The production of triploid landlocked Atlantic salmon (Salmo salar L.) and their potential for aquaculture

Benfey, Tillmann J. (1983) The production of triploid landlocked Atlantic salmon (Salmo salar L.) and their potential for aquaculture. Masters thesis, Memorial University of Newfoundland.

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Abstract

Heat shocks of 5 minutes at 32°C or hydrostatic pressure shocks of 3 or 6 minutes at 7.0 x 10⁴ kPa (10,150 p.s.i.), when completed within 20 minutes of fertilization at 10°C, were found to induce 100% triploidy with 70-90% survival (relative to controls) in eggs of landlocked salmon (Salmo salar L.). The identical heat shock yielded substantially lower numbers of triploids when applied 25 to 45 minutes after fertilization. Pressure shocks of longer duration (9 to 15 minutes at 7.0 X 10⁴ kPa) or higher magnitude (6 minutes at 7.9 X 10⁴ to 10.5 X 10 4 kPa) resulted in 100% mortality prior to hatching. The duration of the effective period within which heat shocks could be used to induce triploidy was found to be temperature dependent, being longer at 6.5°C than at 10°C. Attempts to induce polyploidy with cytochalasin B were unsuccessful. -- Karyotyping was not a useful method for the routine identification of triploids because it was time-consuming and inconsistent. The use of a microspectrophotometer to measure the DNA content of Feulgen-stained erythrocytes, although time-consuming, clearly distinguished triploid from diploid fish. Blood cell sizing by means of a Coulter Counter Channelyzer was a highly effective alternative to screen for triploids, being both fast and accurate. This technique was routinely used to identify triploid individuals. The use of various erythrocyte dimensions measured from blood smears was a valid method for indentifying triploids, but was also time-consuming. No mosaic polyploids were found, and the existence of such fish is questioned. -- Triploid fish had a greater mean erythrocyte volume (MCV) but lower erythrocyte count than diploids; the haematocrit was thus the same in diploids and triploids. Although the total blood haemoglobin content and the mean corpuscular haemoglobin concentration (MCHC) were lower in triploids than in diploids, the mean corpuscular haemoglobin content (MCH) was higher than that of diploids. The increase in triploid MCV was mainly due to an increase in cell length; there was only a minor increase in cell width and no increase in cell height. The nucleus of triploid erythrocytes occupied a greater percentage of the corpuscular volume than did the diploid nucleus. Mean cytoplasmic haemoglobin concentration was found to be the same for diploids and triploids when this was taken into account. -- The rates of oxygen consumption by diploid and triploid fish were the same, as was the oxygen tension at asphyxiation. Growth rates (measured as change in weight with time) prior to spawning were the same for diploids and triploids, but diploids were consistently shorter in fork length, thus having a higher condition factor. The gonadosomatic index (GSI) of diploid females was 13 times greater than that of triploid females, but the GSI of diploid males was only 1.9 times greater than that of triploid males. Triploid ovaries had the appearance of gonads, but triploid testes were well-developed. Gonad histology revealed that diploid ovaries were packed with hundreds of previtellogenic oocytes (stages 1-4), whereas triploid ovaries contained mostly undifferentiated oogonia. Every triploid ovary examined, however, contained at least one oocyte having the typical diploid appearance, indicating that triploid females were not sterile. Triploid testes contained all the elements present in the diploid testes, but development appeared to be delayed in comparison to that of the diploids. -- It is concluded that triploid females may be of benefit to salmonid aquaculture, but that further studies of growth and maturation are required.

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