The upper critical thermal limits for alevins of Arctic charr from a Norwegian lake north of the Arctic circle

Mean values ±95% CL of the upper incipient (T_IL) and ultimate (T_UL) lethal temperatures, determined at five acclimation temperatures ( T_A ), increased for T_IL from 19.2 ± 0.4° C ( T_A 0.5° C) to 21.0 ± 0.4° C ( T_A 20° C), and for T_UL from 22.6 ± 0.1° C ( T_A 0.5° C) to 26.6 ± 0.4° C ( T_A 20° C). Mean values were close to those obtained for Arctic charr alevins from Windermere (north-west England). These comparative data for alevins, and previous data for 0+ year parr, indicate negligible geographical variation in the thermal limits of Arctic charr.     

An automatic device to record the force necessary to compensate plant movements

A device, which automatically and continuously keeps moving plants in fixed positions, has been constructed. The apparatus is based on optical detection of any movements of the plant organ under study. The plant organ is kept in the desired position by means of wires, and the force necessary to apply to the wires to achieve this is recorded. The force reflects any tendency of the plant organ to move. The system is controlled by an Apple II computer. The device can work in one or two dimensions and record compensation forces in a wide range. In the experiments mentioned below, the magnitude of the forces necessary to keep the plants in a fixed position was of the order of 10^-4N.
Circadian leaf movements of Oxalis regnellii Mig. were studied in the device. The leaf rhythm continued, although the leaflets were clamped and the light input on the leaf therefore constant. Circumnutation of hypocotyls of Helianthus annus L. cv. Californicus were drastically reduced in amplitude when the hypocotyls were kept in vertical position by the wires. Since the gravitropic input signal to this system was zeroed by the equipment, the results demonstrated that in the absence of gravitropic inputs the circumnutation reactions drastically diminish. This confirmed that circumnutations of these hypocotyls are influenced by gravity. Finally, the apparatus was used to study phototropic reactions: By clamping phototropically stimulated coleoptiles of Avena sativa L. cv. Seger no gravitational stimulations were involved and the phototropic reaction without interference from gravity could be studied.     

Infection of Salmincola edwardsii (Copepoda: Lernaeopodidae) in an age-structured population of Arctic charr—a long-term study

The parasite-host relationships between Salmincola edwardsii (Copepoda: Lernaeopodidae) and Arctic charr Salvelinus alpinus were studied in lake Takvatn, northern Norway, over an 8-year period. The infection levels were modest, with an overall prevalence of 16·1% and an abundance of 0·5 parasites fish⁻¹. Most of the infected fish (54·1%) had only one parasite attached, whilst only 4·3% had more than 10. In general, parasite infection increased with increasing age, and hence size of the fish. The increase was modest up to age 7, whereafter a steep increment in parasite burdens occurred. Within each age class there was little effect of either fish size, gender, maturation or choice of macrohabitat upon parasite abundance. There was also little year-to-year variation in infection of the different age classes, indicating a high degree of stability of the parasite-host interactions in this system. The infection levels exhibited seasonal changes, being lowest in early summer and highest during winter. The infection rates seemed to be related to seasonal and ontogenetic habitat shifts of the charr.     

Referate

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