Both lack of kelp and crowding by purple sea urchins negatively affect abalone populations. In recent years, there have been unusually warm ocean temperatures which have caused poor growth of kelp and other seaweeds and greatly reduced the amount of food for abalone and sea urchins. Lack of food can cause abalone to die from starvation or to be weakened and reduce their ability to survive predators or strong waves. Lack of kelp greatly reduces the amount of food available for abalone in deeper water and may cause them to move into shallower water where they can be caught in the fishery. Movement to shallow water will make abalone seem more abundant, but the population will be more vulnerable to overfishing. The presence of large numbers of purple sea urchins reduces the amount of food and living space for abalone. When in large numbers, sea urchins can create barrens by eating kelps and seaweeds before they have a chance to grow. Sea urchins are able to withstand starvation conditions and can maintain barrens for many years but abalone cannot survive in barrens. This article provides more information on this issue.