Organizing the echogram measurements along a continuum of measurements or grouping them into a number of acoustically distinct substrate classes is the final step in the process.
Variability and covariance of echogram measurements
Thus significant differences in some of the echogram measurements can be created for the exact same substrate type if users are not careful about ensuring that the 251 sample window of QTC IMPACT matches up well with the first echo length.
Although this analysis demonstrated that there are several strong advantages (gain adjustment, bottom picking, bad data exclusion, and stacking) in using the partially automated echogram classifying software (QTC IMPACT), there are also several potential pitfalls (dependencies among the 166 EMs, lack of standardization, correlation with depth, influence of seafloor slope, and mismatch between 251 sample intervals versus first echo length), such that it does not function as users would expect for distinguishing substrate types.
Instead, our results, with corroborations from independent data sets, indicated the importance of analyzing the echogram measurements before any PCA and K-means analysis so that depth-related and slope-related errors, second echo or echo envelope errors, and variable range or collinearity errors could be caught.
Maximum sampling depth was determined from the echograms and monitored as for the midwater trawl.
Echograms that represented Arctic cod schools as "hollow" masses revealed that acoustic shading was prevalent in our data (see Discussion).
Although the tow-yo sampling pattern mixed the catch across the depth ranges sampled, the samples reflected the zooplankton that occurred where the fish were because sampling was limited to depths where most fish echoes occurred on the echograms.
When compared with records from all other areas examined, the echograms from MIC were also uncharacteristically devoid of echoes typical of those from zooplankton-sized organisms, reinforcing the netting results.
At stations where YOY were abundant, echograms revealed that small plankton-size organisms had coalesced into a dense layer near the thermocline.