Master of Science (M.S.)
Science & Environmental Policy
Spatial and temporal variability in environmental conditions and fishing pressure across a species’ geographic range may result in differences in life history traits, population demography, and abundance of marine species. Understanding spatial and temporal patterns of life history and demographic differences is essential for sustainable fisheries management because stock assessment models are sensitive to differences in stock productivity. The Rosy Rockfish (Sebastes rosaceus) is a small, commonly encountered, and ecologically important species in California whose life history and biological traits are only partially understood. I used central California collections of Rosy Rockfish from 2001-14 and data from the 1980s to revise outdated growth models, determine if changes in growth rates, longevity and survivorship have occurred over the past four decades, identify sex-specific differences in growth, and determine the spatial scale of Rosy Rockfish life history trait variations. Break and burn otolith analysis techniques indicated that Rosy Rockfish live at least 30 years (yr) – over twice as long as previously reported – though validation of age structures is still needed. Mean lengths of Rosy Rockfish collected from 1980-83 (246 mm) were greater than those collected from 2012-14 (209 mm). The asymptotic length of Rosy Rockfish collected in central California from 1980-83 was 31 mm greater than Rosy Rockfish collected during 2012-14 (256 mm vs. 225 mm). Maximum ages estimated for the 1980-83 and 2012-14 periods were 32 and 30 yr, respectively. Total mortality (Z) was reduced and survivorship was greater in the 1980s compared to the 2010s collections. These changes are especially interesting because fisheries do not directly target Rosy Rockfish. Male and female Rosy Rockfish exhibited different life history traits. Collections in central California from 2012-14 were comprised of a greater proportion of smaller females than males, and male Rosy Rockfish were larger than females in Half Moon Bay (2012-14) and Santa Cruz (2001-05). Longevity was greater in male Rosy Rockfish; females had greater mortality and reduced survivorship with age. Age and length at 50% maturity was similar between the sexes as both matured at age 8 and 171 mm and male and female growth parameters (Linf and K) were not significantly different. Central California Rosy Rockfish exhibited greater longevity and maximum age compared to southern California Rosy Rockfish, despite similar sample sizes and sampling techniques. Total mortality was reduced and survivorship was greater in southern California compared with central California, indicating potential differences in predation or fishing pressure between areas. Surprisingly, asymptotic size (Linf) was smaller in central California (225 mm) than in southern California (232 mm). A greater portion of midsized fishes in the southern California sample may explain some of these differences. The use of whole otoliths in previous age and growth studies resulted in age estimates that significantly underestimated the oldest age classes in Rosy Rockfish and future stock assessments should revise growth parameters to reflect new age estimates. Using the break and burn technique, there were significant changes in mean length, longevity, survivorship and growth parameters between the 1980s and 2010s samples. The differences in life history and population demography characteristics are consistent with increased mortality from direct fishing removals or indirect, density-dependent effects of competition and predation. Climate conditions were favorable for rockfish over the lifespan of fishes I collected and were probably not responsible for the temporal patterns observed. Likewise, recreational and commercial removals for Rosy Rockfish steadily declined since the mid 1990s, giving Rosy Rockfish populations nearly 20 years of relief from intense fishing pressure. Changes in Rosy Rockfish life history traits are most likely due to the indirect effect of community level changes caused by intense historical fishing pressure on larger rockfish and the subsequent recovery of many species and their predators including lingcod and pinnipeds. Increases in rocky reef population abundances may have increased density-dependent interactions with Rosy Rockfish such as predation and competition leading to increased total mortality. Small species like Rosy Rockfish are susceptible to changing ecosystems; this study highlights the need to account for changes in life histories and population demography within fisheries management.
Fields, Ryan T., "Spatial and Temporal Variation in Rosy Rockfish (Sebastes Rosaceus) Life History Traits" (2016). Capstone Projects and Theses. 562.