Dr. Ellen Pikitch:

Other than fishing, the biggest factor is the environment. These fish are broadcast spawners. They put their eggs and sperm out in the water and they have to find each other. Then they have settle in an area where the environmental conditions are good so that once those embryos hatch, they have plankton to feed on. They must end up in the right places, where there is a good supply of plankton to feed upon. A lot of things can go wrong in that process and there is not a lot we can do about that, unfortunately. That is part of the problem. The environment is fluctuating and is unpredictable. Some years are fabulous and we get really strong recruitment while other years are not good. By keeping more of these fish in the water, what we are doing is maintaining a safe level and a multi-aged structure. We are basically keeping enough fish around so that one bad year does not completely damage the population. In some of the species that have collapsed, individual fish might live for between two to five years. A population that has all three-to-five year classes present is a much more stable population than one where we are fishing out almost all of the production every year and the population is down to only one age class. Such a population is much more vulnerable to collapse.