Observations on the retention of intragastric transmitters, and their effects on food consumption, in cod, Gadus morhua L

Intragastric implantation is a quick and simple technique of tagging carnivorous fish with transmitters and has been widely adopted. Placement of a transmitter in a fish's stomach might therefore affect food consumption. A group of 20 tank-adapted cod (Gadus morhua ) maintained on a diet of sandeels were starved for 3 days and randomly divided into equal numbers of control and experimental fish. A dummy transmitter of the size used in many acoustic tracking studies (length, 56 mm; diameter, 16 mm; weight in water, 5 g) was inserted via the oesophagus into the stomach of each experimental fish. Fish were first fed at 09.00 hours, 2 days after tagging, and subsequently every other weekday. The experiment was ended when only one fish retained its transmitter. This period was 35 days, the approximate life for acoustic transmitters of the size described. The presence of transmitters did not appear to affect food intake.