Good question, Dave. Maybe the abbreviation "U/S" means "upstream" (from the Lake) spawners?
From: Canadian Science Advisory Secretariat Pacific Region Science Response 2016/021
Summer Run Stocks
In most years, the Summer Run stocks dominate total Fraser Sockeye returns. Six stocks in this timing group are forecast using the standard suite of forecast models: Chilko, Late Stuart, Quesnel, Stellako and the recently added Raft and Harrison (Table 1A).
Harrison (Harrison-River Type CU): Recently re-assigned from Late Run Group to the Summer group Harrison Sockeye have a unique life history and age structure compared to other Fraser Sockeye stocks. They migrate to the ocean shortly after gravel emergence (most Fraser Sockeye rear in lakes for one year after gravel emergence prior to their ocean migration). After two to three years in the ocean, Harrison Sockeye return as three or four year old fish (most Fraser Sockeye return as four and five year old fish). Proportions of three and four year old Harrison recruits vary considerably annually, with four year old proportions ranging from 10% to 90% of total recruits (Grant et al. 2010). Odd brood years, on average, produce a higher proportion of four year old recruits, and even years produce a similar proportion of three year old recruits (Grant et al. 2010). Though the difference in odd versus even year age proportions is accounted for in the Harrison forecast models (MacDonald & Grant 2012), the extreme variation in age-at-maturity for Harrison Sockeye increases the level of forecast uncertainty for this stock.
Late Run Stocks
The Late Run consists of five forecasted stocks (Cultus, Late Shuswap, Portage, Weaver, and Birkenhead) and one miscellaneous stock (miscellaneous non-Shuswap including Harrison stocks that migrate downstream to Harrison Lake as fry to rear in this lake) (Table 1A);
Miscellaneous Harrison/Lillooet Lakes (Harrison (downstream)-L) The 2012 brood year EFS for the miscellaneous Harrison/Lillooet Lake stocks is 1,400 (Table 1B, column C). Populations included in this group include those that rear in the Harrison-Lillooet Lake system, and are not included in the Harrison or Birkenhead forecasts (Big Silver, Cogburn, Crazy, Douglas, Green, Pemberton, Pool, Railroad/Sampson, Ryan, Sloquet and Tipella Creeks).
Weaver (Harrison (U/S)-L CU) The 2012 brood year escapement for Weaver (400 EFS) was the smallest escapement on record, falling well below the cycle average (1968-2012: 18,300 EFS) (Table 1B, column C).