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Marked changes in the distribution and relative abundance of sardine Sardinops sagax populations in the California and Southern Benguela Current systems have been reported (CCS and SBCS, respectively), with these changes occurring over recent decades during periods of steadily increasing population size in both systems. Such distributional changes may be environmentally-mediated. We assess whether mesoscale frontal activity could be a mechanism underlying latitudinal and longitudinal shifts in sardine distribution in CCS and SBCS, respectively. Over the period 1980-1997 sardine in the CCS showed a progressive northward shift in the location of highest relative abundance, suggesting northward changes in the location of favorable conditions for young sardines along the frontal zone where the southward California Current and the inshore, poleward California Countercurrent (CcC) converge. We proxied interannual variability along the frontal zone by developing monthly time-series of the frequency of SST fronts in different areas along the California–Baja California coast using the singleimage, edge-detection method applied to monthly satellite data. The relationship between sardine abundance and SST front frequency indices suggests that recruitment increases where optimal front-frequency levels are found and declines where they are suboptimal. We use a similar approach to assess whether the eastward shift in the distribution of Southern Benguela sardine was also environmentally mediated by examining frontal activity data in the period 1985-2005, from different areas along the Agulhas Bank, where seasonal and interannual variation in interactions between a major surface boundary current (the westward-flowing Agulhas Current) and an eastwardmoving, coastal counter-current also occur.

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