A critical reflection on the success of rearing and culturing juvenile freshwater mussels with a focus on the endangered freshwater pearl mussel (Margaritifera margaritifera L.)

Culturing programmes in North America

Over decades of research, the scientific basis of freshwater mussel culture in the USA was mainly developed by Richard Neves and co-workers at Virginia Tech (Freshwater Mollusk Conservation Center, FMCC). Methods for freshwater mussel rearing, especially with regard to technical solutions for intensive cultivation systems and propagation scale-up, were then developed further by universities (Beaty and Neves, 2004 at Virginia Tech; Barnhart, 2006 at Missouri State University) as well as several government organizations specializing in aquatic conservation such as the Aquatic Wildlife Conservation Center (AWCC) at Marion, Virginia. Today, numerous state and federal facilities in the USA are involved in propagation and culture of the most critically endangered mollusc fauna at regional scales (Gatenby et al., 2009) and new rearing methods are being developed further (Center for Mollusk Conservation in Frankfort, KY; Genoa National Fish Hatchery, WV).

Numerous rearing protocols and propagation techniques for more than 40 endangered mussel species have been developed, particularly in south-eastern North America, which is considered the most prominent hotspot of freshwater bivalve biodiversity (> 250 species; Neves, 2004; Barnhart, 2006; Gatenby et al., 2009). Since the end of the 1990s, more than 6 million juvenile mussels have been produced by the two major freshwater mussel research and rearing institutions AWCC and FMCC and more than 1 million specimens had been released into the wild by 2010. Almost all freshwater mussel culturing efforts have a core focus on conservation, but at least one species, the pink heelsplitter (Potamilus alatus), is also a target for pearl culture (Hua and Neves, 2007). Typically, conservation and propagation programmes are initiated after catastrophic population declines, e.g. caused by sewage or mining impacts on extant freshwater mussel populations. North American mussel cultivation systems can mostly be considered intensive (i.e. with permanent surveillance), with juveniles being propagated in laboratory cultures and bucket and/or raceway systems, partially with water recirculation. In most cases, cultured algae, commercial shellfish diets or re-circulated pond water is used for feeding. In contrast to European approaches, representative numbers of juvenile mussels (sub-adults, usually 1–2 years old) are tagged for monitoring purposes before their release into the wild. Consequently, information on survival and growth rates is available (e.g. see guidelines by Virginia Department of Game and Inland Fisheries, 2010), which is typically not the case for most releases in Europe. In parallel with propagation, translocation of adult mussels is considered an appropriate tool for population restoration in the USA.

Culturing programmes in Europe

In contrast to North America, all European efforts in freshwater mussel propagation and captive breeding techniques have exclusively focused on Margaritifera spp. because of the dramatic and accelerating decline of this genus throughout its distribution. To our knowledge, Table 2 considers all major past and present European freshwater pearl mussel (FPM) rearing activities and summarizes the different methods applied since Hruška's first approach (Hruška, 1992). In addition, it provides information on the FPM rearing success in terms of the overall number of juveniles produced and the survival rates based on both the available peer-reviewed literature as well as published technical reports and direct correspondence with the respective institutions in the period from January to May 2011. In the Czech Republic, Germany, the UK, Northern Ireland, and Luxembourg, a total number of about 80 000 pearl mussels have been reared, with an estimated release of 35 000 several-year-old specimens into the wild. New propagation projects have recently started in Spain, France, and Austria.

Since many European rearing and culturing programmes involve the direct release of young mussels (from early post-parasitic phase to a few years) into receiving streams without tagging, little is known about the success of these projects. Only recently, Wilson (2010) released a sample of tagged juveniles into the wild to document survival and growth. In many European countries, culturing programmes have not been in place for a long time and juvenile mussels are mostly still being kept in controlled systems.

Strategies of FPM glochidia harvesting and captive breeding

Generally, two different strategies can be distinguished with regard to the time-span of parental mussels kept outside their natural habitat and the manipulation and handling of mussels for harvesting viable glochidia and subsequent host fish infection.

The first approach follows the idea of maintaining adult mussels in their natural environment. For collection of glochidia, mussels need to be inspected to determine the time of completion of glochidial development. They are then only removed for a few minutes to hours when they are placed into small containers to influence glochidial release by temperature shock and/or oxygen depletion. The released mussel larvae (usually from several females) can be stored for a few days until infection of fish hosts takes place under controlled conditions in hatcheries (Taeubert et al., 2010). With slight modifications, this method has been carried out in the Czech Republic, Germany and Luxembourg (Hruška, 1999; Schmidt and Vandré, 2010; Thielen, pers. comm. 2011).

