Genetic management of Atlantic cod (Gadus morhua L.) hatchery populations.

Abstract

Intensive aquaculture of Atlantic cod is fast developing in both Northern Europe and Canada. The last six years have seen major improvements in the larval rearing protocols and husbandry techniques for this species. Although breeding programmes are currently being developed by both governmental and private institutions in the main cod producing countries (i.e. Norway, Iceland and Canada), most hatcheries still rely on the mass spawning of their own broodstock. Mass spawning tanks are complex systems where fish are left to spawn naturally and fertilised eggs are collected with the overflowing water, with little or no control over the matings of the animals. Few published studies in other commercial marine species (i.e. turbot and sole) have attempted to analyse the output from such systems using microsatellite markers and several parentage analysis software programs. A review of these publications exposed a lack of consistency in the methods used to analyse such complex datasets. This problem was addressed by carrying out a detailed comparison of two analytical principals (i.e. assignment by strict exclusion and assignment by probabilities) and four parentage software programmes (i.e. FAP, VITASSIGN, CERVUS and PAPA), using the DNA profiles, at 5 loci, from 300 cod fry issued from the mass spawning of a large hatchery cod broodstock tank (consisting of 99 fish). This study revealed large discrepancies in the allocation outcomes between exclusion-based and probability-based assignments caused by the important rate of typing errors present in the dataset. Out of the four softwares tested, FAP (Taggart, 2007) was the most appropriate to use for handling such a dataset. It combined the most conservative method of assignment with the most informative output for the results displayed. In an attempt to study the breeding dynamics in a cod commercial hatchery, parental contributions to five groups of 300 fry (from five single days of spawning and from two commercial mass spawning cod tanks) were analysed, based on the genotyping data from eight loci. The parentage results from the exclusion-based analyses revealed that, on a single day, at least 25 to 30% of the total breeding population contributed to fertilised eggs that resulted in viable offspring at 50 and 83 days post-hatch. Family representations were highly skewed - with the marked dominance of a few males - and effective breeding populations were consistently low (approx. 5% of the total breeding population). Parental contribution to a group of 960 codlings - produced following intensive commercial practices (i.e. including successive size gradings and mixing of batches) and belonging to a single graded group - was also analysed, based on the genotyping data from eleven loci. The effective breeding population size of the juvenile batch (c. 14% of the total broodstock population) was two to three times greater than the effective size observed on a single day of mass spawning. The per-generation rate of inbreeding was however relatively high, for this batch alone, at 2.5%. Based on these results, suggestions were made to manage hatchery cod broodstock populations and implement genetic selection. Early maturation of farmed cod in sea cages (at two or three years old) is a major concern for ongrowers. Understanding the mechanism(s) behind sex determination in cod would probably help the development of a method to control sexual maturation. In an attempt to elucidate sex determination in cod, a protocol to induce gynogenesis was developed. Gynogenetic fish were successfully produced by irradiating cod milt with UV and applying a cold shock (at -6oC) to newly fertilised eggs. However, due to poor survival during larval rearing, only one gynogenetic fish survived long enough to be sexed; not enough to conclude anything on the sex determination mechanism(s) in cod.