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

Abstract

Abstract The abundance and distribution of Northern anchovy (Engraulis mordax) and young of the year (YOY) rockfish (Sebastes spp.) are critical for the survival and reproduction of seabirds, mammals, and predatory fish within the California Current Ecosystem. Traditional detection and quantification of forage fish by trawling can be time consuming and expensive, and may not provide the spatio-temporal resolution needed to examine ecological relationships in quickly-changing marine environments. In an effort to accurately sample forage fish with less expense and higher resolution, this study combined seabird diet and acoustic descriptors to quantify anchovy schools and YOY rockfish in hydroacoustic surveys conducted between 2004 and 2015. Anchovy-like schools were selected from echograms assuming a volume backscattering strength (Sv) 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 fish were significantly correlated with abundance catches from trawl data collected from an ecosystem assessment survey and relative abundance estimated from breeding seabird diet data. Inter-annual and seasonal indices of forage fish indicated strong anchovy occurrence during 2004–2008 and increased YOY rockfish from 2011 to 2015. These observations confirm previously described changes in upwelling and forage fish variability off central California. Importantly, these results provide new information on the spatio-temporal variability of the vertical and cross-shelf distribution of anchovy schools and will benefit the design of habitat preference models for anchovy and predators. To verify the acoustics, indices were compared to trawl data from an ecosystem assessment survey and relative abundance estimated from seabird diet 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 anchovy provide insight on how they distribute across the shelf in opposing ocean phases. In addition, acoustic indices derived for both forage fish species showed a significant coherence with both the trawl surveys and seabird diets. Combining acoustic methodologies with trawl data and predator diet can be used to monitor distribution and temporal variability of forage fish species to benefit conservation of top marine predators.