A list containing all elements of mortality_params with
missing entries filled with defaults: base_mortality = 0.001,
natural_mortality (copied from base_mortality),
fishing_mortality = 0, predation_mortality = 0,
optimal_temp = 15, thermal_tolerance = 5, and
stress_factor = 2. If a reproduction sub-list is present,
a spawn_pattern numeric vector (length 365) is appended.