FISHERY BULLETIN: VOL. 76, NO. 2 



proposed that intertidal species are limited in 

 their upper distributional range by physiological 

 (and presumably biochemical) adaptive abilities 

 to environmental stress. At the lower end of the 

 range, organisms are limited by biotic factors such 

 as competition and predation. Field observations 

 indicated that there was essentially no predation, 

 including that by birds, of mullet <50 mm SL 

 when they occupied the intertidal estuarine tide 

 line and swash zone areas at low tide (Tables 2, 3). 

 At high tide, and during ebb and flood, mullet <50 

 mm SL were exposed to predators, but the poten- 

 tial for being attacked and caught was reduced by 

 occupying the shallowest tide line waters and by 

 the schooling habit. 



The absence of predatory fishes in the shallow 

 intertidal estuarine regions at low tide may be 

 related to 1) a subminimal depth or area of water 

 in which to maneuver, and 2) possibly, although 

 data are lacking, an inability of predators to adjust 

 to rapidly fluctuating thermal and salinity re- 

 gimes. Predators escaped entrapment by remain- 

 ing seaward of the tide line during ebbing tides 

 just as juvenile mullet s^50 mm SL did. In addi- 

 tion, potential invertebrate predators were absent 

 from the shallow intertidal areas presumably es- 

 caping seaward and/or, as often observed or 

 caught, burrowing to a level below the surface of 

 the mud/sand substrate. 



Juvenile mullet ^50 mm SL as well as adult 

 mullet appear to be competitors with individuals 

 <50 mm SL for food resources in the intertidal 

 estuarine region. At high tides the larger fish 

 moved into and fed in the areas used by the 

 younger fish during low tides. With the incoming 

 tide, the younger fish moved shoreward with the 

 tide line. In addition, other species of fishes moved 

 in with the flood tides. It is not known whether 

 these fishes utilized the same food resources as the 

 young mullet. 



Space also may be at a premium in the shallow 

 intertidal estuarine regions, particularly in tide 

 pools. As discussed previously, the volume of 

 water in the tide pools as well as the depth of pool 

 water may be critical. Formation of large schools 

 is characteristic of larger juvenile mullet as it is 

 for the species as a whole and inter- and intra- 

 specific competition for space may occur. Other 

 species of fishes, as well as the larger juvenile 

 mullet, were not observed in the shallowest water 

 during low tide. The ability of small mullet to 

 occupy the shallowest, warmest water may also 

 occur in the northern Gulf of California (Heath 



312 



1967), where they (no length data) are one of two 

 species penetrating the farthest up seawater 

 drainages along the margin of the desert. 



Mullet <50 mm SL formed loose schools with 

 individuals constantly feeding during low tides, 

 particularly in tide pools. At high tides, or in more 

 exposed environments, feeding often ceased and 

 tight dense schools were formed. This was evident 

 when predators were nearby, approaching, or at- 

 tacking. When exposed to predators at high tides 

 and changing tides, the schooling habit confers an 

 increased advantage to the mullet in terms of sur- 

 vival (Major 1977, in press). Most of the attacks by 

 predators on schools that I observed, failed. The 

 formation of schools appears to be yet another 

 adaptive feature in the behavioral repertory of 

 mullet. The schooling habit increases the ability of 

 individual mullet to survive as prejuvenile and 

 juveniles in the intertidal estuarine region and 

 presumably as prejuveniles in oceanic waters. 



Prejuvenile and small juvenile mullet have pre- 

 sumably evolved the necessary biochemical and 

 physiological adaptations to exist successfully in 

 the fluctuating, often near lethal, intertidal es- 

 tuarine environment in the spring months. In 

 Hawaii, this has allowed them to use this interti- 

 dal refugium to escape their predators and com- 

 petitors for food and space, making possible undis- 

 turbed feeding activity. Kinne's (1960) work with 

 Cyprinodon tnacularius and Norris's (1963) study 

 of Girella nigricans suggest that high tempera- 

 tures increase the rate of food uptake and diges- 

 tion. Food conversion (to growth) efficiencies are 

 highest at lower temperatures, however. De Silva 

 and Perera (1976) experimentally determined 

 that young mullet grow more rapidly at 20%o sa- 

 linity than at salinities of 30, 10, or l%o. This was 

 comparable to Kinne's (1960) work with C. macu- 

 larius. 



The widely fluctuating environmental variables 

 in the estuarine intertidal in Hawaii may provide 

 the necessary conditions for rapid grovvi;h in mul- 

 let. This would allow metamorphosis to be com- 

 pleted in all mullet by the time the tidal situation 

 becomes less favorable, as predators gain access to 

 the small mullet and intra- and interspecific com- 

 petitors gain access to their feeding areas as well. 

 The formation of large schools during all stages of 

 life appears to be important in reducing predation 

 and possibly also in competing with other species 

 for food and space in the estuarine intertidal re- 

 gion. 



