Processed EK60 acoustics data used to examine changes in vertical distribution of mesopelagic fish

Marine heatwaves can impact the distribution and abundance of epipelagic organisms, but their effect on deep pelagic communities is unclear. Using fisheries acoustics data collected in the Central California current from 2013-2018, we found that during the warmest years of a large marine heatwave (2015-2016), the estimated center of mass depth of mesopelagic fishes deepened by up to 100 m compared to pre-heatwave conditions. Using a generalized additive model, we evaluated which biophysical factors may have driven these changes and found that light, dynamic height anomaly, and acoustic backscatter explained 81% of the variability in depth. We attribute the vertical shift by mesopelagic fishes into deeper waters to heatwave driven compression of upwelling habitat that indirectly increased the amount of light reaching mesopelagic depths. Our results suggest that mesopelagic fishes are interconnected with and thus sensitive to changes in near-surface oceanographic conditions, which could lead to cascading effects on vertical carbon export and the availability of mesopelagic fishes as prey for top predators under future climate conditions.

Methods
Examining the effects of the 2015-2016 marine heatwave on mesopelagic fishes in the California Current Ecosystem

The project includes the following data:

-Processed EK60 Acoustics Data from central California: daytime acoustics data (2 hours before/after sunrise/sunset, when ship speed > 5knots, bottom depths >725 m). These data were collected as part of the Rockfish Recruitment and Ecosystem Assessment Survey, an annual spring survey by the NOAA National Marine Fisheries Service, Southwest Fisheries Science Center. Acoustics data were processed in Echoview Software (Version 10): noise removed, a -75 dB threshold applied and gridded into 100 m horizontal, by 5 m vertical grid cells. For details on acoustics processing, please see manuscript. 

This project R code is available at: https://github.com/ilyiglesias/Meso_acoustics