DO NATURAL MORTALITY AND AVAILABILITY DECLINE WITH AGE - AN ALTERNATIVE YIELD PARADIGM FOR JUVENILE FISHERIES, ILLUSTRATED BY THE HAKE MERLUCCIUS-MERLUCCIUS FISHERY IN THE MEDITERRANEAN

The paper explores the apparent contradiction between a high trawling pressure on juveniles and sustained production of hake that has occurr ed over the last decade in many Mediterranean fisheries. The practical consequences are followed of assuming rapid declines in natural morta lity rate M in the first few years of life to a low, constant adult na tural mortality, as well as the observation, for small-mesh trawl cod ends, of declining availability with age. Several approaches are propo sed for fitting declining M-with-age with a reciprocal function for ha ke, using criteria based on mean life-time fecundity, mean age at egg production, existing estimates of adult M, and vectors based on stock productivity assumptions. All vectors of M-at-age were similar to MSVP A estimates of North Sea stocks. The implications of the changes in mo rtality with age for stocks harvested by fine-mesh trawls were explore d in yield per reemit calculations under 2 different hypotheses: 1) us ing current estimates of growth and adult mortality, 2) M-at-age vecto rs for juveniles, dropping rapidly from age 0+, and declining availabi lity to trawling for older fish. These hypotheses were compared within yield per recruit analyses. Under the new assumptions, given current F>>M (adults), yield isopleths predict no significant increases in Y/R with stretched mesh >40 mm, but a substantial decline in fecundity pe r recruit with small increases in effort by gill nets or longlines, ai med at mature fish. These results are linked to the refugium concept f or older fish, and it is speculated that this may be in part responsib le for the continued productivity of other sustained fisheries for juv enile resources elsewhere.