MEPS 468:231-243 (2012)  -  DOI: https://doi.org/10.3354/meps09927

ABSTRACT: 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 × 10⁵ 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