Case Study: Determining in situ target strength and biomass of Atlantic mackerel
Shetland Islands, Scotland
The abundance and distribution of many aquatic organisms can be effectively quantified using acoustic surveying methods, and are particularly useful for the assessment of pelagic fish. In order to convert acoustic backscatter into estimates of abundance, it is essential to know the target strength (TS) for the species and size of interest.
In situ TS estimates of Atlantic mackerel (Scomber scombrus) were obtained using a multi-frequency split-beam echosounder and estimates of abundance from a dedicated mackerel acoustic survey around the Shetland Islands, Scotland, in 2014 were made.
A series of arithmetic and logical variables were used to construct virtual echograms using Echoview. Firstly, suitable sample regions (defined as a volume which contains an aggregation with a surrounding margin of open water) were created. Next high density regions (i.e. schools) were identified and removed using a multifrequency thresholding filtering algorithm (adapted from Fernandes, 2009, see Figure 1) in order to minimize multiple-target errors. A single-target detection algorithm (Soule et al., 1997) in Echoview (spilt-beam method 2) was then applied to isolate in situ TS at each frequency within the selected sample regions. Furthermore, the alpha-beta tracking algorithm (Blackman, 1986), implemented by Echoview, was applied to all accepted single-target detections. All single-targets and fish tracks were exported to a .csv format and post-processed outside of Echoview.
To determine biomass of mackerel, schools were firstly detected using the multifrequency detection algorithm described by Fernandes (2009). Alternatively, the SHAPES algorithm in Echoview (Shoal Analysis and Patch Estimation System, Barange, 1994), whereby schools or shoals are selected based on contiguous features between data points determined from the acoustic data works well. Target classification was then based on decibel difference (ΔdB) between the echo strengths recorded simultaneously at several frequencies (Figure 2). Schools classified as mackerel where integrated and then subsequently exported and analyzed outside of Echoview.
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