Transplant/release captive-bred or hatchery-reared species - Transplant/release molluscs
Overall effectiveness category Likely to be beneficial
Number of studies: 7
Background information and definitions
Many populations of subtidal benthic invertebrate species have declined or been depleted due to the multiple threats they are under, such as habitat loss and overharvest (Airoldi et al. 2008; Hobday et al. 2000). To counteract this phenomenon, captive-bred or hatchery-reared subtidal benthic invertebrates can be transplanted or released at a site, either to introduce a species to a new site (where they did not historically occur), to reintroduce a species to a site (where they used to occur), or to enhance the population at a site where the species is already present by increasing its abundance (Hansen & Gosselin 2013). As the outcomes of transplanting/releasing species can vary largely with the type of species, studies have been grouped by broader taxonomic group (e.g: crustaceans such as lobsters or prawns; or molluscs such as abalone, scallops, or mussels).
Here, only direct transplanting/releasing methods have been considered, without added interventions or changes to the methods to improve survival after release. However, when transplant/release was undertaken in predator exclusion cages, evidence has been summarised under “Species management – Transplant/release captive-bred or hatchery-reared species in predator exclusion cages”.
When transplanting hatchery-reared individuals is undertaken for a habitat-forming species, effects on the invertebrates associated with the habitat are reported in “Habitat restoration and creation – Transplant captive-bred or hatchery-reared habitat-forming (biogenic) species”. Evidence from translocation studies of naturally occurring species is summarised under “Species management – Translocate species” and “Habitat restoration and creation – Translocate habitat-forming (biogenic) species”.
Airoldi L., Balata D. & Beck M.W. (2008) The gray zone: relationships between habitat loss and marine diversity and their applications in conservation. Journal of Experimental Marine Biology and Ecology, 366, 8–15.
Hansen S.C. & Gosselin L.A. (2013) Do predators, handling stress or field acclimation periods influence the survivorship of hatchery-reared abalone Haliotis kamtschatkana outplanted into natural habitats? Aquatic Conservation: Marine and Freshwater Ecosystems, 23, 246–253.
Hobday A.J., Tegner M.J. & Haaker P.L. (2000) Over-exploitation of a broadcast spawning marine invertebrate: decline of the white abalone. Reviews in Fish Biology and Fisheries, 10, 493–514.
Supporting evidence from individual studies
A replicated, before-and-after study in 1997–2001 in six sites of soft seabed in west-central Florida, Gulf of Mexico, USA (Wilbur et al. 2005) found that one year after transplanting hatchery-reared bay scallops Argopecten irradians to three depleted sites, populations of wild (not transplanted) bay scallops at the transplant sites and at three adjacent sites had not become genetically more similar to hatchery-reared scallops. A year after transplant, the frequency of wild bay scallops genetically similar to hatchery-reared ones (as number of haplotypes/sample) did not significantly increase in transplant sites (before: 0–3 in samples of 35–249; after: 0–5 in samples of 63–249), or across the region (transplant sites and adjacent sites combined – before: 5–12 in samples of 160–600; after: 13–23 in samples of 512–991). Between 1998 and 2000, hatchery-reared bay scallops (23,000–63,000/site; 20–30 mm in length) were transplanted in cages (50/site) within seagrass beds to three depleted sites during three transplant events (see study for details). Divers collected wild bay scallops (50–300/site; 40 mm in length) before and a year after each transplantation events at all transplant sites and at three adjacent sites (without transplants but benefitting from spill-over effect). Scallops were genetically assessed and compared to hatchery-reared scallops.Study and other actions tested
A replicated study in 2004 at four coral reef sites in the Singapore Strait (Guest et al. 2008) found that after being transplanted in the field aquarium-reared giant clams Tridacna squamosa had a high survival rate and grew over seven months. Of the 144 clams transplanted, 116 were recovered (80.6%), but survival rates differed amongst transplant sites (24–34 out of 36 transplanted clams per site). All recovered clams had increased in weight, shell length and shell height over the seven-month transplant, but rates differed amongst transplant sites (3.3–4.8 mm/month). In April 2004, a total of 144 aquarium-reared clams (eighteen-month old) were equally divided into 24 groups (6 clams/group) and transplanted into four sites (6 groups/site). Clams were released 50 cm above the seabed. Prior to transplant and after seven months, all clams were weighted, and their shell lengths and heights measured.Study and other actions tested
