Noninvasive, mass marking of fish by immersion in calcein: evaluation of fish size and ultrasound exposure on mark endurance

Calcein was administered into the caudal fin rays of three different sized rainbow trout Oncorhynchus mykiss (0.2, 0.3 and 1.0 g). Two different delivery techniques were studied: immersion and immersion preceded by a 30-s ultrasound exposure. The presence of fluorescent marks in the rays of the caudal fins was monitored at different times post treatment up until 12 months, and the delivery techniques were compared by determining the average pixel luminosity of digitally captured fluorescent images of the rays. The experimental fish were also monitored throughout the study period in terms of both growth and survival. Differences were not found in growth rates between control fish and the different treatments within any of the size groups. The same was true for survival rates. Endurance of the marks produced in the fish was found to be controlled by two distinct factors. The more dominant of the two was the size of the fish at the time of treatment, where a positive correlation was found between mark endurance and fish size. Ultrasound exposures were found to increase mark endurance in the small- and medium-sized fish. Although a steady decrease in luminosity was found in the marks of the fish over time, differences were not found between the ultrasound and non-ultrasound treated fish in the large group over the period of sampling. Results of the study will contribute to the development of a non-invasive, mass marking and nonlethal mark retrieval procedure of hatchery-reared fish in both aquaculture and stock enhancement programs. The results will also benefit ongoing research for using ultrasound as a non-invasive method for mass administering compounds into fish. (C) 2002 Elsevier Science B.V. All rights reserved.