No abstract

We evaluated three alternative procedures for tagging juvenile Nile tilapia Oreochromis niloticus (1.9–13.7 g) with passive integrated transponder (PIT) tags: injection with hypodermic syringe injectors (N = 40) and abdominal surgery with or without suturing (N = 55 and 40, respectively). The survival of Nile tilapia tagged with injectors was low (10–50% at 10 d) and proportional to fish size because of the difficulty of controlling the penetration of the syringe into the intraperitoneal cavity following the piercing of the body wall. Surgically implanted fish had much higher survival rates (78–100% at 10 d). Suturing reduced the risks of tag expulsion and protrusion of the viscera through the open incision within the first 3 d following surgery (10% risk in the nonsutured fish). Over the first 3 d after tagging, tagged fish showed depressed growth rates (to a greater extent in smaller fish and proportionally greater in sutured fish) but showed normal growth later on. These results demonstrate that surgery is suitable for PIT‐tagging small juvenile Nile tilapia, which offers opportunities for studies of individual performance.

Seven yellow eels (572-643 mm, 318-592 g) Anguilla anguilla (L.) were tagged with surgically implanted radio transmitters (activity circuit, 1.6-1.7 g) and tracked in the Awirs stream, a small (width < 5 m, depth from 0.1 to 1.2 m), densely populated (ca. 250 kg of eel ha −1 ) tributary of the Belgian River Meuse. The eels were positioned daily from late April to mid-August, and their diel activity was studied during twenty four 24-h cycles. During day-time, the eels were resting in rootwads or in crevices inside stone walls or in crevices in between rocks. They became more active in the late afternoon but generally did not leave their residence before sunset, except under overcast weather. Activity peaked during the first part of the night then progressively vanished, and always ended before sunrise. The area exploited during night-time never extended over more than 40 m 2 , except when the eel changed its residence. The intensity and timing of nocturnal activity and the extent of the daily activity area were dependent on water temperature (respectively P < 0.0001, P < 0.05 and P < 0.0005), with eels showing little or no activity when the diurnal temperature did not exceed 13 • C. Eels showed higher agitation under full moon and maintained their activity later in the night (P < 0.05). The eels showed restricted mobility, and occupied small stream areas (from 0.01 to 0.10 ha) in a non sequential mode, except for two fish which were displaced to the River Meuse by a spate in early June and were never recovered. The length and frequency of net daily journeys were higher (P = 0.005) at water temperatures above 16 • C in late May and June, which also corresponded to the period of immigration of eels from the River Meuse. This study thus shows that large yellow eels may adopt a highly sedentary lifestyle in a continental, fast flowing and densely populated environment, even at periods of the year when these stages usually show upstream migrations.

This telemetry study aimed to document the mobility of Salmo trutta in the River Ourthe sub-basin (tributary of the River Meuse) during summer and autumn, and to analyse the environmental factors which trigger spawning migration or limit their extension. Nine trout (233-2217 g and 26.6-55.2 cm FL) were radio-tagged with intraperitoneal radio transmitters and positioned daily, from 14 August 1996 to 15 January 1997. Until 1 October, fish showed restricted movements: daily journeys never exceeded 300 m and corresponded to displacements by high floods or to routine home range movements. From 7 October to 15 November, seven of the nine trout travelled upstream over distances from 5.6 to 22.95 km, into tributaries and sub-tributaries. Migration speed was fast during the early days, when trout could travel over more than 5 km per night, then progressively decreased as they were approaching putative spawning redds under lower temperature. Both in the River Ourthe and in the Aisne stream, all migrations started within less than three weeks (early October) and were found to be triggered by the combination of three environmental factors: high variations of water temperature and water level between consecutive days, within a thermal range of 10-12 • C. From the trout point's of view, these may be signs that the environment becomes unpredictable, as its variability increases within a thermal range which no longer enables them to achieve high growth rates. These results are discussed within the context of foraging strategies, life history strategies and management of trout population.

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