Electro-fishing (600 V, d. c. current) was carried
out on a quarterly basis except from July 1998
to March 1999 and from December 2000 to
March 2001. Three sampling sites (100 metres
in length) were selected as representative of the
aquatic habitats in the Estor˜aos River (sites E1,
E2 and E3 located at 12 km, 5 km and 2.5
km respectively from the river mouth) (Fig. 1).
These sites were closed with stop nets (mesh size
0.5 cm) before sampling. All fish caught were
anaesthetised (ethylenoglycol-monophenylether,
3 ml/10 L water) and then measured (fork length
to the nearest mm) and weighted (to the nearest
0.1 g). Scale samples were collected during 2001
and 2002, according to Ombredane & Baglini`ere
(1992), for age determination. All fish were
returned to the stream after recovering from the
anaesthesia procedure.