Development of an Atlantic salmon (Salmo salar) genetic improvement program: Genetic parameters of harvest body weight and carcass quality traits estimated with animal models

Abstract

The Atlantic Salmon Broodstock Development Program is a partnership of researchers and producers that aims to breed salmon with an optimal combination of fast growth rate, low incidence of early sexual maturation, and good carcass quality for commercial aquaculture in Atlantic Canada. Estimation of genetic parameters for these traits is an essential step in the development of this breeding program. Four year classes of Atlantic salmon were produced, each consisting of 48–93 full-sib families. Marked fish from each of these families were randomly distributed to several producers to be raised under commercial conditions. At harvest, gutted body weights and sexual maturation level were observed in 812–3471 individuals per year class at processing plants. Additionally, colour score, astaxanthin, canthaxanthin, fat and moisture contents were recorded on 472 immature individuals from one year class. Genetic parameters were estimated with single- and multiple-trait animal models. Body weight, astaxanthin, canthaxanthin, colour, fat and moisture all exhibited moderate heritabilities (0.1–0.2), indicating that these traits should respond to selection. Positive genetic correlations were found between body weight and the carotenoid pigments, colour, and fat. These results indicate that direct selection for weight may have favourable indirect responses of higher colour scores and pigmentation but also an undesirable increase in flesh fat content. A properly weighted selection index should therefore be used to select broodstock for increased harvest weight and flesh colouration, while controlling flesh fat content.

Introduction

The Atlantic Salmon Broodstock Development Program (ASBDP) is a partnership of researchers and commercial producers based in St. Andrew's, New Brunswick, Canada that aims to develop a genetically-improved strain of Atlantic salmon for commercial aquaculture in Atlantic Canada. The goal of the breeding program is to develop salmon with an optimal combination of fast growth rate, good carcass quality, and low incidence of early sexual maturation.

Under current New Brunswick government regulations, only Atlantic salmon descended from the wild population of the Saint John River, New Brunswick can be cultured; therefore, genetic improvement of cultured salmon must rely on selection and breeding within this strain. Genetic parameters for traits in this population, and specifically for the ASBDP stock, must therefore be estimated to predict how the ASBDP stock will respond to various selection and breeding techniques.

Growth rate has generally been the trait of highest economic importance in meat livestock improvement programs and this is also the case in Atlantic salmon culture. Body weight at a fixed age has frequently been used as an indicator of growth rate in aquaculture and, additionally, body weight at harvest determines the price paid to growers by processors. Studies have generally found harvest-age (2–3.5 years) body weights in Atlantic salmon to have moderate heritability (Gunnes and Gjedrem, 1978, Gjerde and Gjedrem, 1984, Standal and Gjerde, 1987, Gjerde et al., 1994, Rye and Refstie, 1995) and are therefore suitable traits for improvement through genetic selection.

As the Atlantic salmon industry has expanded, meat quality traits have become of increased interest to producers. Flesh colour is considered to be an indicator of salmon freshness and quality and processors and retailers will downgrade or even reject product with insufficient colour (Nickell and Springate, 2001). The distinctive pink–red colour of salmonids is caused by carotenoid pigments in their diet. In current culture situations, pigments are usually added to feed in the form of astaxanthin (the most common form; Nickell and Springate, 2001) and canthaxanthin. These pigments are an expensive component of salmonid feeds so, to minimize costs, producers want animals that most efficiently absorb and retain these pigments in the flesh. Flesh carotenoid pigment content has thus also become a trait of interest for breeding programs. Family differences in flesh carotenoid pigment content have been observed in rainbow trout (Torrissen and Naevdal, 1984) and genetic parameters of carotenoid pigment contents were estimated in coho salmon fed a canthaxanthin-supplemented diet (Iwamoto et al., 1990). Flesh carotenoid pigment content genetic parameters have not been estimated in Atlantic salmon. Another carcass quality trait of increasing interest is flesh fat content because excessive fat content may have a detrimental effect on meat texture and also affect further processing; Gjedrem (1997) suggested that fat levels over 18% are too high. Genetic parameters of flesh colour and fat content have been previously estimated in Atlantic salmon (Gjerde and Gjedrem, 1984, Rye and Gjerde, 1996), coho salmon (Iwamoto et al., 1990, Neira et al., 2004), and rainbow trout (Gjerde and Schaeffer, 1989).

In the present study, data were collected on harvest body weight from four ASBDP year classes. Carcass quality parameters were also measured in one year class. The objective was to estimate heritability and genetic and phenotypic correlations among harvest weight and five carcass quality traits: colour score, flesh contents of carotenoid pigments astaxanthin and canthaxanthin, fat, and moisture. These genetic parameters should help predict how the ASBDP stock will respond to various selection techniques.