With the intention to safeguard the regionally most threatened populations from local extinction, the second strategy is fundamentally different from the first one in transferring either some or all of a mussel population permanently into captivity (so-called ‘ark’ projects). Typically, about 50 adult mussels of target populations are kept in gravel baskets within flow-through systems supplied with river or lake water (e.g. Freshwater Biological Association Station, Windermere, UK) or are transferred into artificial streams/raceways filled with gravel in indoor or outdoor hatchery facilities (e.g. Ballinderry River and Fish Hatchery, Northern Ireland). Infection of host fish species is achieved by maintaining host fish populations either directly within the mussel tanks or in the effluent water of the mussel containers. This approach is mainly used in the UK (Northern Ireland, Wales, England) and Ireland (Preston et al., 2007; Moorkens, 2011).

The next stage after completion of mussel larvae development, i.e. glochidia excystment and harvesting, is rearing of juvenile mussels, and this again involves two strategies differing in the intensity of mussel care involved (Table 2).

The original, and still the most successful method of FPM propagation in respect to the overall number of several-year-old juveniles reared, is the method developed by Hruška (1999, 2001). Starting in the 1980s, J. Hruška established methods for semi-natural breeding of FPM and studied the effect of temperature on reproduction, growth, and age structure of the population from the Blanice River in South Bohemia (Hruška, 1992). In brief, the Hruška method can be described as follows: (i) maintenance of infected host fish under controlled conditions; (ii) daily collection of freshly excysted juveniles; (iii) laboratory pre-culture in small containers without flow and feeding of organic detrital suspension; and (iv) transfer of juveniles into cages or containers adapted to mussel size and placed in natural rivers or semi-natural flow channels. By application of this relatively labour-intensive method, 30 000 juveniles more than 3 years old were produced in small wetland ditches (Hruška, 1999; Spisar, pers. comm. 2011).

The Hruška method has been successfully adopted by M. Lange from Saxony, Germany, with a few modifications to the initial diet in the laboratory pre-cultivation of juveniles for a few months (by adding animal protein components due to lower quality detritus to achieve high initial growth rates) but mainly by the use of mussel field cage systems as previously described by Buddensiek (1995). Subsequently, for cultivating juveniles older than 2 years, gravel boxes with 300 µm mesh gauze are used and placed upright in the free-flowing water of local streams. Approximately 35 000 several-year-old juveniles have been released (most of them in the Czech Republic and Saxony) up to 2010; thus the number of semi-naturally reared FPM probably exceeds the number of naturally recruited juveniles in Central Europe. Following the principles of the modified Hruška method, FPM rearing programmes started a few years ago in Luxembourg (EU Life Project) and in federal states of Germany (Bavaria, NRW). However, these projects have not been running for long enough to evaluate their overall rearing success.

In contrast to the relatively labour-intensive methods developed in the Czech Republic and Saxony, Preston et al. (2007) followed a low-intensity approach in Northern Ireland. The method is based on a semi-natural flow-through system supplied by river water with juvenile mussels excysting from the host fish directly into the gravel substratum of a hatchery raceway. Successful rearing of up to 13 000 individuals for single cohorts has been achieved. Approximately 700 juveniles ranging in age from 4 to 9 years were released during a pilot reintroduction study with about 6 500 juvenile mussels (from 1 to 9 years old) remaining in the vivarium raceway (Wilson, 2010). However, it is yet unclear whether this method can be scaled up for long-term propagation (Thomas et al., 2010).

Key factors for success

First, as indicated by the results of FPM propagation studies carried out so far, overall rearing success seems to be strongly determined by the survival rates during the first months following excystment, which appears to be the most critical phase. During this period and especially over the first winter, mortality rates are highest (Buddensiek, 1995; Schmidt and Vandré, 2010). Second, if outdoor sheet cages and sediment boxes are to be used, the following factors are considered crucial, based on the authors' practical experience and those of colleagues (Buddensiek, 1995, F. Elender, pers. comm. 2011): (i) positioning of field mussel cages; (ii) their regular maintenance to maintain exchange with ambient water; (iii) the temperature regime and food/detritus quality of the selected stream (Lange and Selheim, 2011); as well as (iv) the security and recovery rate of cages which strongly depends on the size and flow dynamics of the respective stream (Hastie and Young, 2003).

