Reproductive Complexity in a Long-Lived Deepwater Fish, the Blackgill Rockfish

Fish populations are regulated by complex biological processes and ecosystem interactions. To inform stock assessments and management models, a clear understanding of a species' reproductive biology is necessary, particularly in long-lived species, for which estimates of sustainable yield are sensitive to such parameters. Deviations from traditional views of iteroparity in marine fishes (e.g., prolonged adolescent periods and skipped spawning) complicate estimates of maturity and, in turn, spawning output. The Blackgill Rockfish Sebastes melanostomus, an important component of California's commercial rockfish fishery, is a slow-growing deepwater species whose population is currently thought to be increasing after being fished to below conservation target levels. The objectives of this study were to describe the annual reproductive cycle, update maturity estimates, and detect unusual patterns of ovarian development by using both macroscopic and histological methods. Females were collected between 2010 and 2013 at locations off central and southern California. The annual reproductive cycle was protracted: vitellogenesis was initiated 7 months prior to the first appearance of fertilized eggs, and parturition occurred in December-May. A prolonged adolescent period, potentially lasting up to 12 years, was characterized by the presence of previtellogenic secondary-growth oocytes year-round and abortive maturation events. The cold temperatures and low oxygen levels characteristic of the species' habitat are hypothesized to be the main drivers behind this slow development. Macroscopic staging was found to be sufficient for maturity estimates when samples were temporally restricted. Estimates of length and age at 50% maturity were 33.6 cm and 25.7 years based on histological staging. Estimates of regional maturity based on commercial samples collected between 2001 and 2009 showed a trend of decreasing size at maturity with decreasing latitude. This study highlights the importance of histological examination in accurately assessing ovarian development and consequent baseline maturity information for managed fish populations.