Temporal Resolution of Biological and Physical Influences on Bay Anchovy Anchoa Mitchilli Egg Abundance near a River-plume Frontal Zone

Bay anchovy egg abundance in the Manatee River estuary, Florida, was compared with conditions in the spawning ground before and after spawning to distinguish prey-supply-based spawning responses from post-spawning advective effects. Plume fronts were a prominent feature of the spawning ground. Egg distribution, copepod distribution and front development were surveyed 25 times across a wide range of freshwater inflow conditions. Surface-front strength (maximum interpolated salinity slope) was strongly correlated with the size of the frontal zone, as defined by surface areas having slopes >1 psu km ^-1 . Eggs were most abundant landward of the frontal zone at a location with consistently high abundance of the copepod Acartia tonsa , which is consumed by adult and larval bay anchovies. Large, significant changes in egg abundance were observed over both long (seasons) and short (<3 d) periods (Tukey 95% HSD multiple range tests). An El Niño-associated flood event caused the rapid evolution of a large, seaward-moving frontal system that initially had a negative effect on egg abundance. Spawning resumed in reduced salinity following passage of the front, demonstrating a level of plasticity in spawning response to salinity change. A stepwise regression model explained 92% of the variation in egg abundance and suggested that tidal amplitude was not a significant determinant of abundance. Instead, egg abundance appeared to be determined initially by pre-spawning prey supply and temperature, and subsequently by variable retention behind the frontal zone.