Understanding Smolt Survival Trends in Sockeye Salmon

Abstract

Many populations of Sockeye Salmon Oncorhynchus nerka in the eastern North Pacific Ocean experienced significant productivity declines that began about 1990, but there is no consensus on the mechanisms responsible. To better understand Sockeye Salmon survival trends, we examined the 50-year time series for two age-classes of Sockeye Salmon smolts from Chilko Lake in central British Columbia. Arranging survival time series for both age-classes by ocean entry year and combining them, weighted by a proxy model of sampling variance, reduced the sampling variance in the original age-1 smolt survivals sufficiently to indicate a linear trend of increasing survival from 1960 to 1990 that suddenly changed at or near 1991 to a lower and declining trend from 1992 to 2008. Neither density nor mean length influenced smolt survival. Returns in a given year were not good predictors of siblings returning in subsequent years. Time spent at sea increased linearly beginning around 1970. Although smolt survivals differed between ecosystem regimes, there was only the one clear pattern break about 1991. To improve our understanding of mechanisms, survival trends were compared with environmental indices that included catches and hatchery releases of potentially competing salmon from around the North Pacific Ocean. Smolt survivals were more similar to abundance indices of Sockeye Salmon, Chum Salmon O. keta, and Pink Salmon O. gorbuscha than to indices of global, regional, or local ocean climate. Our results are consistent with the hypothesis that salmon productivity in the North Pacific declined soon after 1990. We present a simple model to illustrate how increased competition at sea, related to the release of large numbers of hatchery salmon, in conjunction with changes in ocean productivity, may have played a significant role in improving Sockeye Salmon survivals while reducing their growth before 1991. After 1991, these factors may have acted to reduce survivals while the growth of survivors showed no effect.Anadromous Pacific salmon Oncorhynchus spp. comprise a multispecies complex of varying productivities. Their recent abundance in the Pacific Ocean, as reflected by commercial catch, is as high as it has ever been (Irvine and Fukuwaka 2011). Global abundances are driven primarily by Pink Salmon O. gorbuscha and Chum Salmon O. keta, as well as, particularly in the eastern North Pacific Ocean, by Sockeye Salmon O. nerka (Eggers 2009;Ruggerone et al. 2010;Irvine and Fukuwaka 2011). The status of Sockeye Salmon populations varies among regions however, and in British Columbia's Fraser River, low Subject editor: Suam Kim, Pukyong National University, Busan, South Korea *Corresponding author: james.irvine@dfo-mpo.gc.ca Received February 8, 2013; accepted July 24, 2013 numbers of returning salmon in recent years are a major concern (Grant et al. 2011;Rand et al. 2012).The Fraser River watershed is one of the world's greatest salmon producers (Northcote and Larkin 1989), although numbers returning annually are highly variable...

Marine growth patterns of southern British Columbia chum salmon explained by interactions between density-dependent competition and changing climate

“…Fukuwaka and Morita (2008) found evidence that following the closure of the Japanese high seas gillnet fishery, the age of maturation in Asian chum salmon increased. Efforts to reduce harvest on coho, pink, and sockeye salmon in North America (Irvine and Akenhead 2013;Irvine et al 2014) may have also resulted in increased body size within some commercial catches (Jeffrey et al 2017). However, because the age of maturation for many Pacific salmon populations has increased and even surpassed historical levels, it is less likely that fisheries-induced evolution has caused directional selection for smaller-sized individuals within these populations (Morita et al 2005).…”

“…populations (Pyper and Peterman 1999;Agler et al 2013;Jeffrey et al 2017), which are often accompanied with increased age-at-maturity (Bigler et al 1996;Morita et al 2005;Morita and Fukuwaka 2007). Relatively high abundances of salmon, especially during periods of low marine productivity, have been hypothesized to cause reductions in size-at-age and salmon survival (e.g., Irvine and Akenhead 2013;Yasumiishi et al 2016). Density-dependent competition for common prey resources is presumably greatest for the three most abundant species, chum (Oncorhynchus keta), pink (Oncorhynchus gorbuscha), and sockeye salmon (Oncorhynchus nerka), each of which is primarily planktivorous (Davis et al 2004;Kaeriyama et al 2004;Ruggerone et al 2012).…”

Habitat Manipulations Confound the Interpretation of Sockeye Salmon Recruitment Patterns at Chilko Lake, British Columbia

“…For example, eggto-smolt survival for sockeye salmon (Oncorhynchus nerka) from Chilko Lake (Fig. 1) is marked by a large increase starting with brood year 2004 (Irvine and Akenhead 2013) and brood years 2005 and 2006 produced record numbers of smolts despite average spawner abundance (Table 1). McKinnell et al (2012) remarked on "recent, unexplained high freshwater survival of the 2005 and 2006 brood years in Chilko Lake" as did Grant et al (2011) who noted "freshwater production has been exceptional in recent years; numbers of outmigrating smolts in the 2005 (77 million age-1 smolts) and 2006 (71 million age-1 smolts) brood years were well above average brood years: 20 million age-1 smolts)."…”

“…These fry swim upstream to Chilko Lake where they live and grow for, typically, one year before migrating to the ocean as smolts during a six-week period in the following April and May. About 4% of the fry stay in Chilko Lake for a second year and migrate as age-2 smolts (Irvine and Akenhead 2013). Age-3 smolts are rare; the fi rst recorded at Chilko Lake was in 2010 (brood 2006) despite many thousands of smolts examined annually since 1948.…”

“…A small fraction return as jacks, precocious males, after one marine winter (Healey et al 2000). Since 1970, an increasing fraction of adults have returned after three marine winters instead of two (Irvine and Akenhead 2013).…”