FISHERY BULLETIN: VOL 76, NO. 1 



DISCUSSION 



Ontogenetic changes in prey utilization by P. 

 scitulus showed an early dependence on plank- 

 tonic or epifaunal prey such as crustacean larvae, 

 copepods, mysids, cumaceans, and gammarid am- 

 phipods. Larger P. scitulus (>90 mm) ate more 

 infaunal organisms such as lancelets and 

 polychaetes. Separation by prey kind was greatest 

 at 90 mm which corresponded to the transition size 

 between immature and mature fishes. 



The greatest percent occurrence of juvenile 

 fish (March-May) coincided with periods of 

 higher utilization of brachyurans, natantians, 

 cumaceans, amphipods, mysids, pelecypods, and 

 polychaetes by adult fish, although lancelets re- 

 mained the dominant prey. Consequently, size dif- 

 ferences in food habits were not biased by seasonal 

 unavailability of certain prey to adults or juve- 

 niles. Also, Ross (1974) demonstrated that 

 changes in food habits with increasing fish size 

 were generally consistent between stations. 



Other studies on food habits of P. scitulus have 

 indicated that small crustaceans and polychaetes 

 were important prey (Reid 1954; Springer and 

 Woodburn 1960; Ross 1977, in press). Ross (1977, 

 in press) found that P. scitulus from offshore of 

 Tampa Bay utilized principally brachyurans, 

 polychaetes, cumaceans, gammarid amphipods, 

 natantians, and lancelets. 



Total food consumption showed an accelerating 

 rate of increase with fish length, but initially this 

 occurred through a rapid rise in the number of 

 prey consumed, rather than through an increase 

 in prey size. Prey size did not increase with in- 

 creasing fish size for searobins <90 mm. Although 

 numerous studies have demonstrated positive cor- 

 relations between prey and predator sizes (e.g., 

 Northcote 1954; Hartman 1958; Wong and Ward 

 1972; Hespenheide 1973), Schoener (1969, 1971) 

 predicted that prey size would decrease with de- 

 creasing predator sizes to a lower horizontal 

 asymptote. Essentially, the energy gained from 

 progressively smaller prey gradually approaches 

 the energy expended in obtaining and digesting 

 prey. Data on prey size-predator size relationships 

 supporting this prediction were reviewed by 

 Schoener (1971), but did not include fishes as 

 examples. 



Prey size (both length and volume) was posi- 

 tively correlated with fish size for searobins 90 mm 

 and larger. The increase in mean prey size relative 

 to predator size occurred primarily through a 



232 



progressive shift to different, larger prey taxa, and 

 only secondarily by size selection within a single 

 prey taxon. 



The transition from numerous small prey to 

 fewer large prey was preceded by rapid growth of 

 jaw size relative to body size and by an increase in 

 intestinal length. Since intestinal absorption may 

 be increased through the development of folds and 

 an increase in length or both (Siankowa 1966), the 

 relative increase in intestinal length of P. scitulus 

 is perhaps a response to increased energy demands 

 of larger fish or to their utilization of larger prey 

 items. 



Growth in fishes may occur as a series of stanzas 

 which are entered by ecological and physiological 

 size thresholds (Parker and Larkin 1959). Growth 

 stanzas may be recognized by changes in weight- 

 length relationships (Ricker 1975). The shift from 

 small to large prey in P. scitulus was accompanied 

 by a change in the weight-length relationship in- 

 dicating the presence of two growth stanzas. 

 Growth efficiency, measured as weight gained per 

 ration weight per unit time, varies extensively 

 with prey kind (Paloheimo and Dickie 1966). For 

 instance, growth efficiency of trout increased as 

 the ration progressed from hatchery mash to 

 gammarid amphipods to minnows. The two 

 growth stanzas in P. scitulus may thus reflect an 

 increase in the proportion of food energy available 

 for growth as small crustaceans are replaced by 

 larger lancelets and polychaetes in the diet. 



Relative prey size showed a parabolic relation- 

 ship with fish size. Consequently, small P. scitulus 

 were, in effect, predators of large prey. Prey size 

 distributions have been shown to follow a lognor- 

 mal relationship in various communities (Whit- 

 taker 1952; Schoener and Janzen 1968; Griffiths 

 1975), so juvenile leopard searobins were utilizing 

 an apparently abundant energy source. However, 

 since mean prey size did not increase with increas- 

 ing fish size for searobins <90 mm, with growth, 

 searobins tended toward being "small" predators 

 due to the continued use of the same-sized prey 

 items. Although prey availability was not moni- 

 tored, P. scitulus between 20 and 90 mm were 

 likely operating as number maximizers (cf. 

 Griffiths 1975). Griffiths presented evidence that 

 juvenile stages of several kinds of vertebrates pass 

 through such a stage during which prey items are 

 utilized in close proportion to their actual occur- 

 rence. 



Searobins >90 mm showed an increase in rela- 

 tive prey size, thus tending again towards being 



