Storage of unfertilized rainbow trout ( Oncorhynchus mykiss) eggs in sealed polyethylene (PE) bags

Abstract

Seven experiments were conducted in an effort to establish ways of optimizing storage conditions for unfertilized rainbow trout eggs. Eggs in coelomic fluid with antibiotics added were stored at 2 ± 1 °C for various periods before being inseminated with cryopreserved spermatozoa and incubated to the eyed stage. The proportion of eggs reaching the eyed stage, relative to freshly collected eggs, was used as the parameter for effective storage. In the first two experiments (Exps. 1 and 2), eggs were stored in vials that were uncovered or covered with a special tissue culture membrane “BioFolie 25” or polyethylene (PE) freezer bag material. Regardless of the material used, covering of the vials did not impair fertilization rate of stored eggs. During subsequent experiments (Exps.3–7), eggs were packaged in sealed PE bags rather than storing them in vials. In bags, no effect on fertilization capacity by various surrounding gas atmospheres (air, exhaled air, oxygen or nitrogen) was observed; carbon dioxide, on the other hand, was extremely detrimental. Heat sealing of packets, size of packet and mass of the eggs on top of those at the bottom of a packet had no effect (P > 0.05). Handling of eggs at room temperature was unproblematic for up to1 hour. If in direct contact with the gas atmosphere, eggs retained their fertilization capacity longer under air than under oxygen; if several layers of eggs were involved, however, oxygen was indicated. Direct exposure to nitrogen and, more so, carbon dioxide brought about a drop in pH and was detrimental. It appears that inside an egg packet there is sufficient oxygen present in a dissolved state to maintain fertilization capacity at a reasonable level for more than three weeks, provided the carbon dioxide concentration does not increase and cause a decline in pH. Due to the selective permeability of PE for carbon dioxide, the pH inside egg packets remained neutral. Therefore chilled storage of unfertilized rainbow trout eggs in sealed PE bags is particularly effective. After 20 days, fertilization rates of more than 50% were recorded. With no necessity for an air space or oxygen supply and no risk of spillage or dehydration, PE egg packets, being space-saving and convenient to handle, are suitable for storing and can be further applied for shipping of unfertilized rainbow trout eggs.