Initial growth and survival was found to be most crucial at the stage of freshly excysted mussels (Buddensiek, 1995) and often is not addressed sufficiently by studies and experiments carried out so far (Schmidt and Vandré, 2010). According to Lange and Selheim (2011) early growth to a minimum total shell length of about 1 mm can be considered a threshold minimum size for survival during the first winter. Based on practical experience, we assume that one of the main reasons for the lower rate of mortality and the higher rearing success in Saxony or the Czech Republic compared with the low success observed by Schmidt and Vandré (2010) in Bavaria is the use of an initial laboratory phase (Hruška, 1992). In the study by Schmidt and Vandré (2010), only freshly excysted juveniles were directly transferred into different types of experimental mussel field cages such as Buddensiek cages or sediment baskets without initial feeding of a detritus suspension. In contrast, the approach by Lange (2005) largely prevents young mussels in weak condition from being transferred into the sheet cages, which are usually highly susceptible to infection by fungi. At the same time, the proposed preparatory culture (i.e. generating young mussels of ~ 1 mm size) allows the earlier application of a gauze with larger mesh width (≥ 300 µm) already in the first field cage phase. This provides a more effective food and oxygen supply for the caged juvenile pearl mussels, leading to a direct beneficial influence on growth and survival.

As is evident from the rearing of juvenile pearl mussels over the last 10 years, the success or failure regarding the overall survival of 1+ FPM juveniles (after initial laboratory culture) depends largely on the experimental setup and on the exact positioning of sheet cages and gravel boxes in the stream flow, as well as their regular maintenance and cleaning. In the present trials, a fixed vertical position of sheet cages at a 90° angle to the flow direction resulted in the highest survival rates. In contrast to the approach by Schmidt and Vandré (2010), burying cages or boxes in the substratum was not performed. Our results show that the distribution patterns of older juveniles (> 3 summer periods) kept in gravel boxes indicate a preference for the gravel surface and the side-wall areas within the cages near the gauze (Figure 1). In these interface areas, detrimental effects of siltation are lowest and fresh water, oxygen, and food supply is highest. Indeed, stream siltation with suspended fine sediments is a fundamental problem for freshwater pearl mussels during their early life stages (Geist and Auerswald, 2007). In Saxony, Lower Bavaria, and Luxembourg, cages or boxes are cleaned weekly from the outside, removing any clogged material and fine sediments. These observations are also supported by McIvor and Aldridge (2008) who report that the high early survival (> 90%) of juveniles in the Welsh FPM rearing programme was probably the result of careful cleaning of the mesh and increasing the mesh sizes to maintain the flow of water past the juveniles. Therefore, technical maintenance of cages and boxes at regular intervals is essential for early survival of juveniles. Moreover, before placing juveniles into the cages, all types of boxes should be placed in the respective streams at least for 1 week (or longer) in order to allow the establishment of a biofilm.

In summary, seemingly minor details may determine the overall success for semi-natural propagation of freshwater mussels in different types of cage systems. In Saxony, about 4000 several-year-old mussels reared in Buddensiek sheet cages and gravel boxes (as described above) were released to the wild in 2009/2010 and viable individuals were found 1 year later at the point of release (M. Lange, pers. observation 2011).

Conservation and management decisions

‘Risk’ is the first crucial management decision that should be considered in the context of FPM rearing and conservation: for example, the risk of keeping the last remaining individuals of relict populations in degraded habitats or transferring them into captivity in order to establish ‘arks’; or the risk of releasing captive-bred juveniles into (still) unsuitable habitats. Geist (2010) discussed risk in the context of developing conservation strategies for the FPM by simultaneously considering genetic and ecological traits. Risk can be reduced by distributing infected host fishes and juvenile mussels to different laboratories and by a spread of juvenile mussels over different stream reaches or streams.

Further important decisions concern the input intensity of the rearing strategy. Whereas most freshwater mussel propagation programmes in North America are carried out in specific rearing facilities (e.g. former fish hatcheries), the propagation of Margaritifera in Europe is more diverse, ranging from intense culture (e.g. Luxembourg) to low-intensity and field-based approaches (Preston et al., 2007; Schmidt and Vandré, 2010). In contrast to the low-intensity approach followed by Preston et al. (2007), semi-natural captive breeding of M. margaritifera following the Hruška method is relatively labour intensive. However, captive breeding of juvenile FPM in mussel field cage systems provides essential baseline data and will contribute knowledge on factors that determine FPM habitat quality. Such information will also greatly enhance recovery programmes which depend on the identification and suitability of locations for the intended release of the reared mussels.

Ultimately, there remain considerable challenges in FPM rearing such as potential loss of fitness of parental mussels and juveniles reared in artificial environments, the lack of knowledge concerning the optimal starting diet (detritus, algae), the development of efficient rearing systems for long-term propagation, the genetic integrity of cultured mussels, and the maintenance of mussel cages in large quantities until juvenile release.