larvae swims at higher Strouhal number but lower Reynolds number. However, the faster larvae swims distinctively at opposite conditions, that is, at lower Strouhal number but higher Reynolds number. Strouhal number is constant over similar speed ranged adult fishes. Strouhal number does not only depend on the small size of the swimmers, but also dependent to the flow regime. As in fishes which swim in viscous or high-friction flow regime, would create high body drag which will lead to higher Strouhal number. Whereas, in high viscous regime, the adults swim at lower stride length which leads to lower tail beat frequency and lower amplitude. This leads to higher thrust for same displacement or higher propulsive force, which unanimously reduces the Reynolds number. Larval fishes start feeding at 5–7 days post fertilization (d.p.f.). And they experience extreme mortality rate (≈99%) in the few days after feeding starts. The reason for this 'Critical Period' (Hjort-1914) is mainly hydrodynamic constraints. Larval fish fail to eat even if there are enough prey encounters. One of the primary determinants of feeding success is the size of larval body. The smaller larvae function in a lower Reynolds number (Re) regime. As the age increases, the size of the larvae increases, which leads to higher swimming speed and increased Reynolds number. It has been observed through many experiments that the Reynolds number of successful strikes (Re~200) is much higher than the Reynolds number of failed strikes (Re~20),. Numerical analysis of suction feeding at a low Reynolds number (Re) concluded that around 40% energy invested in mouth opening is lost to frictional forces rather than contributing to accelerating the fluid towards mouth. Ontogenetic improvement in the sensory system, coordination and experiences are non-significant relationship while determining feeding success of larvae A successful strike positively depends upon the peak flow speed or the speed of larvae at the time of strike. The peak flow speed is also dependent on the gape speed or the speed of opening the buccal cavity to capture food. As the larva ages, its body size increase and its gape speed also increase, which cumulatively increase the successful strike outcomes. Hence larger larvae can capture faster escaping prey and exert sufficient force to suck heavier prey into their mouths. The ability of a larval prey to survive an encounter with predator totally depends on its ability to sense and evade the strike. Adult fishes exhibit