Fitness of hatchery-reared salmonids in the wild

Abstract

Accumulating data indicate that hatchery fish have lower fitness in natural environments than wild fish. This fitness decline can occur very quickly, sometimes following only one or two generations of captive rearing. In this review, we summarize existing data on the fitness of hatchery fish in the wild, and we investigate the conditions under which rapid fitness declines can occur. The summary of studies to date suggests: nonlocal hatchery stocks consistently reproduce very poorly in the wild; hatchery stocks that use wild, local fish for captive propagation generally perform better than nonlocal stocks, but often worse than wild fish. However, the data above are from a limited number of studies and species, and more studies are needed before one can generalize further. We used a simple quantitative genetic model to evaluate whether domestication selection is a sufficient explanation for some observed rapid fitness declines. We show that if selection acts on a single trait, such rapid effects can be explained only when selection is very strong, both in captivity and in the wild, and when the heritability of the trait under selection is high. If selection acts on multiple traits throughout the life cycle, rapid fitness declines are plausible.

Keywords: adaptation; captive breeding; conservation genetics; selection.

Figure 1

Illustration of the relative fitness comparisons made by Araki et al. (2007a, and the spatial and temporal opportunities for domestication selection to occur. Thin arrows indicate where a fish moves over the course of its lifecycle. The thick solid line illustrates where and when in the lifecycle selection could act to reduce the fitness of C[W × W] fish (captive progeny of two wild parents) compared to wild fish. The gray and dashed lines illustrate differences in the lifecycle (and hence opportunities for differential selection) of C[W × W] and C[C × W] fish, respectively. See text for details.

Figure 2

A quantitative genetic model of stabilizing selection after one generation of truncation due to domestication. The solid line represents phenotypic distribution of a quantitative trait at the equilibrium state under stabilizing selection (standard normal distribution), and the dotted line represents relative fitness of individuals with the corresponding trait values (x-axis) when ω² = 2 (variance of the adaptive landscape: ω² + 1 = 3. see Estes and Arnold 2007). Four different levels of truncation are considered and an arrowhead represents the truncation point (Truncation%) in each case such that all fish with trait values to the left of the arrows are viable. Selection differentials (S) after one generation of hatchery rearing and natural reproduction of hatchery fish are shown with three different levels of heritability (h² = 0.2, 0.5, 0.8).

Figure 3

Relationship between relative fitness (RF) and strength of natural selection (ω²). Small ω² represents strong selection in the wild. RF at different ω² was calculated from Eqn (3) and shown with four different levels of truncation and three different levels of heritability (A–C).