Observers were placed on pelagic vessels in the Scottish fisheries for mackerel (Scomber scombrus), herring (Clupea harengus), "maatje" herring (herring caught just before their first spawning) and argentines (Argentina silus) to monitor by-catch composition and discarding practices. A total of 67 days was spent at sea, 11 on the argentine fleet, 28 on the herring fleet, 12 on the "maatje" herring fleet and 16 on the mackerel fleet. The level of by-catch generally ranged from <1% to around 2.5% of the total catch. The argentine fishery took a significant proportion of blue whiting (Micromesistius poutassou) (approx. 10% of the overall catch) but this was landed and sold in the market. The greatest range of by-catch species was found in the argentine fishery, including 11 species of fish and one species of squid. Discarding rates in the fisheries varied, with herring and argentine fisheries showing no discards, the mackerel fishery a discard rate of around 4% and the "maatje" herring fishery a discard rate of around 11%. By-catches included small numbers of gannets (Morus bassanus) but no marine mammals. If the sampled trips were representative, results indicate that marine mammal by-catch events typically occur during less than one in 20 hauls in the fisheries studied. A larger scale study is needed to confirm this. Cetaceans were sighted in the vicinity of the fishing boats during 4% of observed fishing activities.

To understand the ecology of the exotic silver European eel Anguilla anguilla introduced into Japanese waters, the migratory behavior of 106 specimens captured in the coastal waters of Japan between April 1997 and March 2002 was analyzed. Their migratory behavior was apparently correlated with various environmental factors, particularly photoperiod, water temperature, lunar phase, and passage of atmospheric depressions, and was similar to the behavior of the species in European waters. These findings suggest that transplanted European eels retain their ability to respond to environmental cues for seaward migration in similar temperate habitats. The timing of the migration of silver European eels coincided with that of the native Japanese eels A. japonica, suggesting that the silver European eel was synchronized physiologically with the native eel by the same environmental factors.

In the Mediterranean Sea, the fish Sarpa salpa (Sparidae) is the only vertebrate grazer of Posidonia oceanica meadows. In order to gain knowledge about the behaviour of S. salpa and a better understanding of the meadow's primary production recycling, an original study was designed using ultrasonic telemetry to investigate activity patterns and space utilisation in the field. During June–September 2000, we compared diel movements of six adult S. salpa (249–317 mm FL and 313–633 g) in the Bay of Calvi. These fish were tagged by intraperitoneal insertion of ultrasonic transmitters. Their positions were recorded with a directional hydrophone from a small boat with an accuracy between 10 and 50 m, depending on the local environment. The tracking duration ranged from 3 to 22 days (average 11.8 ± 7.3). Locations were performed from at least dawn to dusk or early night, and one fish was tracked during the entire 24-h cycle. Fish were more mobile during the twilight periods, but statistical analysis indicated individual differences for the precise period of mobility. Two major behavioural patterns were observed: first, the fish remained in close vicinity of the harbour during the day and moved away to the north or the south at dusk to access nocturnal sites, occupying a home range of about 4.3 ha. The second behavioural pattern involved persistent occupation of the same sites during day and night within a relatively restricted home range (about 0.8 ha). Great variation in mobility was found and the same individual fish could show both kinds of behaviour.

The genetic structure of two native Silurus aristotelis (one of Europe's oldest surviving freshwater fish species) populations was investigated using eight microsatellite loci. Average values of population heterozygosity were very high, even approaching values reported for marine fishes, possibly due to stable population sizes and the prolonged undisturbed conditions prevailing in the lakes that represent the native distribution of S. aristotelis. The success of an attempt to introduce the species into a new environment was also evaluated. No loss of genetic variability in the introduced population was detected. Additionally, assignment tests and trees based on genetic distances among individuals indicated the low differentiation of the translocated population from its donor population, and, differentiated the two autochthonous populations successfully. Comparisons with previous allozyme and mitochondrial RFLP studies were also made and showed that results on relative levels of genetic variability among populations were in good agreement among all methods. However, microsatellite analysis exhibited higher power of statistical tests for differentiation among population samples compared to allozymes.

Formalin (a 33–38% aqueous formaldehyde solution) is currently used for bath treatments to control ectoparasitic infections of fish. Its effects on nitrification were evaluated in a semi-closed pilot scale saltwater recirculating turbot culture system. Tested treatments included 1 h static exposure to 20, 30, 40, 50, 60, 70, 90 mg l−1 formaldehyde without flushing, and long-term (i.e., 2, 4 and 6 h) exposures with constant 60 mg l−1 formaldehyde in a recirculating system. Formaldehyde has no apparent effect on the ammonia oxidative bacterial community. However, a significant effect on nitrite oxidation was observed in 1 h static exposures of concentrations higher than 40 mg l−1, and in recirculating system exposures of 60 mg l−1 formaldehyde for more than 4 h. Repeated treatments may be hazardous for nitrifying bacteria, which could induce an increase in nitrite concentration. Nitrite concentrations should be monitored when treatments are repeated or when they last a long time in recirculating aquaculture systems.