Life after precocious male maturation in a semelparous salmonid: A physiological perspective in Chinook salmon, Oncorhynchus tshawytscha.

Spring Chinook salmon, Oncorhynchus tshawytscha, are typically semelparous, dying after a single lifetime spawning event. However, observations of survival following maturation in males that mature at age-1 (common names: precocious parr or microjacks) have been documented, but little is known regarding their post-maturation physiology. In this investigation, mature microjacks were fed either a high or low winter ration to examine potential effects of feeding regime on mortality, smoltification and rematuration the following year. We measured a series of physiological factors including survival, size, percent whole body solid (an index of whole body lipid), and gill Na+/K+-ATPase activity (an index of smoltification). We also analyzed a series of maturation indices including plasma 11-ketotestosterone (11-KT), gonadosomatic index (GSI), pituitary follicle stimulating hormone (fshb) and luteinizing hormone (lhb) beta-subunit mRNA, and testicular histology in male fish following maturation as microjacks. In both treatments, the proportion of males that survived (High Ration: 0.91, Low Ration: 0.87) and rematured (High Ration: 1.0, Low Ration: 1.0) was very high, more indicative of iteroparity than semelparity. The low winter ration may have caused a slight delay in rematuration as evidenced by delayed fshb expression and 11-KT secretion in the spring. However, there was no evidence microjacks smolted in the spring after maturation at age-1. From a fisheries conservation perspective, the potential for iteroparity in microjacks (survival to remature at age-2) means it is possible they contribute genetically more than once when breeding in the natural environment.