Reply to MacCall et al.: Acoustic-trawl survey results provide unique insight to sardine stock decline

Abstract

Long preceding our investigation (1) and MacCall et al.’s response (2), MacCall (3) recognized that consideration of environmentally driven low-frequency variability in fish populations is critical to successful fisheries management. He noted that prolonged periods of poor recruitment coupled with fishing rates suited to more favorable environmental conditions may result in depletions of valuable fish stocks (3). Our analysis of the literature and numerous indicators, particularly the results of our 2006–2011 acoustic-trawl surveys (A-T), forecast such a depletion of sardine. Foundations of our analysis are time series of pelagic fish biomasses and biomass-weighted length distributions of sardine, uniquely provided by the A-T. The 2006–2010 A-T results show that the sardine biomass has decreased by more than fivefold; importantly, their recruitment has been low; and the migrating portion of the stock has been composed of a single dominant, rapidly diminishing length class. Also, the 2011 A-T results indicate only a modest recruitment, which is consistent with the increase in spawning stock biomass estimated with the Daily Egg Production Method (DEPM) (Fig. 1). Furthermore, mackerels are now more abundant than sardine in the survey region; and the remaining sardine trend increasingly to be caught with research trawls in areas with mackerels. MacCall (3) claimed that effects of intraguild competition may prolong rebuilding or reduce sustainable yield.