Spermatology and sperm ultrastructure in farmed coho salmon (Oncorhynchus kisutch)

Fish sperm quality assessment is an essential process for commercial production. The goal of this study was to evaluate the quality of fresh milt of farmed coho salmon using a functional and structural approach. A total of 37 males were used to determined parameters such as sperm concentration, pH and osmolality of seminal plasma, viability, mitochondrial membrane potential and fertility. In addition, ultrastructure and morphology were assessed through transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results show that coho salmon release a large volume of milt (55.08 mL +/- 20.14 mL) with a mean concentration of 13.45 +/- 3.40 x 109 spermatozoa mL-1. A positive correlation was found between sperm concentration and spermatocrit (P < 0.05; r = 0.8447). Milt contained a high percentage of motile sperm (85.00-95.00%) with a mean motility period of 45.25 +/- 2.56 s, viability of 95.24 +/- 2.60% and mitochondrial membrane potential of 91.33 +/- 4.03%. The fertility rate was also high (96.00 +/- 4.16%). SEM and TEM images revealed that mature spermatozoa of coho salmon have a total length of 32.85 +/- 1.25 mu m, are anacrosomal, and consist of an ovoid head, a short cylindrical mid-piece and a long flagellum which can be differentiated into a long main-piece and a short end-piece. The posterior part of the nucleus contains the nuclear fossa in which are located the distal and proximal centrioles, perpendicular to each other. There is only one mitochondrion, positioned as a ring in the mid-piece. The flagellum has two lateral side-fins, apparently of unequal length, and shows the typical "9 + 2" axoneme. These findings indicate that most of milt and sperm traits are within the range reported for other salmonids. Similarly, the results of morphology and ultrastructure are consistent with other salmonids, especially rainbow trout and brown trout.