634 



Fishery Bulletin 100(3) 



dance of American shad eggs in 1997. Bongo nets could 

 not be used, and sampling included pushnet surveys in 

 the upper reaches of the rivers (from 31 March through 

 20 May 1998 and from 11 April through 7 May 1999). The 

 weekly sampling on each river consisted of pushnet tows 

 at approximately one meter below the surface at each sta- 

 tion. A pushnet frame fitted to the bow of a 14-foot boat 

 (Olney and Boehlert, 1988) accommodated two plankton 

 nets (333 pm, 60 cm). Catches from both nets were com- 

 bined. In 1998, eight stations per river were systemati- 

 cally sampled bracketing M94 to M120 and P109 to P131. 

 In 1999, two stations at M124 and M128 were added on 

 the Mattaponi River; we added six upriver stations 

 (P135-P154) and one downriver station (P104) on the 

 Pamunkey River (each spaced at 3.2-rkm intervals, Fig. 

 1). Bongo and push nets were fitted with a flow meter for 

 volumetric measurements and tow times were adjusted 

 (three to seven minutes) to meet a lower limit of 50 m^ of 

 water filtered through both nets combined. 



Laboratory procedures and data analysis 



Ichthyoplankton samples were preserved in 10'^ phos- 

 phate-buffered formalin. Ichthyoplankton were sorted 

 and larval fish and eggs were identified (Lippson and 

 Moran, 1974; Jones et al.^), enumerated, and removed 

 from the original, whole sample. Densities were reported 

 as number per 100 m'K Relative abundance in both rivers 

 was calculated by average density of each life stage (egg, 

 yolksac larva, and postyolksac) multiplied by total volume 

 of spawning or nursery area sampled. Total volume of 

 spawning or nursery area sampled was determined sepa- 

 rately for each species by including locations within the 

 sampling region where eggs (spawning reaches) or larvae 

 (nursery reaches) were collected. River volumes were 

 calculated by using bathymetric surveys and correspond- 

 ing areal estimates from a digitized record of the mean 

 high-water shoreline position as shown on the 7.5 minute 

 topographic map series of the U.S. Geological Survey 

 completed by Comprehensive Coastal Inventory, Virginia 

 Institute of Marine Science (Bilkovic, 2000). For purposes 

 of comparison, we used data on the abundance of Ameri- 

 can shad and striped bass juveniles in the Pamunkey 

 and Mattaponi rivers. The data were taken from annual 

 surveys of juvenile abundance conducted by the Virginia 

 Institute of Marine Science (Austin et al., 2000; Olney and 

 Hoenig^). 



in Figures 2 and 3. Average density (total eggs or larvae 

 per total volume filtered) of each species per river is 

 depicted in Figure 4. On the Mattaponi River (1997-99), 

 American shad eggs were collected over a 44-km reach 

 (M81-M124) and the highest densities occurred between 

 M96 and M124. Striped bass spawning occurred over a 27- 

 km reach and the highest densities in the sampled area 

 occurred between M68 and M87, downstream of the pri- 

 mary spawning reaches of American shad (Figs. 2-4). On 

 the Pamunkey River, American shad eggs were collected 

 over a 53-km reach (P98-P150), and the highest densities 

 were found between P104 and P131. Striped bass spawn- 

 ing occurred over a 60-km reach (P72-P131), and the 

 highest densities were found between P72 and P87. There 

 was some spatial overlap in spawning of these species, but 

 the primary spawning reaches were separate. Temporal 

 overlap in spawning of American shad and striped bass 

 occurred throughout the sampled period in both rivers 

 (Figs. 2-3). 



On the Mattaponi River, American shad larvae (total 

 length, 6.1-19.2 mm) were collected from M68 to M124, 

 and the highest densities were observed between M94 

 and M102 — a reach that is downstream of the spawning 

 habitat. On the Pamunkey River, American shad larvae 

 (total length, 6.6-12.2 mm) were collected between P76 

 and P128. Densities were highest at P102, 105, and 124. 

 Larval striped bass were collected from M68 to M94 and 

 from P72 to P109, and peak catches (>l/m') were collected 

 from M68 to M80 and from P72 to P91. In both rivers, 

 we observed overlap in American shad nursery grounds 

 and striped bass spawning reaches. However, the highest 

 densities of larval striped bass were downstream of the 

 primary shad spawning and nursery areas (Fig. 4). 



Average density of individual life stages of American 

 shad was higher in the Mattaponi River than in the 

 Pamunkey River; the opposite pattern was apparent for 

 striped bass (Table 1). Estimates of the relative numbers 

 of American shad and striped bass (average density x river 

 volume) suggested that abundance of American shad eggs 

 and larvae was higher on the Mattaponi River than on the 

 Pamunkey River by a factor of 5.5 and 4.4, respectively. 

 Relative abundance of striped bass eggs and larvae was 

 higher on the Pamunkey River than on the Mattaponi 

 River by a factor of 29 and 9.9, respectively (Table 2). 



Discussion 



Results 



Trends in density (numbers/100 m'') of eggs for each river 

 and species by date and station are depicted for 1997-99 



" Jones, P. W., F. D. Martin, and J. D. Hardy Jr 1978. Devel- 

 opment of fishes of the mid-Atlantic Bight. An atlas of egg, 

 larval, and juvenile stages. Vol. 1, Acipenseridae through 

 Ictaluridae. U.S. Fish and Wildlife Service report FSW/OBS- 

 78/12. Fish and Wildlife Service, U.S. Dep. Commer., Washmg- 

 ton DC 20005. 



Over the three years of surveys, eggs and larvae of 

 American shad were rare compared to those of striped 

 bass (Table 1). Despite our successive efforts to relocate 

 sampling stations upstream of known striped bass spawn- 

 ing habitat (Grant and Olney, 1991; Olney et al., 1991), 

 striped bass eggs and larvae were more abundant (-114 

 times and -38 times, respectively) than those of American 

 shad (Table 2). These differences could be attributed to the 

 relative sizes and egg production of the spawning stocks 

 because the number of mature American shad presently 

 in the York River system is believed to be low in relation 

 to historic run sizes (Nichols and Massmann, 1963; Olney 



