Altering Prey Availability and Hypoxic Extent

For each of the four maximum hypoxic extents (15, 30, 45 and 60%), three prey availability levels were considered (Limited, Default and Unlimited). The controls acting on the population were quite different between these cases. In brief, under food limitation crabs had lower survival, decreased rates of growth and reduced rates of egg production per mature female. When food was not limiting, the primary factor limiting the population was a cannibalism-patch interaction - as seen in the first scenario.

Dissolved Oxygen

Within each of the four maximum hypoxic extents, dissolved oxygen differed little between the Limited, Default and Unlimited prey simulations.

Background Prey and Clam Biomass

Overall, background prey levels were reduced by factors of 3-7.5 between the Unlimited and Limited cases. Under Limited there was little background prey towards the mouth of the estuary relative to either the Default or Unlimited cases. In the Unlimited prey case, the background prey actually increased with increasing hypoxic extent. This was due to the exceedingly large juvenile crab population at low hypoxic extents greatly reducing background prey. At larger hypoxic extents, the crab population was diminished by a cannibalism-patch interaction, and thus background prey biomass increased.

Clam biomass decreased by factors of 2-6.9 between the Unlimited and Limited cases with Default being closer to the Unlimited case. Generally, there was higher clam biomass in the deeper parts of the estuary – although under Limited and 60% hypoxic extent little clam biomass was present anywhere in the estuary.

Clam Density

Under Limited, overall levels of clam recruitment were much lower than under Default or Unlimited. However, in all cases recruitment occurred over the entire estuary, although it was slightly lower along the shoreline of the estuary.

Crab Biomass and Crab Density

As a result of the above changes in prey abundance, overall crab biomass was lower by a factor of ~2 under Limited relative to Unlimited prey availability while Default was in between these two cases. Changes in overall crab densities were similar to the overall changes in crab biomass (Unlimited, Default, Limited). Crab densities were always highest along the shoreline, but decreased between Unlimited and Limited.