atures ▁were ▁elev ated ▁above ▁ 5 7   ° F ▁and ▁ab normal ities ▁and ▁egg ▁mor bid ity ▁rates ▁increased . ▁ ▁Introduction ▁of ▁pre dat ory ▁fish ▁species ▁such ▁as ▁bl u eg ill ▁and ▁various ▁types ▁of ▁bass ▁that ▁pre y ▁on ▁sal mon ▁sm olt ▁are ▁also ▁a ▁major ▁contrib utor ▁to ▁the ▁decl ine . ▁The ▁San ▁Jo aqu in ' s ▁point ▁of ▁diver gence ▁with ▁the ▁distrib ut ary ▁Old ▁River ▁has ▁histor ically ▁created ▁a ▁sort ▁of ▁bott l ene ck ▁for ▁out ▁migr ating ▁sal mon ▁because ▁the ▁Old ▁River ▁in ▁turn ▁branches ▁into ▁many ▁side - ch annels ▁and ▁sl ough s ▁of ▁the ▁delta ▁as ▁well ▁as ▁various ▁canal ▁int akes . ▁Rec ent ▁years ▁have ▁seen ▁work ▁by ▁the ▁California ▁Department ▁of ▁Water ▁Resources ▁and ▁the ▁California ▁Department ▁of ▁Fish ▁and ▁Game ▁to ▁construct ▁and ▁manage ▁temporary ▁rock ▁bar riers ▁at ▁the ▁head ▁of ▁the ▁Old ▁River ▁in ▁order ▁to ▁keep ▁fish ▁in ▁the ▁main ▁channel ▁of ▁the ▁San ▁Jo aqu in ▁River . ▁In ▁the ▁fall ▁of ▁ 2 0 0 9 , ▁just ▁ 2 , 2 3 6 ▁sal mon ▁returned ▁to ▁the ▁entire ▁river ▁system ▁to ▁spawn ; ▁this ▁has ▁led ▁to ▁a ▁government ▁ban ▁on ▁sal mon ▁fish ing ▁off ▁the ▁coast s ▁of ▁northern ▁California ▁and ▁Oregon . ▁Rec ent ▁years ▁have ▁seen ▁efforts ▁to ▁bring ▁back ▁some ▁of ▁the ▁sal mon ▁population ▁to ▁the ▁San ▁Jo aqu in ▁and ▁some ▁of ▁its ▁trib