A biotelemetry experiment was designed to determine whether the selection of a residence area by adult barbel Barbus barbus (Cyprinidae) was dependent on the availability of physical habitat features, on the presence of fish shoals and/or on the presence of specific partners. The experiment took place in 3.0 km stretch of the River Ourthe (Belgium), where the latter features had been characterised by habitat and electric fishing surveys (1 190 fish ≥ 15  cm, at 265 locations). Six barbel (34.8–49.5 cm) belonging to a single shoal of 34 fish were tagged with 40 MHz radio transmitters, transplanted in pairs at different distances (−1 km, −30 m, +1 km) from the capture site and tracked from 3 September to 7 October 1990.

Fish showed variable mobility patterns and home range sizes (200–2 400 m). They selected 17 residence areas of which the cumulative frequency of occupation ranged from 0.1 to 79.5 day × fish. The comparison between the residence areas selected, habitat suitability and the location of fish shoals in the study area indicate that the presence of a shoal of at least 10 resident fish within a suitable habitat (normalised suitability index ≥ 1.0) is necessary but not sufficient to guarantee its long term selection and occupation. Homing movements of transplanted fish were directed towards the activity area rather than to the residence area itself. The fish that did not home, selected residence areas in a part of the river with a higher availability of potential feeding areas. These elements are discussed within the context of habitat utilisation and foraging strategies by a gregarious species. It is suggested that resident fish of the same species may ease the exploitation of a non familiar environment by a naive fish, and that this may positively trade off the benefit provided by home site fidelity, depending on the availability of food resources.

Surgery procedures were used to implant telemetry transmitters into the body cavity of adult (574–1 033 g) tilapias Oreochromis aureus in aquaculture tanks (4 m2, 1.5 m3, 26.5 ± 0.5 °C, ≥ 5.0 mg O2 l−1) and their effect on fish survival, growth and behaviour was evaluated. Only one out of 35 implanted fish died. With one exception, all fish consistently retained their transmitter until the end of the study (up to 30 months). Healing was faster when the incision was sutured with polyamide monofilament (5–14 days) than with other suture materials, due to tunnelling with atraumatic needles for catgut or fouling of braided silk. In all 10 fish sacrificed after 30 and 50 days, the transmitter had become encapsulated by connective tissue. No infection or damage to the viscera was observed.

The activity of four tilapias (903–1 033 g) equipped with motion sensitive transmitters was telemetered during the recovery from anaesthesia and surgical procedures. All four fish maintained a normal diurnal activity rhythm pattern throughout the study but had low levels of activity during the first 12–24 h. Based on the evolution of their resting posture after surgery, it is suggested that tilapias need 3 to 4 days to completely compensate the negative buoyancy resulting from anaesthesia and tagging.

Swimming capabilities and in situ measurement of muscle activity from adult Atlantic salmon (Salmo salar) at two seasonal temperatures were measured using radio transmitted electromyogram (EMG) signals. Forced sustained levels of activity and critical swimming speeds were determined and correlated to radio transmitted EMG signals using a modified Blazka swim speed chamber. There were no differences in swimming performance levels between tagged and untagged individuals. At 18 °C, sustained activity and critical swimming speeds were approximately 70% and 20% higher than at 12 °C, respectively. No differences in burst activity were observed at these temperatures. EMGs recorded from salmon during ascent of an artificial flume at cold temperatures revealed that overall muscle activity is greater than that observed for critical swimming speeds. This implies that white muscle may be recruited at this temperature. However, in contrast, most activity at 18 °C is below that observed during critical swimming speed. Moreover, salmon required almost twice as long to traverse the flume at 18 °C than at 12 °C. Together, our data demonstrates that salmon may recruit white muscle fibres and incur an oxygen debt at colder temperature as a strategy for ascending velocity obstructions at a quicker rate.

