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Mycobacteriosis
David A. Ross Ph.D. Scientist Emeritus
Woods Hole Oceanographic Institution
Woods Hole, MA
Stripers, for some unknown reason have a high susceptibility for mycobacteriosis, which has infected resident fish in Chesapeake Bay beginning in the 1980s. Recent studies, including the recent USGS/NOAA workshop on mycobacteriosis have shown a broad and increasing rate of infection.
Fish with mycobacteriosis develop a bacterial infection that causes inflammation and ugly lesions or sores on their skin; it also causes tissue destruction and formation of scar tissue in one or more organs, especially the kidneys and spleen. The disease has been characterized as a “wasting disease” because it results in a loss of body mass and causes fish to look emaciated.
It is not known how fish actually become infected. Do they get it from their food, from stress, or from the water? And can they pass it on to others that swim nearby or when in a crowd? Among the possible reasons for the increase in mycobacteriosis (up about 62% from 1998 to 2005) are overpopulation and/or poor water quality combined with insufficient food sources.
The relationship between the occurrence of the disease and mortality of striped bass has not been definitely determined but there is evidence that mycobacteriosis may be having an impact on the bass population - more study on this question is needed. Some fishery biologists show that because of mycobacteriosis, natural mortality estimates – number of fish that die from natural causes – have increased dramatically for small bass in recent years, to perhaps as much as triple previous estimates. This could mean that assumptions surrounding current striped bass population levels are flawed.
Most of the fish with the disease are year-round residents of Chesapeake Bay although there have been reports that some infected fish are showing up as far north as New England waters. Other species in the bay, particularly Atlantic menhaden, a prey species of stripers and bluefish, also show increasing evidence of mycobacteriosis infection. One study showed much smaller numbers of infected fish in Delaware Bay to the north. Stripers further from the Bay display signs of the infection less often, perhaps because the migrating fish spend only a few months in the Bay while spawning, or perhaps because the symptoms lessen or are eliminated as fish find better sources of food and cleaner and cooler open water.
The infection is age-dependent and is even found in young-of-the-year fish. Infection rates can be as high as 50% for 3 to 4 year old males and up to 80% for age 6-year fish. Skin lesions are observed more in the fall than in other seasons.
People handling infected fish may develop skin lesions that can be difficult to treat. These fish should not be handled by individuals with cuts or sores on their hands. The workshop participants did not concern themselves with the health implications of handling or eating infected fish. Perhaps this issue is perceived to be too politically charged, but more understanding of the possible health ramifications of delivering infected fish to the seafood consuming public would seem to be of critical importance.
What happens to the sick stripers swimming in Chesapeake Bay? This is an important question since estimates are that Chesapeake Bay and its tributaries, during breeding times, hold about 70 percent of the Atlantic coast breeding stock. Mycobacteriosis usually causes death in an aquaculture environment but the effect on mortality in the wild is not yet certain. While the current mycobacteriosis outbreak may not as bad as it sounds, it certainly is not good. The mycobacteriosis problem combined with the smaller reproduction numbers from recent years, the continuing environmental decline in spawning areas, and the over fishing of breeding age fish doesn’t make the future look very bright.
A fair question to ask is how making striped bass a game fish would alleviate any of these negative pressures. Certainly it will not solve the mycobacteriosis problem, nor will it clean up Chesapeake Bay. On the other hand, when migratory bird populations were threatened, removing the pressure of market gunning provided the remaining ducks and geese the greatest possible opportunity to create strong broods: the results are undisputed.
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