Altering Cannibalism Potential and Hypoxic Extent

For each of the four maximum hypoxic extents (15, 30, 45 and 60%), three cannibalism potentials were considered (Small, Default, and Large). All variables are reported with respect to these three levels. The main finding from this scenario is that Small cannibalism potential leads to higher juvenile and adult crab densities that in turn lead to food limitation and reduced rates of egg production per mature female. Alternatively, Large cannibalism potential increased rates of mortality so much that adult density decreased. Although rates of egg production per mature female were high, adult density decreased so much that overall rates of estuary egg production decreased and limited subsequent recruitment.

Dissolved Oxygen

Within each hypoxic extent, dissolved oxygen differed little between Small, Default and Large cannibalism potentials.

Background Prey and Clam Biomass

Relative to Small cannibalism potential, levels of background prey were ~4 times higher under Large and ~3 times higher under Default. Background prey under Small cannibalism potential was lowest towards the mouth of the estuary and decreased with increasing hypoxic extent.

The overall increases in clam biomass between Small, Default and Large were smaller (factors of 1.25 to 1.5) than those observed for background prey. The increase in clam biomass is explained by differences in crab density. Under Small cannibalism potential (leading to higher crab densities) there was a low clam biomass along the shoreline, while under Large cannibalism potential (low crab densities) the shoreline had a higher clam biomass.

Clam Density

Clam density under Small, Default and Large cannibalism potential was not limited by recruitment.

Crab Biomass and Crab Density

Relative to the Default cannibalism potential, crab biomass was more than 3 time higher under Small and <2 times higher under Large cannibalism potentials. Spatial shifts in the locations of high crab density also occurred. Under Small cannibalism potential, high crab densities occurred in the lower salinity waters far away from the mouth of the estuary – waters which also had the highest background biomass. Under Default, crab density was high along the shoreline in the middle of the estuary, while under Large cannibalism potential crab density was highest in the shoreline nearest to the estuary’s mouth.