This week, we’ve handed the pen to Raewyn Peart, Policy Director at the Environmental Defence Society (EDS). Raewyn joined us at our recent conference as part of the Fit for Purpose Marine Protection panel discussion. While we don’t always see eye to eye with eNGOs, we greatly value the opportunities to come together and collaborate where it makes sense.
 

Let’s join forces to reduce sediment

By Raewyn Peart, Policy Director, Environmental Defence Society
 

Sediment is a pervasive killer of marine life. It destroys juvenile fish habitat. It smothers shellfish and other sedentary species. It clogs the feeding parts of filter feeders and the gills of fish. It reduces the ability of seaweeds and seagrass to photosynthesise. All these impacts reduce the productivity and resilience of fish stocks through reduced larval settlement, reduced juvenile survival and reduced food supply.

Some coastal areas are impacted by sediment more than others. For example, in the Marlborough Sounds, some 259,000 tonnes of suspended sediment is deposited into the Havelock estuary each year and carried out into Pelorus Sound by tidal currents. Around double this amount (509,000 tonnes) is deposited into the Bay of Islands annually and makes it way around to Te Rāwhiti Inlet.

To put this into some perspective, the sediment pouring into the Bay of Islands equates to 50,000 truck loads a year, or just under 1,000 loads a week. These volumes are dwarfed by the 1.9 million tonnes of mud that the Clutha River pours into Otago coastal waters each year. These are just three examples of the multiple places along the coast where sediment is causing damage.
 
Sediment is not deposited evenly throughout the year, but it mainly arrives in heavy plumes when storm events hit, making it even more deadly for marine life. Most people living on the coast have seen chocolate brown sediment plumes pour out of their local river or stream after heavy rain. This will only get worse in a climate changing world where we will see more storms and increased intensity of storm events.
 
The sources of sediment are multiple and arise from activities that remove vegetation and/or expose bare soil. They include road cuttings, earthworks, clear fell forestry harvesting, grazing of steep soils and leaving stream banks bare.
 
Some activities produce a disproportionate amount of silt for the land area occupied. For example, forestry harvesting was found to produce 18 per cent of the sediment deposited in the Havelock estuary whilst only accounting for 1.8 per cent of catchment land use. And this was an underestimate as subsoil erosion from slips and tracks within forestry blocks was not accounted for.

It's not just the type of land activity that is important, but whether water flow is moderated within the catchment, to enable sediment to settle out before reaching the marine area. This is where wetlands and floodplains perform a critical role. We have recently seen shocking pictures in the media of silt covered farms and houses when rivers have broken their banks after weather events. Where stopbanks held, this wall of silt ended up in the marine area, and not on the land.
 
Most wetlands have been drained and many rivers have been straightened and confined to narrow beds by stop banks. Although this frees up land for production, and provides some protection of surrounding land from flooding, it also serves to more quickly channel sediment into the marine area instead of depositing it on land.
 
The commercial fishing and environmental sectors have often ended up in conflict over marine issues. But surely this is something we can both agree on – sediment is bad for fisheries and is bad for the marine environment. So, let’s join forces to do something about it.

How might we do this? First, we could identify a place where sediment is directly impacting fish stocks. We could then jointly develop solutions and jointly promote their implementation. A coalition between the seafood and environmental sectors would get attention. It would help to highlight the issue and to mobilise action. Let’s roll our sleeves up, join forces and get on with it.

Read about the impact of sediment on marine areas and EDS' oceans reform case studies