Size-structured diel use of river banks by fish

We examined the role of body size in the diel use of river banks by fish an d the abundance of piscivorous fishes to determine if predation pressure in fluences diel habitat use. We hypothesised that the mean standard length (S L) of non-piscivorous fish would decrease at night. Fish were sampled via p oint sampling by electrofishing every 3-4 h over three 24-h periods (late A ugust 1992, early September 1993, late May 1994) along two adjacent stretch es of bank (shallow sand, steep boulder) of the River Morava, Czech Republi c. Gill netting in mid-channel adjacent to the sand bank was also used in 1 993 to determine predator abundance. Analysis of variance and size-class or dination revealed higher numbers of smaller fish were captured at night alo ng the banks than during the day, being in most cases significant. In almos t all cases (all years and along all bank types), the standard length (SL) of non-piscivorous fishes was significantly lower during hours of darkness, in particular near midnight. Conversely, the SL of the non-piscivorous fis hes in August 1993 was significantly higher in the gill nets during hours o f darkness. Size class ordination and electivity indices revealed that bitt erling Rhodeus sericeus in August 1992 were; distributed according to size along the sand bank (smaller size classes) and the boulder bank (larger siz e classes); also, equivalent or adjacent size classes of Rutilus rutilus (4 1-50, 51-60, 61-70, 71-80, 81-105 mm SL), a large bodied species, and Gobio albipinnatus (26-50, 51-60, 61-70, 71-80, 81-90 mm SL), a small-bodied spe cies, co-exploited both types of river bank, moving to the sand bank at nig ht, a possible example of recruitment bottleneck. In May 1994, smaller size classes of Leuciscus leuciscus (78-84, 85-90, 90-108 mm SL) and L. cephalu s (63-99, 100-149, greater than or equal to 150 mm SL) preferred the sand a nd boulder banks, respectively, with the largest sizes of the two species o ccurring together more often than expected along both banks. The results ge nerally support our hypothesis, suggesting that shallow river shorelines re present important night-time refuges from predation for the smaller size cl asses of fish, thus constituting crucial life-history habitat important for the recruitment of many riverine fishes.