In the "index of biological integrity," scientists are trying to find a way to rank an estuarine river based on fish assemblages - or entire communities - rather than using a single indicator species to serve as a barometer of the system's health.
To do that, they divide the catch data for each river into three categories, each of which have three subcategories.
Diversity or Richness. This is measured by:
- The total number of species present.
- The results of the bottom trawl, which helps assess the quality of deep water habitats.
- Species dominance. Dominance is determined by finding the number of species it takes to account for 90 percent of the total catch. The more species it takes to do that, the more diverse the population.
Abundance. This is measured by:
- The total number of fish present minus Atlantic menhaden, which can be present in such overwhelming numbers in some rivers when they spawn that they skew the data.
- The number of estuarine spawners such as bay anchovy, killifish and silversides. These fish can live year-round in a tributary and may adapt to degraded conditions. If their numbers are diminished, it can be a sign of stressed conditions.
- The number of anadromous spawners, species that live most of their lives at sea but return to fresh water to spawn, such as shad, striped bass, blueback herring and white perch. Anadromous fish use tributaries as spawning grounds and nursery ar eas for their young. These life stages are sensitive to poor habitat conditions.
Trophic relationships. This is a measure of the fish community based on individual species' roles in the food chain. Fish were placed in three categories:
- Planktivores, species which feed primarily on algae such as menhaden, bay anchovies and silversides
- Carnivores, species which feed primarily on other fish, such as striped bass and bluefish
- Benthivores, species that feed primarily on benthic organisms, such as spot and croaker.