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. According to data from the National Institute of Fisheries Science (NIFS), fish are most active in water temperatures of 12–18 °C and salinity of 30–35 ppt.
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'.
NIFS’s 2023 coastal fish habitat survey found that even within the same temperature range, a salinity difference of more than 2 ppt reduces fish density by an average of 18 %.
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 phenomenon was confirmed in the Korea Maritime Institute (KMI) 2022 analysis of temperature variation by depth, showing differences of less than 0.3 °C.
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, regions where temperature shifts abruptly by more than 1 °C see fish avoidance rates increase by over 30 % (KMI data).
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 only 5–7 % of all points meet 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 typically adapt to a new environment within an average of 45 minutes, providing sufficient exploration time.
When recording precise data, it is important to log timestamps along with GPS. In a 2022 KMI experiment, measuring the same GPS coordinate three times at 10‑minute intervals showed that when temperature variation stayed within ±0.2 °C, fish response rates increased by 22 %.
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, improving fish attraction efficiency by more than 15 %.