Seasonally varying importance of abiotic and biotic factors in marsh-pond fish communities

Abstract

A series of field and laboratory experiments examined the factors that control fish- assemblage 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, surf- zone fishes typically died within 24 h. Levels of dissolved oxygen and temperature frequently ex- ceeded the tolerance limits of the surf-zone fishes (demonstrated in a series of laboratory experi- ments), 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 evalu- ate the survival and condition of a common surf-zone fish, Membras martinica (the rough silverside): (1) 8 M. martinica (14 fish m -2 ), (2) 16 M. martinica (28 fish m -2 ), and (3) 8 M. martinica + 8 Fundulus heteroclitus (28 fish m -2 ). After approximately 1 mo during the spring, fall and winter, survival and condition of fishes in Treatments 2 and 3 were significantly reduced, suggesting that biological inter- actions were an important cause of surf-zone fish mortality. A temporal environmental stress gradi- ent may result in seasonally varying importance of abiotic and biotic structuring factors.