On The Bayfront
I received e-mail last week from a fisherman who came across a problem and wondered if I knew the answer. Am I the “Dear Abby” of the fishing world? Well, I guess he thinks so!
The e-mail went like this: “I was fishing in the bay and came across a few large striped bass that had sores and appeared distorted. They look like they lost weight. When I opened them I saw they had sores and tumors on their organs. They were disgusting.”
As it turns out, this wasn’t the first time I’ve heard of this. Toward the end of the fishing season for the last few years I’ve heard of encounters with such sick fish. At this time, it appears to only affects the older striped bass. A few minutes of research pinpointed a very serious problem that is growing bigger by the year.
There is a large striped bass study group in Chesapeake Bay, Maryland. Biologists are trying to find out why the survival rate of striped bass, whose numbers rebounded under strict catch limits, appears to be falling. There’s an increased concern that we could be looking at some future crash.
Pollution, disease or starvation could be at fault, biologists say. Some scientists are also beginning to ask whether the recreational-catch limits and commercial-harvest quotas, which helped the striped bass recovery, are too restrictive.
“We’ve got a rare case of a species coming back to high abundance and are now seeing things that may be problems caused by this high abundance,” said Desmond Kahn, a biologist with the Delaware Division of Fish and Wildlife. If the trend continues, the decline would be the first threat striped bass have faced since the 1980s, when overfishing whittled down their numbers to such a point that the federal government banned fishing for the species from North Carolina to Maine.
The Atlantic States Marine Fisheries Commission, composed of fifteen states, engineered the moratorium. The Atlantic commission declared the striped bass “fully recovered” in 1995.
Striped bass now support a multimillion-dollar sports fishing industry and are regularly pursued by commercial netters. About 75 percent of the coastal striped bass population is spawned in the Chesapeake Bay. Most of the fish live there for several years until they mature. They then join the adult population that generally migrates offshore from North Carolina to Maine’s coastal rivers, returning to the bay each year to spawn. The commission’s estimates continue to show robust population growth.
Yet, biologists are trying to figure out why young striped bass in the bay aren’t living as long as they once did. The bay’s striped bass enjoyed a survival rate between 60 and 70 percent through the mid-1990s, Kahn said. That rate, however, dropped to 40 to 50 percent in 1998 and has remained about the same since then. Kahn said an analysis of tagged fish indicates the numbers are declining because the striped bass are dying - not because they are being harvested. One possible cause could be a disease called mycobacteriosis, which was discovered in the Chesapeake Bay’s striped bass in 1997, a year before striped bass survival rates fell. At first, an estimated 10 percent of the bay’s resident striped bass were believed infected with the disease, which often leaves red sores on a fish’s flanks and attacks its internal organs. Now, more than 70 percent of the fish are infected, said Vogelbein. Another problem could be pollution. Nutrients from sewage-treatment plants and polluted runoff from farms and development create algae blooms that leave a vast “dead zone” in the bay’s depths devoid of oxygen. Striped bass normally seek summer refuge in deep water but now must make do in shallower water that contains oxygen but is warmer than the fish prefer.
Mycobacteriosis is a generic term that describes diseases caused by a group of bacteria known as mycobacteria, which are widespread in aquatic environments. A small fraction of mycobacterial species causes disease in animals and humans. The current outbreak is most commonly associated with a new species of mycobacteria called Mycobacterium shottsii . M. shottsii was first identified by VIMS scientists in 2001, and is present in 76% of infected bass. Infected Bay bass also harbor other species of mycobacteria, sometimes within the same fish that contain M. shottsii . Mycobacteriosis of Chesapeake Bay striped bass primarily infects internal organs such as the spleen and kidneys. Internal signs of the disease typically include small grayish white nodules in these organs. A small percentage of the infected fish also exhibit unsightly skin ulcers. Loss of scales is common in these ulcers. Infected fish sometimes exhibit significant weight loss. These disease symptoms are mainly observed in the summer and early fall. Fish exhibiting the unsightly skin ulcers are of greatest concern to anglers. There is a slight potential for human infection from handling striped bass infected with M. shottsii and other mycobacteria. Concern is warranted because M. shottsii is closely related to M. marinum , a species known to pass from infected fish to humans via handling. ( M. marinum has also been isolated from some Chesapeake Bay striped bass). Anglers should therefore : return any fish with skin lesions to the water, wear gloves when handling striped bass, take particular care if they have a cut, scrape, or abrasion on their hands or arms, and wash thoroughly with soap and water after coming into contact with fish or open water. Do not keep or eat a fish that you would not buy in a fish market. Any fish that are consumed should be cooked thoroughly: greater than 75°C (~170°F) for 20 minutes. Anyone who suspects they may have been exposed to mycobacteriosis from handling infected striped bass should contact their physician and inform them of the nature of the exposure.