Hand harvest instead of using a dredge

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

45%
Certainty

18%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

Dredging, for instance for bivalves, normally involves towing a heavy steel frame along the seabed, which negatively impacts subtidal benthic invertebrates due to direct physical damage, and changes to the seabed structure and topography (Currie & Parry 1996). Hand harvesting, (for instance using hand-pushed rakes, hand-dredge, dip nets, harvesting-knife, or direct manual harvesting) instead of dredging can have fewer negative impacts on subtidal benthic invertebrates and the surrounding seabed (Bishop et al. 2005; Leitão & Gaspar 2007; Narvarte et al. 2011).

_{Bishop M., Peterson C.H., Summerson H.C. & Gaskill D. (2005) Effects of harvesting methods on sustainability of a bay scallop fishery: dredging uproots seagrass and displaces recruits. Fishery Bulletin, 103, 712–719.}

_{Currie D.R. & Parry G.D. (1996) Effects of scallop dredging on a soft sediment community: a large-scale experimental study. Marine Ecology Progress Series, 134, 131–150.}

_{Leitão F.M.S. & Gaspar M.B. (2007) Immediate effect of intertidal non-mechanised cockle harvesting on macrobenthic communities: a comparative study. Scientia Marina, 71, 723–733.}

_{Narvarte M. González R. Medina A. & Avaca M.S. (2011) Artisanal dredges as efficient and rationale harvesting gears in a Patagonian mussel fishery. Fisheries Research, 111, 108–115.}

Study locations

Key messages

Two studies examined the effects of hand harvesting instead of using a dredge on subtidal benthic invertebrate populations. Both were in San Matías Gulf, South Atlantic Ocean (Argentina).

COMMUNITY RESPONSE (2 STUDIES)

Unwanted catch community composition (1 study): One replicated, controlled study in San Matías Gulf found that, when harvesting mussels, the community composition of the unwanted catch was similar by hand harvesting and by using a dredge.
Unwanted catch richness/diversity (1 study): One replicated, controlled study in San Matías Gulf found that, when harvesting mussels, hand harvesting caught fewer species of unwanted catch compared to using a dredge.

POPULATION RESPONSE (2 STUDIES)

Unwanted catch abundance (1 study): One replicated, controlled study in San Matías Gulf found that, when harvesting mussels, hand harvesting caught fewer unwanted sea urchins and brittle stars compared to using a dredge.
Unwanted catch condition (1 study): One replicated, controlled study in San Matías Gulf found that, when harvesting mussels, the damage caused to unwanted sea urchins and brittle stars was similar by hand harvesting and by using a dredge.

OTHER 1 STUDY)

Commercial catch abundance (1 study): One replicated, controlled study in San Matías Gulf found that more commercially targeted mussels were caught by hand harvesting than by using a dredge.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A replicated, controlled study in 2007 on a mussel bed in the San Matías Gulf, South Atlantic Ocean, Argentina (Navarte et al. 2011 - same experimental set-up as Navarte et al. 2012) found that hand-harvesting mussels caught fewer unwanted species including fewer unwanted sea urchins and brittle stars than with standard artisanal dredges. In total, hand-harvesting caught 27 species of unwanted catch, while dredging caught 47. The percentages of unwanted sea urchins Arbacia dufresnei and brittle stars Ophioploccus januarii caught (% by numbers of total catch) were lower by hand-harvesting (sea urchins: 2%; brittle stars: 32%) than dredging (sea urchins: 9%, brittle stars: 68%). More commercially targeted mussels were caught by hand-harvesting (76%) than the dredge (57% of total catch). Nineteen tows (5 min duration) were conducted in May 2007 on the mussel bed at 14–20 m depth with a standard artisanal dredge (1.6 m mouth width, 80 mm net bag mesh size). Four 40 kg commercial bags of catch hand-harvested by divers in the same area were obtained for comparison. All species were sorted (mussels; unwanted catch), counted, weighed and identified. Average proportions of mussels and unwanted catch (mostly invertebrates) were estimated for each sample. Apart from mussels, sea urchins and brittle stars dominated all samples.
Study and other actions tested
Referenced paper
Narvarte M., González R., Medina A. & Avaca M.S. (2011) Artisanal dredges as efficient and rationale harvesting gears in a Patagonian mussel fishery. Fisheries Research, 111, 108-115.
A replicated, controlled study in 2007 on a mussel bed in the San Matías Gulf, South Atlantic Ocean, Argentina (Navarte et al. 2012 - same experimental set-up as Navarte et al. 2011) found that hand-harvesting mussels caught a similar community composition of unwanted catch, and damaged similar numbers of unwanted sea urchins or brittle stars, compared to standard artisanal dredges. The percentages of total sea urchins Arbacia dufresnei and brittle stars Ophioploccus januarii that were damaged (lightly or severely) were similar by hand-harvesting (sea urchins: 67%; brittle stars: 65%) and dredging (sea urchins: 76%, brittle stars: 75%). Nineteen tows (5 min duration) were conducted in May 2007 on the mussel bed at 14–20 m depth with a standard artisanal dredge (1.6 m mouth width, 80 mm net bag mesh size). Four 40 kg commercial bags of catch hand-harvested by divers in the same area were obtained for comparison. All species were sorted (mussels; unwanted catch), counted, weighed and identified. Average proportions of mussels and unwanted catch (mostly invertebrates) were estimated for each sample. Apart from mussels, sea urchins and brittle stars dominated all samples, and were placed into damage categories: undamaged, lightly damaged or severely damaged (combined under ‘damaged’).
Study and other actions tested
Referenced paper
Narvarte M., González R., Medina A., Avaca M.S., Ginsberg S. & Aliotta S. (2012) Short term impact of artisanal dredges in a Patagonian mussel fishery: comparisons with commercial diving and control sites. Marine Environmental Research, 73, 53-61.

Please cite as:

Lemasson, A.J., Pettit, L.R., Smith, R.K. & Sutherland, W.J. (2020) Subtidal Benthic Invertebrate Conservation. Pages 635-732 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Subtidal Benthic Invertebrate Conservation

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