Nutritional quality of macrophytes eaten and not eaten by two temperatezone herbivorous fishes: A multivariate comparison

Abstract

The mean annual chemical compositions (ash, lipid, carbohydrate, protein, nitrogen and carbon) of 23 species of macrophytes (22 seaweeds and 1 seagrass) from a rocky intertidal habitat on the central California coast were determined from December 1981 through December 1982. These data were used to test the hypothesis that the 13 red and green seaweeds eaten by the two principal herbivorous fishes (Cebidichthys violaceus andXiphister mucosus) at the site are higher in nutritional quality than the 9 red and brown seaweeds and the seagrass not consumed by these fishes. A MANOVA using the ash, lipid, carbohydrate and nitrogen data showed that the centroids of the dietary and nondietary species groups were significantly different. In a two-group discriminant analysis that followed, only two species were misclassified as members of the opposite group. Multigroup discriminant analysis of the 23 macrophytes resulted in some overlap among dietary and nondietary species. Species were discriminated on the first canonical axis by ash content and on the second and third axes by lipid and carbohydrate contents. Nitrogen contributed little to the overall discrimination of species in the analysis. The nondietary red algaCorallina vancouveriensis, with its high ash content and therefore relatively low nutritional quality, was clearly separated from all other species in the analysis. Brown algae were of higher nutritional quality, but are not eaten by the two fishes, possibly because these seaweeds produce indigestible carbohydrates and secondary compounds. However, the exclusion from the diets of several red algal species that were virtually indistinguishable from the dietary red algae remains unexplained.