The Mortality/Growth ratio of larval fish and the slope of the zooplankton size‐spectrum

Abstract

Abstract Predicting recruitment remains a fundamental problem in fisheries science. The Mortality/Growth ratio ( M / G ) of larval fish summarizes the competing rates of M and G in relation to cohort survival and potential recruitment. The larval cohort biomass initially declines after spawning ( M > G ) until transition ( M / G = 1), after which the cohort may grow in biomass ( M < G ). A major challenge is to obtain routine fine‐scale measurements of M / G and integrate them over broad scales. Here, we propose the M / G size‐spectrum hypothesis, whereby the M / G ratio of larval fish is correlated with the slope ( S ) of the zooplankton size‐frequency distribution. S represents the tension between faster and more efficient growth (flatter S ) and mortality (steeper S ) among particles the size of larval fish. S is explicitly driven by the ratio of mortality to growth of zooplankton, and the range of published values of S (−0.4 to −2) is consistent with M / G ratios reported for larval fish. As S can be measured at fine and large spatial scales using towed optical plankton counters, the M / G size‐spectrum hypothesis provides a context to help quantify the recruitment potential of larval fish.