Habitat use by an intertidal salt-marsh fish:trade-offs between predation and growth

Abstract

Biological and physical factors interact to determine habitat use by animals. While sessile organisms inhabiting intertidal areas have been well studied, the processes underlying the use of intertidal areas by highly mobile animals such as fishes remain poorly understood. This study examines whether an intertidal fish, the mummichog Fundulus heteroclitus, preferentially uses habitats where growth rate is maximized or, like most sessile animals, is restricted to habitats providing escape from strong negative biological interactions but compromising growth. I compared growth rates and predation rates on adult F. heteroclitus at 3 Rhode Island salt marshes in 2 seasons (summer and fall). Salt marshes varied in the types of habitat (creek, pond, channel, mudflat) available to fishes within each marsh. Growth rates of marked mummichogs held in cages were determined in each habitat and each marsh for 6 wk. Predation risk was assessed by determining mortality of F. heteroclitus tethered in each habitat in each of the 2 seasons. Habitat use was quantified by trapping fish in each habitat in each season. Both growth rate and predation risk influenced habitat use by F. heteroclitus in these intertidal salt marshes. Overall, salt marshes offered opportunities for both higher growth rates and shelter from predation compared to areas outside salt marshes. However, mummichogs often faced trade-offs between predation refuge and growth in specific habitat types. When predation risk was high across all habitat types in a marsh, habitats associated with the highest growth rates were used. If a predation refuge was available, habitat use increased, even when growth rates in the habitat were low. A habitat characterized by both shelter and high growth rates, e.g. Coggeshall Cove ponds in fall, were used almost exclusively. Variation in quality of the same habitat type in different marshes led to differences in overall marsh quality. Determination of profitability of habitat for fishes cannot be assessed through habitat use alone; measurement of factors, such as predation risk and growth, which influence habitat quality are also necessary.