Vertical distribution of a demersal fish species and its implication on the acoustic detection and assessment of argentine hake (Merluccius hubbsi)

The distribution of fish in the water column, particularly of demersal species near the seabed, is relevant to the detection and assessment of fish populations. Here, acoustic data recorded during six consecutive seasons on the Patagonian stock of argentine hake (Merluccius hubbsi) provided detailed information about the vertical distribution of the species along the acoustic track. It was determined that during the day, about 95% of individuals are located within the first ten meters above the bottom, and that almost half of them are concentrated in the first meter from it. An exponential function was found to represent this near-bottom distribution, with a marked positive gradient in the vicinity of the seabed. The height of the Acoustic Dead Zone (ADZ) was estimated and employed together with the true vertical distribution of fish to calculate correction factors for the nautical area scattering coefficient (NASC). Considering the closeness of fish to the bottom, a distribution variable is proposed, the demersality coefficient, which allows mapping the behavior of fish over the area of distribution, providing a tool for the analysis of distributional aspects of ecological importance.