SHELF Florida Middle Grounds (FMG) echosounder data collected from 2017-07-13 to 2017-07-14

After collecting egg survey samples, active-acoustic transects were collected to examine the spatial distribution of reef fish in the Florida Middle Grounds (FMG). Active acoustic data were collected with calibrated multi-frequency split-beam echosounders (Simrad EK80) operating at nominal frequencies of 38, 70 and 120 kHz. Pulse repetition rates were set to maximum, with a pulse duration of 128 ms. The 38 kHz transducer was deployed on the hull of the vessel, and the 70 and 120 kHz transducers were deployed on a pole along the side of the ship. During the survey, water temperature and conductivity were collected throughout the water column at standardized locations using a Sea-Bird 19+ CTD (Sea-Bird Electronics, Inc.) to characterize water chemistry, vertical water column structure, and derive sound-speed profiles. Sound speed and absorption coefficients were calculated to account for the effects of temperature and salinity on acoustic transmissions. Acoustic data were visualized and manually edited in Echoview 8. The echo-integral was computed for the acoustic backscatter in cells 10 m long by 10 m deep, comprising each elementary sampling distance unit (EDSU). To account for positional differences in the location of the transducers, offsets were measured and incorporated to correct the spatial location of each sample for the three transducers. A bottom-detection algorithm with a 0.5 m backstep was applied to the data and was manually edited to ensure the seafloor and reef structure were excluded from the analysis. Standard output following echo integration includes the acoustic estimate of fish density, SA (NASC, nautical area scattering coefficient, in m2 nmi−2). The echo integral was quantified for each elementary distance sampling unit to examine reef-fish density associated with each station with an echo integration threshold of -60 dB. Following echo integration, estimates of target strength (TS, in dB), which is proportional to fish length, were computed for each station to describe the length distributions of detected fish. Single targets were detected with a TS threshold of -47 dB; targets exceeding the algorithm criteria were accepted into the analysis. The single target algorithm was tuned to accept targets with echo envelopes between 0.6 and 1.7 times the pulse length, with a maximum beam compensation of 6 dB. Given the mixed-species assemblage often associated with natural and large artificial reefs in the Gulf of the relationship defined in McCartney and Stubbs (1971) was used for TS as a function of SL (in cm) following: TS = 24.5 * log10 (SL) − 66.84. Fish density estimates were calculated as mean NASC divided by mean σbs of a known volume of water given (EDSU) the relationship, fish density (fish m−2) = NASC/(4π*σbs).