The Impact of Different Hatchery Rearing Environments on Smolt‐to‐Adult Survival of Spring Chinook Salmon

Abstract

The aim of this study was to assess the effect of differing hatchery rearing environments on smolting, early male maturation, and smolt-to-adult return rates (SARs) in Chinook Salmon Oncorhynchus tshawytscha. In this investigation, SARs were compared for spring Chinook Salmon from Hood River stock reared at three different hatcheries and then released into the Hood River, replicated over three separate years (2010)(2011)(2012). Differences in SARs were found both between rearing groups and release years. Smolts reared at Pelton Ladder had consistently higher SARs than those reared at either Parkdale or Carson hatcheries. Smolt length at release was positively correlated to SARs across years and rearing groups. In addition, multiple regression models suggested that attributes related to smolt quality were positively related to SARs while the minijack rate was negatively related to SAR. Migration of smolts downstream after release was also assessed. Larger smolts were faster downstream migrants and migration rate was positively related to subsequent SAR. Differences in smolt quality and SAR between rearing groups is attributed to differing patterns of growth in the different hatchery environments. The best-performing smolts came from rearing groups that experienced a strong seasonal change in growth rate: relatively high growth in the summer, reduced growth and depletion of energetic stores in the autumn and winter, and then increased growth again in the spring. This pattern of seasonal anabolic and catabolic changes has been termed the wild fish template, and generating this physiological pattern generally may be applicable to hatchery-reared Chinook Salmon.

Survival of prey growing through gape-limited predators

“…Theories have suggested fish need to reach a critical size by a particular life stage [33,34] while others have demonstrated the importance of early marine growth rates [31,35,36]. Furthermore, other studies suggest freshwater growth prior to migration affect adult survival [6,12,37,38]. The properties noted in these studies and others are intrinsic to the model.…”

“…For example, the SAR of hatchery reared Chinook salmon was found to be correlated with the environmental conditions prior to juvenile migration. The mechanism is thought to involve the physiological development of growth factors and smoltification prior to juvenile migration to the ocean [12]. These mechanisms might be readily incorporated into the model, and therefore into adult survival projections by linking the marine growth rate parameter to the early life physiological development.…”

“…Susceptibility to predation typically depends on the size of the prey relative to the gape size of the predators [4,5]. The mechanism, denoted as size-selective mortality [3,[6][7][8][9][10], involves the interplay of the predator and prey sizes and prey growth rate, which leads to larger fish surviving better than small ones to adult stages [11,12].…”

“…To illustrate the dramatic contrast of mortality with size consider the marine survival of spring Chinook salmon (Oncorhynchus tshawytscha) from Snake River hatcheries in the Columbia River Basin of the Pacific Northwest, USA. Using data extracted from [11], salmon enter the ocean at lengths of [10][11][12][13][14][15] cm and return as adults at lengths of up to a meter. The ratio of the number of adult in a cohort returning to the river to spawn to the number of the juveniles in the cohort that entered the ocean, denoted the smolt-to-adult-ratio (SAR), is on the order of 1% and the average mortality rate over the ocean residence is ~0.00002/d.…”

“…However, the process is complex. Salmon SAR characteristically increases with smolt length at marine entrance [11,12] but not in all populations [16][17][18]. In some Chinook salmon populations, size-selective mortality was detected during early ocean residence [14,15,19], but only after the first 30 days of marine residence in others [8] and in freshwater stages in yet other populations [6].…”