A series of field and laboratory experiments examined the factors that control fishassemblage structure in barrier islands ponds on the Eastern Shore of Virginia. We investigated why surf-zone fishes, which are introduced to the island ponds during periodic wash-over events, are unable to persist for extended periods. We simulated over-wash events by collecting fishes from the shallow surf-zone and introducing them into 0.56 m 2 field enclosures. During summer months, surfzone fishes typically died within 24 h. Levels of dissolved oxygen and temperature frequently exceeded the tolerance limits of the surf-zone fishes (demonstrated in a series of laboratory experiments), suggesting that these variables were responsible for fish mortality. In non-summer months, when dissolved oxygen and temperature stresses were ameliorated, surf-zone fishes could survive for months in the ponds. We conducted field-enclosure experiments at 3 fish densities in order to evaluate the survival and condition of a common surf-zone fish, Membras martinica (the rough silverside):