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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

Published online by Cambridge University Press:  15 September 1997

Alvaro J. Abella
Agenzia Regionale per la Protezione Ambientale della Toscana Via dell'Ambrogiana, 2 57127 Livorno, Italy
John F. Caddy
FAO, Via delle Terme di Caracalla 00100 Roma, Italy
Fabrizio Serena
Agenzia Regionale per la Protezione Ambientale della Toscana Via dell'Ambrogiana, 2 57127 Livorno, Italy
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The paper explores the apparent contradiction between a high trawling pressure on juveniles and sustained production of hake that has occurred over the last decade in many Mediterranean fisheries. The practical consequences are followed of assuming rapid declines in natural mortality rate M in the first few years of life to a low, constant adult natural mortality, as well as the observation, for small-mesh trawl cod ends, of declining availability with age. Several approaches are proposed for fitting declining M-with-age with a reciprocal function for hake, 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 MSVPA estimates of North Sea stocks.

The implications of the changes in mortality with age for stocks harvested by fine-mesh trawls were explored in yield per recruit calculations under 2 different hypotheses: 1) using current estimates of growth and adult mortality, 2) M-at-age vectors for juveniles, dropping rapidly from age 0+, and declining availability 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 per recruit with small increases in effort by gill nets or longlines, aimed at mature fish. These results are linked to the refugium concept for older fish, and it is speculated that this may be in part responsible for the continued productivity of other sustained fisheries for juvenile resources elsewhere.

Research Article
© IFREMER-Elsevier, Paris 1997

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