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Estimation of striped bass (Morone saxatilis) diets using fatty acid signature analysis Edward J. McGinley Accurate estimates of diets are essential for the management of fisheries, especially when this information is used to construct food webs for a system. Traditionally, these studies have relied on examining the stomachs contents through direct observation of sacrificed fish, or using instruments to “flush” the items from the stomach. These methods only provide information on the recent feeding history. Fatty acid analysis is a biochemical technique that offers promise for examining diets in fish over a longer time scale than just the last few prey species consumed. The goal of this dissertation was to examine the feasibility and efficacy of using fatty acid signature analysis to record striped bass (Morone saxatilis) diets. I examined how collection location (Upper or Lower Chesapeake bay), species, and season affected the fatty acid signature (a compilation of all fatty acids present) and lipid content of four common striped bass prey items: Atlantic menhaden (Brevoortia tyrannus), bay anchovy (Anchoa mitchilli), spot (Leiostomus xanthurus), and blue crab (Callinectes sapidus). Demersal species (spot and blue crab) were separated from pelagic species (menhaden and bay anchovy) based upon their fatty acid signature. Spot and blue crab were also grouped by season, with summer blue crab and spot distinct from fall blue crab and spot. Blue crab and spot within a season had very similar fatty acid signatures. Anchovy and menhaden did not show the same type of seasonal grouping as the demersal species. Anchovy and menhaden had the highest lipid content followed by spot, and blue crab had the lowest lipid content. Collection location did not appear to play a role in structuring the fatty acid signature. These results necessitate the collection of prey species at the same time as collection of predators for fatty acid signature analysis. Fatty acids are deposited in tissues based upon the needs of that particular tissue. I assessed the diet history of striped bass based on two different tissues, adipose and liver tissue. Striped bass were held in flow through tanks at the NOAA Fisheries James J. Howard Laboratory in Sandy Hook, New Jersey, with water being pumped directly from the Sandy Hook Bay. Fish were fed a diet of spot for six weeks, at which point the spot diet was switched to a diet consisting of menhaden. Lipid levels in both tissues increased after the diet was switched to menhaden, a prey that had approximately twice the amount of lipid. The entire fatty acid signature did not change to mimic the prey as reported in a study in which the authors demonstrated that the fatty acid signature of cod (Gadus morhua) significantly changed to a squid signature in approximately three weeks. However, in the present study, certain marker fatty acids specific to the prey were able to distinguish the diet switch from spot to menhaden. The change in marker fatty acids and lipid levels was evident after a period of 31 days. Both liver tissue and adipose tissue demonstrated the change in diet, but adipose tissue may offer a more surgically feasible and non-lethal sample in striped bass. The effect of striped bass size on fatty acid incorporation was analyzed for three different size classes; small (150 – 200 mm), medium (300 – 380 mm) and large (fish greater than 460 mm). Fish were housed in flow through tanks at the NOAA Laboratory in Oxford, Maryland, with water being pumped directly from the Tred Avon River. Striped bass were fed a diet of spot for four weeks, at which time the diet was switched to menhaden for four weeks. Lipid levels for these fish indicated that there was little to no deposition of lipids throughout the experimental feeding of menhaden. Fatty acid signatures also indicated that the entire fatty acid signatures, nor marker fatty acids, were able to determine the diet switch. Based upon these findings and negligible growth, the most likely cause was a lack of consumption by striped bass. Due to high turbidity, feeding was difficult to observe. One of the most promising aspects of fatty acids analysis is the ability to estimate the proportional contribution of different prey items to the diet using prey fatty acids and their respective lipid levels. The statistical program, quantitative fatty acid signature analysis (QFASA), can perform this type of analysis, and also takes into account the effect of predator metabolism of each fatty acid by using calibration coefficients. I tested this model using striped bass fed diets containing mixtures of spot and menhaden and a control diet of just menhaden. In this experiment, the striped bass were fed spot for six weeks before the menhaden feeding experiment began to allow sufficient time for the fatty acids to become homogenized within the striped bass tissues. The model correctly quantified the contribution of spot after six weeks, but it was unable to correctly assess the inputs from the mixed diets or menhaden diet alone. Recent studies have shown that fatty acids may take 12 – 14 weeks to stabilize in fish, which is twice as long as this experiment ran. Most of the work performed with QFASA has tested the model for homeotherms, e.g. marine mammals and seabirds. The fact that fish are poikilotherms may necessitate the duration of fatty acid incorporation to be on the scale of several months rather than weeks. Poikilotherms regulate the internal body temperature based upon ambient water temperatures, while homeotherms require a constant energy source to maintain a set body temperature. Fatty acids may be mobilized quicker and have a shorter retention time in homeotherm tissues. However, this situation is improbable for a generalist fish species like striped bass that will consume a variety of prey items and have the potential to be highly mobile. Lastly, I tested the QFASA model on wild caught striped bass to determine the possibility of using this model on wild fish with prey items caught at the same time. Fish were caught during the fall when they are most likely to be consuming a high proportion of menhaden. Percent biomass of the stomach contents for these fish was compared to previous studies that collected similar sized fish (age-3) during the same season. The stomach contents of fish for this experiment, and fish from previous studies, showed that menhaden made the bulk of the diet (greater than 70%). The QFASA estimated the contribution of menhaden to be minimal (less than 2%). It is possible that striped bass were not feeding on menhaden for a long enough duration for the fatty acid signatures of menhaden to become predominant.