© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Managing marine genetic diversity: time for action?
a Centre for Marine Biodiversity, Bedford Institute of Oceanography 1 Challenger Drive, PO Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2
b Institute of Marine Research PO Box 1870 Nordnes, N-5817 Bergen, Norway
c Adaptive Dynamics Network, International Institute for Applied Systems Analysis A-2361 Laxenburg, Austria
d Danish Institute for Fisheries Research, Department of Inland Fisheries Vejlsøvej 39, DK-8600 Silkeborg, Denmark
*Correspondence to E. Kenchington; tel: +1 902 426 2030; fax: +1 902 426 1862. e-mail: kenchingtone{at}mar.dfo-mpo.gc.ca.
Keywords: genetic diversity, management, monitoring, reference points, selection
Received 12 March 2003; accepted 28 May 2003.
The 1992 Convention on Biological Diversity has established an international framework for broader conservation objectives for the management of ocean use activities (cf. Sainsbury and Sumaila, 2003). The Convention calls for preservation of biological diversity, including genetic, species and ecosystem diversity, thereby creating a demand for developing management forms that can cope with this issue. A number of initiatives have been developed to address this and other related international agreements. Notably, Ecosystem-Based Fishery Management (EBFM; cf. Brodziak and Link, 2002; Sainsbury and Sumaila, 2003) has emerged as a holistic approach to maintaining ecosystems and sustainable fisheries. EBFM should incorporate all levels of diversity, but in practice has focused on species and ecosystem diversity (e.g., Brodziak and Link, 2002). The application of EBFM must be broadened to include conservation of genetic diversity, including intraspecific diversity, which are not necessarily protected by maintaining diversity at higher levels (cf. Kenchington, 2003).
Scientific justification for conserving genetic diversity stems from several sources including: (1) maintaining adaptability of natural populations; (2) the future utility of genetic resources for medical and other purposes; and (3) changes in life history traits and behaviour that influence the dynamics of fish populations, energy flows in the ecosystem, and ultimately, sustainable yield. The challenge is to formulate appropriate management actions for the preservation of genetic diversity (e.g., Sainsbury and Sumaila, 2003). This will require consensus on what it is we are trying to preserve (e.g., alleles, traits, population structure) and some means of assessing genetic "status". This paper endeavours to outline a process to develop management advice for marine genetic diversity. We challenge fish conservation geneticists to consider their work in a more applied context so that management actions can be developed to preserve genetic resources.
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We propose a three-phase approach to the development of this advice: (1) identification of management objectives, (2) definition of acceptable risk and/or identification of appropriate reference points (when possible), and (3) development of a monitoring program (Figure 1). Only when the management objectives are clear, can we recommend management actions, some of which may impose social and economic hardship.
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Management objectives
Any management regime requires clear management objectives that can be rendered operational. Considerations for defining management objectives for maintaining intraspecific genetic diversity include the following:
- Population structure;
- Genetic diversity among populations;
- Genetic diversity within population.
In examples from the literature, genetic diversity itself (e.g., number of alleles or genotypes) is not directly "managed", but the elements that influence it are. Thorpe et al. (1995) have suggested that the first priority should be to maintain populations in a natural setting to which adaptation may have occurred, and in which evolutionary forces continue to act. Taylor and Dizon (1999) describe two similar objectives used by the US Southwest Fisheries Science Centre: (i) maintain populations and (ii) maintain the full geographic range of a species. Both of these examples address Consideration (1) and to a certain extent Consideration (3), however, they do not directly address loss of genetic diversity within populations due to selective fishing (cf. Stokes et al., 1993; Conover, 2000; Law, 2000; Stokes and Law, 2000) or the relative abundance of populations. The latter is important in maintaining migration patterns (gene flow) and population structure, both potentially disrupted by exploitation. Table 1 provides examples of management objectives that match our list of considerations.
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With respect to genetic impoverishment caused by fisheries-induced selection, the options can be broken down further to slow/stop/reverse selection. It is necessary to specify which component of selection is being addressed, e.g., selection on maturation, sex, etc. Also, the management actions need to be specifically targeted if a reversal of selection pressure is desired, as opposed to slowing selection down. This may involve gear modification (e.g., mesh size) or spatial allocation of effort, so that the "unpacked" objective could become: stop fisheries-induced selection on age-at-maturation.
Reference points?
ICES defines reference points as "specific values of measurable properties of systems (biological, social, or economic) used as benchmarks for management and scientific advice" (ICES, 2001). Their purpose is to flag decision points, and therefore, the consequences of not taking an action at a particular reference point should be clear. Two types of reference points are distinguished. Target reference points are "properties of stocks/species/ecosystems which are considered to be desirable from the combined perspective of biological, social, and economic considerations" (ICES, 2001). Limit reference points are "a value of a property of a resource that, if violated, is taken as prima facie evidence of a conservation concern. By ‘conservation concern’, ICES means that there is unacceptable risk of serious or irreversible harm to the resource..." (ICES, 2001).
Are scientists in a position to formulate reference points addressing genetic diversity objectives? Here the first challenge is to understand the consequences of losing genetic diversity. For example, one of the difficulties with determining minimum acceptable levels of genetic diversity is that we do not know precisely what aspect of genetic variability will be important for a species to adapt to environmental change in the future. We can deduce that genes under selection (e.g., quantitative trait loci) will be important, however very few of these have been identified for any species. When phenotypic traits are used as a proxy of genetic diversity, it is easier to quantify the short-term outcome of following the management advice. Here, modelling – in particular, modelling that incorporates population and/or quantitative genetics – will probably have an important role in predicting the consequences of decisions. Yet model predictions may be sensitive to biological characteristics of a particular population and may not be easily generalized. However, we do know that specific actions will generally lead to a negative effect, and these can be avoided. For example, loss of populations will generally result in loss of genetic diversity, although we cannot say that losing 1 of 5 is acceptable but losing 2 is not.
