abundance). We also hypothesize that the relative importance of benthic vs. pelagic trophic pathways to sport fish will differ spatially based on the prey abundance in different habitat types (oyster reef, salt marsh, along salinity gradient, etc.). Finally, we hypothesize that these spatial and temporal differences in trophic connections are important to understanding the effect of ecosystem stressors such as coastal hypoxia, increased nutrient loads, loss of salt marsh and changes in freshwater flow on sport fish production and how such stressors can and should be integrated into management decisions. We will describe spatial and seasonal variability in benthic vs. pelagic trophic connections and its importance to EFM and coastal planning. These data will be used to develop food web modules (sensu de Ruiter) to quantify ecosystem effects based on network modeling approaches (e.g., ECOPATH).
We propose to examine spatial and temporal variability in the stable isotope signature of C. nebulosus in coastal Mississippi Sound. Our objectives are to 1) quantify individual variation in isotopic signature and 2) examine how individual variation may change both between sample sites and between seasons. We will also 3) examine differences in the seasonal turnover rate of isotopic signatures in two tissues types and address which tissue type is best suited to quantifying trophic pathways in Mississippi Sound.
Fish samples will be collected from existing sport fish surveys conducted by USM Center for Fisheries Research and Development and Mississippi Department of Marine Resources. Fish will be collected four times per year (Dec-Jan, Mar-Apr, Jun-Jul, and Sept-Oct) from two locations (Bay St. Louis and Biloxi Bay) with two sample sites within each location. We will collect 25 individual C. nebulosus from each location during each sample period. In the lab, collected fish will be weighed to the nearest 1 g and measured to the nearest 1 mm (total length). Muscle and liver tissue will be removed and dried at 70o C to a constant weight. The dried samples will then be ground to a fine powder with a tissue grinder. Powdered samples will be treated with 10% hydrochloric acid and then rinsed with deionized water to remove any remaining carbonates that might interfere with organic carbon isotope analysis. Samples will then be dried at 70 o C and stored in clean 6x4 mm tin cups for shipment to the contract lab for stable isotope analysis. Stable isotope data will be analyzed to quantify individual variation in the isotope ratio of all three isotopes by location, season, and tissue type. Isotope ratio values will also be compared for differences between location, season, and tissue type with either an ANOVA analysis or with appropriate non-parametric comparison tests (e.g., Mann Whitney U-test). Seasonal patterns in the differences in isotope ratios between tissue types will be compared to seasonal patterns in fish diet to assess tissue-specific turnover rates in response to seasonal changes in diet. Both estimates of individual variation in stable isotope ratios and estimates of tissue turnover rate will be used to assess the level of discriminatory power possible in stable isotope data for detecting seasonal opportunistic shifts in fish diet in an estuarine environment.
Stable isotope ratios reflect shifts in the isotopic signature of a fishes diet, however diet varies between individuals and the turnover rate in isotopic signature is growth rate dependent (Herzka 2005, Perga and Gerdeaux 2005).
In particular the turnover rate of isotopic signatures in muscle tissue is known to be a function of both fish size and growth rates (Mac Avoy et al. 2001). In order for isotope data of important sportfish to be useful in a given ecosystem we need to first quantify individual variation in isotopic signature and examine seasonal change in the isotopic signature of various tissue types with different turnover rates. A characterization of these two sources of variation will allow for a clearer interpretation of how opportunistic changes in fish diet may be reflected in seasonal/spatial variance in isotopic signature. This analysis will provide a baseline dataset for analysis of larger stable isotope datasets collected to address specific questions about a coastal ecosystem, such as the relative importance of benthic vs. trophic pathways to sportfish production.