Relative abundance and size composition of Red sea urchin, Strongylocentrotus franciscanus, populations along the Mendocino county coast, 1991

Abstract

Underwater surveys were conducted in the summer of 1991, as part of a three year survey, to determine density and size composition of red sea urchin, Strongylocentrotus franciscanus, populations along the Mendocino coast at three different depth zones. The study consisted of two parts: i) a broad scale survey, with 12 systematically chosen sites from Gualala to Mendocino and ii) a fine scale survey, with nine sites in the vicinity of Fort Bragg. The fine scale sites were selected to represent different habitat types and levels of commercial exploitation. The sites included the Point Cabrillo Marine Reserve (PCMR) as an unfished control and the Caspar Commercial Urchin Closure Area, established in 1989 to assess the effects of closure upon recovery of fished areas. The broad scale mean density was 0.71 red urchin m-2 (SD 1.9), a decline from the 1.3 and 1.1 red urchin m-2 found during the 1988 and 1989 surveys, respectively. The 4.6-m depth zone yielded only 0.17 m-2. No site in the broad scale survey bad greater than 2.2 red urchin m-2. Fine scale fished site mean density declined to 0.34 (SD 1.1) and the PCMR control site density increased to 7.0 m-2 (SD 6.2). Abundance was variable; however, as in past surveys the highest densities were generally found at the 10.7-m and 15.2-m depth zones. The presence of a mode in the 15-35 mm size interval indicated a recent recruitment event. However, continued declines in legal-sized (>89 mm) red sea urchins survey-wide demonstrate the need for more effective fishery management. (79pp.)