Existing modelling studies on shellfish Laboratoire Ressources Halieutiques IFREMER Port-en-Bessin, France
Worldwide production of cultured shellfish GENIMPACT February 2007
Main species and producers worldwide GENIMPACT February 2007
Main species and producers in the EU GENIMPACT February 2007
Good knowledge of the genetics of cultured shellfish stocks Quantitative genetics Life history traits during larval and juvenile stage (developmental rate, growth rate, survival; e.g. Crassostrea gigas and Mytilus edulis) Production traits during spat and adult stage (growth rate and survival; e.g. Crassostrea gigas, mussels of the Mytilus genus and scallops of the Argopecten genus) Disease resistance (e.g. Ostrea edulis) Genetic impacts of aquaculture Effect of inbreeding (e.g. Crassostrea gigas, Ostrea edulis, Pecten maximus ) Loss of neutral genetic variability in selected strains (e.g. Crassostrea virginica and Ostrea edulis) Neutral and quantitative genetic impact of rearing practices (e.g. Crassostrea gigas) Polyploidy Way of producing triploids Effect of polyploidy on somatic growth and reproduction Two noticeable gaps The potential loss in quantitative genetic variability due to selection and its implications for the potential for adaptation The potential fitness consequences of artificial selection and their implication for local adaptation
A few empirical studies focusing on genetic impacts of escapes The invasion of introduced non-native species and their effect on endogenous specie through competition (Crassostrea gigas), The hybridisation between non-native species and closely related endogenous species (Crassostrea genus, Mytilus genus and Pinctada genus), The differences in terms of local adaptation between cultured strains originating from different geographical locations (Crassostrea gigas), The loss of genetic structure and potentially of local adaptation in wild populations due to the translocation of individuals between different geographical locations for farming purposes (Pinctada margaritifera and Pecten maximus).
No modelling study on the genetic impact of escapes Strategy Identify specificities of shellfish as compared to finfish in terms of biology and farming practices Identify related pressing questions regarding the risk of genetic impacts of cultured stocks on wild populations in shellfish Review existing models of shellfish population dynamics and ecology Propose ways to further develop these models to include genetics and use them to assess the risk of genetic impacts of cultured shellfish stocks on wild populations
Shellfish specificities Biology: Dispersal can only occur during the pelagic larval stage since later stages are sessile influence on gene flow. Dispersal depends on 4 main determinants: Hydrodynamics, Developmental and survival rate of larvae (which in turn depend on temperature, salinity and food availability) Availability of suitable substrate for settlement when metamorphic stage reache Sexual determinism: most shellfish are hermaphrodites affect sex ratio and reproductive success Either protandric sequential hermaphrodite (male and then female) Or simultaneous hermaphrodite Mating: most shellfish are mass spawners with external fertilization high degree of gene mixing and spread in the population Farming Most of the production rely on natural settlement, shellfish aquaculture is almost fishing main effect of farming: modifying selective pressures experienced by juveniles and adults Most farming in the wild: massive introduction of hatchery produced individuals (among which selected strains and triploids) even if still low percentage compared to natural settlement high probability of interaction between cultured and wild populations (not just a few escapees)
Pressing questions in the case of shellfish The impact of the introduction of non-native species on local populations (e.g. Crassostrea genus, Pinctada genus and Mytilus genus) either through competitive exclusion of endogenous species or through hybridisation with closely related endogenous species The potential loss of local adaptation of wild populations due to changes in their genetic make up and fitness by hybridisation with unintentionally (domestication, changes of selective pressures due to farming) or intentionally (breeding program) selected strains The estimation of gene flow between wild populations and partially sterile triploids that are massively introduced in the wild as well as with potential escapes of tetraploids from hatcheries.
Existing modelling studies on shellfish population dynamics Deal with the interactions with the environment and the consequences on population dynamics in order to assess the carrying capacity of the ecosystem (typically a bay, an estuary or a lagoon) in terms of aquaculture production and to optimise rearing practices and management Most of these models based on three main modules: Module for the spatio-temporal dynamics of hydro-biological conditions: Hydrodynamics; Hydrological parameters: temperature, salinity, oxygen Biogeochemical cycle of nutrients: ammonium, phosphates, nitrates and silicates Primary production: micro-phytoplancton Module for the bioenergetics of shellfish which determine life history traits (growth, reproduction and survival) coupled with hydro-biological module through The effect of temperature, salinity and oxygen on physiology; Food availability (micro-phytoplankton) Module for the population dynamics: Scaling up on the bases of the life history traits emerging from the bioenergetic module.
Further development of existing models Existing models for polyculture could be used as a basis for modelling the risks of genetic impacts of cultured stocks on wild populations since modelling several species is rather similar to modelling several populations One strong limitation though is that current model do not consider closed life cycles: neglect reproduction and natural recruitment making the simplifying assumption that farmers determine seeding density Four components should be added to close the life cycle the reproductive processes at the individual level (sex determinism and fecundity) the probability of gametes encounter according to spatial location pelagic larval life history (developmental rate, growth rate and survival rate) dispersal and settlement locations The main difficulty lies in the probability of gamete encounter, dispersal and settlement The hydro-biological underlying module should allow to be able to predict these processes Recent developments have shown that it is possible to predict dispersal and geographical distribution of shellfish populations from such hydro-biological models
Modelling genetic impacts of escapes Models with closed life cycles coupled with a genetic module should be able to to model the interactions between cultured stocks and wild populations: gene flow mostly depends on: Dispersal which would be included thanks to the hydro-biological module Farming practices (translocation, introduction of polyploids, selective harvesting, etc.) which can be introduced as forcing variables However, some specific empirical knowledge is still missing to parameterise models in order to adress genetic impacts of cultured stocks on wild populations through modelling: genetic variability in terms of both neutral markers and quantitative traits within wild populations, although well documented for cultured stocks, differences in terms of life-history traits between wild populations and cultured stocks differences in terms of population dynamics and fitness life history traits of hybrids population dynamics and fitness Technical developments and the empirical limitations have to be tackled if any progress is to be expected.