Impact of scallop dredging – Opito & Hahei surveys

Field experiments were conducted on 2 subtidal sandflats to identify the short-term impacts of commercial scallop dredging on macrobenthic communities. The two sites (Opito and Hahei) had similar physical and biological characteristics. Opito Bay is exploited by commercial scallop fishers, while the Hahei site is within a marine reserve. Purpose: To describe the impact of habitat disturbance by scallop dredging.

Sediment grain size, macrobenthic community, habitat type and densities of large epifauna observations.

Optio Bay and the Hahei marine reserve

Optio Bay – E2762382 N6496636, NZMG, 1:50000, Point marine reserve – Marine reserve – 2760722, 6482175, NZMG, 1:20,000, AREA

April – July 1991

Two representative sites (70 x 20 m) were selected based on local knowledge of scallop dredging activity, and for similarity in water depth and exposure. A transect line marked at 1m intervals was used to select random sampling locations. Prior to the experiment 15 random cores (10 cm diameter x 15 cm deep) were collected by SCUBA to describe the macrobenthic community and determine spatial variation within each site. Haphazard sediment samples (0-5 cm depth) were collected. A commercial box dredge was run through half of each study site to create a dredged plot and adjacent reference plot. At both sites core samples were collected by SCUBA within 2 hours of dredging and again 3 months later. On both these sampling occasions 12 core samples were collected from random locations in each of the dredged and reference plots. Observations of sediment conditions and epifauna were also made. Macrofauna were sieved through a 0.5 mm mesh and identified to the lowest practical taxonomic level. Data from Hahei and Opito were treated different statistically because the macrobenthic communities at each site were distinctly different. Frequency of collection: One off survey

The experimental assessment of the short-term effects of scallop dredging was quite conservative. Commercial fishers work over much larger areas and repeatedly dredge the same region of the seabed on any one fishing trip, hence resulting in a far higher level of bottom disturbance than that used in the experiment. Also, macrobenthic communities resident at the experimental sites were composed almost entirely of small, short-lived species. No assessment of large-scale changes. Gaps in collection: Not reported Data quality: Not reported Attribute accuracy: Not reported Completeness: Not reported

report, electronic Digital Format: MS Word, MS Excel

Fishing management

See Department of Conservation or NIWA Sensitivity/Confidentiality: See Department of Conservation or NIWA

Finished

Funded by DoC 3.12/518 and FRST CO1429. Additional comments: Small, short-lived species are less likely to show effects of fishing pressure than other community types.