The age structures of populations of African annual Nothobranchius spp. were examined for the first time. Daily increments in sagittal otoliths of Nothobranchiusjurzeri, Nothobranchius kadled, No/hobranch ius orthollotus and Nothobranchius rachovii from southern and central Mozambique were used for age determination. Four hypotheses were tested: (I) timing of hatching is consistent with the calendar onset of the rainy season, (2) hatching is synchronized within a population in a pool , (3) there is a difference in hatching date between geographical regions differing in mean total annual precipitation and (4) sympatric Nothobranchius spp. hatch at the same time. The results show that daily increment analysis represents an applicable method for age determination in Nothobrall chius spp. Despite a significant positive relationship between age and size of fishes, a pronounced variation in fish size at an age precluded the use of fish size as a valid age marker. Timing of hatchi'ng was 110t consistent with the calendar onset of the rainy season. Interpopulation variability was observed in the degree of hatching date synchronization within a population. Hatching dates were relatively uniform in some populations, while there was considerable variability in others. Differences in timing of hatching date were found in on ly I of 2 years within the three regions investigated (Chefu, lower Limpopo and Sofala regions), each of which differed in mean total annual rainfall. The hatching dates of sympatric Nothobranchius spp. were marginally different, but further testing on a larger sample is needed for conclusive results.
1. Burbot larvae (Lota lota) perform a substantial diel vertical migration (DVM) of increasing amplitude in the pelagic zone during a 3-month period before migrating to the littoral zone as early-juveniles. We hypothesised that feeding in the warm surface layers at night and then spending the day in cold water below the thermocline reduces metabolic costs and earns burbot larvae an energetic advantage. 2. To test our hypothesis, we mimicked the temperature conditions experienced by vertically migrating burbot in the pelagic zone. We also simulated three further scenarios, in which temperature remained constant. 3. Burbot showed the best performance (defined as specific growth rate multiplied by the probability of survival) in the treatments simulating DVM. The high temperature treatment, simulating permanent residence in the warm epilimnion, resulted in high growth combined with high mortality. At a permanently low temperature, simulating life in the hypolimnion, growth was poor and activity reduced. 4. In a deep, temperature-stratified lake, where the apparently beneficial overall medium temperature is found in a restricted layer within the thermocline, DVM optimises performance in young burbot. Various ultimate factors might act synergistically in selecting for DVM in larval and early-juvenile burbot.
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