Weisberg and Burton Ichthyoplankton abundance and distribution in the Delaware River 



789 



River and its tidal tributaries, integrate data from the 

 many ichthyoplankton collections conducted during the 

 1970s, and compare these data to describe how spawn- 

 ing and nursery activity in the area has changed. 



Methods 



Ichthyoplankton samples were collected weekly from 

 April to June in 1987 and 1988, following a stratified 

 random design. Sampling in 1987 was conducted be- 

 tween Riverton, New Jersey (river kilometer [rkm| 174) 

 and the fall line at Trenton, New Jersey (rkm 214) for 

 nine weeks beginning on 22 April (Fig. 1). The sam- 

 pling area was stratified into four 10-km regions. Each 

 region was stratified further into three habitats: shoal 

 (river depth of less than 8m), channel bottom (within 

 1 m of the bottom in areas with an overall depth >8 m ), 

 and channel mid-water. Three randomly placed samples 

 were collected from each habitat in each region during 

 each week. Sampling in 1988 was conducted between 

 Bristol, Pennsylvania (rkm 191), and Artificial Island, 

 New Jersey (rkm 87), for eight weeks beginning on 18 

 April (Fig. 1). Allocation procedures in 1988 were the 

 same as in 1987, except that the area was stratified 



PENNSYLVANIA 



TRENTON 



NEW JERSEY 



RANCOCAS CBEB< 



UPPER REGION 



CHRISTINA RIVER 



CSD CANAL 



MID-RIVER 

 REGION 



LOWER REGION 



1987 SAMPLING AREA 

 fil 1988 SAMPLING AREA 



Figure 1 



Map of the study area. 



into nine equally-sized regions, and in the channel bot- 

 tom habitat, only two samples were collected. 



The sampling of the river in 1987 also included tribu- 

 tary stations in Martin's Creek, Neshaminy Creek, 

 Dredge Harbor, and Rancocas Creek. In 1988, tribu- 

 tary stations were established in the Schuylkill River, 

 Raccoon Creek, Oldman's Creek, the Christina River, 

 and at two stations in the Chesapeake Bay and Dela- 

 ware River (C&D) Canal. Tributary stations were lo- 

 cated approximately 1 km upstream of the confluence 

 with the Delaware River, except for the second C&D 

 Canal station, which was located near the Rt. 13 bridge, 

 about 4 km from the river. One mid-water sample was 

 collected from each tributary station each week, ex- 

 cept for the C&D Canal stations, where both mid-wa- 

 ter and bottom samples were taken weekly. 



Samples from the bottom habitat were collected by 

 using a 1-m diameter epibenthic ichthyoplankton sled; 

 water column and shoal habitats were sampled using 

 stepped oblique tows with a 0.5-m bongo net. Both 

 types of gear were fitted with 505-^m mesh plankton 

 nets. The volume of water that each net filtered was 

 estimated with a General Oceanics digital flowmeter 

 mounted at the mouth of the epibenthic sled and to 

 one side of the bongo net. Tows were conducted against 

 the current for five minutes at a speed of 

 approximately l.Om/sec. The catch was 

 preserved in 101 formalin, stained with 

 rose bengal, and taken to the laboratory 

 for analysis. 



Physico/chemical data (temperature, 

 dissolved oxygen, pH and conductivity) 

 were collected weekly with a calibrated 

 Hydrolab Surveyor II in each of the river 

 regions, the C&D Canal, and at each of 

 the tributary stations. Measurements in 

 the river and in the C&D Canal were 

 taken at surface, mid-depth, and bottom. 

 In the tributaries, measurements were 

 taken at surface and bottom. Water trans- 

 parency was measured with a 20-cm 

 Secchi disk. 



In the laboratory, all eggs with diam- 

 eters greater than 1.0 mm were removed 

 from the collections and inspected to de- 

 termine if they were M. saxatilis or A. 

 spidissima based on the size of the 

 perivitelline space relative to the diameter 

 of the egg. Small eggs ( less than or equal 

 to 1.0mm in diameter) consisted mostly of 

 M. americana, Alosa aestivalis (blueback 

 herring), or Alosa pseudoharengus (alewife), 

 but were not differentiated quantitatively. 

 M. americana yolk-sac larvae were dis- 



