By Janet Krenn, Staff Writer
When a mathematical model said that catch limits for Chesapeake Bay striped bass could be increased, managers knew something was fishy. Usually, catch limits increase only if fish abundance has increased, but striped bass have been in steady decline for the past seven years.
It was a troubling contradiction. Fishery managers use these models to set fishing effort, and accuracy is essential. If managers accepted the erroneous model results, fishing would increase and be unsustainable.
In short, says Matt Smith, “The models were producing screwy results.”
Smith, Sea Grant-National Marine Fisheries Service Population Dynamics Fellow, joined advisor John Hoenig at Virginia Institute of Marine Science to check the math.
In these models, little numbers make big differences. To calculate how many striped bass are out there, the model needs to know the rate at which striped bass die of natural causes and the rate at which they are caught by fishermen. Because data showed that the number of fish caught by fishermen was on target, Smith suspected something else was at play. Could fish be dying of natural causes more quickly than originally estimated?
“Historically, there’s been a single default natural mortality rate used for all striped bass,” says Smith, and for a while, that number worked well in the model. “Then, along came myco, and abundance started trickling downward.”
Myco, short for mycobacteriosis, causes skin infection in striped bass. The disease was first observed in the Chesapeake Bay around 2000 and could be the reason for the models’ errors.
“Disease prevalence throws a wrench in the stock assessment,” says Beth Versak, fisheries biologist at Maryland Department of Natural Resources who specializes in striped bass. Versak is also a member of the Atlantic States Marine Fisheries Commission’s Striped Bass Tagging Sub-Committee that manages the mathematical tag-based models. The more widespread the disease, the more it affects the survival of a whole population. In the case of myco, she says, “Striped bass are a little unique because of the level of disease prevalence in the population.”
On behalf of the Sub-Committees, Smith and numerous colleagues worked with existing striped bass tagging projects to investigate how long diseased fish could live. To accomplish this task, researchers rated the fish’s level of infection, then tagged and released the fish. When fishermen reported catching tagged fish, Smith was able to use this data to calculate disease-associated reductions in survival. In some cases, fishermen actually held recaptured fish on ice and returned them to the research team. These fish were reevaluated for signs of disease allowing Smith to calculate how quickly the disease progressed.
Smith’s research estimates that myco is responsible for a 60% increase in the natural mortality rate for Chesapeake Bay striped bass. Of course, survival depends on the severity of the infection. Fish released with the earliest signs of disease showed no reduction in survival while those released with the most severe signs of disease survived half as well as healthy striped bass. The study also found that on average, it takes about three-and-a-half years for the disease to progress from its earliest stages of the disease to the later stages with substantially increase mortality.
The Striped Bass Tagging Sub-Committee agreed with Smith’s results and accepted that a shift in striped bass natural mortality rate occurred around 1998. Now the model uses the original natural mortality rate for fish tagged before 1998 and a new rate for fish tagged after that year—a big accomplishment considering the Sub-Committee members from different states need to reach consensus to enact such a change.
As scientists improve their understanding of striped bass and what happens to them in the wild, the models should become more accurate over time. These improvements will happen incrementally. Says Versak, “And Matt helped us to tease that out a little bit more.”