Estuarine movement patterns of adult thin-lipped grey mullet, Liza ramada (Risso) (Pisces, Mugilidae), observed by ultrasonic tracking

The movements of adult thin-lipped grey mullet, Liza ramada (Risso), were studied in the River Mira estuary (southwest coast of Portugal) by ultrasonic tracking. The fish did not show any preference for a particular time of day to perform their movements. Movement direction was strongly dependent upon the tidal state, upstream movements being made during the flood and downstream movements during the ebb. During a complete tidal cycle, a fish covered a median distance of 6245 m, travelling at an average ground speed of 0.81 body lengths per second. Directional movement was the most commonly observed behaviour although fish also were stationary for up to at least 3 h. The upper reaches of the estuary were the areas most commonly used by this species. Moving in the direction of the prevailing tidal current enables these fish to cover an extensive area of estuary with a minimum energy cost.