Observations on fish grazing of the cultured kelps Undaria pinnatifida and Saccharina latissima (Phaeophyceae, Laminariales) in Spanish Atlantic waters

Despite the great importance of grazing by herbivorous fishes in the marine environment, the knowledge of this phenomenon on seaweeds cultivation is very limited. Indeed, there is not available information on the effects of fish grazing in the farming of economically valuable kelps. During an investigation of the cultivation of commercial kelps Undaria pinnatifida and Saccharina latissima in North Western Atlantic coast of Spain (Galicia), it was found that grazing by fishes was sometimes responsible for removing of algals material from culture ropes. Fish grazing were generally rare and with a minor influence, usually its adverse effects are limited to a small decrease in the yield in some culture ropes, however, in some cases, it can also involve the lost of virtually the entire crop of culture rope. Most of herbivore grazing was associated to sparid fishes such as bogue (Boops boops), and salema (Sarpa salpa). Preferably, herbivorous fishes consumed blade of both cultured kelps when they were still young fronds (juvenile stage of algae). Indeed, blade of adult fronds was clearly less affected, and in S. latissima adults there was even not observed any fish grazing effects. Herein, different biotic and abiotic factors are discussed as possible causes of the influence of fish grazing on cultured kelps.