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• Collaborative research shows decline in key fishery indicators for CA rock crab. • Depletion was detected using regression-based assessment of empirical data. • Stock decline varied substantially by species, sex, and location. • Data-limited assessments provided useful insight into a complex multispecies system. • Approach could facilitate adaptive, empirical management of data-limited fisheries. The southern California rock crab fishery targets stocks comprised of three species: red (Cancer productus), yellow (Metacarcinus anthonyi), and brown rock crab (Romaleon antennarium). Fishers have expressed concern about the sustainability of the fishery due to increased fishing effort over the past decade, and because it is managed as one assemblage despite distinct life history differences among the species. We collaborated with fishers to test for stock-specific declines in key fishery-dependent indicators by replicating a 2008 study in 2016–2017 and comparing indicator values between years using multiple regression techniques. Indicators included spatially explicit species-level data for size, catch-per-unit-effort (CPUE), discard rate, sex composition, and trap location and depth across the heavily fished Santa Barbara Channel and Northern Channel Islands. Results showed significant declines in male size, overall CPUE, and proportion of crab landed versus discarded for heavily targeted stocks, translating to fewer pounds per trap and potential financial losses for fishers. Fishing and environmental conditions may have both contributed to stock declines. Evidence of decline differed substantially across space, species, and sex. We suggest that a spatially explicit and adaptive approach to empirically managing southern California rock crab may help to protect fishers from financial loss and avoid continued depletion of certain stocks, and we show that relatively simple collaborative approaches can provide defensible insight into complex systems. [ABSTRACT FROM AUTHOR]