When do humans switch from aerobic respiration to anaerobic respiration ?

Humans switch from aerobic respiration to anaerobic respiration when they lack oxygen to breathe.

Explanation:

When humans and other animals lack sufficient oxygen, they'll create ATP through a form of anaerobic respiration.Anaerobic means respiration under low oxygen level.When you are doing heavy work your heart beats faster to deliver you oxygen. if this oxygen is not enough even at the highest heart beat then your body switches to anaerobic respiration. In anaerobic respiration you take glucose and break it down into carbon molecule called as lactic acid.  

They include lobsters and crabs

Ithink its because more have died recently than in the past.