In India, monsoon not only replenished ground water, but plays a significant role in ecological and productive cycle of the Indian Ocean. Southwesterly monsoon winds displace surface layers offshore along Somali and west coast of India, which brings subsurface nutrient-rich water to the surface (upwelling). This process re-fertilises the overfished upper 100-120 meters, and has a bearing on fish production, its distribution and abundance pattern. The upwelling promotes growth of microscopic plants (phytoplankton) in the surface photic zone which serves as fodder to the fish and other organisms down the food web.
A fishing ban of 61 days within 12 nautical miles from coastline is enforced every year from June 1 to July 31 by governments of Karnataka, Tamil Nadu, Gujarat, Maharashtra and Goa, with the key objective of conserving fish resources, while commercially important species spawn, so that the fishermen can get optimal benefit. It also provides adequate protection to fishermen and their vessels from inclement weather at sea. Though, the breeding takes place throughout the year which is different for different species depending on the environmental conditions such as temperature, salinity, oxygen in water and coastal topography.
There are three largest fisheries on the west coast of India which are Indian oil sardine (Sardinella longipes), Bombay-duck (Harpadon nehereus) and shrimp/prawn fisheries. The Indian oil sardine fishery is concentrated in large shoals along the southwest coast of Kerala and Mysore.
The coastal states of Gujarat and Maharashtra contribute the largest catches of Bombay Duck which has a wide and discontinuous distribution along both coasts. Most shrimp fisheries are subject to exploitation throughout their life cycle. According to the studies, the overall stocks of shrimps and Bombay Duck are believed to be fully exploited to maximum sustainable yield.
The breeding months vary for other commercial fish. Studies indicate that Indian Mackeral (Rastrelliger kanagurta) spawns in the coastal waters between 70-100 m depths during June to September on west India coast. The economically important migratory fish Kingfish (Scomberomorus commerson), Barramundi or Chonak (Lates calcarifer), Cobia or Mudoshi (Rachycentron canadum) and squid breeds during March-May and October-November. Silver pomfret (Pampus argenteus) spawns during March-May, Tiger prawns (Panaeus sp.) prefer April-June and Nov-December period, prawns (Metapenaeus sp.) breed during September to December, and Bombay duck spawns from January to March and October to December.
It is clear from above that most of the commercially important varieties have prolonged breeding period with a staggered for laying eggs. Therefore, it is very difficult to enforce bans in a staggered manner. The Central Marine Fisheries Research Institute, Kochi, has undertaken a couple of detailed scientific analyses to arrive at definite periods when the ban can be enforced.
