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



791 



Temperature, dissolved oxygen, and salinity were not 

 vertically stratified in the water column and did not 

 differ substantially between tributaries and the near- 

 est mainstem water quality sampling locations. 



Average dissolved oxygen values decreased over the 

 sampling period during each year of the study, rang- 

 ing from greater than 10 mg/L during the first week to 

 less than 8 mg/L during the last week. Dissolved oxy- 

 gen differed by as much as 3 mg/L among regions, with 

 the lowest values occurring downstream of Philadel- 

 phia. However, only a single measurement below 5 mg/ 

 L was recorded in 1988, and no measurements below 

 6 mg/L were recorded in 1987. 



The river was consistently fresh upstream of Pea 

 Patch Island (km 98); salinity averaged less than 2 ppt 



in the regions downstream of Pea Patch Island. The 

 highest salinity (5 ppt) was observed in the C&D Ca- 

 nal during the second week of sampling in 1988. A 

 storm and subsequent freshwater runoff during the 

 sixth week of the 1988 sampling turned the entire 

 mainstem sampling area fresh but had little effect on 

 salinity in the C&D Canal, which never fell below 2 ppt. 



Distribution of eggs and larvae 



Eighteen taxa, comprising 15 families, were collected 

 from the Delaware River during 1987 and 1988 (Table 

 1). Seventeen of these taxa also were found in the 

 tributaries (Table 2); no taxon occurred only in the 

 tributaries. Most taxa were not abundant. Only three, 



