Genetic improvement in coho salmon (Oncorhynchus kisutch). II: Selection response for early spawning date
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Phenotypic records of within-year spawning date in two cultivated populations of coho salmon, named as even (n=2272) and odd (n=1347) year classes are analyzed. Both populations were selected for harvest weight and early spawning for four generations, using independent culling levels. Hat-vest weight was selected using breeding values obtained from an animal model in males and females, while early spawning was selected phenotypically only in females. A post-selection analysis to estimate breeding values in both characters allowed comparison of the phenotypic response (R) and genetic selection response (GR) to selection for early spawning, narrow sense heritability (h(2)), and genetic Correlation with harvest weight (r(g)). Mean spawning dates were 13 and 15 days earlier after four generations of selection in the even and odd year class, respectively. This represents a phenotypic response to selection of -2.74 +/- 0.7 (P = 0.03) and -3.23 +/- 1.3 (P = 0.09) days per generation in the even and odd year classes, respectively. The heritability estimates, by regression of the selection differential on phenotypic change, were h(2) = 1.78 +/- 0.37 (P < 0.05) for the even year class and h(2) = 1.90 +/- 0.42 (P < 0.05) for the odd year class. In both cases, calculated values were greater than the highest possible value for heritability (h(2) = 1) which indicates that the response is overestimated due to a positive environmental effect which was not quantified. The estimates of narrow sense heritability using an animal model were high in both populations (even year class h(2) = 0.40 +/- 0.05, odd year class h(2) = 0.44 +/- 0.06). Breeding value analysis showed the genetic selection response was -0.62 and -1.13 days per generation in the even and odd year classes, respectively. The genetic correlation between spawning date and harvest weight was low in both year classes (even r(g) = 0.25 +/- 0.13; odd r(g) = -0.02 +/- 0.24). The genetic selection differentials (GS) were lower in males than in females (even year class GS(f) = -1.12 y GS(m) = -0.13; odd year class GS(f) = -1.46; GS(m) = -0.36), which reflects selection practiced only on females.