The California Department of Fish and Game (CDFG) completed its final CWT release of juvenile Chinook from the Merced River Hatchery (MRH, RM 52) on May 4, 2007.  As before, we observed an increase in catch of wild Chinook along with CWT fish at our traps at Hatfield State Park (RM 2).  Trap efficiency decreased with increased flows for the Vernalis Adaptive Management Program (VAMP), resulting in lower catch numbers at our traps (Figure 1).  MRH personnel reported the hatchery's remaining juvenile Chinook (approximately 40,000 unmarked fish) were released during the week of May 14^th.  This release is likely responsible for the last peak in catch observed on May 16^th.

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

As of May 27^th, a total of 223 CWT Chinook have been captured.  The largest daily catch of CWT for this sampling period was 34 fish which occurred on May 8^th.  Individual fork lengths ranged from 85 mm to 106 mm.  The mean length of CWT captured was 96 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 191.  The largest daily catch of wild Chinook was 15 fish which occurred on May 7^th.  Individual fork lengths ranged from 55 mm to 104 mm (Figure 2).  Mean length for wild Chinook captured was 86 mm.  Due to equipment malfunctions, weights were not recorded for the majority of this sampling period.

 

Figure 2. Photo of field technician Tyson Mutoza (left) processing a  wild Chinook salmon smolt (right) caught on 5/19/07. [click on the photo for enlarged view]

Flows began to decrease on May 8^th and reached their pre-VAMP levels (~300 cfs) at Hatfield State Park on May 21^st.  The decrease in flow greatly reduced water velocity at the Hatfield traps, creating problems with trap operations (i.e. frequent stops), and thus reducing trap efficiency.

Two trap efficiency tests were conducted during this sampling period (Table 1).  Juvenile hatchery Chinook salmon were dye-marked and released at night above the traps.  The first release occurred on May 7^th during the peak of VAMP flows.  The second release occurred on May 15^th just as flows began to decline.  Our trap efficiency was found to be low at 0.6% on May 7^th and 0.2% on May 15^th.  This is a likely consequence of higher flows.

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

During this sampling period a total of twelve 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, warmouth and numerous bass too small to identify to species.

Environmental Data: 

Instantaneous water temperature measured at the Hatfield rotary screw trap ranged from 61°F to 72°F.  Instantaneous turbidity measured at Hatfield rotary screw trap ranged from 1.71 to 4.89 NTU.  During this sampling period flows at Snelling (MSN; RM 46) ranged from 257 to 1156 cfs, and at Cressy (CRS; RM 27) from 284 to 988 cfs.  Hourly flows in Dry Creek (DSN; confluence at RM 31.5), which is a significant tributary to the Merced River, ranged from 1 to 32 cfs.