For certain aspects of genetic diversity, target reference points can be established – at least from the biological perspective (Table 2). Here the biological target would be no loss of populations, however, this would need to be modified by social and economic considerations to produce the final target.
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Alleles are lost from populations through genetic drift and selection and gained through mutation and migration. A conservation concern arises when rate of allele loss exceeds rate of gain. While trying to establish limit reference points, it is important to acknowledge that loss of alleles from a species may represent an irreplaceable component of genetic diversity, at least within management timeframes of years and decades. The irrevocability of genetic loss combined with our inability to assess the consequences of not taking action, result in greater potential risks associated with any decision making process that allows for loss of diversity. Loss of alleles may qualify as a conservation concern if the risk is judged unacceptable, however determining the limits at which the resource is "harmed" will be problematic for the reasons discussed above. In this case the limit reference point may be very high and close to the target reference point, largely reflecting our ignorance.
Selection and genetic drift also influence allele frequencies. Although changes in allele frequency do not represent strictly irreplaceable loss, they still may be very difficult to reverse in practice. Thus, limit reference points are likely to have to be set very conservatively because the negative consequences of exceeding the limit reference point will be difficult if not impossible to subsequently rectify. Nevertheless, limit reference points could be defined for some objectives, especially those applicable to within population genetic diversity (Table 2): recent theoretical work suggests that successful breeding population sizes of 1000 to 5000 are required for long-term population viability (Lynch and Lande, 1998). If limit and/or target reference points can be established, genetic risk assessment (Currens and Busack, 1995; Allendorf et al., 1997 as discussed in Nielsen and Kenchington, 2001) may provide a framework for decision making in the light of uncertainty and consideration of other factors (e.g., biological, economic and social).
Monitoring genetic changes
Methods selected for monitoring genetic diversity will depend upon the management objective. An effective monitoring program requires three phases: identifying monitoring questions, identifying monitoring methods and the analysis and interpretation of information for integration into management strategies and the refinement of management objectives (Gaines et al., 1999). Examples of monitoring questions include: What is the genetic diversity within a population or among populations? How has habitat fragmentation affected the genetic structure of a population or species?
Once the questions are established, the monitoring methodology can be determined. This includes both sampling design and choice of genetic markers and phenotypic traits as well as consideration of derived indices. Genetic diversity can be measured at many different levels. Different types of markers or combinations of markers can be used to address specific questions related to the management objectives. Markers that are ideal for identifying population structure (e.g., neutral markers) are not useful for monitoring traits under selection. With the development of high-throughput equipment with low operating costs, genetic monitoring programs have become affordable. However, an important constraint on addressing monitoring questions is the lack of historical data. Even where tissue exists, it is often preserved such that the extraction of good quality DNA is difficult. It is recommended that tissue samples from research vessel survey catches be routinely archived.
In monitoring phenotypic traits, existing biological data from fisheries surveys are generally adequate to identify potential cases where fishing may have caused selection. However, in order to disentangle the genetic component of variation, direct environmental effects need to be accounted for. This requires either monitoring, in addition to phenotypic traits, the relevant environmental variables, or monitoring phenotypic traits that are robust to environmental variations. Reaction norms for age- and size-at-maturation are an example of the latter (e.g., Heino et al., 2002).
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Conclusions |
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Managing genetic diversity in marine populations requires serious attention. Evidence that fisheries-induced selection is causing genetic changes in fish stocks is currently unfolding, and the conventional wisdom that the large size of marine populations protect them from loss of genetic diversity is being challenged (e.g., Hauser et al., 2002). Furthermore, our views on population structure are changing rapidly, with previously unrecognized microgeographical structure being detected for classical marine species. It is clear that loss of genetic diversity, be it due to small effective population size or directional selection, can have consequences for renewable marine resources that are undesirable from the human perspective. These consequences range from loss of aesthetic or cultural values to potential loss of productivity.
Although the appreciation of conserving local populations is widespread in management of certain freshwater fisheries (i.e., migratory salmonid fishes), these ideas are yet to spread to management of marine resources. In many cases, management units do not reflect biological units (cf. Stephenson and Kenchington, 2000). We believe that the precautionary management of marine resources requires adding genetic diversity to the agenda. While the first steps of the process to develop management advice for preservation of genetic diversity have already been taken, this process is still very much at its infancy. Within the ICES framework, this process has been initiated in the Working Groups on the Application of Genetics in Fisheries and Mariculture (WGAGFM) and Ecosystem Effects of Fishing Activities (WGECO), and these working groups can provide a good forum for further developing the management framework for marine genetic diversity. We hope that, by clarifying the management needs, marine scientists in general and fish geneticists in particular will rise to the challenge of carrying their research through to the consideration of management implications of marine genetic diversity.
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Acknowledgements |
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We thank K. Brander, U. Dieckmann, O. R. Godø and the members of the WGECO and WGAGFM in 2002 for discussions on the issues formulated here. An early version of this paper was presented at the ICES Annual Science Conference, Copenhagen, 2002, by E. Kenchington (Kenchington and Heino, 2002). MH acknowledges the financial support from the Academy of Finland (Project 45928).
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