Only eat fully grown fish, not babies!
For the majority of fish, mollusks and crustaceans, there is a minimum size under which they cannot be fished or sold. Regional regulations are supposed to protect the resources, but all too often they are ignored by those who are fishing, selling and buying. Eating young fish (or newborns, as is the case with whitebait) obstructs the natural reproductive cycle. Only adult fish can reproduce and ensure the continuity of the species.
The oldest fertile females are essential to renewing stocks. The example of the red snapper (Lutjanus campechanus, found in the Gulf of Mexico and off the southeastern coast of the United States) is very eloquent: Just one 61-cm-long female contains the same number of eggs (9,300,000) as 212 42-cm-long females (figures from Nature magazine). In an article published in August 2008 by Fisheries Research, scientists emphasize that stocks would be seven times larger if, before fish were caught, they were allowed to reach adult age and to spawn several times.
The EU funded INCOFISH project is a source of information about sustainable fish with a database organized by country on the size of adult fish. Different diagrams (fish rulers) illustrate the minimum size of the most common fish from the North Sea, the Baltic, Senegal, the Philippines and Peru.