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Wild striped bass Morone saxatilisin Chesapeake Bay are experiencing a high prevalence of mycobacteriosis, which produces granulomatous lesions of the skin and visceral organs. Culture-based studies have indicated that the newly described species Mycobacterium shottsiiand M. pseudoshottsiiare the dominant isolates from diseased fish. The classical fish pathogen M. marinumis also found, albeit at much lower frequencies. Both M. shottsiiand M. pseudoshottsiiare extremely slow-growing on standard selective media, and up to 12 months may be required for isolation and characterization. Epidemiological studies of mycobacteriosis in Chesapeake Bay would therefore benefit from rapid molecular assays with which to detect these species in fish. In this paper, we describe the development of polymerase chain reaction–restriction fragment length polymorphism (PCR–RFLP) assays capable of detecting M. shottsii, M. pseudoshottsii, and, in most instances, coinfections thereof in striped bass tissues. In addition, PCR–RFLP assays were designed to detect M. marinumand other as-yet-undescribed Mycobacteriumspp. present in Chesapeake Bay striped bass. Comparison of these molecular assays with culture-based techniques using splenic tissue from wild striped bass yielded generally concordant results and demonstrated the applicability of these techniques to the study of wild fish.

Eighty striped bass Morone saxatiliswere obtained from Delaware Bay using commercial gill nets set adjacent to Woodland Beach (n= 70) and Bowers Beach (n= 10) in December 2003. Fish were examined for gross lesions. Total lengths (TLs) and eviscerated weights were determined to calculate condition factors (K). Portions of spleens were aseptically harvested for bacterial culture, and portions of spleens, kidneys (anterior and posterior), livers, and gonads were obtained for histological examination. The size distribution of the striped bass was relatively homogeneous; the mean TL was about 600 mm for all samples. Mean Kexceeded 0.95 in all samples and was not significantly different (P> 0.05) among samples. Significant differences in mycobacterial infection prevalence (P= 0.05) were observed among samples; samples obtained at Woodland Beach (WB) on December 10 (53.8%, n= 13) and December 17 (7.1%, n= 42) exhibited the most striking differences in prevalence. Mycobacterial infection intensity ranged from 1 × 102to 1 × 107colony-forming units per gram of spleen. Acanthocephalan infection prevalence and intensity, non-acid-fast bacterial infection prevalence, and fish sex ratio were also significantly different among the samples (P= 0.05). Similar to the mycobacterial infections, differences in sex ratio, acanthocephalan infection, and non-acid-fast bacterial infection were observed between the WB samples taken on December 10 and 17. However, no significant associations (P> 0.05) were observed between sex ratio or these infections and mycobacterial infection. The differences in bacterial and parasite infection prevalence and intensity and fish sex ratio in some samples indicate that these fish had a different history and that the epizootiology of mycobacterial infection in striped bass from Delaware Bay may be relatively complex.