The initial elevation of the seagull is at 15 3/8 feet. (+ 15 3/8 feet)
The seagull then descended 23 1/6 feet to catch the fish. (- 23 1/6 feet)
Therefore the elevation of the fish is:
elevation of fish = 15 3/8 – 23 1/6
elevation of fish = 123 / 8 – 139 / 6
elevation of fish = - 374 / 48 = - 187 / 24 = - 7 19/24
negative sign means it is below sea level