A replicated, controlled study in 2005–2006 in one area of muddy sandy seabed with in Northwest Harbor, North Atlantic Ocean, New York, USA (Tettelbach et al. 2011) found that over six months after transplanting hatchery-reared bay scallops Argopecten irradians irradians, abundance (indicating survival) decreased in plots where they were free-planted and in suspended bags, and that scallop growth and formation of shell biofouling varied with transplantation method. In both years, abundance of free-planted scallops decreased over time (2005: from 81–110/m2 to 18–37/m2; 2006: from 65–253/m2 to <1/m2). Authors report maximum mortalities of 0–1.5%. In both years, abundance of suspended scallops decreased over time (data presented on a logarithm scale), and typically did not vary with stocking densities (7 of 11 sampling dates/year; data not shown). Changes in abundances was not compared between transplanting methods. Transplanted scallops grew in both methods over 6.5 months but grew more in free-planted plots (2005: +20–21 mm; 2006: +30–33 mm) compared to suspended bags (2005: +13–14 mm; 2006: +21–22 mm). Growth rate of scallops in bags did not vary with stocking densities (data not shown). Over the 4.5 months after transplantation, free-planted scallops developed less biofouling than suspended scallops (2005: 0.62 vs 1.98 g/scallop; 2006: 0.91 vs 2.5 g/scallops; data extracted from the text). Two methods of transplantation were tested: free-planting and suspended bags. Free-planted scallops were distributed directly on the seabed in four 25 x 25 m plots at 1.3–3 m depth. Suspended scallops were placed in 36 floating units (2 m below the surface), each consisting of three bags of 50, 100 and 200 scallops/bag. Scallops were deployed in March/April of 2005 and 2006. From May–September/October scallop abundances were monitored monthly and growth was quantified biweekly. Monthly survival was estimated by counting live free-planted scallops in 12–16 quadrats (1 m2)/plot and counting live scallops/bag. Growth (shell height increase) was assessed for 20 scallops/methods/sampling date. In August 2005 and 2006, biofouling organisms growing on 156–159 scallop shells were scrapped and weighed.Study and other actions tested
Referenced paperTettelbach S.T., Barnes D., Aldred J., Rivara G., Bonal D., Weinstock A., Fitzsimons-Diaz C., Thiel J., Cammarota M.C., Stark A., Wejnert K., Ames R. & Carroll J. (2011) Utility of high-density plantings in bay scallop, Argopecten irradians irradians, restoration. Aquaculture International, 19, 715-739.
A replicated, controlled study in 2009 in one area of seabed of Vancouver Island, North Pacific Ocean, Canada (Hansen & Gosselin 2013) found that transplanting hatchery-reared northern abalone Haliotis kamtschatkana into the wild reduced survivorship after seven days. Survivorship was lower in transplanted abalones (average survivorship: 0.58) compared to non-transplanted hatchery-reared abalone kept in tanks (average survivorship: 0.97–0.99). In 2009 a total of 1,680 hatchery-raised abalone (4.2–6.5 cm shell length) were used in a project assessing the survivorship of transplanted abalone. Seven groups of 20 tagged abalone were transplanted onto the seabed at 10 m intervals (9 m water depth). Seven days after transplanting, surviving abalone were searched for and counted during circular surveys (5 m radius around each of the transplant locations). Seven groups of 140 abalone were kept in hatchery tanks (not transplanted) for comparison. After seven days, the number of surviving abalone in tanks was determined.Study and other actions tested
A replicated, before-and-after study in 2005–2010 of 23 sites across five areas of in Peconic Bays, North Atlantic Ocean, New York, USA (Tettelbach et al. 2013 - same expeimental set-up as Tettelbach et al. 2015) found that over four years after initiating transplantation of hatchery-reared bay scallop Argopecten irradians irradians, larval recruitment increased across all areas. Larval recruitment across all five areas was higher after restoration (2010: 29–118 spat/collector/day), compared to before (2005: 2–10 spat/collector/day), including two areas where no scallops had been transplanted, suggesting larval transport from restored sites to unrestored sites. A restoration programme aimed to increase scallop reproductive success was initiated in 2006 by transplanting several millions of hatchery-reared bay scallops in nets or directly on the seabed (100–200 scallops/m2; see paper for details). Larval recruitment was monitored at 23 sites across five embayments (three with transplanted scallops, two nearby without to assess larval transport) for 6 years: 2005–2006 (before intensive restoration) and 2007–2010 (after commencement of intensive restoration). Spat collectors were deployed (3/site) at 1–6 m average depth before 1st June to sample bay scallop larvae. A second set of collectors was deployed three weeks later. Every three weeks thereafter, a new set of collectors replaced those that had been in the water for six weeks. After retrieval, all scallops in the spat collectors were counted and shell heights were measured.Study and other actions tested
