In order to estimate daily ration of juvenile Potamoschistus lozanoi and to evaluate the effect of tide and daylight on its feeding activity, two 24h samplings were performed, one (in 1985) with high water around midnight, the other (in 1986) with low water around midnight. Incidently, the sampling in 1985 was around neap tide and in 1986 it was around spring tide. The fish were observed to feed more intensively during the night. Daily ration in 1985 was estimated to be about 1% of body ash-free dry weight (ADW). For 1986, the estimate is about 5% of body ADW in spite of similar water temperatures at both sampling dates. It is suggested that a semilunar rhythm, either in the fish or in the invertebrate prey, is the basis of the observed difference. This phenomenon may also explain the prominent 14-ring bands observed in the otoliths. Because of the influence of the various rhythms on feeding activity in fishes living in a strongly tidal environment, an accurate estimate of daily ration from production estimates and a P:C ratio established in the laboratory may be logistically less demanding than an estimate from stomach-content data.
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