Semester

Summer

Date of Graduation

2007

Document Type

Thesis

Degree Type

MS

College

Davis College of Agriculture, Natural Resources and Design

Department

Animal and Nutritional Sciences

Committee Chair

P. Brett Kenney.

Abstract

Recirculating aquaculture systems enable increased fish production per volume of water. Unfortunately, recirculation systems also provide conditions for bacterial growth. Aeromonas, a genus of bacteria ubiquitous in the environment, contains fish pathogens, human pathogens, and/or spoilage organisms. Of the salmonid species, rainbow trout are the most susceptible to Motile Aeromonad Septicemia (MAS), the fish disease caused by Aeromonas spp. In humans, illnesses such as gastroenteritis, septicemia, and wound infections are caused by ingestion or exposure to Aeromonas spp. The objective of this study was to survey Aeromonas spp. present in the production system, on the fish, and on the butterfly fillet. Psychotropic plate counts of fillets were also determined as part of a 6-d shelf life study.;Rainbow trout and water samples were collected from The Conservation Fund's Freshwater Institute (TCFFI) in Shepherdstown, WV over a period of eight month. Source water, a natural spring, and three production systems (flow-through, 80% reuse, and 95% reuse) were sampled. Three fish from each system were rinsed in 100 mL Butterfield's Phosphate Buffer (BPB) and analyzed for Aeromonas spp. and psychrotrophic plate count. Three additional fish per system were filleted, divided into equal halves, and randomly assigned to 0 or 6-d storage. Core samples were taken following 0 and 6 d storage at 4 +/- 2°C, blended with 25 mL BPB, and processed. Psychrotrophic plate counts were performed for whole-fish rinsates and fillet cores on d 0 and 6 of refrigerated storage.;Presumptive positive Aeromonas isolates were obtained from the fillets, rinsates, and water samples via enrichment in Tryptic Soy Broth with Ampicillin (TSBA) and isolation on Starch Ampicillin Agar (SAA) incubated at 28 +/- 2°C for 24 h. The oxidase test, resistance to the vibriostatic agent 0/129, and starch hydrolysis with Lugol's solution were determined for isolates before species identification with API 20NE strips. Water samples were enumerated for Aeromonas spp. by filtering 100 mL through 0.45 mum filters and placing the filter on SAA plates. Water SAA plates were flooded with Lugol's solution after incubation at 28 +/- 2°C for 24 h, and presumptive positive Aeromonas spp. were counted.;Fifty-seven Aeromonas isolates were collected over an eight month period. The highest frequency of positive samples (P < 0.05) occurred in January (16/21), whereas the lowest occurred in November and February (4/21). In January, two Aeromonas spp. were isolated from the source water, two isolates from the 80% reuse water, and one isolate from the 95% reuse water. Four rinsates were positive, two from each the 80% reuse and 95% reuse systems. Two core samples were positive at d 0 from the 95% reuse, and five were positive after 6 d, three from the 80% and two from the 95% reuse systems.;The November sampling generated one positive each from the source, 80% reuse water, 80% reuse fish rinsate, and 95% reuse rinsate. In February, three positive samples were isolated from the source and one positive from the 80% reuse rinsate. Psychrotrophic plate counts of fillets increased (P < 0.05) with refrigerated storage, while counts for fish rinses were highest (P < 0.05) for the 95% reuse system. The flow-through system produced fish that gave the highest (P < 0.05) percent fillet yield. Frequency of Aeromonas spp. detection was affected by sampling period and water reuse. Of particular note, prevalence of Aeromonas spp. increased in January, coinciding with discontinuation of UV light and ozone treatments for the 95% reuse system. Implementing pre-harvest controls for Aeromonas can decrease fillet contamination, resulting in a longer shelf-life and a safer product for consumers.

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