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MAINTAINING RAZORBACK SUCKER IN LAKE MOHAVE 131 TABLE 1.-Razorback sucker production and survival in ephemeral backwaters in Lake Mohave, Arizona-Nevada, during 1991-1994. The number stocked represents larvae unless otherwise noted. Number juveniles Average size Surface areas Number stocked harvested Yield (cm) at Year and location (ha) (size, cm) (% survival) (number/ha) harvestb 1991 Yuma Cove 0.82 100 spawners 0 0 1992 Yuma Cove 0.82 88 spawners 296 361 35.4 1993 Yuma Cove 0.82 200,000 eggs 17 (<0) 21 32 420 (2.6) 386(92) 470 23.2 Willow Cove 0.17 500(l.5) 26(5) 153 21.9 Nevada Larvae 0.10 2,000(l.1) 250(13) 2,500 13.8 Arizona Juvenile 0.17 2,010 (1.5) 198(10) 1,165 17.4 69 (13.5) 55(80) 324 30.7 1994 Yuma Cove 0.82 3,000(2) 358(12) 407 32.0 Willow Cove 0.17 1,000(2) 160(16) 941 20.8 Dandy Cove 1,000(2) 562(56) 19.8 Nevada Larvae 0.10 500(2) 217(44) 2,190 22.4 North Chemheuvie 0.16 1,000(2) 812(81) 4,476 19.4 South Sidewinder 0.05 500(2) 201(40) 3,960 20.1 a Surface area at reservoir elevation 195 m. 'Harvest dates ranged from August to November juvenile razorback suckers. After the water level fell, 420, 2.6-cm-laboratory-reared fry were stocked into Yuma Cove. This stocking produced 386 juve- niles; survival was 91.5% (Table 1). Three other coves were stocked with laboratory-reared fish of various size groups. Fish stocked at lengths of 1.1- 1.5 cm showed a 5.2-12.5% survival rate; the sur- vival of stocked fish larger than 2.5 cm was 80% (Table 1). Four hundred eighty-seven juvenile fish were tagged with PIT tags and released into the reservoir; the remaining smaller fish were stocked into Davis Cove. Intensive larval collections from the reservoir be- gan in January 1994. Larval razorback suckers were collected each week at multiple locations until the last week in March. The 3-month effort yielded over 11,000 larvae. Larvae were held in laboratory tanks and fed until they reached 2 cm TL and then dis- tributed to all nine ephemeral backwaters. Stocking rates ranged from 4,000 to 10,000 larvae/ha. The USFWS received 2,000 larvae to rear for future broodstock at Dexter National Fish Hatchery. Six of the nine backwaters produced young razor- back suckers. The absence of razorback suckers in three backwaters was attributed to berm failure (reconnection to the reservoir) or poor water qual- ity. Approximately 2,200 young suckers were har- vested, tagged with PIT tags, and released into Lake Mohave in the fall of 1994. Survival rates for fish in backwaters ranged from 12 to 76% (Table 1). Davis Cove was intensively sampled prior to stocking to remove resident fish. Electrofishing and gill netting harvested over 600 kg of nonnative fish (largemouth bass, striped bass, green sunfish Lepo- mis cyanellus, bluegill, channel catfish, yellow bull- head Ameiurus natalis, and common carp Cyprinus carpio) from the 1.2-ha cove. Scuba divers removed additional fish with spear guns and concluded that few large (>30-cm-long) predators remained. The cove was stocked on 24 June 1992, with 10,000 juvenile razorback suckers from Dexter National Fish Hatchery and 143 fish reared in an ephemeral backwater. Although the hatchery- and cove-reared fish were young-of-the-year, hatchery fish were about half the size (average 6.8 cm) of cove-reared fish (>12 cm). Predation of smaller hatchery fish by Lepomis sp. and juvenile largemouth bass was wit- nessed by divers. Subsequent sampling during the following 18 months yielded only 2% of the hatch- ery fish and over 50% of the larger cove-reared fish. Recovery of Cove-Reared Fish The spring monitoring effort recovered 5 of the 153 cove-reared fish released into Lake Mohave during autumn 1992 (Table 2). These fish were collected as far as 34 km from their release points. All five fish were males. Release of 487 fish in 1993 brought the total number of fish stocked into Lake Mohave to 640. Ten 28-52.9 cm (TL) cove-reared