CHAO and MUSICK. LIFE HISTORY OF JUVENILE SCIAENID FISHES 



channel stations. In spring months (March-May), 

 bottom temperatures increased rapidly, and the 

 upper portion of the York River had slightly 

 higher temperatures than the lower portion. The 

 shoal stations also showed a slightly higher mean 

 bottom temperature than the channel stations. In 

 summer months (June-August), the bottom 

 temperature of the upper portion of the river was 

 higher than the lower portion. The shoal stations 

 also showed a higher mean bottom temperature 

 than the channel stations. In fall months 

 (September-November), bottom temperature de- 

 creased rapidly. The upper portion of the river had 

 slightly higher temperatures than the lower por- 

 tion in the early fall (September-October). In 

 early winter (December), bottom water tempera- 

 ture was slightly higher in the lower portion of the 

 river (Figure 2). No apparent differences were 

 found among the shoal and channel stations. 



Dissolved Oxygen 



Dissolved oxygen in the York River (Figure 2) 

 was generally lower in warmer months (May- 

 October) and higher in colder months 

 (November- April). In the warmer months, dis- 

 solved oxygen was lowest at the deeper channel 

 stations. There was no apparent difference be- 

 tween the upper and lower portions of the York 

 River. In colder months, dissolved oxygen was 

 slightly higher in the upper portion of the river 

 and no apparent difference was found among shoal 

 and channel stations. 



Salinity 



Salinity decreased toward the upper portion of 

 the York River (Figure 2). Lower salinities usu- 

 ally were found in spring ( March-May) and winter 

 (December-February). The extremely low 

 salinities of June to August 1972, were caused by 

 hurricane Agnes (Anderson et al. 1973). Salinity 

 at channel stations was usually higher than at 

 shoal stations, especially in the lower portion of 

 the river from March to June. 



Temporal and Spatial Distributions 



Young sciaenids are among the most abundant 

 migratory finfishes in the York River (Massmann 

 1962; Colvocoresses 1975; Markle 1976). Tem- 

 poral and spatial distributions of juveniles of the 

 four most abundant sciaenids, Cynoscion regalis, 



Bairdiella chrysoura, Micropogonias undulatus, 

 and Leiostomus xanthurus, are compared (Figures 

 3-5) to determine ecological partitioning during 

 their estuarine life. The relative abundance of 

 each species is expressed by the geometric mean, 

 logio (x + 1), of the individual catches per tow 

 within the substrata, where x is the mean number 

 of individuals per tow. Four months (July, Oc- 

 tober, January, and April) were selected to repre- 

 sent the seasonal abundances from different parts 

 of the York River (Figure 3). Monthly mean 

 catches per tow by river distance (stratum) and 

 depth (substratum) were compared (Figures 3-5). 

 Fishes caught in the Mattaponi and Pamunkey 

 rivers were compared only by river distance (Fig- 

 ure 4). 



In July 1972 and 1973, all four species of 

 juvenile sciaenids were present in all parts of the 

 estuary except the upper part ( Figures 3-5). Rela- 

 tive abundance varied among species (Figure 3). 

 Bairdiella chrysoura was more abundant in the 

 lower and middle part of the river, while C. regalis 

 and M. undulatus were more abundant in the 

 upper part of the river (Figure 5). Leiostomus 

 xanthurus was ubiquitous. Micropogonias un- 

 dulatus gradually declined in abundance up- 

 stream in both the Mattaponi and Paumkey rivers 

 (Figures 4, 5). Leiostomus xanthurus catches were 

 quite variable in the Pamunkey River. This may 

 have been caused by the contagious distribution of 

 this species. Sciaenids were more abundant in 

 shoal stations (Figure 3) than channel stations, 

 especially in July 1972. Colvocoresses (1975) and 

 Markle ( 1976) noted a general decline in the mean 

 number of species and individuals of fishes caught 

 per month in the summer from channel stations. 

 This may be attributed to a reduction in the dis- 

 solved oxygen concentration, usually below 5 mg/1 

 at the bottom of the channel (Markle 1976; 

 Brehmer see footnote 5), and was apparently the 

 case in the present study (Figure 2). Catches of C. 

 regalis did not decline in channel stations, but this 

 species is the best adapted for pelagic life of the 

 four species studied (see "Correlation of Feeding 

 Structures and Food Habits" section), and may 

 have been captured in midwater where dissolved 

 oxygen values did not decline. 



In October (1972, 1973) juveniles of all four 

 species of sciaenids were present in all parts of the 

 estuary (Figure 3) and all reached their highest 

 total abundance (Markle 1976). Cynoscion regalis 

 was more abundant in the lower parts of the York 

 River; B. chrysoura and L. xanthurus were more 



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