Environmental influence on the behaviour of Indian mackerel and their availability to fishing gear along the Malabar coast

The upwelling phenomenon observed along the Malabar coast regularly from March to October, produces a plankton bloom. This helps in increasing the reproductive output of mackerel and effect successful spawning and recruit­ ment. During the monsoon period the new recruits concentrate at the surface as the thermocline is very shallow. During this period they become highly vulnerable to the surface gear. With the sinking of thermocline they move to deeper waters reducing their vulnerability. Mackerel prefers a temperature around 27°C and hence stays immediately above the thermocline. Hence, there is a demersal phase in the life history of the fish during summer. The fish is a prolific breeder always ready to make use of a favourable environment for spawning.