In order to elucidate the 24 hr (temporal) pattern in the consumption of prey items (food), in relation to the availability of these prey items to Pomatoschistus lozanoi and P. minutus (pisces, Gobiidae) an investigation was carried out in the Westerschelde tidal estuary. A beam trawl net was used to sample the fish while a Van Veen grab and a hyperbenthic sledge net were used for macrobenthos and hyperbenthos respectively. The catches of gobies as well as those of hyperbenthos were compared to some of the environmental factors recorded in the field during sampling. The macrobenthos was dominated by amphipods. Bathyporeia species contributed about 89% in numbers. Numerically, the hyperbenthos was dominated by Mysids. Neomysis integer and Mesopodopsis slabberi accounted for more than 80% of all hyperbenthos. The composition of prey species available as food for P. lozanoi and P. minutus was compiled from macrobenthos and hyperbenthos data after imposing some specific assumptions. Neomysis integer, Mesopodopsis slabberi and Bathyporeia species were the most abundant preys available in the environment. Stomach content analysis revealed that P. minutus consumed a wider range of prey species than P. lozanoi. The most important prey species in both fish were Neomysis integer. Mesopodopsis slabberi and Bathyporeia species, the most numerous species in the environment. However, the relative importance of these preys varied with time of the day. The fish seem to feed during the day as well as at night. Most intense feeding was observed at 14 hrs and lowest between 06 and 08 hrs in both fish species. Bathyporeia species were eaten less frequently by P. lozanoi than expected from their occurrence in the environment, while P. minutus consumed them in accordance to their occurrence in the environment. Neomysis integer were eaten in accordance to their occurrence in the environment at night, but they were preferred during the day time. Mesopodopsis slabberi were eaten in accordance to their occurrence in the environment by P. lozanoi throughout, but they were avoided by or inaccessible to P. minutus at night. In both fish species the average gastric evacuation rate (E) (as derived from the Andersen (1984) equation) was estimated to be 0.29 h-1 and estimates for the size classes were 0.33, 0.30, 0.27 and 0.24 h-2 for 30-34, 35-39, 40-44 and 45-49 mm classes respectively. The daily food consumption was estimated using Eggers (1977) and Elliott & Persson (1978) models. These models gave comparable daily food consumption for both fish species of about 1.4 % of body weight for P.lozanoi and just under 1 % of body weight for P. minutus. Food consumption differed between size classes, smaller fish consumed more food per unit body weight (about 5% for 30-34 size class) than bigger fish (about 0.4 % for 45-49 size class). For a given size class, food consumption of P. lozanoi was similar to that of P. minutus. Factors that may be responsible for the observed catch rates of hyperbenthos and fish are discussed together with possible sources of errors in quantification of available food in the field. Shortcomings of stomach content methodologies and their influences on observed food and feeding habits have been reviewed. The importance of estimating food consumption is discussed. From the size frequency distributions of the prey in the stomachs, in comparison to those in the field, it seemed as if the fish were selecting slightly smaller than average animals of the major prey types. Despite the hint shown by frequency of occurrence of prey items and food selection of prey items, the question of whether P. lozanoi and P. minutus compete for Neomysis integer, Mesopodopsis slabberi and Bathyporeia species could not be answered because pre-re |
