376 



Fishery Bulletin 94(2). 1996 



egories (fish, crustaceans, and cephalopods) between 

 two size (FL) groups of juvenile cobia, small (236- 

 338 mm) and large (340-440 mm), that represented 

 all juveniles with food in their stomachs. The x 2 test 

 and Fisher's exact test were used to identify signifi- 

 cant differences in frequency occurrence of principle 

 prey categories between two geographic areas of juve- 

 nile collection (east of long. 89"W; west of long. 89°W), 

 essentially described as east and west of the Missis- 

 sippi River delta. The diet was also examined for sea- 

 sonal differences in the occurrence of principal prey. 



Results 



Stomach contents of 49 juvenile R. Canadian rang- 

 ing from 236 to 440 mm FL (mean 335 mm) and from 

 0.08 to 0.81 kg TW (mean 0.34 kg) were examined. 

 Thirty-nine of the 49 (80% ) stomachs examined con- 

 tained prey. The diet comprised three principal cat- 

 egories: fish, crustaceans, and cephalopods (Table 1). 



Fishes occurred in 31 (79.5%F) of the stomachs 

 containing food and represented 40.9% of the total 

 number of prey and 37.7% of the total volume of prey 

 (Fig. 2). The fish category dominated the %IR1 

 (60.0%) (Fig. 2). Fish prey were represented by nine 

 species (Table 1). Other fishes, owing to various 

 states of digestion, could be identified only to ge- 

 nus (Anchoa sp. and Symphurus sp.) or family 

 (Serranidae). 



Anchovies were by far the predominant fish 

 prey items, accounting for 19.7%iV and 18. 4% V of 

 all items in the diet ( Table 1 ). The striped anchovy. 

 Anchoa hepsetus, was the dominant species by % V 

 (9.2 ),//?/( 163.8), and %IRI (4.9). The bay anchovy, 

 Anchoa mitchilli, exhibited the highest numeric 

 percentage (8.4) among fish in the diet; however, 

 all specimens of A. mitchilli occurred in one stom- 

 ach. The dominance of anchovies would be even 

 greater if one considered that unidentifiable fish 

 remains (15.1%N, 6.9%V; and 1AA%F) were pri- 

 marily anchovies. 



Crustaceans ranked first in importance numeri- 

 cally (42.7%7V) and second by volume (37A%V) 

 and occurred in 59.0% of the stomachs (Fig. 2). 

 Decapods were the most common crustaceans en- 

 countered (Table 1). The roughback shrimp, 

 Trachypenaeus sirnilis, was the dominant food 

 item in the diet on the basis of %N ( 19.2 ), but all 

 of these specimens occurred in only two stomachs. 

 Other decapod species were brown rock shrimp. 

 Sicyonia brevirostris, lesser rock shrimp, Sicyonia 

 dorsalis. and iridescent swimming crab, Portunus 

 gibbesii. Other decapods were identifiable only to 

 Portunidae, Xanthidae, or Brachyura. Sicyonia 



brevirostris and P. gibbesii contributed 9.1%V and 

 5.9% V to the total diet, respectively (Table 1). Deca- 

 pod remains (2.2%V) were categorized as being either 

 those of shrimp ( 11.8%AT. 33.3%F) or crab (0.8%>A0. The 

 mantis shrimp, Squilla empusa ( Stomatopoda I, contrib- 

 uted 6.4% V but only 2.1%JV to the diet. 



Cephalopods were present in 17.9% of the stom- 

 achs and comprised 16. 3% N and 24.9% V of prey or- 

 ganisms (Fig. 2). This group was represented by two 

 species, the slender inshore squid Loligo plei, and 

 the longfin inshore squid, Loligo pealei. Overall, 

 Loligo plei dominated the diet among identifiable 

 prey ( 17.0% V, 15 A%F, 494.3 IRI and 14.9 f ; IRI) and 

 was the second most important food item based on 

 % N ( 15. 1 ) (Table 1). Loligo pealei was the fifth rank- 

 ing food item by volume, contributing 7.7%. 



The proportion of empty stomachs was significantly 

 greater for juveniles collected during the night (40% ) 

 than during the day ( 12%) <x 2 =3.518, df=l, P<0.05). 

 Mean stomach fullness was found to vary signifi- 

 cantly (with 95% confidence) between daytime (3.13) 

 and nocturnal ( 0.92 ) collections ( ANOVA, df= 1 , P<0.05 ) 

 (Fig. 3). When further measured for specific time in- 

 tervals of capture, mean fullness values were 0.08 

 (0001-0400), 2.00 (0400-0800), 3.29 (0800-1200), 2.45 

 ( 1200-1600), 3.70 ( 1600-2000), and 0.73 (2000-2400) 



100 



80 



on 



4d 



20 



%i 



, IRI 





Fish 



Crustaceans Cephalopods 



Figure 2 



Percent numerical abundance (%N), percent total volume 

 i'r\'i. percent frequency of occurrence (%F), and percent in- 

 dex of relative importance CH IRI) for primary prey categories 

 in the diet of juvenile Rachyccntron canadum from the north- 

 ern Gulf of Mexico. 



