Hybridization between different fish species is frequently performed to enhance production in fish culture. Previous work has examined the feasibility and morphological characteristics of hybrids between common carp (Cyprinus carpio) and bighead carp (Hypophthalmichthys nobilis) primarily in pond culture. Due to the differences in chromosome numbers between common carp (n=100) and bighead carp (n=48), this cross has asymmetric viability only producing surviving offspring when common carp eggs are utilized. Recent investigations identified four different morphotypes and three different ploidy outcomes (diploid, triploid, and tetraploid) in 10 months old fish produced from a cross of these two species. The present work produced Two progenies to evaluate production metrics of the intergeneric hybrid of Common carp (Koi) x Bighead carp from early development up to 6 months old. A half-sibling Koi carp control group was reared in parallel to compare growth performance of the first hybrid progeny. The ploidy of fish produced were examined at 5 days post fertilization (dpf) using whole larvae and after 67 dpf from fin clips using flow cytometry.
The first progeny of Koi carp x Bighead carp was produced from 50 g of eggs from a single Koi carp fertilized with a mixture of sperm from 5 bighead carp males. Only swimming larvae (n = 260) were initially stocked to 10L tanks in four replicates at 6dpf. Survival (Table 1) was significantly lower compared to controls during the first two weeks of feeding with live prey items (Brachionus plicatilus and Artemia nauplii). Large size variability was observed at 14 days of feeding (dof), which led to each initial tank being split into small and large size categories before each was randomly restocked. Significant differences (Welch’s t-test) in weight (Table 1) were only observed for Koi carp and Koi carp x Bighead carp hybrids between small size category fish at 35 dof. Analysis of ploidy for larvae produced found that almost all larvae examined (n = 40; p = 0.976) were true hybrids parent (c-value = 1.41 ± 0.03 pg) with one chromosome set from each parent. A single larva (p = 0.024) was identified as triploid (c-value = 2.23 pg) having two chromosome sets of Koi carp (2n c-value = 1.75 pg) and a single bighead carp (2n c-value = 1.00 pg) chromosome set. Ploidy analysis of surviving fish at 56 dof identified a relationship between size of fish and whether they were true hybrids (0.84 ± 0.39 g) or triploid individuals (8.35 ± 6.24 g) with most fish which were classified as large being triploids and those categorized as small being true hybrids. The second progeny was produced several months after the first progeny and data are still being assessed. However, initial flow cytometry results of 5 dpf larvae (n = 30, p = 1.0) were similar with those from the first progeny with almost all larvae being true hybrids (c-value = 1.39 ± 0.02 pg).