2.2 Traditional method (electrofishing) for fish investigation
Fish were collected from the headwaters to the estuary (Fig. 1) during the rainy season (from June to July) in 2022; each site was sampled three times over a rainy season following basic guidelines (Barbour et al., 1999; Hauer and Lamberti, 2007). Electrofishing equipment consisted of a 24-kW generator, a 12 V-160 A lithium battery, a silicon-controlled inverter, and two continuously adjustable voltage and frequency regulators. This equipment was used to effectively stun and collect fish (individual weight < 10 kg) in a 2 m wide × 2 m long × 3.5 m deep water column. Due to varying water levels, two electrofishing operations were conducted as follows: 1) At wadeable sites, single-pass backpack electrofishing was performed by two operators moving in a zig-zag fashion. Electrofishing was adjusted at low voltage and mixed frequency, and the walking speed was controlled to ensure a sampling effort of approximately 8 m2 min-1over 30 minutes. 2) At nonwadeable sites, personnel and equipment were loaded with a welded hull boat, and a bamboo quant was used to propel the boat to eliminate noise disturbance to fish. Electrofishing was adjusted at a high voltage and main frequency, and the paddling speed was controlled to ensure a sampling effort of approximately 6 m2 min-1. Due to the high water depth, a large scoop net was used by a sternward auxiliary to collect the stunned benthic fish that floated slowly upwards. Boat-electrofishing was conducted over a distance of 500 m, spanning both riverbanks at a depth of 1-3 m (Flotemersch et al., 2006). The number and biomass (weight mass) of fish individuals were collected as traditional fishing data.