Combining seabird diet, acoustics and ecosystem surveys to assess temporal variability and occurrence of forage fish

Abstract The abundance and distribution of Northern anchovy( Engraulis mordax ) and young of the year (YOY) rockfish( Sebastesspp.) are critical for the survival and reproduction of seabirds, mammals, and predatory fishwithin the California Current Ecosystem. Traditional detection and quantification of forage fishby trawlingcan be time consuming and expensive, and may not provide the spatio-temporal resolution needed to examine ecological relationshipsin quickly-changing marine environments. In an effort to accurately sample forage fishwith less expense and higher resolution, this study combined seabirddiet and acoustic descriptors to quantify anchovyschools and YOY rockfishin hydroacousticsurveys conducted between 2004 and 2015. Anchovy-like schools were selected from echograms assuming a volume backscattering strength ( S v ) range of −47.6 to −42.9 dB. YOY rockfish-like single targets were selected considering a target strength( TS ) range between −52.8 to −50.9 dB, calculated from lengths of fish consumed locally by three piscivorous seabirds. Acoustics indices of forage fishwere significantly correlated with abundance catches from trawldata collected from an ecosystem assessment survey and relative abundance estimatedfrom breeding seabirddiet data. Inter-annual and seasonal indices of forage fishindicated strong anchovyoccurrence during 2004–2008 and increased YOY rockfishfrom 2011 to 2015. These observations confirm previously described changes in upwelling and forage fishvariability off central California. Importantly, these results provide new information on the spatio-temporal variability of the vertical and cross-shelf distribution of anchovyschools and will benefit the design of habitat preference models for anchovyand predators. To verify the acoustics, indices were compared to trawldata from an ecosystem assessment survey and relative abundance estimatedfrom seabirddiet data collected from locally-breeding piscivorous seabirds. Results add to the existing knowledge of how these species distribute in the water column, and with regards to anchovyprovide insight on how they distribute across the shelf in opposing ocean phases. In addition, acoustic indices derived for both forage fishspecies showed a significant coherence with both the trawlsurveys and seabirddiets. Combining acoustic methodologies with trawldata and predator diet can be used to monitor distribution and temporal variability of forage fishspecies to benefit conservation of top marine predators.