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It is well established that environmental factors influence the development of organisms and in particular that the temperature causes changes in growth in response to ambient pressure on the metabolism. This has been verified by comparing temporal and spatial changes in length at age, identifying differences in the ontogenetic growth pattern. The present work assesses inter-annual variability of the average standard length (SL) on age-1 group fish caught in Bahia Magdalena, BCS, relative to the environment [Sea Surface Temperature (SST), Level Sea and the Pacific Decadal Oscillation (PDO)], during 1982 to 2009. The biological material is the result of monthly sampling of commercial catch, registering a total of 15.704 organisms with sizes between 105 and 225 mm SL and 0 to 8 years old. Significant differences in size structure between the sexes (P> 0.05) were not detected. Multiple regression analysis between the annual average SL on environmental variables throughout the study period did not show any significant relationship. However, we identified three periods in which the annual changes of SL were negatively associated with the environment (P <0.05): a) from 1982 to 1995 with the sea level, b) 1996-2000 with the SST, and c) 2001 -2009 with the SST and sea level. Supported by evidence of the seasonal presence of two stocks of sardine in Bahia Magdalena (temperate and warm stocks) with different size structure, we analyzed the inter-annual variability of the average SL by stock in relation to environmental variables lagged one year. The SL of individuals of temperate stock was calculated using only the organisms captured from May to June of year t and was related with the environmental variables of the year t-1 from January to June. For the warm stock, the SL was estimated using size data from November to January of year t, and related to environmental variables from July to December of year t-1. Multiple regression analysis for the warm stock identified a significant relationship between the annual average SL on sea level (P <0.05); while the inter-annual changes of SL of the temperate stock was significantly related on the sea level and the PDO (P <0.05). The results support the seasonal presence of two sardine stock in Magdalena Bay, which respond, for its latitudinal distribution, at sea level (warm stock) and the decadal oscillation (temperate stock).

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