Mass mortality events are ephemeral phenomena in marine ecosystems resulting from anthropogenically enhanced and natural processes. A fish kill in King Harbor, Redondo Beach, California, USA, in March 2011 killed ~1.54 × 10 5 kg of fish and garnered international attention as a marine system out of balance. Here, we present data collected prior to, during, and following the event that describe the oceanographic conditions preceding the event, spatial extent of hypoxia (dissolved oxygen < 1.4 ml l −1 ), and subsequent recovery of the harbor. In situ sensors within the harbor revealed rapid decreases in dissolved oxygen in surface waters from 7 to 9 March 2011, coincident with the mortality event on 8 March. Continuous observations provided evidence that respiration of a large population of fish within the harbor, potentially exacerbated by an incursion of upwelled low-oxygen water, resulted in significant oxygen reduction in the harbor and ultimately caused mortality of the fish population. The hydrodynamically constrained northern basin transitioned to nearly anoxic conditions, while spatially variable hypoxia was observed throughout the harbor and adjacent bay for >10 d following the event. Initial recovery of dissolved oxygen in the harbor was facilitated by storm-mediated mixing. No connection was apparent between increased algal biomass or phycotoxins within the harbor and the mortality event, although the fish showed evidence of prior exposure to the algal neurotoxin domoic acid. Our findings underscore the essential role of ocean observing and rapid response in the study of these events and the role that oceanographic processes play in hypoxia-driven fish mortalities. Alterations in upwelling regimes as a consequence of climate change are likely to further increase the frequency and magnitude of upwelling-driven hypoxia and mortality events.
KEY WORDS: Fish kill · Hypoxia · Nearshore embayment · Upwelling · Mixing
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 468: [231][232][233][234][235][236][237][238][239][240][241][242][243] 2012 may also represent a severe depletion of local fish populations, and Lowe et al. (1991) documented 86 individual mortality events in which >1 million fish were killed from 1980 to 1989 nationwide in the USA.It is often difficult to directly attribute causation to mortality events due to their patchiness in time and space and potential forcing by multiple underlying processes; indeed, the complexity of such events can lead to great debate about the mechanisms underlying mass mortality events (e.g. Paerl et al. 1998, Burkholder et al. 1999. Surpassing the temperature limits of aquatic species has been implicated in some seasonally localized mortalities (Bohnsack 1983, Hoyer et al. 2009, Balayla et al. 2010. Phytoplankton biomass can also be a causative factor behind many mass mortality events, either through biomass accumulation and consequent oxygen de pletion during the night or termination of a bloom (Paerl et a...