Two main circumstances govern the relationship of living things in the sea : the unbelievably lavish fruitfulness of marine life forms, and the utter ruthlessness with which the larger creatures eat the smaller ones. Somebody has calculated, for instance, that if all the eggs laid by codfish were hatched and grew to maturity, the Atlantic would be packed solid with codfish within six years. But nature does not let this happen. Only an infinitesimal fraction of all codfish eggs ever become full-sized cod, and wastage among other fish is as great. One sea creature in about 10 million escapes the usual violent death inside another sea creature. 

[from Leonard Engels (ed.) 'The Great Pyramid of Life' in The Sea, Time-Lift: Books.]

a) Under what circumstances would the Atlantic be packed with codfish within six years?

b) How does nature prevent this from happening?