Some marine resources, such as the wedge-clam D. trunculus, have shown great fluctuations in time and short- and long-term alterations have been generated. These variations have produced a decrease in the commercialization of the resources and, consequently, a decline in the socioeconomic conditions of the extractive fishery sector in Spain, and especially in Galicia. To increase the production of exhausted natural bivalve beds in a sustainable way several practices
are possible. One option is the introduction of adult individuals from other genetically similar areas where this species is abundant (Camara and Vadopalas, 2009). Repopulation could be a way to maintain exploitation of natural stocks and promote the development of the fishing sector.
Other possible and common action for restocking and enhancement of bivalve species is the release of seed obtained in breeding facilities or hatcheries (Camara and Vadopalas, 2009). In the case of D. trunculus, it has been recently demonstrated that there is a real possibility of obtaining seed in hatcheries for release in natural beds to reach commercial size (Louzán et al., 2016).
Furthermore, other measure should be implemented to relieve some fishing pressure on this species and apply appropriate mating schemes for the restocking programme (Miller et al., 2003).
In the face of the collapse of a fishery, while the object species recovers, a suitable strategy is the exploitation of a similar species. Such exploitation should be carried under appropriate management practises to avoid transferring the problem to the new species. As it was indicated in a previous study by our group, D. vittatus could be a potential alternative exploitable resource for the D. trunculus fisheries (Fernández-Pérez et al., 2017a). However, it must be seen as an important but temporary emergency measure to rescue and maintain genetically unique populations of D. trunculus and their variability until their natural bank can be restored.
In this way, the conservation and restoration of natural clam beds requires an intense genetic evaluation using several molecular markers to assess the risk of different strategies such as restocking programmes or the translocation of individuals from some areas to others. This study corroborates the fact that D. trunculus displays a considerable level of genetic diversity with population differentiation along the Iberian Peninsula. However, in this case, the F type mtDNA did not detect the differentiation of localities from the Alboran Sea (FU), in contrast to the M type and previous studies based on microsatellites. The characteristics of different molecular markers could explain the difference observed, maybe because M genome evolves typically faster, potentially allowing for insight into more recent events in population history (Śmietanka et al., 2013), as microsatellite markers do. This reason makes the M genome sequences more suitable to investigate the genetic structure in D. trunculus populations. From a conservation point of view,
the three confirmed genetically divergent groups, Atlantic Ocean, Alboran Sea and Mediterranean Sea, should be considered as different management units. This genetic differentiation should be taken into account in relation to restocking strategies to avoid putting at risk the genetic diversity of the species.
Genetic analysis plays an important role in fisheries management, helping identify stock resources and providing potentially useful management suggestions. Maintenance of genetic diversity in exploited populations is a main goal in conservation biology. In the short-term, genetic variation is relevant to the preservation of reproductive success and to population productivity, while in the long-term, it contributes to adaptive potential (Frankham et al., 2002; Allendorf et al., 2008).
Genetic differentiation among populations and correct delineation of marine species boundaries are also essential for species conservation and management (Avise, 1998; Sweijd et al., 2000) because these aspects provide information about the genetic exchange among them and their evolutionary history (Allendorf et al., 2013). Moreover, evaluation of population differentiation is fundamental to assess the genetic risk associated with restocking programmes (Ward, 2006) and translocation of individuals between natural populations (Johnson, 2000). Restocking programmes must consider the genetic structure of the species, since the introduction of genotypes that are not very representative of the natural population can have negative effects.
Therefore, individuals used as breeding stock in the hatchery should preferably come from the population to be repopulated, but where this is not feasible (when it is not possible to take broodstock from the stock to be enhanced or the local population is very small and suspected to be highly inbred), the most genetically similar population should be used (Ward, 2006). In this study, genetics data revealed three independent management units (i.e. Atlantic Ocean, Alboran Sea and Mediterranean Sea). Therefore, these three management units need separate management strategies, especially those relevant to translocation and propagation, and an effort to avoid homogenization should be made by managing each unit separately and restricting transfers of seed across units.
This study provided valuable information on the genetic structure of 18 localities across Iberian Peninsula of the wedge clam D. trunculus. The data can be useful to the sustainable management of wild stocks as well as in promoting successful restocking actions based on aquaculture production. Localities formed three groups that we suggest should be treated as separate management units. In addition, transfers of seed or broodstock across these units should be restricted.
Acknowledgements
We would like to thank Dra. S. Nóvoa from Centro de Cultivos Marinos de Ribadeo – CIMA (Xunta de Galicia) and Dr. M. B. Gaspar from Instituto Português do Mar e da Atmosfera – IPMA (Portugal) for providing specimens, and Dra. L. Martínez and K.M. Hein for her constructive and valuable comments. The authors wish to thank Professor John Baxter for helpful remarks and suggestions that improved the quality of the manuscript. We also thank two anonymous referees for useful comments during manuscript review. This work was supported by the Ministerio de Economía y Competitividad (Spain) through project AGL2016-75288-R AEI/FEDER, UE.