Calculating an index of abundance
Data collected every year is used to determine the size of the spawning runs in Virginia. A catch index is an assessment of the population size measured by catch and effort data. It is usually measured as number (or weight) of fish caught per standard unit of fishing effort. In the case of the VIMS American shad monitoring program, it is the weight of pre-spawning fish captured per total days of sampling. Post-spawning females captured are not calculated in the catch rates since the historic shad fishery was a roe fishery. A seasonal catch index is calculated by estimating the area under the curve of daily catch versus time of the year. With several years of data, the results can provide an informative picture of a species' health and spawning success in Chesapeake Bay.
A benchmark for stock assessment
Using historical data, restoration targets were set for American shad in the York, James, and Rappahannock rivers. Catch-per-unit-effort data were compiled from logbooks that recorded landings by commercial fishermen using staked gill nets at various locations throughout the middle reaches of the three rivers. Fishing data from 1980-1993 were used to produce the average catch index for the James and Rappahannock rivers. Data from the 1950s was used for the York River catch index. These historic indices are the current restoration targets. Each river has a specific catch index that must be met before the fishing ban will be lifted. Monitoring data for the James, York, and Rappahannock rivers are available. Historic and current indices are graphed along with restoration targets for each river.
Spawning Stock Assessment
Catch data is used to calculate a daily and seasonal catch per unit of effort (fish/meter of net/hour) per species. The 2014 catch rates will serve as the starting point for future comparisons to determine annual relative abundance of river herring. Age composition, mortality, sex ratio, and other attributes of the spawning stock of each species, are also calculated.
Juvenile Index of Abundance
Catches, mean length, mean weight, and the mean fish per tow from the nighttime surface trawls on the Chickahominy River are calculated. An index of abundance of juvenile river herring and American shad is obtained through the annual VIMS Juvenile Striped Bass Seine Survey in the James, York, and Rappahannock rivers. Data from this survey is reported, as this survey provides greater spatial coverage within the tributaries of the Chesapeake Bay. Catches from different years are standardized by calculating a juvenile abundance index (JAI, geometric mean catch per tow), which allows for a relative comparison of catches among years and between rivers.