AbStraCt.-Water quality in the 

 tidal freshwaters of the Delaware 

 River has improved substantially 

 over the last decade. Areas near 

 Philadelphia that were once anoxic 

 now rarely experience dissolved oxy- 

 gen concentrations lower than 3 

 ppm. To assess how fish spawning 

 and nursery activity in the river has 

 changed following these water qual- 

 ity improvements, ichthyoplankton 

 were collected from April to June in 

 1987 and 1988 in the tidal freshwa- 

 ters of the Delaware River and com- 

 pared to data available from studies 

 conducted during the mid-1970's. 

 Eighteen taxa were collected in the 

 present study; larval Morone 

 americana (white perch), Alosa 

 sapidissima (American shad), and 

 Alosa spp. (river herring) were the 

 most abundant. Total density of eggs 

 and larvae was highest upstream of 

 Philadelphia, where water quality 

 has historically been highest. 

 Ichthyoplankton density and taxo- 

 nomic composition in the tributar- 

 ies were similar to the most produc- 

 tive portions of the mainstem, except 

 in the Schuylkill River. There, wa- 

 ter quality is still poor, and 

 ichthyoplankton density was an or- 

 der of magnitude less than in the 

 mainstem. In comparison to data 

 from the previous decade, the 

 present assemblage of the Delaware 

 River was more diverse. Nine taxa, 

 most notably A. sapidissima, were 

 significantly more abundant in at 

 least one portion of the river in the 

 present study, though total 

 ichthyoplankton density was no 

 higher than prior to the water qual- 

 ity improvements. Five taxa, most 

 notably Cyprinus carpio (carp), were 

 significantly less abundant in the 

 present assemblage. 



Spring distribution and abundance 

 of ichthyoplankton in the tidal 

 Delaware River 



Stephen B. Weisberg 

 William H. Burton 



Versar, Inc. 



9200 Rumsey Road 



Columbia, MD21045 



Manuscript accepted 15 June 1993. 

 Fishery Bulletin 91: 788-797 1 1993). 



The tidal freshwater portion of the 

 Delaware River was once among the 

 most polluted estuaries in the United 

 States. Water quality was so poor dur- 

 ing the 1940s that gases released from 

 the water reportedly tarnished metal 

 and corroded engine parts of naval 

 vessels moored near Philadelphia, PA 

 (Albert, 1988). Conditions were poor- 

 est in the Philadelphia area, where 

 much of the river became anoxic dur- 

 ing warmer months. This so-called 

 pollution block substantially reduced 

 the upriver migration of anadromous 

 fishes such as A. sapidissima and 

 Morone saxatilis (striped bass) 

 (Chittenden, 1971, 1974). 



Installation of secondary treatment 

 at municipal sewage treatment 

 plants along the river has reduced 

 organic loading by more than 75% 

 since the early 1970's (Albert, 1988). 

 As a result, dissolved oxygen concen- 

 tration in the river has increased, and 

 the magnitude and duration of the 

 pollution block near Philadelphia has 

 been reduced. In areas that were once 

 anoxic, dissolved oxygen concentra- 

 tions during the spring are now 

 rarely lower than 5 ppm, or 3 ppm 

 during the summer (Albert, 1988). 

 Fishery resources appear to have re- 

 sponded to the improved water qual- 

 ity conditions; the size of the A. 

 sapidissima spawning population has 

 increased substantially (Maurice et 

 al., 1987) and average catch of young- 

 of-year M. saxatilis has increased by 

 more than an order of magnitude 1 . 



Despite the size and importance 

 of the Delaware estuary, little is 

 known about which fish use the 

 tidal freshwater region of the river 

 as a spawning and nursery 

 ground. The most recent regional 

 ichthyoplankton survey was con- 

 ducted in 1981, but covered 100 

 km of river with only 20 samples 

 per week and only data for M. 

 saxatilis were reported 2 . Numerous 

 ichthyo-plankton collections were 

 conducted in the mid-1970s as 

 part of power plant or industrial- 

 related impact studies, but these 

 were individually limited in spa- 

 tial extent and have never been 

 integrated to provide a regional 

 description. Most important, how- 

 ever, no regional study has been 

 conducted since the dramatic im- 

 provements in water quality oc- 

 curred. In this study we provide 

 new data to describe present 

 ichthyoplankton distribution and 

 abundance in the tidal Delaware 



1 Weisberg, S. B., W. H. Burton, and H. A. 

 Wilson. 1991. Delaware River striped bass 

 studies: population estimate of the 1990 

 year class and an evaluation of the young- 

 of-year index of abundance. Rep. to the 

 Delaware Basin Fish and Wildl. Manage. 

 Coop., Trenton, NJ. 



- Himchak, P. J., J. Carlson, and R. Tilton. 

 1981. Spawning and recruitment of the 

 striped bass, Morone saxatilis, in the Dela- 

 ware River. Rep. available from New Jer- 

 sey Dep. Environ. Protection, Trenton, NJ, 

 Bureau of Marine Fisheries. 



788 



