Trends in Relative Abundance and Early Life Survival of Atlantic Menhaden during 1977-2013 from Long-Term Ichthyoplankton Programs

The Atlantic Menhaden Brevoortia tyrannus, a commercially important clupeid, supports one of the oldest and largest commercial fisheries on the U.S. East Coast. Despite recent increases in adult biomass, juvenile indices have declined coastwide and have remained particularly low in Chesapeake Bay. In order to understand the underlying causes of this decline, knowledge of larval recruitment is essential. We developed an index of larval abundance by using larval data collected from two large-scale ichthyoplankton sampling programs that occurred from Nova Scotia, Canada, to Cape Hatteras, North Carolina, during 1977-1987 and 1999-2013. Larval abundance data were standardized to a day-0 age by applying an age-length key from a study of larval ingress into Chesapeake Bay; a delta-lognormal model was used to account for spatial and temporal changes in sampling. We found that Atlantic Menhaden larval abundance increased from 1977 to 2013 and was highest in the winter; most individuals were detected at nearshore stations. Over our time series, larval abundance corresponded closely to adult spawning stock biomass. Due to the lack of a direct relationship between our larval abundance estimates and a coastwide juvenile index, we examined several environmental factors (temperature, Chesapeake Bay discharge, Atlantic Multidecadal Oscillation [AMO], wind speed, and wind direction) for potential effects on the relative survival of larvae. Larval abundance had a significant negative effect on relative survival. Temperature and to a lesser extent wind speed and AMO appeared to exert the greatest effects on the early life survival of Atlantic Menhaden: cooler temperatures, intermediate wind speeds, and negative-phase AMO were the most favorable for survival. Our findings suggest that the observed reduction in recruitment is not a problem of larval supply but rather is limited by survival between the larval and juvenile stages.