European hake (Merluccius merluccius) were caught alive in the northern Bay of Biscay in June/July 2002, using a codend specially designed to avoid crushing fish and also to retain water while hauling the trawl. In all, 1307 fish were tagged with anchor T-bar tags, injected with tetracycline, then released. The length range of the tagged and marked fish varied from 13 to 58 cm, and the modal size was 28 cm, at which length they were assumed to be 2–3 years old. A mean survival rate of 68.2% was estimated. Mortality was mainly caused by stress of capture and physical damage, and depended on the size of the catch and the depth of trawling. By the end of March 2003, 32 fish and three tags had been returned to the laboratory (a 2.7% return rate), the recapture data indicating that the probability of survival after release does not depend on catch depth or depth at location of release. For combined sexes, the somatic growth rate was estimated at 0.033 ± 0.019 cm day−1 (n=15). Males and females did not differ significantly in somatic growth rate, which were, respectively, 0.028±0.018 (n=6) and 0.033 ± 0.012 cm day−1 (n=6). This pilot experiment represents the first recorded mass tagging of European hake, which is known to be a very fragile species. The preliminary results indicate that it would be possible to carry out a large-scale tagging experiment of the European stocks in order to improve assessment and subsequent management decisions.

The aim of this project was to study the behavioural response and movement of artificially reproduced and reared fish after being released to open waters. Further, the possibility to track the migratory behaviour and site fidelity of these fish under the specific conditions of the River Po by using acoustic telemetry. Between the summers 2002 and 2004, 12 Adriatic sturgeons (A. naccarii), body weight from 11 to 26 kg, were released in the River Po at different times of the year, with acoustic transmitter fixed externally to the dorsal scutes. Seven releasing actions and 49 localization campaigns (one day each) have been carried out during the period over a stretch of about 63 km from Isola Serafini's dam (releasing point) to Castel San Giovanni, of which 35 campaigns resulted in localization success. During the first hours, all fish stayed close to the releasing point, and successively moved upstream, some of them for several kilometres (up to 52); others moved upstream and downstream, selecting in some cases a particular place for staying. All the fish movements were related to the river hydrological conditions, moving downstream when the flow increased. Although the study is not exhaustive, it has been possible to determine that reared sturgeons are able to survive when released into open waters, at least for the period of observation. Fish exhibited a normal behaviour, swimming mainly upstream and selecting specific sites for sheltering.Acoustic telemetry has proven so far to be a suitable monitoring system to follow sturgeon movements, while the variability of the hydrological regime of the River Po can represent a serious constraint to the execution of localization campaigns. Using stationary hydrophones and receiving stations fixed along the riverbanks may obtain better results.

The end effect in trawl catches is defined as the proportion of the fish catch taken during shooting and hauling of the net, a period excluded from that nominally referred to as haul duration. If important, this effect will lead to biased abundance estimates, because the swept area will be underestimated. An experimental survey was carried out to compare catch numbers obtained in standard research 30-min hauls with those from 0-min hauls, the latter referring to the trawl being hauled as soon as the trawl geometry stabilized on the seabed. Average catch ratios (0-min/30-min hauls) ranged from 0.05 (s.d. 0.06) for sole to 0.34 (s.d. 0.64) for hake, indicating that the end effect might be more important and more variable for highly mobile species. As a consequence, the bias in abundance indices derived from swept area estimates that ignore end effects will be species-dependent.