Overwintering habitat selection by the mummichog, Fundulus Heteroclitus, in a Cape Cod (USA) salt marsh

This study tracked the seasonal distribution and winter habitat selection of the mummichog, Fundulus heteroclitus (Linnaeus), in a Cape Cod, Massachusetts salt marsh. Fish (mean size = 43.1 mm total length, range = 10-93 mm) were collected with a 1 m(-2) throw trap and by excavating sediments. In fall, F. heteroclitus began migrating upstream in creeks and eventually moved into upstream pools where they remained throughout winter. F. heteroclitus burrowed into the sediments of these pools at a density of 0.5 fish m(-2), but was not found burrowed in the sediments of downstream pools or any creeks. Sediments in upstream pools were composed of a higher proportion of fine-grained particles and organic content than other marsh pools and creeks, and winter temperatures in upstream pool sediments remained above 1 degrees C. Temperatures in the water column and sediments of downstream pools regularly dropped below -1.8 degrees C, exceeding the lethal limit for F. heteroclitus. These results support other recent work showing that F. heteroclitus migrates upstream in salt marshes in fall and overwinters in salt marsh pools. Moreover, this study demonstrates that F. heteroclitus does not utilize all available pools as overwintering habitat but apparently selects pools with sediments that offer a thermal refuge from lethal winter temperatures.