MAJOR: ASPECTS OF ECOLOGY OF STRIPED MULLET 



oceanic waters, and appear to be "preadapted" to 

 the near lethal conditions inshore. This may indi- 

 cate the existence of an ontogenetic biological 

 rhythm (cued by slight monthly changes in photo- 

 period or water temperature?) in these mullet, 

 which biochemically and physiologically pre- 

 adapts these fish for life in the intertidal estuarine 

 regions, while they are still in oceanic waters. 

 Seasonal and daily acclimation to existing ther- 

 mal and salinity regimes may then occur after the 

 prejuveniles arrive inshore. 



Factors affecting the distribution of striped mul- 

 let 3^50 mm SL are more complex. In Hawaii, field 

 acclimated fish behaviorally select or prefer water 

 temperatures well below their experimental CTM. 

 The natural fish kill observed in March also indi- 

 cates that the larger juveniles (and most pre- 

 juveniles) were not able to survive exposure to 

 high temperatures (at least for relatively long 

 periods of time). Behavioral selection of tempera- 

 ture regimes well below CTM has also been ob- 

 served in the estuarine goby, Gillichthys mirabilis 

 (de Vlaming 1971). 



Mullet &50 mm SL in Hawaii moved seaward to 

 pools with open connections to deeper water with 

 ebbing tides during the spring. During the non- 

 tide-pool forming low tides in the late summer, the 

 larger juveniles may be acclimated to tolerate 

 higher ambient seawater temperatures in Hawaii 

 as they are in the northern Gulf of California. In 

 Hawaii, however, intertidal waters reach their 

 maximum temperatures during the period from 

 late winter to early summer when shallow tide 

 pools are formed, although ambient oceanic temp- 

 eratures are higher during late summer. Heath 

 (1967) indicated that inshore water temperatures 

 in the northern Gulf of California are highest dur- 

 ing the late summer-early fall period. It is not 

 known whether CTM's are similar for field accli- 

 mated striped mullet ^50 mm SL in the spring in 

 Hawaii and in the late summer in the northern 

 Gulf of California, or whether mullet from both 

 locations have CTM's parallelling seasonal 

 changes in ambient (oceanic) seawater tempera- 

 ture. 



Selected temperatures are lower for young mul- 

 let ^50 mm SL compared with smaller fish at each 

 salinity, and much lower at decreasingly lower 

 salinities (Figure 3). Presumably, physiological 

 changes mediated hormonally/biochemically 

 occur during metamorphosis, resulting in a pref- 

 erence for reduced temperatures by the larger 

 juveniles. This decrease in temperature "toler- 



ance" with metamorphosis (age) is somewhat con- 

 trary to the discussion thus far. It may only be a 

 behavioral trait not directly correlated with CTM 

 (i.e., CTM may actually be increasing). Behavioral 

 selection of lower temperatures appears to be 

 adaptive in the field during the period between 

 April and August. In addition to having widely 

 varying salinity values, many of the tide pools 

 formed during this period may have been too shal- 

 low for larger juveniles to feed and swim. Many of 

 the pools were shallower than the body depth of 

 the older juveniles. Thus, those individuals that 

 remained seaward of the tide line as they com- 

 pleted metamorphosis may have reduced or es- 

 caped the possibility of entrapment and exposure 

 to lethal conditions in tide pools and shallow wa- 

 ter; conditions observed once during this study. 



These relationships may indicate the existence 

 of an endogenous rhythm involved in the move- 

 ment of (behavioral selection by) juvenile mullet 

 towards deeper, relatively cooler, more saline 

 water during or after metamorphosis. This 

 rhythm may be acting in opposition to the pre- 

 sumably increased acclimation to higher ambient 

 (oceanic) seawater temperatures. The change in 

 behavior with metamorphosis may be a result of 

 endogenous rhythms perhaps coupled with 

 exogenous factors, such as the slight monthly 

 changes occurring in seawater temperature 

 and/or photoperiod, or it may be due directly to 

 these exogenous factors. The reproductive cycle of 

 striped mullet appears to be coupled with both 

 these environmental variables (Kuo et al. 1974; 

 Kuo and Nash 1975), so presumably younger indi- 

 viduals could use these same cues as well. It is 

 difficult to separate cause from effect, but the shal- 

 lowness and volume limitations of tide pools may 

 be critical. Thus, selection may have favored those 

 metamorphosed individuals with reduced physio- 

 logical tolerance to high fluctuating temperatures 

 and low fluctuating salinities as found in the es- 

 tuarine intertidal (i.e., those individuals that be- 

 haviorally moved away from such conditions). 



If selection favored those metamorphosed 

 (metamorphosing) individuals that moved into 

 deeper intertidal waters, what selection pressures 

 may have favored individuals able to survive the 

 kaleidoscopic conditions of the estuarine interti- 

 dal tide pools? 



Experimental and field evidence demonstrate 

 the importance of refugia for species from their 

 competitors and/or predators (Cause 1934; Crom- 

 bie 1946; Connell 1961; Paine 1969). Connell 



311 



