Indeed, there is evidence from many fisheries of earlier maturation; such changes take place fast enough to be detected over the decades. For example, early in the twentieth century, you would not find a mature cod in the North Sea less than about 50 centimeters in length, whereas by the 1980's mature cod were as small as 15 centimeters.
在很多渔场也确有鱼群提早成熟的实例。这样的变化快到足可以在过去的几十年间得以见证。比如，二十世纪初，在北海一条成熟的鳕鱼一般不短于五十厘米，而到了八十年代，一条成熟的鳕鱼就只有十五厘米那么长了。所以海里有很多鱼没有足龄就开始交配。通过保护大鱼、成鱼，人工选择的目的由缩短鱼类成熟期，转变为加快生长速度，安全渡过最不该被捕捞的阶段。