RELATIVE EFFECTS OF TURBIDITY AND LIGHT-INTENSITY ON REACTIVE DISTANCE AND FEEDING OF AN ESTUARINE FISH

Gulf killifish Fundulus grandis were allowed to prey on daggerblade gr ass shrimp Palaemonetes pugio in clear water with bright light, turbid water containing bentonite clay, and clear water treatments where the light intensity was adjusted to match that in the bottom of the turbi d tanks. Significantly fewer shrimp were consumed in the turbid tanks relative to the clear and shade treatments where predation rates did n ot differ significantly. The results suggested that the influence of s uspended particles on predation rates was a consequence of light scatt ering and was not related to a decrease in light intensity. Reactive d istances were subsequently determined for human observers viewing a sm all target in elongated tanks containing turbid water (7.3-60.5 NTU) u nder conditions of both low (8-10 mu E m(-2) s(-1)) and high illuminat ion (153-1249 mu E m(-2) s(-1)).). Reactive distance was primarily gov erned by turbidity while light intensity had little influence except a t low turbidities. The shape of the relationship between reactive dist ance and turbidity for humans resembled curves reported for a variety of fish species.