Identifying the contribution of wild and hatchery Chinook salmon (Oncorhynchus tshawytscha) to the ocean fishery using otolith microstructure as natural tags

Abstract

Quantifying the contribution of wild (naturally spawned) and hatchery Chinook salmon (Oncorhynchus tshawytscha) to the mixed-stock ocean fishery is critical to understanding their relative importance to the persistence of salmon stocks. The inability to distinguish hatchery and wild salmon has inhibited the detection of declines or recoveries for many wild populations. By using Chinook salmon of known hatchery and wild origin, we established a baseline for separating these two sources using otolith microstructure. Otoliths of wild salmon contained a distinct exogenous feeding check likely reflecting an abrupt transition in food resources from maternal yolk not experienced by fish reared in hatcheries. Daily growth increments in otoliths from hatchery salmon immediately after the onset of exogenous feeding were wider and more uniform in width than those in wild fish. The discriminant function that we used to distinguish individuals reared in hatcheries or in the wild was robust between years (1999 and 2002), life history stages (juveniles and adults), and geographic regions (California, British Columbia, and Alaska) and classified fish with ~91% accuracy. Results from our mixed-stock model estimated that the contribution of wild fish was 10% ± 6%, indicating hatchery supplementation may be playing a larger role in supporting the central California coastal fishery than previously assumed.

Life history effects on hatchery contributions to ocean harvest and natural-area spawning

“…This is made readily apparent by comparisons of different metrics -the proportion hatchery origin among ocean catch (pHOC) and among natural-area spawners (pHOS) show opposite responses to hatchery cohort size, whereas measures of individual fish fates (the proportion straying (pS) or caught (pC)) and metrics of hatchery performance that are independent of the relative sizes of hatchery and natural populations such as catch per stray spawner (CPS) show no response. It seems important to note here that empirical measures of pHOC (e.g., Barnett-Johnson et al 2007;Kormos et al 2012) and pHOS (e.g., Johnson et al 2012;Kormos et al 2012;Hinrichsen et al 2016) are common in the literature, but we are not aware of estimates of the latter quantities (i.e., pS, pC, CPS). This suggests that consideration of empirical measures of the latter metrics may be helpful in informing hatchery managers on their success in trading off between competing goals, given the overall size of the hatchery program.…”

“…Because initial cohort size (N 1 ) and juvenile survival (p 1 ) only appear in our model as a product, we varied their joint outcome N 2 by applying a cohort ratio (f N = N 2,h :N 2,w ) varying from 0.1 to 8 to allow for a range of scenarios between a small hatchery program in conjunction with a small stock to a hatchery-dominated system where over 90% of fish are of hatchery origin (e.g., Barnett-Johnson et al 2007). We assumed that the maturation multiplier b applied equally to all ages, with the exception that no value of b a could exceed 1.0 (since maturation rates cannot exceed 100%) and b 5 was fixed at 1.0 (b x,h = min{b x,w , 1}).…”

“…Many studies track hatcheryorigin proportions in fisheries harvest (e.g., Zaugg et al 1983;Hilborn and Eggers 2000;Barnett-Johnson et al 2007) and among natural-area spawners (e.g., Seelbach and Whelan 1988;Kostow et al 2003;Nickelson 2003). Stray rates vary widely among natural and hatchery populations (Quinn 1993), and straying of hatchery fish has been implicated in wild salmon declines (e.g., Hilborn and Eggers 2000), but much remains to be learned about the interactive effects of hatchery practices and their relative influences on total production, on the proportions of hatchery-origin fish in ocean and instream harvests, and on the contribution of hatcheryorigin fish to natural-area reproduction.…”

Weakening portfolio effect strength in a hatchery-supplemented Chinook salmon population complex

“…In stark contrast with Bristol Bay's diverse and productive salmon fishery and other high latitude stocks with limited anthropogenic impact to freshwater habitat is California's Central Valley fall-run Chinook (CVC) salmon (Oncorhynchus tshawytscha) stock complex (Griffiths et al 2014). The Central Valley freshwater habitat is highly regulated and modified, and recent work suggests that the fall-run populations breeding in the different river systems are now genetically indistinguishable (Williamson and May 2005), in contrast with other studied salmon complexes that exhibit geographic structuring (e.g., Habicht et al 2007), and that hatchery-produced fish contribute the majority of total production (Barnett- Johnson et al 2007; Kormos et al 2012;Mohr and Satterthwaite 2013;Palmer-Zwahlen and Kormos 2013). …”

Stable isotope marking of otoliths during vaccination: a novel method for mass-marking fish

“…Fish identification markers, whether artificial or natural, are an essential tool for population-based ecological research, particularly for studies of population connectivity (Swearer et al 1999, Thorrold et al 2006, Almany et al 2007), stock identification (Campana 2005, Barnett-Johnson et al 2007, fish migratory patterns (Kalish 1990, Jones et al 1999, Kennedy et al 2002, Elsdon & Gillanders 2004, Walther & Limburg 2012 and stock discrimination (Adey et al 2009, Glover 2010. However, the reliability of a mark or marker-based data can be uncertain depending on the type of identification used.…”