Size structure, age, and diets of introduced Chinook salmon (Oncorhynchus tshawytscha) inhabiting the Palena River, Chilean Patagonia

Chinook salmon (Oncorhynchus tshawytscha) were recorded in two extensive areas during a survey conducted between February 2012 and June 2013 to obtain information regarding the seasonal distribution and population structure of a self-sustained salmonid population inhabiting the Palena River. A total of 83 Chinook salmon were captured, which corresponded to 8.3% of the total salmonids collected in both areas. Juveniles of less than 15 cm were recorded in the upstream section of the river, from which fry-parr were less than 5 cm, and pre-smolt ranged between 5 and 10 cm aged 0+. Smolts displaying the characteristic silver coloration with an absence of parr-markings ranged between 10 and 15 cm and were aged 1+. The diet of the juvenile salmon was composed mainly of aquatic insects. Mature salmon returning from the ocean were collected in the middle section of the river, with weights ranging between 4.6 and 28 kg and their age ranged from 4+ to 6+ years in the ocean, plus 1+ year’s growth in freshwater (stream type ecotype). All the adult salmon were devoid of food. Immature adults, with ages between 3+ and 4+, were collected from a small lake (Claro Solar), displayed a piscivorous diet mainly based on native Galaxid species. Adult salmon returning from the ocean were infested with the cestode Hepatoxylon trichuri, attached to the pyloric caeca.