I. Lein1, Y. Barr2, S. Helland1 and H. Tveiten1
1Nofima, Sjölseng, N-6600 Sunndalsøra, Norway. 2 Ullstindveien 307, Vågnes, 9022 Krokelvdalen, Norway
E-mail: ingrid.lein@nofima.no
Introduction
Various species of cleaner fish are increasingly used to control the number of salmon lice (Lepeophteheirus salmonis) in salmon and trout cages in Norway. Due to disease problems in wild caught cleaner fish, restricted catching seasons and considerations regarding the sustainability of wild stocks has led to the initiation of commercial farming of cleaner fish. Production of ballan wrasse (Labrus bergylta) for use in salmon cages started 4-5 years ago. Farming of new species always comes with challenges, either biological or technical. The present study describes initial works on the propagation of ballan wrasse in captivity carried out within the joint research project LeppeProd.
Materials and methods
Mid-June 2011 broodstock of ballan wrasse were caught by net off the coast of mid-Norway and transferred to tanks at Nofimas' research station at Sunndalsøra. The fish were treated with antibiotics and iodine to heal catch wounds. Weight and length development was followed for 10 month. All ballan wrasse start out as females (protogynous hermaphrodit), and dependent on size distribution some individuals change sex (Muncaster, 2008). For maximum egg production a high frequency of females is desired in a farming situation. Gender determination was done either by ultrasound or biopsy of the gonad using 12 cm sterile cat catheters.
Natural spawning is the method used by the industry. However, a lot of eggs are lost, and the sticky eggs become covered by debris from the tank water. Attempts were done to strip eggs from females and milt from males and to determine spawning rhythms in females. Desticking procedures are commonly used for other fish species like the common carp. Incubation and disinfection of single eggs can be done in a simpler and more efficient way than for sticky eggs. Different methods for desticking of eggs were tested including treatment with protease before and after fertilization.
Ballan wrasse males release very small amounts of milt, and running milt is not always available at the same time as eggs are stripped. Dilution of milt and short-term storage of milt was therefore tested to improve the utilization of the milt. Initial tests were done to synchronize the spawning by use of gonadotropin releasing hormones (GnRH). Broodfish were injected with the commercial product Ovaprim®. Biopsy of oocytes was used to determine the maturity status of oocytes. Performance of fish treated with GnRH was compared to non-treated fish.
Results and discussion
Adult ballan wrasse tolerated anesthetics and handling well, and wrasse broodstock caught by net healed within a month when treated with antibiotics and iodine shortly after catch. The survival during the two first months was 80%, thereafter few fish died. Females increased their mean body weight by 17% and males 14% during 10 months while the increase in length was minor. Hence, the condition factor increased for both sexes. Length is not a good measure for growth and condition in adult fish >25 cm nor for sex determination as there is an overlap between females and males.
Ultrasound gave >80% precision of gender determination. However, this method requires expensive equipment and trained personnel. Fish under 28 cm TL are most probably females. Cat catheters inserted into the gonads through the urogenital opening gave 100% precision for gender determination. In females the catheter is easily inserted all the way into the gonad while in males the catheter stops half way in. This is a simple method that can be used by hatchery staff without special training. However, good hygiene is important to prevent inflammation of the gonads.
Eggs were easily stripped from some females while others retained the eggs even though they appeared ripe. No regular spawning rhythms were observed in females. Males released small volumes of milt (0.05-0.8 ml), and not all males had running milt. Fertilization tests resulted in 90-100% fertilization, indicating that stripped eggs were mature. Diluted milt gave significantly higher fertilization rates than undiluted sperm, and can be stored for at least five days without any reduction in fertilization capacity.
Attempts to remove the sticky layer from naturally spawned and fertilized eggs did not succeed. Treatment of unfertilized eggs with protease however, successfully prevented the development of a sticky layer. Replacing the ovarian fluid with Ringers' solution combined with protease treatment was most successful indicating an interaction between proteins of the ovarian fluid and proteins on the egg surface (Mansour, 2009).
The initial tests with injections of GnRH to induce spawning gave varying results from massive spontaneous spawning to no obvious effect. A large variation in oocyte diameter was observed, which is typical for a multiple spawner, but few oocytes had reached the final maturation stage (migrated germinal vesicle). For the final test females with the highest proportion of large oocytes were injected with GnRH resulting in more females releasing eggs than in the control group.
Stripping an fertilization of ballan wrasse eggs/milt worked well, but some additional research are needed to confirm that eggs without the sticky layer develop normally and also to determine correct timing for injections with GnRH.
References
Mansour, N., Lahnsteiner, F. and Patzner, R.A., 2009. Ovarian flid plays an essential
role in attachment of Eurasian perch, Perca fluviatilis eggs. Theriogenology 71, 586-593.
Muncaster, S., 2008. Reproductive Physiology of Ballan wrasse (Labrus bergylta). PhD
dissertation, University of Bergen, Norway. ISBN 978-82-308-0532-9.