The California Department of Fish and Game (CDFG) released 100,000 CWT Chinook on April 20^th from the Merced River Hatchery (RM 52). A second group of 100,000 CWT Chinook was released on May 4^th. The first catch of CWT Chinook occurred during the evening of April 24^th at Hatfield State Park (RM 2). The catch peaked on April 26^th and has since decreased. The second group of CWT Chinook is expected to begin to appear at our traps sometime during the second week of May. Along with CWT fish we also observed a significant increase in the number of wild Chinook caught in our traps (Figure 1)

Figure 1. Chart showing Chinook catch to date vs. flow at the Hatfield traps.

As of May 6^th, a total of 147 CWT Chinook have been captured. The largest daily catch was 62 fish which occurred on April 26^th. Individual fork lengths ranged from 82 mm to 111 mm. The mean length of CWT captured was 95 mm. No weights were recorded for CWT Chinook.

During this sampling period a total of 133 wild Chinook were caught bringing the season total to 151 wild Chinook. The largest daily catch was 61 fish which occurred on April 25^th. Individual fork lengths ranged from 64 mm to 120 mm. Mean length for wild Chinook captured was 81 mm and mean weight was 5.5 grams (Figure 2).

 

 

 

 

 

 

Figure 2. Photos of wild (left) and coded wire tagged (right) Chinook salmon smolts caught on 4/26/07 and 4/28/07, respectively. [click on the photo for enlarged view]

The peak flow for the Vernalis Adaptive Management Program (VAMP) on the Merced River has been adjusted to ~1100 cfs due to recalibration of the gauge at Vernalis by USGS. The traps have continued to operate well under the increased flow conditions due to the hard work of the field technicians who monitor the traps a minimum of twice a day seven days a week.

Two trap efficiency tests were conducted during the sampling period (Table 1).  Juvenile hatchery Chinook salmon were dye marked and released at night above the traps. The first release occurred on April 24, 2007, four days after flows were beginning to increase for VAMP. The second release occurred on May 1, 2007 as flows continued to increase.  Our trap efficiency was found to be low, a likely consequence of the of higher flows.

Table 1. Marked fish releases conducted at Hatfield State Park to evaluate trap efficiency.

 

 

 

 

 

 

 

 

Figure 3. Photo of technicians Will Clayton and Joe Deppen preparing the holding site for efficiency fish on April 24 (left) and technician Will Clayton releasing efficiency fish at night on April 24 (right). [click on the photo for enlarged view]

During this sampling period a total of eleven different types of incidental species were captured including white catfish, channel catfish, bluegill sunfish, prickly sculpin, Sacramento sucker, Sacramento pikeminnow, Sacramento splittail, spotted bass, largemouth bass, Hardhead, and warmouth (Figure 4).

    

 

 

 

 

 

  

 

Figure 4. Photo of a warmouth captured on May 1^st (left) and a spotted bass captured on May 4^th (right). [click on the photo for enlarged view]

Environmental Data: 

Instantaneous water temperature measured at the Hatfield rotary screw trap ranged from 58°F to 69°F.  Instantaneous turbidity measured at Hatfield rotary screw trap ranged from 2.21 to 7.3 NTU.  During the sampling period flows at Snelling (MSN; RM 46) ranged from 434 to 1173 cfs, and at Cressy (CRS; RM 27) from 350 to 947 cfs.  Hourly flows in Dry Creek (DSN; confluence at RM 31.5), which is a significant tributary to the Merced River, ranged from 28 to 37 cfs.