Jellyfish 37 



On subsequent visits to Wolf Fork (16 August and 12 September 1995) 

 I collected medusae with an aquarium dip net from a small boat. I measured bell 

 diameter of live specimens under a dissecting microscope by placing a ruler 

 under a clear plastic petri plate containing a few medusae and a small amount of 

 water. I judged measurement error to be ±1.0 mm. 



RESULTS 

 On 16 August 1995, I observed a dense swarm of medusae near the 

 head of Wolf Fork. Medusae decreased greatly in abundance with distance from 

 the swarm. Density of medusae in three bucket samples was 1.2, 1.9, and 4.8 

 medusa/L. This is approximately equivalent to 1,000-5,000 medusae/m 3 in the 

 upper 0.5 m of the water column within an area of about 25 m 2 . At 100-200 m 

 from the swarm (toward the main channel of the reservoir) there were 10-50 

 medusae/m 3 ; at 300-400 m medusae were scarce (<1 medusa/m 3 ). I did not 

 observe medusae in lower Wolf Fork, the main channel, or in other coves of Skin 

 Creek, although I did not make an exhaustive search. My conversations with 

 anglers, reservoir managers, and local fish biologists suggest that this is the first 

 record of C. sowerbyi at Stonewall Jackson Lake. The origin of C. sowerbyi in 

 the drainage is unknown. 



Table 1. Selected water quality measurements for Wolf Fork, 16 August 1995. 

 Site distances are from the swarm toward the main channel. All values for sam- 

 ples collected at the water surface. DO=dissolved oxygen. 



Site 



Temp 



DO 



PH 



Conductivity 



Alkalinity 



so 4 



Ca 



Medusa 



(m) 



C 



(mg/L) 





nSlcm 



mg/L 

 (CaCO,) 



(mg/L) 



(mg/L) 



abundance 



(#/m<) 



-100 



30.0 



7.7 



7.0 



98.3 



19.1 



21.9 



12.1 



<1 







30.0 



8.1 



7.0 



100.5 



18. 2 



21.6 



12.8 



1000-5000 



100 



30.5 



8.3 



7.1 



99.8 



17.8 



22.0 



12.5 



10-50 



200 



30.5 



8.3 



7.2 



99.6 



17.5 



23.7 



12.6 



<1 



300 



30.5 



8.3 



7.2 



98.1 



17.2 



22.6 



12.7 



<1 



Water chemistry did not differ appreciably among sites within Wolf 

 Fork (Table 1). Dissolved oxygen decreased with depth to 5 mg/L at 2.1 m and 

 1.8 mg/L at 2.7 m. Depth of the channel in Wolf Fork was 2.5-3.5 m. Flow from 

 the Wolf Fork watershed (the stream) into Wolf Fork (the cove) could not be 

 detected at the culvert. 



On 18 August 1995, medusae collected ranged in size from 6 to 20 mm 

 (Fig. 2). Mean (SE) size and size class distribution of medusae collected from 

 the swarm location were different from medusae collected at three other stations 

 in Wolf Fork. Swarm medusae were 13.3 (0.2) mm in diameter; medusae from 

 outside the swarm were 15.2 (0.3) to 16 (0.4) mm in diameter depending on col 



