Estimation of potential productivity in Eastern Boundary Currents using remote sensing

This study provides a satellite-based estimate of potential primary production in the four Eastern Boundary Currents (EBCs), i.e. the California, Humboldt, Canary, and Benguela currents, from the first 24 months of the Sea-Viewing Wide Field of View Sensor, SeaWiFS. Within each EBC, production was estimated for the area of high chlorophyll concentration (>1 mg m−3) or active area, which is likely to determine the production that can be utilized by higher trophic levels. Primary production decreased with latitude within each EBC while the extent of the active area was related to the magnitude of offshore transport. The most productive EBC was the Benguela Current (0.37 Gt C yr−1), followed by the Canary (0.33 Gt C yr−1), Humboldt (0.20 Gt C yr−1), and California (0.04 Gt C yr−1) Currents. Interannual differences between 1997, 1998, and 1999 were largest for 1997 (measured by the Ocean Color Temperature Scanner, OCTS), which may be due primarily to the different sensor and algorithm. The Humboldt Current was more productive, and the Canary much less, during 1997 than in the two following years. The El Niño of 1997–1998 led to smaller annual production in 1998 in the Pacific EBCs. The upper bound of sustainable fish yield was estimated assuming a food chain of 2.6 links and an average trophic efficiency of 10%. The resulting values are 4–150 times larger than the observed fish catch from 1990 through 1997. Actual catch data in the Benguela Current were 20 times smaller than in the Humboldt Current. The most likely explanations for the differences in potential and observed fish catch are related to differing trophic structure and spatial accessibility in different EBCs. If the estimated yield is an upper bound that will be decreased to 10% or 20% by environmental accessibility, the small pelagic fishery in all four EBCs is likely to be food-limited.