Peter G. Wells, James N. Butler, and Jane S. Hughes, Eds., American Society for Testing and Materials, Philadelphia, 1995. Impact is defined as a statistically significant difference between samples exposed to oil and reference samples.
Recovery is then the disappearance through time of such a statistical difference.
Both impact and recovery must be assessed in relation to the background of natural variation that characterizes marine environments. The strengths and weaknesses of these approaches are discussed. In many situations, the third approach may be most useful. Following the Exxon Valdez oil spill in March 1989, over 35,000 dead birds were retrieved. Recovery for some populations was projected to take decades.