Broodstock contribution after mass spawning and size grading in barramundi (Lates calcarifer, Bloch)

Appropriately designed selective breeding programs are needed to limit the loss of genetic diversity and control levels of inbreeding, and to base selection decisions on data collected from many offspring of many families. Achieving a relatively even contribution by broodstock to subsequent generations is necessary and for many aquaculture species this is possible to control through mate pairing. Barramundi (Lates calcarifer) provides an exception, because it is a species that mass spawn in small groups and whose offspring are repeatedly size graded in an effort to avoid cannibalism. Following mass spawning a large broodstock group of 33 barramundi, levels of parental contribution and multiple measures of genetic diversity were estimated over the course of repeated size grading events. Parentage was inferred using 17 microsatellite DNA loci. Twelve dams and twenty-one sires were artificially spawned over two nights and sampled at 1, 18 and 90 days post hatch (dph). Broodstock contributions were skewed and the contribution by individual dams and sires was as high as 48 and 16% respectively at 1 dph. Despite the unequal contribution and high variance in family sizes, 31 broodstock were detected as contributing to the spawning events and as a result up to 103 full-sibling families were detected (18 dph, n = 472). A reduction in allelic richness (Ar) was identified from broodstock to offspring at 1 dph, (Ar was 3.94 among broodstock and 3.52 among offspring sampled). However, no further loss of Ar or genetic diversity was detected in the offspring from 1 to 90 dph, which included the period of metamorphosis, multiple size grading events and losses through size culling, mortalities and the sale of juveniles. The effective census population size ratio (Ne/Nc) ranged from 0.31 to 0.51 at times of sampling, (Ne was calculated between 10.1 and 16.7, well below the broodstock census size of 33) and the rate of inbreeding was less than 5%. This research provides valuable baseline data that can be used to make recommendations for the maintenance of genetic diversity and control of inbreeding for a barramundi selective breeding program. It also provides an example of what considerations need to be made for the genetic management of mass spawning and/or cannibalistic species