Why does it take so long for fish stocks to recover from over-fishing? This problem has been worrying both scientists and fishery managers who expect stocks to quickly rebound when fishing stops. Now a research team from Stony Brook University believes they have an answer: continually harvesting the largest and oldest fish (as fishing regulations typically require) alters not only size but also numerous other genetic characteristics that are harmful to the overall population. As reported in Ecology Letters, the researchers experimentally harvested captive groups of a marine fish named the Atlantic silverside. Removing the largest fish over several generations gradually caused a "Darwinian debt": the fish that remained in these populations became progressively smaller but surprisingly many other traits also changed including fewer and smaller eggs with lower survival and growth. Even behavioural traits like foraging and feeding rate declined. Collectively these changes hamper population recovery and because they are genetic, they don't immediately go away when fishing ceases. How long it would take to undo the debt is now being studied by the research team.