Minimal body size for tagging fish with electronic microchips as studied in the Nile Tilapia

Individual identification of fish is often desirable for the smallest possible size, but it is crucial that tagging does not interfere with fish survival, physiology, or behavior. We evaluated radio-frequency identification (RFID) tags (10 mg) and PIT tags (PIT; 32 mg) in fish of two different size-classes of Nile Tilapia Oreochromis niloticus: 200-500 mgand 640-1,600 mg, wet mass (WM). This produced four categories of tag load for each type of tag: 5.0, 3.3, 2.5, and 2.0% of WM. We tested 30 fish per category. Survival averaged 95.8% for RFID tags and 98.3% for PIT tags. Tag retention after 35 d was 99.1% for RFID tags and 96.6% for PIT tags. Tagged fish grew more slowly than controls. Growth penalty was proportional to tag load, but restricted to the first 4 d after tagging and compensated by catch-up growth, except in fish <300 mg presumably due to greater difficulties of handling and tagging. Small PIT tags can thus be used confidently in tilapia of about 1.3 g and RFID tags in tilapia of about 0.4 g. If growth is not a premium, the corresponding minimal sizes are 1.0 (for PIT) and 0.3 g (for RFID)

Regulation of the glucocorticoid receptor mRNA levels in the gills of Atlantic salmon (Salmo salar) during smoltification

The regulation of the Glucocorticoid Receptor (GR) transcript was investigated in the gills of Atlantic salmon (Salmo salar) during the parr-smolt transformation. Sampling of parr and smolt fish was performed between December and July and in particular during the smoltification period occurring in spring. Quantification of GR transcripts revealed differences between the two groups in March and at the beginning of April. During these dates, the amounts of GR mRNA in parr gills were respectively three and two fold lower than those measured in smolts. In order to determine which factors are responsible for these differences, we studied the long-term effects of prolactin and Cortisol treatments on GR transcript in the gills of presmolt fish. The plasma levels of these two hormones respectively drop and rise during smoltification. Contrary to Cortisol long-term treatment which did not modify the amount of gill GR transcript, short-term treatment induced a significant decrease within 12 hours. Prolactin long-term treatment caused a significant increase of GR transcript abundance after 13 days of implant treatment. This result is unexpected with regard to those obtained in the smoltification analysis but is in agreement with previous studies performed in mammary gland revealing a positive control of PRL on GR in epithelial cells. Our data suggest that the regulation of the GR transcript during the parr-smolt transformation probably involves several hormonal factors