Ecosim simulation exercise was carried out for predicting over 10 years the changes in fishery yields in the multi-species and multi-gear marine fisheries of the Arabian Sea off Karnataka. The present study elucidates that in all the gears (multiday and single day trawl; purse seine, drift gillnet, hook and line and artisanal) the key resources such as mackerel, sardines, seerfishes, tunas, sharks and skates and rays showed rapid decline in yields within 5 years due to a consistent increase in fishing effort (@ 17% per annum). The shrimp yields showed an increasing trend in trawls as they seem able to sustain the high fishing pressure as long as their predators are also harvested. In all gears excepting hook and line, there is no ecological and economic advantage in increasing the fishing effort. Also increasing the effort can result in rapid declines of many important marine resources. This will have a serious effect on the ecosystem functioning. Attempt has been made to model changes in the marine ecosystem due to fishing as part of the effort to move towards ecosystem based fisheries management. This is the pioneer effort for the same.