Abstract
There are over 3,000 swing moorings for boats in the Marlborough Sounds. Some seabed habitats are highly sensitive to any type of disturbance caused by swing moorings while others may be more resilient. Historically, some of Marlborough’s swing moorings are likely to have been placed over areas of the seabed that are sensitive to damage. Marlborough District Council (MDC) has identified a need for a review of the effects of swing moorings on different subtidal habitats, and for guidelines to manage effects.
Block-and-chain swing moorings, which represent the large majority of moorings currently present in the Marlborough Sounds, consist of an anchor (usually a concrete block) to which is attached a heavy-gauge ground chain, an intermediate chain and a top (‘riser’) rope, with a large surface float. MDC’s Mooring construction guidelines recommend that the length of chain used be equal to the depth of water at mean high water of spring tides, with one third of this chain to be ground chain. Conventional swing moorings can impact the seabed surrounding the anchor via the arc swept by the chain. In situations where a single mooring anchor is used, the mooring chain may be dragged repeatedly across the seabed through an arc of 360º around the anchor with changes in tidal movement and wind direction. Even in areas without conspicuous surface features, chain-scour will loosen sediments, making them more vulnerable to erosion and alteration of texture by water movement.
Habitats and species of particular ecological, cultural or conservation significance that are particularly sensitive to the effects of block-and-chain moorings include:
- rocky reefs and cobble fields (moorings are not likely to be located on these substrata but may be close enough that the reef is within the area swept by the chain)
- macroalgal beds (where these are growing on reef, moorings are not likely to be located within them but may be close enough that the bed is within the area swept by the chain)
- beds of rhodoliths, hydroids, bryozoans, shellfish, brachiopods, burrowing anemones or sea grass (eelgrass)
- sponge and bryozoan gardens
- tubeworm mounds, reefs and beds
- areas of shell hash (shell hash can provide important habitat diversity in soft sediments and chain sweep will enhance rates of breakdown of the hash)
- fish spawning and nursery areas not included in above (through direct destruction and through loss of structures to which eggs are attached and in which juveniles may shelter).
The simplest method for limiting adverse effects of swing moorings on significant seabed habitats and organisms is to restrict them to locations where they are absent, such as areas of muddy or sandy sediment. However, disturbance to these habitats will impact the animals living within the sediment and could have adverse ecological effects, such as loss of feeding areas for fish. It is also important to note that current absence of significant habitats or species from an area of seabed may reflect the effects of past activities, such as trawling or dredging, and that removal of all these sources of disturbance would allow recovery of valued ecological features.
Alternative types of mooring can be use that are designed to avoid damage to the surrounding seabed. These typically employ an elastic component in the mooring line that takes up slack in the line at low tide or calm conditions, preventing the line/chain from lying on the seabed. Several studies in Australia have shown that these designs can prevent damage to sea grass beds and, when they replace block-and-chain moorings, allow recovery of the habitat.
We suggest the following guidelines for assessing consents for moorings:
- No consents for new moorings in ecologically significant marine sites (ESMS) where the mooring will adversely affect the values on which the significance of the site is based.
- Existing moorings in ESMS to be removed or converted to environmentally friendly moorings where the mooring has adversely affected the values on which the significance of the site is based.
- Applications for all new consents or renewal of existing consents shall include a description of habitats in the vicinity of the mooring and identification of significant habitats or species present (to be documented with, for example, video or drop camera images).
- New consents in locations outside ESMS but where significant habitats or species are present shall require environmentally friendly moorings.
- Existing moorings located in areas with significant habitats or species nearby shall be converted to environmentally friendly moorings or removed if damage is occurring. This includes cases where a significant species or habitat, such as eelgrass or horse mussels, is present within the mooring field but outside of the areas of chain sweep, or in areas around the mooring field, when such species or habitats may be expected to recolonise the impacted areas if ground chains are removed.
- New consents to have moorings preferentially in areas of mud or sand seabed with no specific ecological, conservation or traditional value.
Application of these guidelines requires information on the nature of the seabed in the vicinity of the proposed (or existing) mooring and identification of significant habitats and organisms present. Guideline 3 is intended to ensure that suitable information is provided with the consent application to allow application of the other guidelines and this, and the second guideline, have already been used in recent consent approvals. MDC has recently mapped and classified benthic habitats in Queen Charlotte Sound and Tory Channel using ground-truthed multibeam sonar. This information will inform consent decisions in terms of the physical nature of the seabed environment.
We also recognise that, were larger-scale forms of disturbance (dredging, trawling, etc.) to be reduced in areas of soft-sediment seabed, mooring chain disturbance would appear relatively more important as a driver of seabed health.
Given the higher cost of environmentally friendly moorings (by a factor of two or more), demonstrations of their reliability and effectiveness, both in holding vessels and in protecting the seabed, is likely to be a significant factor in their uptake by vessel owners. Standards and quality-management systems for the manufacture and installation of moorings would provide confidence to vessel owners and insurers. MDC may also wish to consider facilitating demonstrations of potentially suitable moorings, in collaboration with mooring manufacturers and installers, vessel owners and other stakeholders.