Calculate the difference between the 1990-1997 average blue crab population and the 1998-2009 average blue crab population predict the average blue crab population for 2010-2020 and explain

The difference of the blue crab population From 1990-1997, the average was (800+750) / 2 = 775; and from 1989-2009: (350+350) / 2 = 350. I predict that from 2010 to 2020 the population will increase to about 500 because it seems to go on trends of increasing and decreasing.