Ulcerative Dermatitis In Striped Bass
| The following is a summary of the meeting held at the U.S. Fish and Wildlife Chesapeake Programs Office in Annapolis, Maryland on November 20, 1997.
Summary of Striped Bass anomalies during Fall 1997 Tagging Project by DNR
The Striped Bass Project has been sampling fish from pound nets throughout the Chesapeake Bay and Potomac River over the last several months. The pound nets range from Pt. Lookout in the lower Bay to the Still Pond Creek area in the upper Bay. The nets on the Potomac range from Pt. Lookout up river to the Herring Creek (Tall Timbers) area. Sampling was done in three rounds:
Round 1 (8/11/97 - 8/16/97)
Round 2 (8/23/97 - 8/29/97)
Round 3 (9/27/97 - 10/3/97)
All Striped Bass were measured and examined for overall health, and the majority were tagged and released. Any anomalies we saw were classified as follows: hemorrhagic, necrotic, petechia, abrasions, and other. The First three are thought to be associated with the bacterial infections that Striped Bass tend to get during the summer, and the last two were used to describe net and other physical damage.
Approximately 700 phone calls from the public have come in to the fish health hotline since mid-September and the majority of the calls identify these bacterial problems in Striped Bass. We contacted John Coll of the Fish Pathology Unit of the U.S. Fish and Wildlife Service. This unit has one of the best equipped fish health labs in the United States. John accompanied us during Round 3 tagging and sampled several dozen Striped Bass from pound nets in the Potomac River and middle Bay. Samples were also taken by our Oxford fish health unit, and sent to the University of Maryland fish health labs.
A total of 4,275 Striped bass have been sampled by the Striped Bass Project over all three rounds with the following anomalies observed. Rounds one and two are combined because very few fish were sampled in Round 1.
Total # sampled
Total with any type of anomaly
Total with bacterial related anomalies
|Main Bay: Rounds 1 & 2
|Potomac: Rounds 1 & 2
|Main Bay: Round 3
|Potomac: Round 3
Interstate management of Striped Bass has restored and maintained a high biomass in the 1990's with high size limits and low fishing mortality rates. Striped Bass biomass has increased greatly, while biomass of other two major inshore piscivores along the Atlantic Coast has decreased. An extended size structure in Chesapeake Bay now exists that was absent under low size limits and high mortality rates prior to 1985. Dominant year classes were produced in 1993 and 1996 and Striped Bass abundance is very high in the Bay. During 1997, a considerable fraction of legal-size Striped Bass (> 18 inches) were observed with ulcerative dermatitis and weight-at-length in September - October was significantly lower than in the early 1990's.
We have formed five working hypotheses to test as underlying causes of poor condition and ulcerative dermatitis. These are:
- Unusual warming patterns or warmer than usual temperatures in 1997
- Development of a temperature-dissolved oxygen (DO) squeeze because of high temperatures and high nutrient levels
- Forage depletion from adverse environmental
- Forage depletion from high predation
- A combination of some or all of the previous factors
Larger Striped Bass, primarily members of the 1993 year-class, might also be at a disadvantage in competing for forage with the 1996 year-class because younger fish could have continued to feed and grow in higher water temperatures that exacted a high metabolic cost on large fish. By Fall, the predatory demand of these two year-classes may have outpace the supply of their main forage species. We have begun developing approaches for testing these hypotheses and have approached other agencies and universities to help us.
The University of Maryland Eastern Shore has a Striped Bass bioenergetics study underway this year to look at forage demand. This model should be adaptable for addressing the effect of temperature at differing levels of feeding success on fish condition.
The Department of Natural Resources' Tidewater Ecosystem Assessment Division has queried their long-term Chesapeake Bay water quality data base to see how main Bay temperature-DO conditions in 1997 differed from other years. It is possible that the amount of habitat with suitable temperature-DO for large Striped Bass has diminished or the population has expanded beyond what is available.
We examined trends of major forage species relative abundance in our Juvenile Striped Bass Seining Survey and indices of the two major Striped Bass prey, Atlantic menhaden and bay anchovies, are at the lowest levels ever observed. Atlantic silverside and Spot abundance is also low. Blueback herring and Gizzard shad abundance are at reasonable levels; however, these species would mostly be available in fresher regions of the Bay where Striped Bass do not typically reside. White perch are the only prey species whose abundance has increased in recent years. These data support a hypothesis of depleted forage abundance, but the role of Striped Bass in this depletion is unclear. Examination of trends in baywide zooplankton abundance (using Bay Program monitoring data) may provide insight on whether major forage fishes have decreased because of major shifts in productivity or predation.
The field sampling for 1997 coincided with the Striped Bass stock assessment group's fall tagging survey which takes place during the fall Striped Bass season in the Chesapeake Bay. Fish were collected from pound nets located at Tall Timbers in the Potomac River, the main Bay north of Point Lookout, Kent Island, and an area near Rock Hall. To date a total of 55 fish have been collected and delivered to Maryland Department of Agriculture's Animal Health Laboratory in College Park, Maryland. Streptococcus and Mycobacterium were observed in several of the internal organs. The lesions on the fish were of a different characteristic than those seen in 1995; early in the sampling the lesions were severe and the skin more eroded on some of the fish to the point that the muscle layer was visible (about 15-20%). Later in the fall the lesions seemed to heal on a lot of the fish because the number of individuals showing lesions had diminished significantly (5-10%) probably due to the drop in water temperature. The incident is also species specific which was not the case in the 1988 Streptococcus event. This alone makes this incident even more unique.
A necropsy of the fish revealed an enlarged spleen with white nodules throughout; the head kidney's confirmation was spongy and pale (not a deep red color). Body fat was non-existent in the majority of fish examined along with empty stomachs and lower GI track. The brain on some fish was hemorrhagic and the meninges were clear, probably due to the bacterial infection. In addition, this year parasites were numerous in the lower intestine of several individuals.
- Is this problem with a disease in the Striped Bass an
- Is this a forage or population problem?
- Do any of these fish die or are they recovering from this disease?
- Why are the fish able to live with a Streptococcal infection of this
magnitude when in 1988 many fish died?
- Has this Mycobacteriosis been causing this problem all along and we
have just missed the bug each year?