01The concept of a prime spot, scientifically?
'Prime spot' refers to locations where specific species concentrate, but in reality it is linked to the principle that fish move according to the water’s physical and chemical conditions. Fish generally tend to be more active when water temperature and salinity fall within a range suited to their habitat.
Therefore 'prime spot' is more of a dynamic area that forms when the temperature, depth, and current of a region align, rather than a fixed coordinate. In the 30–80 m depth range, areas with stable temperature tend to retain fish strongly, which can be called a 'potential prime spot'.
However, even within the same temperature range, a larger salinity gap tends to lower fish density.
In other words, considering the combined conditions of temperature and salinity simultaneously is necessary to secure the reliability of calling a location a 'prime spot'.
Also, at depths shallower than 30 m, diurnal temperature swings cause rapid mixing of water layers, whereas in the 30–80 m zone the temperature profile is relatively flat, allowing fish to stay for extended periods.
This tendency has also been observed in various field conditions.
02Point selection, three key variables
The first variable is water temperature. Temperature directly affects fish metabolism and prey searching, and zones of rapid temperature change become avoidance areas. For example, fish tend to avoid zones where water temperature changes abruptly.
The second is current and tidal flow. When currents are at 0.3–0.6 m/s, prey is carried downstream and fish concentrate. Areas with abundant tides have currents crossing vertically, leading to active prey redistribution and a higher probability of prime spot formation.
The third variable is depth and water pressure. At water pressures of 0.3–0.5 MPa (30–50 m depth), temperature and salinity differences between layers are minimized, allowing fish to inhabit stably.
According to KMI 2021 data, even within the same temperature range, water pressure above 0.6 MPa reduces fish density by an average of 12 %.
Zones that satisfy all three variables are called 'triple coincidence points', and in practice, very few points meet all these conditions. Therefore precise measuring equipment and data logging are essential.
03Practical application, finding usable points anytime
To quickly assess temperature and current on site, use a handheld thermometer and a radio receiver (current meter). Measuring at 5‑minute intervals and averaging yields reliable data that minimizes momentary fluctuations.
If the measurements fall within the ranges presented above, set the area as a target point and stay at depths of 30–80 m for 1–2 hours. Fish gradually adapt to a new environment over time, providing sufficient exploration time.
When recording precise data, it is important to log timestamps along with GPS. Repeated measurements at the same coordinates tend to show a clearer fish response when temperature variation is small.
Also, because the current meter drains battery quickly, calibrate it every 30 minutes and use it together with a wind vane to visualize prey movement paths according to current direction.
Based on this data, adjusting line placement to 'vertical 1 m × horizontal 5 m' spacing lets the bait flow naturally in the water, which tends to improve fish attraction efficiency.