Role of the Trunk Musculatures in Oviposition of Chum Salmon, Oncorhynchus keta

Abstract

An electromyographic study was conducted on the female chum salmon which engaged in natural spawning in a small tank. Electrodes were implanted in the dorsal, ventral, and dark portions of the lateral trunk musculature (LTM) of the fish. It was found that every site of recording gave a strong burst of potentials (500-800 μV) lasting several seconds (3.45-6.50 s), simultaneously with extrusion of eggs by the fish. This indicates that almost all LTM keep on contracting convulsively during oviposition. Presumably, the continuous and synchronous contractions of LTM produce an increase in the abdominal pressure, by which the ovulated eggs are squeezed out through the genital opening.
The muscular and neural mechanisms involved in oviposition of salmon are different from those in swimming, when short, alternative contractions of the left and the right LTM, instead of a long spasmodic, take place. It may be that in the higher CNS there is a center (or centers) which is superior to the spinal locomotive centers and functions only when the fish oviposits.