Determining age is an important step when assessing growth, mortality, and yield ofcultivated and wild populations, but studies linking shell growth marks in the pearloyster Pteria sterna with the age of individuals are lacking. Thirtyjuveniles (20.0 ± 1.2 mm shell height), collected from a winter spatfall, were marked withthe fluorochrome calcein and kept in the field in culture containers. After day 16, thejuveniles were cleaned and their shells cut along the sagittal axis to determineperiodicity of micro growth bands formed in the inner shell layers and to estimate age.During this trial, fluorescent calcein marking succeeded in individuals larger than 20 mmshell height; these formed an average of 15 micro growth bands over the 16 days,representing 1 band per day. The marker created a wide fluorescent band containing threemicro growth marks, suggesting that calcein was incorporated into the shell over the firstthree days. The use of calcein was found to be an accurate method for validating the microgrowth band frequency of formation in P. sterna juveniles, which in turncan help to estimate age.

Seaward migration of immature salmonids (smolts) may be associated with severe mortality in anthropogenically altered channels. Few studies however, have identified distinct behaviours that lead to exposure to adverse habitats or even unsuccessful migration. This study used high resolution telemetry to map migration routes of Atlantic salmon (Salmo salar) smolts approaching a water withdrawal zone associated with an aquaculture facility in a lowland river. Individual smolts were tagged with an acoustic transmitter and released upstream of the water withdrawal zone. A trap was installed downstream of the water withdrawal zone. The trap captured all smolts that passed the water withdrawal zone. The tracking results confirmed previous studies on Pacific salmon showing that Atlantic salmon smolts may perform milling behaviours (i.e. upstream excursions and circular swimming behaviour) in anthropogenically altered channels. Non-milling and milling smolts were compared. Smolts performing milling behaviours covered a larger area (m2) and experienced an increased probability of entering the water withdrawal zone, considered an adverse habitat. Finally, smolts were identified as either passing (67%) or non-passing (33%) the water withdrawal zone based on the recapture data from the trap. In total, 20% of the non-passing smolts entered the aquaculture facility. Several behavioural traits differed between the remaining (80%) non-passing smolts and the passing smolts. In particular, time spent near the water withdrawal zone correlated negatively with the probability of passage. These links between individual behaviours and exposure to adverse habitats and passage probability may be applied to improve management of salmonid populations.

Acute toxicity of ammonia was investigated in four life stages of juvenile chub, Leuciscus cephalus (cyprinid fish): 1, 10, 20 and 30 days after the first feeding. The fish used for the toxicity test were reared intensively in a closed recirculation system. Each acute toxicity test duration was 96 h and lethal concentration LC1, LC50 and LC99 values were calculated for 24, 48, 72 and 96 h. The susceptibility of chub to acute ammonia toxicity decreased linearly with age and stage of development. The LC50 (48 h) values ranged from 0.62 mg L-1 of unionized ammonia nitrogen for one day after first feeding larvae to 1.73 mg L-1 for 30 days after first feeding ones. A significant linear relationship between chub larvae susceptibility to ammonia toxicity and both body weight and length was found. The critical level of unionized ammonia nitrogen for chub larvae was suggested as 0.49 mg L-1.

Cryopreservation is a valuable tool for genetic improvement programs. Several bivalvemollusc species have already been the subject of such programs and the Tahitian blackpearl oyster industry is now planning the development of selective breeding for desirabletraits in Pinctada margaritifera. The ability to cryopreserve spermatozoawould, therefore, offer significant benefits to the cultured black pearl industry.Spermatozoa were cryopreserved with cryoprotectant agent (CPA) 0.7 M trehalose in 0.8 MMe2SO and a two-step freezing process was used: straws were first maintainedin nitrogen vapour for 10 minutes, then directly plunged into liquid nitrogen and storedfor one week before use. The viability of thawed sperm was 23% lower than that of freshsperm. When using thawed sperm, therefore, a higher sperm/egg ratio of 100 000:1 wasrequired to reach 80% oocyte fertilization, compared with 100:1 for fresh sperm.Nevertheless, this first demonstration of cryopreserved sperm fertility in black pearloyster confirms the hatchery applicability of the cryopreservation technique defined here.Monitoring for larval viability during the first 23 days of life revealed no significantdifferences between the progeny produced with cryopreserved sperm and that produced usingfresh sperm.