A replicated, before-and-after study in 2005–2012 in seven areas of seabed in Peconic Bays, North Atlantic Ocean, New York, USA (Tettelbach et al. 2015 - same expeimental set-up as Tettelbach et al. 2013) found that over five to six years after initiating transplantation of hatchery-reared bay scallop Argopecten irradians irradians, abundance of juvenile bay scallops typically increased, but not that of adult bay scallops. In five of seven areas (including one area where no scallops had been transplanted, suggesting larval transport from restored sites to unrestored sites), juvenile (<1-year-old) scallop abundance was higher after restoration (2011–2012: 0.07–2.8 scallops/m2), compared to before (2005: 0.002–0.08 scallops/m2). adult (>1-year-old) scallop abundance was statistically similar before (2005: 0.01–0.06 scallops/m2) and after transplantation (2011–2012: 0.004–0.2 scallops/m2 scallops/m2). A restoration programme aimed to increase scallop reproductive success was initiated in 2006 by transplanting several millions of hatchery-reared bay scallops in nets or directly on the seabed (100–200 scallops/m2; see paper for details). Juvenile and adult scallops were monitored annually in autumn at 23 sites across seven embayments (five with transplanted scallops, two nearby without to assess larval transport) for 8 years: 2005–2006 (before intensive restoration) and 2007–2012 (after commencement of intensive restoration). Divers counted all scallops within 2–4 transects (50 m2)/site.Study and other actions tested
Referenced paperTettelbach S., Peterson B., Carroll J., Furman B., Hughes S., Havelin J., Europe J., Bonal D., Weinstock A. & Smith C. (2015) Aspiring to an altered stable state: rebuilding of bay scallop populations and fisheries following intensive restoration. Marine Ecology Progress Series, 529, 121-136.
A replicated, randomized, controlled study in 2012 in one oyster reef area in Chesapeake Bay, North Atlantic Ocean, USA (Steppe et al. 2016a) found that restoring oyster reefs by releasing hatchery-reared larvae of Eastern oyster Crassostrea virginica using a direct setting technique resulted in higher average initial spat (young oyster) settlement (2.4–8.4 spat/shell) compared to using a traditional remote technique (0.6–4.6). In addition, using the direct technique 61% of the settled spat survived the winter, resulting in higher spat abundance at the restored site (189/m2) compared to an adjacent non-restored site (6/m2). No comparison of survival was made with spat released using the traditional remote technique. Larvae were released in summer 2012. Direct setting consisted of placing twelve trays (32 x 24 x 15 cm) filled with 30 oyster shells in one area of oyster reef at 2–3 m depth, and releasing approximately 2 x 106 hatchery-reared Eastern oyster larvae directly over it. Remote setting consisted of adding approximately 104 larvae to two tanks, each with six spat-collector bags (55 x 20 x 1.5 cm) containing 20 shells each. Three days after larval release, five shells/tray or /bag were retrieved, and the number of spat/shell counted. After winter 2012/2013, spat were counted on 20 shells/tray for the direct technique. Spat on nearby non-restored reef were counted in six 24 x 36 cm quadrats.
A replicated, randomized, controlled study in 2012 and 2013 of twelve plots (trays) in one oyster reef area in Chesapeake Bay, North Atlantic Ocean, USA (Steppe et al. 2016b) found that after release, hatchery-reared larvae of Eastern oyster Crassostrea virginica settled and survived equally on cleaned and natural oyster shells for a month. In 2012 and 2013, three days after release, initial spat settlement was similar on cleaned (2012: 8.4 spat/shell; 2013: 3.1) and natural shells (2012: 2.4; 2013: 4.9). After a month, the number of surviving spat was similar on cleaned (2012: 1.3; 2013: 2.9) and natural shells (2012: 1.0; 2013: 1.4). Twelve trays (32 x 24 x 15 cm) filled with 30 oyster shells were placed in one area of oyster reef at 2–3 m depth. Six contained cleaned shells, six contained natural shells. In summer 2012, 2 x 106 hatchery-reared Eastern oyster larvae were released using a direct setting technique. Shells were retrieved after 3 days (5/tray) and one month (20/tray), and the number of spat/shell counted. Shells were replaced afterwards. This was repeated in 2013.Study and other actions tested