Short communication Fish stocks of the A à ncora River, northern Portugal: production N. Formigo a , T. Penczak b,* a Departamento de Zoologia-Antropologia ``Dr. Augusto Nobre'', Faculdade de Cie Ãncias da Universidade do Porto, Prac ,a Gomes Teixeira, 4050 Porto, Portugal b Department of Ecology and Vertebrate Zoology, University of èo Âdz Â, 90-237 èo Âdz Â, 12/16 Banacha Str., Poland Received 3 September 1998; accepted 7 September 1998 Abstract The mean biomass (  B) and production (P) of four ®sh populations (brown trout, Salmo trutta m. fario (L.)), Iberian roach, Rutilus arcasii (Steindach.), Iberian nase, Chondrostoma polylepis (Steindach., ssp. duriensis (Coelho)), and eel, Anguilla anguilla (L.)) were assessed in the A à ncora River, north Portugal. At four sites located along the course of the A à ncora River and one site on its tributary  B and P were in the range 36.5±283.9 kg ha 1 and 34.0±608.5 kg ha 1 yr 1 , respectively. Lowest values were recorded at sites in the middle course of the main river, isolated by numerous dams and waterfalls. Greatest values were recorded in the natural tributary (Gondar Creek) and in the out¯ow section of the main river. # 1999 Elsevier Science B.V. All rights reserved. Keywords: North Portugal; Small river; Waterfalls and dams; Mean biomass; Production; Turnover ratio; Isolation stress 1. Introduction Fish production has been estimated in numerous European rivers (Mann and Penczak, 1986; Mann, 1991; Randall et al., 1995). Iberian Peninsula esti- mates were obtained in the source sections of small tributaries of the upper courses of large rivers (Lobo Ân- Cervia  and Penczak, 1984; Lobo Ân-Cervia  et al., 1986; Rodriguez-Ruõ Âz and Granado-Lorencio, 1991). These included natural rivers and these weakly or strongly modi®ed by pollution, regulation, dams, over®shing, and introductions. Production estimates in rivers where ®sh stocks are separated by impassable water- falls are very desirable, as it is known that isolation can cause genetic differences in growth and life history (Northcote, 1981; Gibson et al., 1996). The aim of this study is to estimate the mean biomass, production, and turnover ratios of ®sh in a small river in northern Portugal, which has numerous waterfalls and dams for water mills and irrigation. 2. Materials and methods The A à ncora River, 19 km long, ¯ows into the Atlantic Ocean in north Portugal, and has four tribu- taries, two of which are intermittent (Fig. 1). In its ®rst 7 km, there are two waterfalls, 100 and 50 m high (Formigo and Penczak, 1998). Further downstream the slope of the river is more level (8 m km 1 ) and Fisheries Research 40 (1999) 91±96 *Corresponding author. Tel.: +48-42-6781364; fax: +48-42- 6781364; e-mail: penczakt@biol.uni.lodz.pl 0165-7836/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0165-7836(98)00216-1