Water quality implications for intensive farming of pacú (Piaractus mesopotamicus) in floating net cages in the Middle Paraná River system

Abstract

Floating cages have recently been introduced for intensive pacú (Piaractus mesopotamicus) farming in the Middle Paraná River system (Argentina). However, the reciprocal influence between water quality and fish farming, critical for sustainable management, has received limited attention. We assessed (1) water quality suitability for cage farming in major channels with varying connectivity to the main river; and (2) the impact of cage farming on water quality. We hypothesized that (1) water quality is more suitable in highly connected channels due to reduced influence from the floodplain and polluted tributaries; and (2) small-scale farming has minimal water quality impacts. We monitored water quality 1 m upstream, within, and 1 m downstream of four cages, at 1 m depth. Dissolved oxygen, pH, and NH3 levels outside optimal ranges for pacú occurred in less-connected channels. Oxygen declined with chromophoric dissolved organic matter, whereas NH3 rose with pH. Although slight water quality changes occurred downstream of cages, values remained within typical ranges, indicating minimal impacts from small-scale farming (300 fish per cage). Our results suggest that branches directly fed by the main river provide better water quality for pacú farming. Further studies are needed to assess impacts considering different depths and higher farming scales.