Deborah J Martin-Robichaud

The relationship between sperm density, spermatocrit, sperm motility and fertilization success in Atlantic halibut, Hippoglossus hippoglossus

Abstract The commercialization of Atlantic halibut, Hippoglossus hippoglossus , aquaculture has been hampered by a failure to obtain consistently high fertilization rates. The principal goal of this research was to determine the optimum sperm density for successful fertilization of Atlantic halibut eggs. Sperm densities ranged from 2×10 11 to 6×10 11 spermatozoa/ml at 23% and 99% spermatocrit, respectively, for 36 milt samples collected from 17 males. Regression analysis showed a significant positive linear relationship between sperm density and spermatocrit, supporting the use of spermatocrit as a rapid estimator of sperm density in this species. There was no relationship between sperm density and sperm motility (defined as time elapsed from activation until 8 to 9.4×10 5 spermatozoa/egg, respectively.

Application of DNA markers to the management of Atlantic halibut (Hippoglossus hippoglossus) broodstock

Abstract For many aquaculture finfish species, the current broodstock have been collected from the wild or have undergone only a few generations of domestication. The Atlantic halibut ( Hippoglossus hippoglossus ) aquaculture industry in Atlantic Canada has retained F 1 juveniles ( n =145) from the 1996 spawning of wild adults for candidate broodstock. Through the development and use of a five-microsatellite DNA marker multiplex, we determined the parentage of these 1996 F1 individuals, which are being reared at one government and two industry hatcheries, and evaluated the change in genetic variation between the wild and the 1996 F 1 stock. In the three groups of F 1 fish, single parental pairs were assigned to 98%, 96% and 100% of individuals. Large full- and half-sibling groups were identified within and across F 1 groups and, overall, only 36% of attempted crosses were represented in the retained fish. Effective population size in the parental group decreased from 27 to 13 when variance in family size was accounted for and to 12.5 when changes in gene diversity (compared to the combined F 1 stocks) were considered. Statistically significant differences in measures of genetic variation were not widely observed between groups (original wild sample, parental group, three F 1 groups and combined F 1 ). However, the F 1 population shows a 26% decrease in total allele numbers compared to the wild sample. These observations demonstrate the utility of genetic tools in the evaluation of genetic diversity and determination of pedigree during the establishment of new broodstock. They also emphasize the necessity for closely monitoring future matings among these fish and suggest the need to introduce additional genetic variation into this group of Atlantic halibut broodstock.

Hormonal sex reversal of Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract Atlantic halibut (Hippoglossus hippoglossus L.) females grow larger and mature later than males, making monosex (all-female) culture economically advantageous. The objective of this research was to develop methods for the production of monosex populations of halibut through the use of sex steroids. Six hundred postlarval halibut, mean size 30.0 mm fork length (FL), were divided evenly among eight tanks and fed a formulated diet supplemented with either 17α-methyldihydrotestosterone (MDHT; 1 and 5 ppm) or 17β-estradiol (E2; 10 ppm) for 45 days. Fish were then further reared on a standard halibut diet until sex ratios could be determined through histological and macroscopic observations. The MDHT treatments yielded 97–100% males, while the E2 treatment yielded a 70–74% female population. This demonstrates that the direct feminization of halibut can be achieved, and also allows for the evaluation of indirect feminization through the crossing of hormonally masculinized females (“neomales”) to normal females.

GONADAL DIFFERENTIATION IN ATLANTIC COD, GADUS MORHUA L., AND HADDOCK, MELANOGRAMMUS AEGLEFINUS (L.)

The aquaculture industry in North Atlantic nations has experienced rapid growth in recent decades. However, in both northern Europe and Canada this industry remains largely dependent on the production of Atlantic salmon, Salmo salar L., a species for which overproduction and global competition have decreased the market value. On Canada’s east coast, the farming of Atlantic salmon is also limited by seawater site availability because of lethally low winter temperatures throughout most of the region. Salmon farmers in eastern Canada and northern Europe are currently looking to alternative coldwater species, such as the Atlantic cod, Gadus morhua L.; haddock, Melanogrammus aeglefinus (L.); and Atlantic halibut, Hippoglossus hippoglossus (L.), to maintain the value of their industry and to expand the range of possible farm sites. Due to the increased market demand for fish products and the diminishing supply available from traditional fisheries, these species remain good choices for commercial aquaculture in the North Atlantic. It has been suggested that in 15–20 years, cultured Atlantic cod may reach production levels similar to Atlantic salmon (Rosenlund and Skretting 2006). However, there are still some challenges, which need to be addressed if gadoid farming is to be a success. One of the primary constraints to commercially viable production of farmed Atlantic cod and haddock is early (pre-harvest) sexual maturation (Kjesbu et al. 2006). The flesh quality of farmed fish is affected when sexual maturation begins, as protein and lipid energy stores are transferred from the muscle to the developing gonads. The fact that both male and female Atlantic cod and haddock mature before harvesting is of added concern because spawning can occur in sea cages, leading to the release of offspring from domesticated stocks into the environment where they may have negative effects on local populations (Bekkevold et al. 2006, Jørstad et al. 2008). The production of monosex stocks would eliminate successful fertilization of gametes in open cages. Furthermore, it can be combined with triploidy induction to produce sterile populations of female triploids (e.g., Felip et al. 2001), but to date only mixed-sex populations of triploid cod have been produced (Peruzzi et al. 2007, Trippel et al. 2008). ACTA ICHTHYOLOGICA ET PISCATORIA (2008) 38 (2): 127–133 DOI: 10.3750/AIP2008.38.2.07

Freeze susceptibility in haddock (Melanogrammus aeglefinus)

Haddock (Melanogrammus aeglefinus) is a promising new species for aquaculture in Eastern Canada. Because haddock aquaculture could involve overwintering of fish in sea cages in icy near-shore areas, haddock freeze resistance was studied. Measurements were done on the blood of wild haddock collected during the winter from Georges and St. Pierre Banks as well as from cultured haddock originating from Northeast Bank in the Bay of Fundy. Freezing points were within the normal range for teleost blood, suggesting no freeze resistance adaptations. Glycerol in blood samples from Georges Bank and from most cultured haddock was not above normal physiological levels whereas St. Pierre Bank and Sandy Cove fish showed elevated glycerol levels. However, glycerol in the latter group was far lower than amounts required to depress the blood freezing point to a safe level in icy seawater. Thermal hysteresis measurements revealed no antifreeze protein (AFP) activity in any of the samples. However, ice crystal morphology revealed small amounts of AFP in a haddock sample collected in sub-zero seawater. The haddock antifreeze differed from the antifreeze glycoproteins (AFGPs) of other gadids in that it was active in the presence of peanut lectin and inactivated by EDTA. Neither the glycerol levels nor the trace level of AFP shown here would be sufficient to protect this species from freezing in icy seawater during winter.