Salinity Preference in the Estuarine Teleost Fish Mummichog (Fundulus heteroclitus): Halocline Behavior

Mummichogs prefer seawater (SW) but have wide ability to acclimate to extreme temperatures and salinities. In the field, minnow trapping revealed that mummichogs move progressively into low-salinity warmer water during early spring after ice melt and show significant aversion to colder temperatures and high salinity. First appearance in estuarine shallows occurred above 10°C, and catch increased to 21°C over 4 wk. Three-spine sticklebacks (Gasterosteus aculeatus) also preferred warmer low-salinity locations but preferred slowing streams, whereas mummichogs preferred tidal ponds. In the laboratory, artificial haloclines tested isothermal salinity preference, between 28‰ full-strength SW (below) and 10% SW (3.0‰; above). Mummichogs of both sexes acclimated to 5°C in SW strongly preferred SW. Freshwater (0% SW)-acclimated mummichogs at 21°C also preferred SW, but of sexually mature fish acclimated to 21°C SW, only the males preferred SW; the females showed no significant preference for SW, meaning they freely entered low salinity. SW preference was manifested by a stereotypic passive aversion to the dilute upper layer at the halocline. We conclude that the overall movement of mummichogs into summer breeding grounds of low salinity is driven by maturation of females and their preference for warmer water regardless of salinity.