Do YOU remember what you had for lunch last Thursday?
The shark being interrogated has eaten a person, and the cops are showing a picture of the person they ate to them. The cops accuse the shark of eating the person on Thursday, and, in defense, the shark retorts that he doesn't remember, and justifies this by saying that it's normal to not recall that type of detail: the cops also probably couldn't remember what they had for lunch a few days ago. The joke is that lunch isn't a small unimportant detail for the shark because they killed someone.