Short communication Influence of the time to egg stripping on eyeing and hatching rates in rainbow trout Oncorhynchus mykiss under cold temperatures Azin Mohagheghi Samarin a , Mohammad Reza Ahmadi b, ⁎, Teruo Azuma c , Gholam Reza Rafiee a , Bagher Mojazi Amiri a , Mohammad Reza Naghavi d a Department of Fisheries and Environmental Science, Faculty of Natural Resources, University of Tehran, 31585-4314, Karaj, Iran b Department of Health and Aquatic Diseases, Faculty of Veterinary Medicine, University of Tehran,14155-6453, Tehran, Iran c Freshwater Fisheries Research Division, National Research Institute of Fisheries Science, Fisheries Research Agency, Chugushi 2482-3, Nikko, Tochigi 321-1661, Japan d Department of Plant Breeding, Faculty of Agriculture, University of Tehran, 31587-77871, Karaj, Iran ABSTRACT ARTICLE INFO Article history: Received 11 October 2007 Received in revised form 7 March 2008 Accepted 19 March 2008 Keywords: Oncorhynchus mykiss Retention time Cold temperature Fertilization Ovulation The effects of egg retention time in the abdominal cavity after ovulation on eyeing and hatching rates were studied in rainbow trout, Oncorhynchus mykiss, at cold temperatures of 2 and 8 °C. Two types of experiments were performed: 1) eggs with different retention times were grouped by 3-day intervals until 30 days post- ovulation (DPO) and fertilized with mixed milt from the same sources, and 2) partial volumes of eggs were obtained from individually identified females and stripped and fertilized with mixed milt at 7-day intervals for 35 days. The first experiment showed that the eyeing and hatching rates remained greater than 70 and 60%, respectively, throughout the 30-day experimental period except in the shortest DPO group which exhibited rates of 37.6% and 31.5%. The second experiment clarified that deterioration in egg quality did not occur before 30 DPO at 2 °C and 14 DPO at 8 °C. The eyeing rate also did not show any marked reduction during the experimental period (83%) at 2 °C during the period 28–35 DPO in marked contrast with the sharp drop to 3.6% at 8 °C during the same period. The present study demonstrated that the best stripping time was estimated in the range from 30 to 40 degree-days, and over-ripening of the eggs occurred from 224 degree- days after ovulation at least under cold temperature than in studies reported so far. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The time interval between ovulation and stripping has frequently been regarded as one of the most important factors in the successful breeding of rainbow trout Oncorhynchus mykiss (Craik and Harvey, 1984; Gaudemar and Beall, 1998; Aegerter and Jalabert, 2004). The acceptable retention time of ovulated eggs in the abdominal cavity appears to be between 5 and 15 days post-ovulation (DPO) in the temperature range 10 to 17 °C (Sakai et al., 1975; Escaffre and Billard, 1979; Bry, 1981; Springate et al., 1984; Yamazaki et al., 1989; Lahnsteiner, 2000; Bonnet et al., 2003; Azuma et al., 2003; Aegerter and Jalabert, 2004). To date, however, little information has been available on effects of much colder temperatures on successful breeding except for in vitro studies (cf. Babiak and Dabrowski, 2003 and their review). The present study was performed to examine the effects of egg retention time on the eyeing and hatching rates at temperatures of 2 and 8 °C in vivo. 2. Materials and methods 2.1. Fish Three-year-old female rainbow trout were used as parental fish. The fish were held in outdoor ponds at Kelardasht Hatchery Center in Mazandaran, Iran. Cold water at temperatures of 2 °C or 8 °C was prepared by mixing waters obtained from the nearby river and spring. The fish were fed a commercial diet until satiation once per day except for a starvation period 6 weeks before the expected time of ovulation and spermiation. To confirm ovulation, fish were anaesthetized with 100 ppm tricaine methanesulfonate (methyl-aminobenzoate, MS222) to minimize stress. Two types of experiments were performed as follows. 2.2. Experiment-I One hundred 3-year-old females expected to ovulate in the near future were selected from the brood stock. The state of ripeness was judged by gentle palpation of the abdomen and from the body color of the fish. Serial judgments of ovulation were performed in this way at 3-day intervals for 30 days. Ten groups of fish, each comprising of three females ovulating simultaneously, were held in separate tanks at Aquaculture 278 (2008) 195–198 ⁎ Corresponding author. Tel.: +982161117092;fax: +98 2166933222. E-mail address: mahmadi@ut.ac.ir (M.R. Ahmadi). 0044-8486/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.03.034 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online