Rules: Sources of Mortality

Mean annual natural mortality rate is $\approx$ 0.375 (Eggleston 1998; Rugolo et al. 1998) and only about one out of every million eggs survives to become an adult (van Engle 1958). Although crabs can live as long as 8 years, the average age in the Chesapeake population is $\approx$ 1.5 yrs and $\approx 97$% of all individuals are less than 3 yrs of age (Rugolo et al. 1998).

In the model, a crab dies either because of aggression from other crabs, temperature extremes, starvation, a lack of oxygen, or when it reaches its life expectancy. Rules governing crab death as a result of interactions between crabs were discussed in Appendix A.5.3. If the temperature at the crab's location goes below 5 $\ensuremath{^\circ\text{C }}$ or above 35 $\ensuremath{^\circ\text{C }}$ (the min and max temperatures for the crab's metabolic processes (Tagatz 1969, Table 1)) the crab dies immediately due to thermal stress. A crab dies of starvation if its mass drops below 40% of its maximum weight, $G_{\text{max}}$. Likewise, a crab dies due to a lack of oxygen if the DO in the environment encountered by the crab is continuously below 1 mg/L for more than 3 days. The criteria for death due to hypoxia are kept simple because in the actual estuary, crabs will likely swim up to the oxygen rich surface waters if the bottom waters become hypoxic (Das and Stickle 1994). This condition is included as a way to ensure that the crab movement algorithm is sufficient for getting crabs out of hypoxic waters in a timely fashion. Lastly, crabs may die due to other causes not explicitly modeled. The life expectancy of crabs (assuming no predation) is assumed to follow a Beta distribution. The life expectancy of a crab (assuming the do not die from specific model causes) is generated when the crab is first instantiated into the estuary (Appendix A.5.13). Let $X \sim$   Beta$(m_1=3.0, m_2=1.5)$ then the time when dies due to other causes is given by $Y = 8 X$. Thus, we assume crabs don't live beyond 8 years.