An overabundance of the pacific purple sea urchin (Strongylocentrotus purpuratus ) along the central coast of California has led to an ecosystem phase shift from kelp forests to urchin barrens. Human intervention in the form of collecting purple urchins from barrens offers some relief to kelp forests, but urchins collected from barrens often do not contain a high yield of high- quality uni. These empty urchins do not have a high market value, and divers do not have an economic incentive to collect them. Urchin ranching, the collection of empty urchins from barrens and raising them in an aquaculture setting, gives divers and aquaculture specialists a way to increase uni production in empty urchins and incentivizes people to help restore kelp forests by collecting purple sea urchins from barrens.
This study aims to determine an effective, sustainable, and accessible diet for the uni development of purple urchins collected from barrens along the central coast of California. Locally collected purple sea urchins were fed an assigned diet for a period of 15 weeks. The five diets included spinach, romaine lettuce, carrots, artichoke, and giant kelp. Uni yield was determined by measuring the gonad somatic index of each urchin to reveal gonad growth relative to body mass. Uni quality was measured in terms of color, firmness, and texture. Objective measurements of color were achieved by photographing and breaking down the color of the uni on a three-dimensional color scale in order to compare it between treatments. Firmness and texture were measured subjectively by a single individual using a 1-4 scale, 1 being exceptional and 4 being unacceptable.
The results from this study will help to establish a feeding plan using diets that are economically and environmentally sustainable. By determining an optimal diet for the uni development of wild-caught purple urchins, this study will encourage urchin ranchers to continue collecting urchins from barrens with the confidence that they can raise them effectively and efficiently.