Journal ofFish Biology zyxwvut (1991) 39,433-440 zyxwv Age determination in zyxw Pomatoschistus minutus (Pallas) and Pomatoschistus microps (Kreyer) (Pisces: Gobiidae) from the upper Tagus estuary, Portugal F. MOREIRA, J. L. COSTA, P. R. ALMEIDA, C. ASSIS AND M. J. COSTA Faculdade de Cigncias de Lisboa, Departamento de Zoologia e Antropologia, Bloco C2. Camp0 Grande, 1700 Lisboa, Portugal (Received 28 January 1991, Accepted zyxw 1 April 1991) The use of scale inspection as a means of age-determination in Pomatoschistus microps and Pomatoschistus minutus in the Tagus estuary is confirmed, but annulus formation pattern is different from that described for P. microps in the area of the British Isles (Miller, 1975). In Britain, two annuli are laid down on the scales each year, but in the Tagus, only one annulus is formed. One possible reason for this difference is the fact that in the Tagus the peak breeding season and the lowest water temperatures coincide. Therefore, the two causes of annulus formation coexist at the same time of year. Population structure in both species is dominated by the 0-group, which appears earlier (April/ May) than reported for other areas. Maximum age seems not to exceed 26 months in P. microps and 32 months in P. minutus. Key words: Pomatoschistusmicrops;Pomatoschistus minutus; age determination; Tagus estuary; Portugal. I. INTRODUCTION The common goby, Pomatoschistus microps (Krryer), and the sand goby, Pomatoschistus minutus (Pallas), are small gobiid fishes very common in European estuaries (Wiederholm, 1987). In the Tagus estuary, these species are among the most abundant teleosts (Costa, 1980), playing an extremely important role in the estuarine food web (Costa, 1988~). In the present study, regular monthly sampling of both species over a period of 1 year in the upper Tagus estuary was undertaken to study some aspects of their phenology. The aim of this paper is to test the adequacy of scale use for age-determination in these species. 11. MATERIAL AND METHODS Samples of P. microps and P. minutus were collected monthly from March 1988 to February 1989, in the estuary of the River Tagus (Fig. zyxw 1). This estuary is the largest in western Europe (about 320 km2)(Costa, 19886). Its intertidal area is 41 % of the total area and mean depth is 10.6 m (Bettencourt, 1979). The main characteristics of the two sampling stations can be seen in Table I and mean monthly temperatures in Fig. 2. All fish were caught at low water with a beam trawl (20 mm mesh in the cod end) and trawl hauls lasted 15 min (constant speed). The common goby and the sand goby were the most numerous fish in the samples; the crustaceans Crangon crangon (Linnaeus) and Carcinus maenas (Linnaeus) were also abundant. All fish were preserved in 70% alcohol immediately after capture and kept in this preservative until examination. 433 0022 zyxwvutsr -1 112/91/090433+08 $03.00/0 0 1991 The Fisheries Society ofthe British Isles