CHRISTENSEN: TROPHIC RELATIONSHIPS OF SPARID FISHES 



cal teeth to grasp prey (Figure IOC). Multicusped, 

 incisiform teeth begin to break through when the 

 fish are about 20 mm long (Figure lOD). The 

 pointed teeth are completely replaced by the time 

 the fish are 35 mm long, after which they feed 

 predominantly on algae ( approximately 60% CFI). 

 The multiple cusps wear away and the teeth are 

 bicuspid incisiform by the time the fish are 65 to 75 

 mm long (Figure lOE). These can nip at algae and 

 the diet is composed of 65 to 117c plant matter at 

 this stage. 



The gut shows a corresponding change in length 

 with diet, a long gut being characteristic of a her- 

 bivore. The G/S ratio increases from 0.86 in a 

 20-mm fish (Figure llC, typically omnivorous) to 

 1.36 in a 39-mm fish (Figure IIB), and 2.66 in a 

 99-mm individual (Figure llA). This latter value 

 is typical of a herbivore (Nikolsky 1963), although 

 not as pronounced as in some other herbivorous 

 fish species. 



DISCUSSION 



A number of fish species occur as juveniles or 

 spend their entire life cycle in the eastern Cape 

 intertidal (see description under Study Area). The 

 family Sparidae includes the largest number and 

 the present investigation of the trophic relation- 

 ships of three of these was initiated as competitive 

 interaction is often most vigorous in closely re- 

 lated fish ( Fryer and lies 1972). There is an intense 

 dietary overlap in some cases and the available 

 resources are subdivided in two main ways. Re- 

 cruitment of juveniles of the three species takes 



Figure ll. — Lateral views ofSarpa salpa with the gut unravel- 

 led and displayed to illustrate the increase in gut length with 

 size. A. 99 mm SL IRUSI 74-323). B. 39 mm SL (MSC 75-37). C. 

 20 mm SL (MSC 75-39). 



place at different times of the year and this re- 

 duces competition between those size groups in 

 which the greatest feeding overlap was observed. 

 The remaining size classes were separated as their 

 diets were different. 



Small juveniles of the three species have the 

 most similar diets of all size classes studied. The 

 resulting competition is reduced by two 

 mechanisms. Firstly, juveniles of Sarpa salpa 

 occur in the tide pools primarily from July to Sep- 

 tember (Figure 5) whereas those of Diplodus cer- 

 vinus were found during October and November 

 (Figure 4) and D. sargus was present throughout 

 the year (Figure 3). Secondly, at the time of maxi- 

 mal competition (July-November), the diet of 

 small D. sargus includes food items not taken at 

 other times of the year, e.g., chironomid larvae 

 and cirripede nauplii (Figure 7). This may be due 

 to either the presence of these prey items only at 

 that time of the year and/or to the effects of com- 

 petition forcing D. sargus to include them in its 

 diet. A combination of both factors would appear to 

 be operative in the case of the chironomids as the 

 larvae were obtained in bottom samples taken in 

 October-November and not in March. No data are 

 available for cirripede nauplii, crab zoaea, and the 

 unidentified trochophore larva as plankton sam- 

 ples were not taken. 



Competition for food is greatly reduced by the 

 time the three sparids are about 25 to 30 mm long. 

 At this stage, S. salpa feeds mainly on diatoms and 

 red algae (Figure 9); D. cervinus ingests Palaemon 

 pacificus, harpacticoid copepods, and isopods (Fig- 

 ure 8); and D. sargus takes green algae, harpac- 

 ticoids, chironomid larvae, and cirripede nauplii 

 (Figures 6, 7). The separation is equally distinct in 

 subadult fish as S. salpa is then a herbivore, D. 

 cervinus takes polychaetes, some amphipods, and 

 isopods, while D. sargus feeds on amphipods and 



399 



