Age and Growth of Two Herbivorous, Kelp Forest Fishes, the Opaleye (Girella nigricans) and Halfmoon (Medialuna californiensis)

Abstract

Opaleye (Girella nigricans) and halfmoon (Medialuna californiensis) are herbivorous sea chubs (Perciformes: Kyphosidae) that occupy an ecologically important role in kelp forests off southern and Baja California. This study provides information on length-weight relationships, age, and growth of these two ecologically important species. Opaleye and halfmoon were collected from throughout the Southern California Bight to evaluate these life history characteristics. Length-weight relationships were described by the equations W5 0.00002L3.081 for opaleye and W5 0.000003L3.454 for halfmoon. Sagittal otoliths were used to age opaleye from ages 3–10 and halfmoon from ages 0–8. In addition, age classes 0-II for opaleye were determined from length frequency analysis of preserved specimens. Von Bertalanffy growth curves were fitted to mean standard length (mm) at age for each species. Opaleye were aged up to 10 years whereas halfmoon was recorded up to eight years of age. Standard length-at-age growth curves were typical of nearshore marine fishes with rapid growth in the first few years, reaching an asymptote quickly thereafter. This study demonstrates opaleye and halfmoon are short-lived, fast growing species, and this information combined with other life history characteristics shows the importance of opaleye and halfmoon and the need for ecosystem-based management in kelp forest communities.