FISHERY BULLETIN: VOL. 76, NO 2 



mullet are most abundant), reduces insolation and 

 thus reduces the potential for the attainment of 

 lethal conditions in tide pools. These relationships 

 are particularly important since young mullet 

 congregate in areas of springs and freshwater 

 run-off. However, cloud cover during February 

 through May varies more than during any other 

 period of the year and points to the fact that the 

 environment in which mullet <50 mm SL occur 

 fluctuates tremendously within a season and from 

 year to year. 



Seasonal rainfall is maximal during winter- 

 spring in Hawaii. Rainfall in the mountains 

 (Kaneohe Mauka Station) exceeds that in nearby 

 shore regions (Hawaii Institute of Marine Biology 

 Station). Fluctuations in rainfall from month to 

 month and year to year are great and appear to be 

 unpredictable, particularly during the season 

 when mullet <50 mm SL are abundant in the 

 intertidal estuarine region. Run-off is maximal 

 during this season due to the heavier mountain 

 rainfall. This run-off could contribute to poten- 

 tially lethal or near lethal conditions in the inter- 

 tidal region by reducing its salinity, particularly 

 during periods when low tides occur during noon. 

 Intertidal spring water temperatures, on the other 

 hand, were recorded to be as much as 10° cooler 

 than surrounding water of higher salinity (Table 

 2). This cooler water may serve to reduce overall 

 intertidal estuarine temperatures, at least during 

 nontide-pool forming tide levels, but at the same 

 time it increases the thermal and salinity heter- 

 ogeneity of the environment. 



Returning to a discussion of adaptations of mul- 

 let, experimental studies indicate that tempera- 

 ture acclimation is important in the ability of 

 striped mullet ( at least larger juveniles ) to survive 

 higher temperatures (Heath 1967; Sylvester 1974, 

 1975; Sylvester et al. 1974). Heath, although not 

 providing fish length or salinity data at which 

 tests were made, reported critical thermal max- 

 ima (CTM) of 42.4°-43.rC for mullet in the north- 

 ern Gulf of California. Sylvester (1974, 1975) and 

 Sylvester et al. (1974) demonstrated increased 

 CTM (29.0°-41.6°C) with increasing acclimation 

 temperatures at a salinity of 32%o for juvenile 

 striped mullet, 70-125 mm SL, in Hawaii. At a 

 lower acclimation temperature, and a salinity of 

 0%o, the CTM's were reduced. In general, CTM's 

 were lower at lower salinities. Sylvester (1974) 

 also found that juveniles adjusted or acclimated 

 faster at higher temperatures, and that their 

 thermal resistance to lethal temperatures de- 



310 



creased slightly when they were exposed to fluc- 

 tuating low, rather than constant, temperatures. 



Sylvester (1975) also demonstrated the exis- 

 tence of increased CTM at noon and lower CTM in 

 the morning and afternoon for fish 78-122 mm SL. 



There is good evidence that underlying bio- 

 chemical changes are responsible for acclimation 

 to changing thermal regimes (reviews in 

 Hochachka and Somero 1971, 1973; Haschemeyer 

 1973; Somero 1975). Hochachka and Clayton- 

 Hochachka (1973) provided some evidence that 

 this may also be the case for striped mullet about 

 120 mm long in Hawaii. 



Whether the ability of mullet to tolerate in- 

 creasingly higher temperatures, seasonally and 

 daily, is a result of interacting endogenous factors 

 (biological rhythms) as is indicated in other ani- 

 mals (Sweeney and Hastings 1960; Wilkins 1965), 

 and/or exogenous factors (direct exposure to in- 

 creasing temperatures) is not known. 



Sylvester's (1974, 1975) studies were conducted 

 between August and January and those of Heath 

 ( 1967) in March and September. As the discussion 

 above and Figure 4 indicate, ambient seawater 

 temperatures are highest during August to Oc- 

 tober and lowest during February to April in the 

 tropical-temperate Northern Hemisphere. Al- 

 though the seasons varied during which the exper- 

 iments of Sylvester and Heath were conducted, the 

 CTM data obtained were similar for the experi- 

 mentally acclimated fish. Doudoroff (1957) and 

 Allen and Strawn ( 1971) reported that relatively 

 brief exposure to high nonlethal temperatures 

 usually increased heat resistance in a number of 

 species of fish. This increased resistance was not 

 readily lost when fishes were subsequently ex- 

 posed to low temperatures. This also appears to be 

 true for striped mullet as Sylvester's ( 1974) study 

 indicates. Thus, striped mullet appear to have an 

 ability to modify their thermal tolerance in direct 

 response to prevailing environmental conditions 

 (exogenous factors). The ability to increase their 

 heat resistance even after a brief exposure to high 

 nonlethal temperature would be especially advan- 

 tageous to mullet in the estuarine intertidal, at 

 least in Hawaii. 



If exogenous factors are solely responsible for 

 the ability of striped mullet to survive high tem- 

 peratures, it is difficult to explain the differences 

 in the distribution of striped mullet presented in 

 this report. 



Prejuveniles enter the intertidal estuarine re- 

 gions from the far more environmentally uniform 



