Effects of ocean thermal structure on fish finding with sonar

1. The width of the sound beam trapped in the surface channel depends, besides on MLD and INGRAD, also on SST. The latter relation is brought about by nonlinearity in the sound speed dependence on temperature and depth (Fig. 4). 2. The bottom bounce beam width depends primarily on the temperature gradient and magnitude of change in the thermocline. With limited sonar beam width, the area near the ship below the thermocline is not accessible to horizontal beams (Fig. 5). 3. The distance to the "partial shadow zone" depends primarily on MLD, but is also affected by SST (Fig. 6). 4. If the INGRAD is more negative than -0.15°C per 30 m, there is no surface channel propagation. The distance at which the sonified field in the surface channel ends, depends on INGRAD and MLD (Fig. 7)