Conservation implications of deformity and reproductive variance in the hatchery stock of greenback cutthroat trout

Authors: Love Stowell, Sierra M*, Kennedy, Chris M, Monroe, Evan G, Martin, Andrew P

Year: 2015 (xlvii)

Abstract

Hatchery propagation is a valuable tool for fish conservation. Rare or declining populations can be supplemented with hatchery-raised individuals to maintain both population size and genetic diversity. However, hatchery propagation can also exacerbate the genetic issues of small populations: initiating a hatchery broodstock imposes a genetic bottleneck and adaptation to hatchery conditions happens within as little as two generations, leading to a stock with decreased genetic diversity and fitness that will introduce maladaptive alleles if used for supplementation. A hatchery broodstock of greenback cutthroat trout (Oncorhynchus clarkii unnamed subspecies) was initiated in 2008 when 67 fish from the single remnant wild population were brought into the hatchery. Of those 67, 42 were spawned in 2010 to make the first generation of greenbacks maintained in the hatchery system. This broodstock has declined rapidly from 8,010 eyed eggs in 2010 to 1,757 fry in 2011, 784 fish in 2012, and 173 fish in 2015. Deformities such as missing eyes, reduced opercula, and extra &#34chins&#34 were common (65% of females, 70% of males) and correlated with smaller body size (length, 11mm shorter, p=1.17x10^-6; mass, 40 g smaller, p=3.3x10^-9); bilateral asymmetry using 20 landmarks was also significant. The rapid decline of the stock and the frequency of deformities are both high compared to other hatchery stocks of salmonids and are indicative of inbreeding depression. In the past, individuals were spawned 1 female : 1 male or 1 female : 2 males and then all eggs, fertilized or not, were pooled together. To assess variation in gamete production and fertilization success, we conducted 112 controlled crosses with 55 females and 91 males (spawned 1:1 or 1:2) and maintained each family group individually. Females had a wide range of fecundity (71-1572 eggs), egg mass (1.3-6.5mg), and lipid provisioning. Fertilization success measured by percent of eggs that developed eyes varied from 0 to 78% with a strong combinatorial effect (i.e. the same female could have or low fertilization success depending on the identity of the father). The variation in individual reproductive success has important implications for the genetic diversity of the stock as such variation lowers the effective number of breeds in the population. We are following the fate of the families through hatching and growth and testing for a relationship between the genetic relatedness of the parents and the fitness of the offspring. These data are being used to inform sustainable reintroduction efforts of greenback cutthroat trout into their native range and are illustrative for the management of small fish populations in other systems.