Acoustic target strength of chub mackerel (Scomber japonicus) measured in situ using split beam acoustics

摘要

Acoustic abundance estimates of the stock size of chub mackerel (Scomber japonicus) in Moroccan waters have shown a continuous increase over the last ten years. Knowledge of the backscatter properties or accurate target strength of the fish is neededfor converting the integrated echo energy to biomass. The chub mackerel has a gas-filled swim-bladder. Despite of this, the backscatter strength of this fish is by experience known to be weak compared to other swimbladdered schooling pelagic species likesardine and horse mackerel. This difference is actually used to separate schools of chub mackerel from other schooling pelagic fish during the echogram scrutinizing processes. This paper reports in situ target strength measurements of chub mackerel carried out onboard the R/V Al Amir Moulay Abdallah during a cruise off Morocco. The vessel is equipped with two split-beam echo sounders of respectively 38 and 120 kHz and the measurements were conducted after a trawl sample taken at the same place and depth showing that 99.6 percent of the catch consisted of chub mackerel. The estimated mean target strength for chub mackerel appears significantly lower than the one currently applied for abundance estimation in the region, but not significantly different from reported target strength values of chub mackerel measured in situ in other areas.