Weisberg and Burton. Ichthyoplankton abundance and distribution in the Delaware River 



795 



Alosa sapidissima eggs 



1987 

 O 1988 



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Alosa sapidissima larvae 



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 o 



z 



LU 



o 

 w 



UJ 



I 



* A a* ,«P .A ,<* * A <(• * 



>A/osa spp. larvae 



Morone americana larvae 



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S. 



LU 



X 





Perca flavescens larvae 



a 



UJ 



E 



E 



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Morone saxatilis larvae 



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WEEK 



Figure 4 



Temporal patterns of abundance for the dominant larval fish of the tidal Delaware River. 

 Solid and dashed lines represent 1987 and 1988, respectively. 



stream (Glebe and Leggett, 1981; Maurice et al., 1987), 

 could get past Philadelphia before hypoxic conditions 

 developed. Second, only fish migrating out of the river 

 late in the season, that is fish spawned in the most 

 upstream reaches, arrived in the Philadelphia area 

 after the lethal dissolved oxygen conditions dissipated. 

 Our data suggest that the spatial extent of A. 

 sapidissima spawning habitat in the Delaware Rivet- 

 has expanded over the last decade, coincident with the 



water quality improvements near Philadelphia. While 

 we found no evidence of spawning or nursery activity 

 in tidal freshwater during the 1970s, prior to water 

 quality improvements, by the late 1980s the density 

 of larvae there was as high as in non-tidal upstream 

 reaches. The suggestion of expanded spawning ground 

 for shad is consistent with Maurice et al. (1987), who 

 documented substantial increases in spawning activ- 

 ity during the last decade in the lower reaches of the 



