The cryptic responses of hatchery-reared sole to a natural sand substratum

In laboratory experiments, both reared and wild sole Solea solea selected a sand substratum in preference to a hard substratum. Reared sole with no previous experience of sand buried quickly when placed on sand. Light motivated burial, and the motivation of reared sole to bury was as strong as that of wild sole. The burial efficiency (the proportion of the ocular side covered by sand after a single burial attempt) of reared sole was lower than that of wild sole, but increased to that of wild sole after a period of 12 days maintenance on sand. Motivation to bury is therefore innate, but efficiency is affected by experience. The reactive distance of reared sole to a standardized predation threat was the same as that of wild sole and was shorter when buried (6 cm) than when not buried (15 cm). Burial therefore affected the response to a predation threat, indicating that burial is a cryptic behaviour. Munsell colour charts were used to determine the time required for reared sole to match the skin colour 'tone' of wild sole after placement on sand. Adaptation of colour value (lightness) took 4–7 days, but adaptation of chroma (intensity of colour) and hue took 33 and 69 days respectively. It is therefore recommended that flatfishes reared for stock enhancement exercises are conditioned to sand prior to release due to the relatively long time required for crypsis to improve through colour adaptation and burying.