An obvious truism is that all phases of the life cycle are essential to a population. If a spawning habitat is made unavailable or a crucial nursery area is destroyed, no degree of fishing restriction or improvement for another stage in the life cycle will help the population. Most species involved in Atlantic coastal marine fisheries are dependent on estuaries for rearing habitat. Estuaries can usually be used for harbors, which leads to construction of cities. The resultant pollution and dredging and filling to improve harbors and provide real estate for development destroys the function of the estuary as a rearing habitat.
Critical habitat is lost to forces the fisheries manager has very little influence on.
In 1986 the CA Department of Fish and Game used rotenone to treat a stretch of Big Chico Creek with the intent to remove competitors and predators and improve the survival of juvenile spring chinook. No improvement in the fishery ensued because passage and entrainment problems for migration were more critical to the population than were limits to juvenile rearing. Environmental filters such as tolerance ranges. It is assumed that each species occupies it's own niche, to the exclusion of other species. Deterministic communities are persistent and resilient.