Size-structured diel use of river banks by fish

Abstract

We examined the role of body size in the diel use of river banks by fish and the abundance of piscivorous fishes to determine if predation pressure influences diel habitat use. We hypothesised that the mean standard length (SL) of non-piscivorous fish would decrease at night. Fish were sampled via point sampling by electrofishing every 3-4 h over three 24-h periods (late August 1992, early September 1993, late May 1994) along two adjacent stretches of bank (shallow sand, steep boulder) of the River Morava, Czech Republic. Gill netting in mid-channel adjacent to the sand bank was also used in 1993 to determine predator abundance. Analysis of variance and size-class ordination revealed higher numbers of smaller fish were captured at night along the banks than during the day, being in most cases significant. In almost 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 fishes in August 1993 was significantly higher in the gill nets during hours of darkness. Size class ordination and electivity indices revealed that bitterling Rhodeus sericeus in August 1992 were distributed according to size along the sand bank (smaller size classes) and the boulder bank (larger size classes); also, equivalent or adjacent size classes of Rutilus rutilus (41-50, 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, 81-90mm SL), a small-bodied species, co-exploited both types of river bank, moving to the sand bank at night, 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. cephalus (63-99, 100-149, ≥ 150 mm SL) preferred the sand and boulder banks, respectively, with the largest sizes of the two species occurring together more often than expected along both banks. The results generally support our hypothesis, suggesting that shallow river shorelines represent important night-time refuges from predation for the smaller size classes of fish, thus constituting crucial life-history habitat important for the recruitment of many riverine fishes.