Primijetio sam u više navrata rasprave oko toga vidi li se liganj na fishfinderu ili ne, pa evo nešto izdvojenog sadržaja iz studije o tome. Zaključak je - uređaji velike snage i visoke frekvencije (kraćeg vala) mogu uspješno detektirati lignje na dubini na kojoj ih tradicionalno tražimo/lovimo.
IZVOR:
http://www.fao.org/tempref/docrep/fao/0 ... 000e09.pdf"...When squid are too close to the bottom, there is a
shadowing effect that makes detection difficult. Chances of detection can be
increased by increasing transmitter power and decreasing pulse length. As a rough guide, it is generally
impossible to detect squid that are closer to the surface than twice the wave height or closer to the bottom than one-half the pulse length of the transmitted signal. There is a trade-off between detecting squid near the bottom, and workable water depths, however. The shorter pulse lengths that are needed to separate squid echoes from bottom echoes require higher frequencies. The greater signal attenuation at higher frequencies limits the depth at which squid can be detected. Echointegration of organisms with low target strengths (such as squid) that are near the bottom is most practical in water depths less than 200 m.
A limitation to simultaneous collection of target strength and echointegration data is that
single echoes are required for estimates of target strength. In most cases, squid are in schools
sufficiently dense to preclude collection of individual echoes. Target strength estimated in these cases will be
from animals on the periphery of the school, and may not accurately represent the target strength of squid in the middle of the insonified school. Another problem of echointegration is that when squid are in an extremely dense school, the lower level of the school may be in the acoustic shadow of the upper
layers. This results in an acoustic signal that is not proportional to squid density, a primary premise of echointegration.
Several researchers are investigating the assumption of linearity (see Traynor et al.1990). Foote (1983) showed linearity was not a problem for herring in densities as high as 60 fish m. Although Starr (1985) estimated squid densities as high as 83.9 squid m, and Vaughan and Recksiek (1978) observed densities as high as 99.6 squid m, with the reduced reflective energy of squid, these densities should not affect the assumption of linearity. This is an area that may require further research. In practice, it is best to avoid target saturation if possible. The potential for saturation is reduced if surveys are conducted when school density is not extremely high; also
transmitting at a high power setting will reduce the chance of encountering a shadowing effect. In'the worst case, when shadowing does occur, there will be an underestimate of school abundance. Acoustic surveys are limited by water depth. When squid are in water that is too shallow, the proportion of the water that is surveyed acoustically is minimal and squid may avoid the vessel, thus avoiding detection
by the echosounder. Although Sauer et al. (1992) saw little evidence of squid avoidance of fishing vessels when Loligo vulgaris reynaudii were spawning, fish avoidance of su
rvey vessels has been well documented (e.g. Olsen et al.1983, Misund 1990, Ona and Godo 1990).
In cases where water depth is less than 20-30 m, the chances of detecting squid will be greater if a small boat is used and the transducer is towed or placed on a boom off the beam. To maximize the volume of water insonified, the transducer should be placed as close to the surface as possible..."