The effect of fish farming on dissolved amino acid concentrations, bacterioplankton abundance and exoproteolytic activity was assessed in 3 experimental marine ponds. Different standing stocks of fish were introduced (semi-intensive pond: 250 g.m−2, semi-extensive pond: 50 g.m−2, control pond: 0). Sea bass farming increased dissolved combined amino acid (DCAA) concentrations only in the semi-intensive pond. Bacterial standing stock was unaffected by fish food supply. However, bacterial exoproteolytic activity was strongly stimulated by aquaculture intensification; the average maximal rate of dissolved protein hydrolysis (Vm) increased with intensity (control pond: 1 500 nM.h-1; semi-extensive pond: 2 600 nM.h−1; semi-intensive pond: 5 100 nM.h−1). DCAA fluxes through bacterial exoproteolytic activity ranged between 16 (semi-extensive) and 11% (semi-intensive) of the daily nitrogen input by fish food. Bacterial exoproteolytic activity allowed a substantial part of the increased supply of dissolved amino nitrogen to be incorporated into bacterial biomass, then available for transfer to higher trophic levels within the ponds. It also significantly decreased dissolved organic nitrogen export from the ponds to the surrounding environment.

Two specific Enzyme Linked lmmunosorbent Assays (ELISA) for vitellogenin were developed using two different antisera anti-plasma vitellogenin purified by SDS polyacrilamide gel electrophoresis and electroelution. Purified vitellogenins migrated as single bands under SDS conditions with an apparent molecular mass of 154 and 152 kDa for Chrysichthys nigrodigitatus and Heterobranchus longifilis respectively.The ELISA was performed on 96 well microtiter plates using specific antibodies raised on rabbits and a second antibody (Goat anti-rabbit) labeled with peroxidase. Parallel displacement curves were obtained between purified vitellogenin and female plasma as well as plasma from estradiol treated fish. Both antibodies cross-reacted with the heterologous vitellogenin but the curves were not parallel, indicating different antigenic structures of the two vitellogenin molecules from the two catfish species. The reliability and the specificity of these assays allowed us to quantify vitellogenin in plasma samples during vitellogenesis of the two species. The annual vitellogenin plasma levels presented a good correlation with mean oocyte diameter and reproduction period in Chrysichthys nigrodigitatus. In Heterobranchus longifilis the mean oocyte diameter is almost constant all year round, but vitellogenin levels presented significant variations with higherlevels corresponding mainly to the low gonado-somatic index values observed during the dry seasons.

The proportion of vibrio E, dominant in healthy turbot larvae, was artificially increased in the rotifer enrichment medium. The main effect of this vibrio enrichment was to improve the resistance of larval turbot challenged with a pathogenic strain of Vibrio splendidus, vibrio P. The improvement of the survival rates was significant with 1.5 × 106 CFU of vibrio E added per ml of enrichment medium, and it was still maximum at the highest dose tested (5 × 107 CFU.ml−1; 80 ± 2% of survivors after 48 h of challenge, versus 50 ± 2% for the control group).

Further in vitro characterisation showed that vibrio E was able to grow in the presence of the chelating agents EDDHA and EDTA, while producing siderophore, as revealed by chrome azurol S dyeing. The pathogenic vibrio P did not grow on iron-depleted medium, and it did not seem to produce siderophore. Consequently, a second experiment attempted to appraise the role of iron availability in the interaction between the pathogenic and probiotic vibrios. To this end, turbot larvae were fed with rotifers treated by the bacterial siderophore deferoxamine. This treatment significantly increased the resistance of larvae chal-lenged with vibrio P in comparison with the control group, but to a lesser extent than the group enriched with vibrio E. There was also an increase of the growth rate due to either the siderophore treatment or thevibrio enrichment, as well. No further improvement was observed when the rotifers were simultaneously both treated and enriched. It was concluded that the probiotic effect of vibrio E may be at least partly due to competition for iron with the pathogen.

Short-term storage over several days as well as cryopreservation of turbot (Scophthalmus maximus) sperm were studied. Two extenders, Ringer 200 and artificial seminal liquid (ASL2), are suggested for semen collection in order to avoid the deleterious effect of urine contamination, and for the purpose of short-term storage between 0 and 15 °C. Oxygen atmosphere is not suitable for turbot sperm storage. Turbot spermatozoa undergo cryopreservation with a high rate of success especially in a sucrose solution with 10% dimethyl sulfoxide (DMSO) and 10% egg yolk.