ASPECTS OF ESTUARINE INTERTIDAL ECOLOGY OF 

 JUVENILE STRIPED MULLET, MUGIL CEPHALUS, IN HAWAII^ 



Peter F. Majors 



ABSTRACT 



Behavior and distribution of schools of young striped mullet, Mugil cephalus. were examined in the 

 field and laboratory. Prejuvenile fish approximately 20 mm standard length leave the open ocean to 

 enter intertidal estuarine regions, where they select the shallowest water, areas with extensive diel 

 temperature and salinity fluctuations. At about 50 mm standard length, the mullet move into deeper 

 intertidal waters. It is at this size that mullet are thought to have completed their metamorphosis to 

 juveniles. In a vertical thermal gradient, fish generally <50 mm standard length selected final mean 

 temperatures of 30.0°-32.4°C at the salinities tested (0, 15, 34%o). In the field, they were found in water 

 with high ( 34. 0°-37.2°C), often near lethal (39.0°-42.5°C), temperatures in shallow pools with salinities 

 of 2-30%o, Juveniles generally >50 mm experimentally selected final mean temperatures of 29.0°C at 

 34%o salinity to 19.5°C at 0%o salinity. In the field, fish ^50 mm remained seaward of the tide line in 

 water of lower and more uniform temperature and higher and more uniform salinity than those 

 recorded for mullet <50 mm. Mullet <50 mm occur seasonally when there are a maximum number of 

 low tides sO.O mm and a minimum number of high tides &0.6 m. This allows the mullet increased time 

 to feed undisturbed in areas where there are no predators and intraspecific and possible interspecific 

 competitors for food and space. By the time fish reach 50 mm standard length, the tidal situation 

 changes, allowing predators and competitors access to the shallow areas during low tide. When in the 

 presence of predators, the schooling habit increases chances of survival for individual mullet. 



The marine environment includes the highly 

 complex estuarine and intertidal habitats, which 

 undergo continuous fluctuation. Organisms dwell- 

 ing within these areas must be able to tolerate or 

 escape from the consequences of extreme tempera- 

 ture and salinity oscillations brought about by 

 tidal and meteorological changes. Coral and rocky 

 intertidal tidepools and the estuarine environ- 

 ment serve as nursery and feeding grounds for the 

 young of many species of fishes (Randall 1961; 

 Norris 1963; Lauff 1967; Carr and Giesel 1975). 

 The purpose of this study was twofold: to deter- 

 mine whether young striped mullet, Mugil 

 cephalus Linnaeus, select specific environmental 

 conditions, particularly with respect to tempera- 

 ture and salinity, as found in intertidal estuarine 

 environments in Hawaii, and to explore the possi- 

 ble causal mechanisms that might lead to the 

 selection of such conditions. An experimental ver- 

 tical thermal gradient (use of such gradients was 



'Oceanic Institute Contribution No. 118. From part of a thesis 

 submitted in partial fulfillment of requirements for the degree of 

 Doctor of Philosophy, University of California at Santa Cruz. 



^Center for Coa.stal Marine Studies, University of California 

 at Santa Cruz, Santa Cruz, Calif; present address: P.O. Box 

 23720, L'Enfant Plaza Station, Washington, DC 20024. 



reviewed by Mantelman 1958; Ivlev and Lei- 

 zerovich 1960; Fry 1964) in a tank was used to 

 study the relationship between salinity and temp- 

 erature and the distribution of schools of young 

 striped mullet, and field observations were made 

 of the distribution, feeding, and predator-prey be- 

 havior of schooled mullet. 



METHODS AND MATERIALS 



Field Sites and Capture of Fish 



Young striped mullet were observed and col- 

 lected in estuarine intertidal habitats at a number 

 of locations on the island of Oahu ( Figure Din the 

 Hawaiian Archipelago during 1972 and 1973. All 

 experimental fish were captured with hand or 

 beach seines near stream mouths or springs and 

 on tidal mud flats in Maunalua Bay on the south- 

 east side of the island. Schools were usually 

 caught in the morning at low tide and transported 

 <16 km to the Oceanic Institute, Makapuu, Oahu. 



Observations were made along Wailupe Beach, 

 Wailupe Stream, and Kuapa Pond Streams (Ha- 

 waii Kai Development drainage culverts) in Mau- 

 nalua Bay, and along Kahana River and a silted 

 Hawaiian fishpond in Kahana Bay on the east side 



Manuscript accepted July 1977. 



FISHERY BULLETIN: VOL 76. NO. 2. 1978, 



299 



