The fluctuations in abundance of a demersal fish, the Malabar sole (Cynoglossus macrostomus Norman) and a pelagic fish viz., oil sardine (Sardinella longiceps) in the Malabar upwelling ecosystem (MUE) were analysed in relation to long-term trends in rainfall in Kerala, onset time of the southwest monsoon, sea level off Cochin and solar periodicity. The abundance of these species showed a clear decadal trend comparable to such trends existing in rainfall, sea level and solar periodicity. The variations in the rainfall showed a 35-year rhythm in its intensity. A fishery forecast of Malabar sole and oil sardine fishery has been attempted based these trends and is a posse ad esse. The fluctuations in abundance of oil sardine, beyond doubt, are dearly influenced by the trends in rainfall during southwest monsoon. Periods of good rainfall have positive effect on its abundance and vice versa. There appear to be a decadal or long-term trend in the productivity of the upwelling zone itself. The spawning, recruitment, survival, feeding, growth, migration and the abundance of various organisms are influenced by rhythms exiting in this large marine ecosystem of the southeastern Arabian Sea.