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Fishery Bulletin 103(2) 



It should be noted that graphic representations of the 

 IRI values are more accurate in describing the diet of 

 fish species (Cortes, 1997). Our results (Fig. 1) demon- 

 strate that the three indices representing the relative 

 importance of each food item highlight the influence 

 that the percentages of occurrence, by number and by 

 weight, have on the overall IRI values. 



The temporal analysis of the tropic spectrum of C. 

 nannus showed that during October, November, and 

 December this species fed mainly on penaeid shrimps. 

 Fish prey were abundant in stomachs collected only 

 during March, April, June, and November. Stomato- 

 pods were present all year round, but only abundant 

 during May. Low Ba values in October were due to the 

 prevailing consumption of Solenocera spp. Monthly dif- 

 ferences in the diet of C. nannus were most probably in 

 accordance with the seasonal variations in prey species 

 abundance, which in turn determined their availability. 

 Lucena et al. (2000) found that temporal variations 

 in the diet of C. guatucupa from southern Brazil are 

 related to seasonal production cycles of prey, mainly 

 fish and crustaceans, thus supporting the view that 

 sciaenids can generally be considered opportunistic 

 species. 



Results of this study showed ontogenetic variations 

 in the trophic spectrum of C. nannus. The smallest 

 individuals (7 cm <rTL <;10.9 cm) feed mainly on sto- 

 matopods, whereas larger individuals (all cm TL), con- 

 sume less stomatopods and more penaeid shrimp and 

 fish. Merriner (1975) also found ontogenetic variations 

 in the diet of C. regalis, where the smallest individu- 

 als (age group "0") fed on crustaceans and small fish. 

 The relative importance of shrimp, however, decreased 

 as C. regalis increased in size, and individuals of age 

 group "2" generally consumed different species of clu- 

 peids, depending on the local abundance of each prey 

 species. The measure of percentage similarity among 

 size classes (Table 2) shows that C. nannus share a 

 limited number of resources. Only fish belonging to 

 intermediate lengths feed on the same prey types in 

 percentages greater than 50% for the total number of 

 food resources used. 



Ontogenetic changes in the diet of C. nannus ob- 

 served in the present study are due to differences in 

 diet composition and proportions of consumed prey. 

 These results suggest that food types are ingested un- 

 equally as fish grow and that morphological and physi- 

 ological changes take place. As fish grow, the size of 

 their mouth increases proportionally, their swimming 

 capacity is modified, and their energetic requirements 

 vary. Thus, larger fish have different feeding require- 

 ments than smaller ones and will attempt to satisfy 

 them by consuming a larger variety of prey types. As C. 

 nannus grow, Ba values increase and the trophic spec- 

 trum of the species grows wider (Fig. 2). Our results 

 indicate that there is a pattern of differential use of 

 food resources throughout the different size classes of 

 C. nannus, and suggest a possible ecological strategy to 

 reduce intraspecific competition for food in the popula- 

 tion (Schoener, 1974; Werner, 1979). 



The increasing variety of food resources used as 

 predators increase in size is a common pattern among 

 marine organisms, including invertebrates (Rangeley 

 and Thomas, 1987; Mascaro and Seed, 2001). These 

 ontogenetic variations in food preferences can be ex- 

 plained by changes in foraging behavior where preda- 

 tors of certain size classes actively select their prey 

 (Jubb et al., 1983; Allan et al., 1987). Alternatively, 

 they can be the result of passive mechanisms that do 

 not involve individual decisions associated with age or 

 life stages, such as differences in the predator's mouth 

 structures, changes in movement velocity of both prey 

 and predator, and spatial or temporal variations in 

 habitat as predators increase in size (Hughes, 1979; 

 Rodrigues et al., 1987). 



To show that Ba values are affected by the type of 

 prey distribution function used, we calculated the mean 

 diet diversity index using 1) the proportion of the num- 

 ber of prey (N; Ba = 0.03), 2) the percent frequency of 

 occurrence of prey (F; Ba = 0.16), and 3) the proportion 

 of prey biomass (W; Ba = 0.32). The values obtained 

 were then compared to those calculated by considering 

 the proportion of individuals (2V*; Ba = 0.41) that use a 

 certain food resource for the total number of stomachs 

 analyzed. Ba values calculated by using N, F, and W are 

 markedly lower than the Ba value obtained by using 

 N*. These differences serve to underline the importance 

 of complying strictly with the property of statistic inde- 

 pendence of sampling units when the feeding habits of 

 a species are being studied. 



Given the numerical importance of C. nannus as part 

 of demersal assemblages, observations on the trophic 

 spectrum of this and other species can help to generate 

 a conceptual model of the trophic webs and dynamics of 

 the feeding relations among communities found on the 

 continental shelf of Jalisco and Colima. an area that 

 has received little attention in the past. 



Acknowledgments 



J. Arciniega, R. Garcia de Quevedo, and V. Landa-Jaime 

 kindly verified the taxonomic status of prey. We also 

 thank L. E. Hidalgo-Arcos for his technical support and 

 S. Bowers who edited the text. 



Literature cited 



Aguilar-Palomino, B., J. Mariscal-Romero, G. Gonzalez-Sanson, 

 and L. E. Rodriguez-Ibarra. 



1996. Ictiofauna demersal de fondos blandos de la plata- 

 forma continental de Jalisco y Colima, Mexico, en la 

 Primavera de 1995. Cienc. Mar. 22:469-481. 

 Allan, J. D„ A. S. Flecker, and N. L. McClintock. 



1987. Prey preference of stoneflies: sedentary vs. mobile. 

 Oikos 49:323-331. 

 Allen, G. R., and R. Robertson. 



1994. Fishes of the tropical Eastern Pacific, 380 p. Univ. 

 Hawaii Press, Honolulu, HI. 



