El pez que encogió

Hace mil años, los colonos nórdicos de la ciudad de York, en la que vivo, comían bacalaos que llegaban a pesar ocho kilos. Lo sabemos por los arqueólogos y la fascinación que sienten por los vertederos medievales de basuras, pero hoy sería afortunado quien encontrara un bacalao que pesara más de dos kilos. ¿Qué hemos hecho a nuestras poblaciones de peces para provocar cambios como éste? ¿Y tiene importancia?

La política de pesca está concebida para permitir que crezcan los peces pequeños. Se debe a la razón más importante: los peces capturados cuando son demasiado pequeños rinden poco. Además, esos peces no han tenido tiempo de reproducirse, lo que socava la población de peces totalmente. Por ejemplo, la intención manifiesta de la Política Común de Pesca de la UE es la de "proteger los recursos pesqueros regulando la cantidad de peces capturados en el mar, permitiendo a los peces jóvenes reproducirse y velando por la observancia de las medidas".

Sin embargo, en el estado actual hemos reducido con la pesca nuestras poblaciones de peces hasta el extremo de que quedan pocos especímenes grandes y viejos. Para entenderlo, veamos el caso del bacalao del mar del Norte. Supongamos que comencemos con diez mil especímenes de un año de edad. Si sólo hay mortalidad natural, unos mil de esos especímenes sobreviven hasta la edad de ocho años. Un nivel moderado de mortalidad por pesca reduce el número de supervivientes a unos cien especímenes, pero los niveles de mortalidad por pesca que hemos estado aplicando en los veinte últimos años reducen el número de supervivientes a tres, aproximadamente.

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