Research Product

Categorical time series regression models were developed to evaluate the importance of natural and anthropogenic environmental changes to the determination of fish stock status in the Choptank and Rappahannock estuaries. Time series from 1929-1988 for the commercial landings of six species and associated fishing effort were reconstructed for the two river basins. Relative catch-per-unit effort (CPUE) was determined and used as an indicator of stock size. Hydrographic data concerning freshwater discharge, temperature, precipitation, and wind speed and direction were collected, and time series were constructed to establish relationships between the variability in stock abundance and climatic variation. Time series for the same period were developed for indices representing pollutant loadings, developmental activities, and habitat alterations for the two targeted river basins. These indices included representations of demography, sewage and industrial loadings, habitat factors, and nutrient, carbon, and suspended sediment inputs. The compilation of these data is the result of intensive data collection efforts using county, state, and federal records to ascertain activities along individual water basins through time. Direct measures of loadings were used when available, and indirect indicators of loadings using materials-balance approaches were utilized where appropriate. Results of model regressions indicate that variation in the natural environment explains most of the variation in relative stock levels for most of the target species in the two Chesapeake Bay tributaries. The inclusion of significant natural variables in the models is generally consistent with existing knowledge on species biology. Pollutant variables such as pesticides, two heavy metals, PCBs, and nitrogen also contributed significantly to explaining observed variation in relative stock abundance of several species in both river systems. Further investigation is required to establish whether cause and effect relationships between pollutant loadings and stock levels may exist.