Distribution of Pacific Sardine Spawning Habitat within Waters of the Mexican and U.S. EEZs

Pacific sardine (Sardinops sagax) eggs were collected by CUFES (Continuous Underway Fish Egg Sampler) systems on joint cruises in Mexico and the U.S during the months of April and May from years 2000 through 2013. The objective was to estimate the distribution of the spring spawning habitat between the two Exclusive Economic Zones (EEZs). We employed a random forest model to interpolate the distribution of eggs obtained over the regional sampling grid. The model employed variables of 1) chlorophyll, 2) line and station numbers in Mexico (by IMECOCAL program) and the U.S. (by CalCOFI program), 3) date and year of sampling, and 4) pumping time of CUFES for collection of the egg samples. Given the results for the area of spawning, then SST ≤16.4ᵒC was used to distinguish the proportion of the northern sardine stock within the CalCOFI region compared to that in the IMECOCAL region. Results from that the habitat probability model indicate:

1. The optimal spawning conditions are found in the CalCOFI region.

2. 4.1% of the northern stock was found in the IMECOCAL region during April, 2002, while only 1.1% was found there in April, 2003.

3. The results also suggest that there exists a mixture of the northern and southern stocks as defined by the SST threshold of 16.4ᵒC, with the southern stock, inferring that it extends northward into U.S. water during the Summer-Fall period with a different spawning schedule than the north stock, whereas the northern stock extends southward during Spring during its spawning season.