Natural Variability in Alewife Abundances on the East Coast of North America: Long-Term Trends and Links with Climatic and Biological Indices

Natural variability occurs in all ecological systems and can potentially enhance or mask system changes caused by other factors. Long-term natural variability may obscure the detection of tipping points through dampening and resiliency effects even when systems are close to transition. Biological monitoring programs are relatively brief and often are unable to be linked in useful ways to identify natural variability to tease apart underlying relationships of populations to suspected system drivers. We assembled river herring abundance (counts and landings) data from 24 rivers along the east coast of North America spanning 1865-2011. Eight different count methods were used. Both counts and landings were connected by calculating proportional variability thereby mitigating methodological differences. Proportional variability was investigated as a potential variability metric, and was used as a response variable to study the relationships of measures of abundance to long-term climatic and biological indices. Proportional variability was found to be sensitive to abundances that fluctuated by magnitudes, but showed promise as a variability metric and indicator of rapid or transitional change when abundances were within typically natural ranges.