Fish behaviour and implications for abundance estimates

Fish vertical distribution and individual swimming behaviour is essential for their acoustic detectability and properties. The project will address under which conditions semi-demersal fish live closely assocoated with the sediment and under which conditi ons they ascend into the water column, increasing their recorded abundance in acoustic surveys. Correspondingly, we aim at quantifying how much fish that at night ascend into the acoustic blind zone for a hull-mounted transducer (particularly relevant for clupeids). This will be done by submerged, upward-looking transducers. Individual fish swimming behaviour and effects on their Target Strength will be studied by acoustic tracking. By applying software that can allocate echoes from subsequent pings to a given organism, individual organisms can be tracked through the acoustic beam, even in deep water when using submerged systems. The 3-D swimming path of individuals will be established, and associated variations in acoustic properties characterized.