Density, growth and annual food consumption of gobiid fish in the saline Lake Grevelingen, The Netherlands

Abstract

Within the scope of a study of the carbon budget of the 108 km 2 saline Lake Grevelingen, investigations were made on density, mortality, growth and food consumption of the main gobiid fish during the period 1980 to 1982. In August 1980 the O-group of Pomatoschistus minutus was estimated at 424 million individuals (on average 3.9 fishes per m 2) with a biomass of 203 tons FW. In 1981 and 1982 peak numbers were less high. O-group P. microps accounted for 282 million individuals (2.6 fishes per m 2) and 133 tons FW in September 1981. By far the highest density was found in the 0 to 0.6 m zone, 15 common gobies per m 2 (7 g FW·m −2). With approximately 5.1 million individuals (13 tons FW) Gobius niger was most abundant in 1982. For adult G. niger a monthly mortality of 27% was estimated. Mortality rates in P. minutus and P. microps were found to be fairly constant over the year. The estimated rates of annual mortality of 99.9% (∼46% per month) and 99.996% (∼57% per month), respectively, appear to be much higher than recorded for estuarine populations. Approximately 60% of the decline in numbers of demersal gobiid fish could be accounted for by the predation of two species of flatfish and two species of piscivorous birds. Young of the year of over 20 mm total length of both species were first caught in June. At the end of the first growing season, the average length and weight of P. microps was 39 mm and 0.6 g FW. In their second year they attained an average size of 51 mm. In O-group P. minutus , the 1980 and 1981 year classes reached an average length of 45 mm and 57 mm, respectively. In their second year, however, the difference disappeared and the mean length in both classes approximated 62 mm. Juvenile G. niger were first caught in August at a length of approximately 3.5 cm. They attained an average size of 4 to 5 cm in the first year, 8 to 8.5 cm in the second and 11 to 12.5 cm in the third year. The maximum production of P. minutus and P. microps , although reached in alternate years, amounted to 1.1 g and 0.8 g ADW·m −2·a −1, respectively, of which 92 to 98% was realized before the moment the maximum numbers (of individuals larger than 20 mm) were actually caught (August/September). In 1980, the diets of P. microps and P. minutus up to a size of approximately 40 mm consisted largely of copepods. With increasing length, they switched to larger epibenthic crustaceans. Annual food consumption for the 1980 P. minutus year class and 1981 P. microps year class was estimated at 1.95 g and 1.6 g ADW·m −2, respectively. The former species derived 91% of its annual food demands from copepods, and in the latter it was even 95%.