淡水魚廣義的說，系指能生活在鹽度為千分之三的淡水中的魚類就可稱為淡水魚。 狹義的說，系指在其生活史中部分階段如只有「幼魚期」或「成魚期」，或是終其一生都必須在淡水域中渡過的魚類。世界上已知魚類約有26000多種，淡水魚約有8600余種。據湖北胖頭魚批發廠家了解到我國現有魚類近3千種，其中淡水魚有1000余種。

世(shi)界上(shang)已(yi)知(zhi)魚(yu)類約(yue)有26000多種(zhong)，是脊椎(zhui)動(dong)物中(zhong)種(zhong)類比較多的一大類，約(yue)占脊椎(zhui)動(dong)物總(zong)數的48.1%。它(ta)們(men)絕(jue)大多數生活(huo)在海洋里，淡水(shui)魚(yu)約(yue)有8600余種(zhong)。我(wo)國(guo)現有魚(yu)類近3千種(zhong)，其中(zhong)淡水(shui)魚(yu)有1000余種(zhong)。

生活于鹽度(du)含量在(zai)0.05%以下(xia)水(shui)域(yu)的(de)魚類(lei)。一般分為原(yuan)生與次(ci)生2大類(lei)。世(shi)界(jie)(jie)共有(you)淡(dan)(dan)水(shui)魚30多(duo)(duo)(duo)個(ge)目，8400多(duo)(duo)(duo)種(zhong)(zhong)(zhong)(zhong)，其中(zhong)有(you)11個(ge)目、4050余(yu)種(zhong)(zhong)(zhong)(zhong)屬原(yuan)生淡(dan)(dan)水(shui)魚。中(zhong)國有(you)淡(dan)(dan)水(shui)魚13個(ge)目、850多(duo)(duo)(duo)種(zhong)(zhong)(zhong)(zhong)。世(shi)界(jie)(jie)淡(dan)(dan)水(shui)魚種(zhong)(zhong)(zhong)(zhong)類(lei)較(jiao)(jiao)多(duo)(duo)(duo)的(de)有(you)鯉(li)形(xing)(xing)目(2420余(yu)種(zhong)(zhong)(zhong)(zhong))、鲇(nian)形(xing)(xing)目(2210余(yu)種(zhong)(zhong)(zhong)(zhong))、脂鯉(li)目(1330余(yu)種(zhong)(zhong)(zhong)(zhong))。中(zhong)國較(jiao)(jiao)多(duo)(duo)(duo)的(de)有(you)鯉(li)形(xing)(xing)目(約600種(zhong)(zhong)(zhong)(zhong))、鲇(nian)形(xing)(xing)目(80多(duo)(duo)(duo)種(zhong)(zhong)(zhong)(zhong))。