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Study identifies water quality thresholds to protect fish during dredging

18 June 2018

Dredging operations worldwide are forecast to intensify in the future

A global study has assessed the potential risk from dredging to coastal fish and fisheries and identified guidelines that could protect 95 per cent of fishes from dredging‐induced mortality.

Dredging operations worldwide are forecast to intensify in the future to meet the demands of an increasing rate of coastal development and shipping activities and up to 20 per cent of fish species are likely to experience lethal and sub-lethal impacts as a result, according to results published in Conservation Letters.

The Western Australian Marine Science Institution Dredging Science Node brought together a team of researchers from universities and management agencies in Australia, led by Dr. Amelia Wenger at the University of Queensland, to develop evidence-based management guidelines to protect fish and fisheries from impacts associated with dredging.

The study found that more than 2,000 ports worldwide are within the range of at least one threatened species, while 97 ports are located within the range of five or more threatened species.

 

Figure 1: The global overlap between coastal ports and threatened marine fishes. The map shows the spatial distribution of threatened species, with the colors denoting the number of threatened species within particular areas. The black crosses indicate the presence of a port. The graph indicates the number of ports that fall within the geographic range of one or more threatened species (Wenger et al.)

 

It also determined that globally, between 2010 and 2014, 40.9 million tons of global commercial fisheries catch and 9.3 million tons of small-scale fisheries catch were extracted within five kilometres of a port, including many species known to be sensitive to sediment.

 

Figure 2: The spatial distribution and quantity of fishing activity that occurs within 5 km of a port. (a), (b) The location of commercial and small‐scale fishing activities and the quantity of catch in tons for each country where fishing activity occurs within 5 km of a port. (c) The countries where fishing of species known to be sensitive to sediment (see Table S3) occurs within 5 km of a port and the quantity of the catch. (d) The proportion of the fisheries catch of sediment‐sensitive species compared to the total fisheries catch that comes from within 5 km of a port for each country (Wenger et al.)

 

Dr Wenger said fish larvae were most likely to be affected by dredging sediment but that there were measures that could be taken to markedly increase the survival rate.

“While adult fish are unlikely to experience lethal impacts during dredging activities, we found that fish during early life history stages are at risk to lethal and sublethal impacts at suspended sediment concentrations and exposure durations regularly occurring during dredging operations,” she said.

“We found that maintaining suspended sediment concentrations below 44 mg/L  and for less than 24 hours would protect 95% of fishes from dredging‐induced mortality.

“Seasonal restrictions during peak periods of reproduction and recruitment could also protect species from dredging impacts,” Dr Wenger explained.

The thresholds developed in the study are considered to be a starting point for an adaptive management framework, to be used in conjunction with a monitoring program that evaluates the effectiveness of different management strategies at mitigating impacts to fish and fisheries.

 

Wenger A, Rawson C, Wilson S, Newman S, Travers M, Atkinson S, Browne N, Clarke D, Depczynski M, Erftemeijer P, Evans R, Hobbs JP, McIlwain J, McLean D, Saunders B, Harvey E (2018) Management strategies to minimize the dredging impacts of coastal development on fish and fisheries. Conservation Letters https://doi.org/10.1111/conl.12572

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.

 

Category: 
Dredging Science