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6 days after they were tagged. They had remained off- shore an average of 55 days. Similarly, 11 cui-ui marked with sonic tags in early April remained off- shore for 6 to 8 weeks, where they remained fairly active. Four that were tracked for 24 h traveled distances of 7 to 12 km, moving mostly at night (Fig. 3). Activities during such night movement probably included feeding and seeking access to the river. During daylight and at any time of day just before peak river migration, cui-ui with sonic tags remained just offshore, usually within 1 km from either the delta or the fishway (Fig. 3). No sonic- tagged cui-ui entered the FPB, however, and by 6 June 1982 we had lost the last of the offshore signals, Presumably the tagged cui-ui either spawned in the fish way or delta, where turbulence prevented detection, or returned to deeper water and resorbed their gametes. In November 1982, 7 months after Fig. 3. Movement of cui-ui tagged with sonic tags, Shore reference points (solid dots 1 to 6) are spaced at O.S-km intervals, Upper sketch, Movement of a member of the prespawning aggregation over a 24-h period beginning at 1130 h on 21 April 1982, Lower sketch: Positions (asterisks) of 10 sonic-tagged cui-ui at 1000-1200 h, 26 April 1982. tagging, we relocated 7 of the 11 sonic-tagged fish along the periphery of Pyramid Lake-indicating that survival was high, History of Recent Spawning Migrations From the time when the Truckee River Delta was formed in the 1930's until the completion of MB dam and the fishway in 1976, there was no documentation of cui-ui spawners entering the river. They were observed on several occasions attempting to do so, seemingly without success Oohnson 1958; La Rivers 1962), Successful migra- tion was observed only after completion of MB dam and the fishway. Except in 1984, virtually all cui-ui migrants captured in the FPB had entered through the fishway, However, cui-ui did not use the fish- way during the first 2 years of operation (1976 and 1977), There was some concern that water veloc- ity and perhaps water turbulence in the fishway were unfavorable, and early fishway history seemed to support this concern. Cui-ui entered the fishway in both 1976 and 1977 but did not ascend it beyond the lower reach of ladder 1 (SonnevilI978). During the next three spawning seasons, after this ladder had been partly modified to reduce velocity and tur- bulence, successively larger numbers of cui-ui used the ladder and were captured at the FPB: 33 in 1978, 139 in 1979, and 4,984 in 1980 (Table 1). This trend was disrupted in 1981, when only 47 cui-ui used the fishway, even though all four ladders had been modified. The relatively small run in 1981 after the large run in 1980 suggested that ladder modifica- tion alone had not influenced the size of the cui-ui run. Sonnevil (1981) observed that the numbers of cui-ui in the prespawning aggregation off the mouth ofthe river were greatest in high-water years. Runs up the fish way or river were also large when flows in the Truckee River were high in March through May (in 1980 and 1982-84; Table 1). Fishway flows have remained almost unchanged since it opened. During the 9 years after installation of the MB dam and fishway complex in 1976, cui-ui entered the FPB by way of the Truckee River in 3 years (1982, 1983, and 1984). Except in 1984, however, the size of the river runs was inconsequential (1 fish trapped in 1982 and 20 in 1983). In both years, most cui-ui traveled through the fishway (13,804 in 1982 and 5,994 in 1983). In 1984, however, all cui-ui taken