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Providing evidence to improve practice

Action: Leave a fallow period during fish/shellfish farming Subtidal Benthic Invertebrate Conservation

Key messages

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  • Three studies examined the effects of leaving a fallow period during fish farming on subtidal benthic invertebrate populations. Two studies were in the Tasman Sea (Australia), and one in the North Pacific Ocean (USA).

 

COMMUNITY RESPONSE (3 STUDIES)

  • Overall community composition (2 study): Two replicated, before-and-after, site comparison study in the Tasman Sea found that after a fallow period invertebrate community composition became similar to that occurring before the fish were added but remained different to communities at sites without fish farms.
  • Worm community composition (1 study): One replicated, before-and-after, site comparison study in the North Pacific Ocean found that after a fallow period polychaete worm community composition changed but remained different to communities at sites without fish farms.
  • Worm richness/diversity (1 study): One replicated, before-and-after, site comparison study in the North Pacific Ocean found that after a fallow period polychaete worm diversity did not change and remained lower compared to sites without fish farms.

POPULATION RESPONSE (2 STUDIES)

  • Worm abundance (2 studies): Two replicated, before-and-after, site comparison studies in the Tasman Sea and the North Pacific Ocean found that following a fallow period, abundances of pollution-indicator polychaete worms decreased, but remained higher compared to sites without fish farms.

Supporting evidence from individual studies

1 

A replicated, before-and-after, site comparison study in 2003–2004 at two soft seabed locations in the Tasman Sea, southeastern Tasmania, Australia (Macleod et al. 2006 – same experimental set-up as Macleod et al. 2007) found that after a three-month fallow period, invertebrate community composition had changed at farmed sites. After the fallow period (no fish in cages), communities were different to that of the pre-fallow period (fish in cages), and similar to communities present before fish were added (empty cages). Community data were reported as statistical model results and graphical analyses. In addition, although similarity in invertebrate community between farmed sites and sites without fish farms (natural seabed) increased after fallow (from 25% to 31% similarity at one location, and from 11% to 27% at the other location), communities remained different. Sediment samples were collected using a grab (0.07 m2). At each of the two locations, five samples were collected at farmed and unfarmed sites before fish were added, following nine months of fish farming (pre-fallow period), and following the three-month fallow period. Invertebrates (>1 mm) were identified and counted. This was repeated over a second farming/fallowing cycle.

2 

A replicated, before-and-after, site comparison study in 2001–2003 at two soft seabed locations in the Tasman Sea, southeastern Tasmania, Australia (Macleod et al. 2007 – same experimental set-up as Macleod et al. 2006) found that after a three-month fallow period invertebrate community composition had changed at farmed sites. After the fallow period (no fish in cages), communities were different to that of the pre-fallow period (fish in cages), and similar to communities present before fish were added (empty cages), but not to that of nearby sites without fish farms (natural seabed). Community data were reported as statistical model results and graphical analyses). Although not tested for statistical significance, at one location, abundances of three pollution-indicator polychaete worms tended to be lower after the fallow period (Capitella capitata pre-fallow: 17,248 post-fallow: 2,621; Neanthes cricognatha pre-fallow: 199 post-fallow: 94; Maldanidae sp. pre-fallow: 54 post-fallow: 0 individuals/m2), but remained higher than at sites without fish farms (Capitella capitata 5; Neanthes cricognatha 4; Maldanidae sp. 0 individual/m2). At the second location, abundances of the opportunistic worms Capitella capitata and Nebalia longicornis tended to be lower following the fallow period (Capitella capitata pre-fallow: 7,470 post-fallow: 5,525; Nebalia longicornis: pre-fallow: 14,902 post-fallow: 1,791 individuals/m2) but remained higher than at sites without fish farms (Capitella capitata: 19; Nebalia longicornis: 0). Sediment samples were collected using a grab (0.07 m2) at 20 m depth. At each of the two locations, five samples were collected at farmed and unfarmed (located 150 m away) sites before fish were added, following nine months of fish farming (pre-fallow period), and following the three-month fallow period. Invertebrates (>1 mm) were identified and counted.

3 

A replicated, before-and-after, site comparison study in 2001–2007 in four sandy seabed locations off the coast of Hawai’i, North Pacific Ocean, USA (Lin & Bailey-Brock 2008) found that after a six-month fallow period polychaete worm diversity, abundances and community composition changed at farmed sites, but remained different from that of sites without fish farms. Community data were reported as statistical model results and graphical analyses. The cumulative relative abundance of three pollution-indicator worms, Capitella capitata, Neanthes arenaceodentata, and Ophryotrocha adherens, tended to be lower after the fallow period (5%), compared to before (70%), but remained higher than at sites without fish farms (0%) (results not statistically tested). Worm species diversity at farmed sites was not different at the end compared to the start of the fallow period, and remained lower than at sites without fish farms (data reported as a diversity index). Four aquaculture locations were surveyed, each with four farmed sites and two unfarmed. Sediment samples were collected 16 times between November 2001 and August 2006 (before the fallow period), twice during the fallow period (between August 2006 to March 2007), and once in May 2007 (after fish were restocked). Divers collected three to five sediment samples/collection/site using hand tube corers (11 cm diameter, to 5 cm depth) at 39–45 m depths. Polychaete worms (>0.5 mm) were identified and counted.

Referenced papers

Please cite as:

Lemasson, A.J., Pettit, L.R., Smith, R.K., and Sutherland, W.J. (2019) Subtidal Benthic Invertebrate Conservation: Global Evidence for the Effects of Interventions. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.