Appendix 1d - Jellyfish Study.pdf - Enviro Dynamics Namibia

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E N V I R O N M E N T A L I M P A C T A S S E S S M E N T R E P O R TD r e d g i n g o f m a r i n e p h o s p h a t e s f r o m M L 1 7 0J E L L Y F I S HStatus (+ of -)Significance(no mitigation)MitigationSignificance(with mitigation)Confidence levelNegative to individual jellyfish, possibly positive for fisheriesLowIn the case of blockage, jellyfish will have to be physically removed or flushedfrom the system.Sailing the vessel to areas with less dense aggregations of jellyfishForward looking sonar could be installed on the vessel to identify dense massesof sub-surface jellyfish during operations. A “jellyfish observer” on deck shouldbe able to identify jellyfish aggregations at the surface.Very LowHighHydrogen sulphide released from dredge sediments causing mortalities to jellyfish.Nature of theimpactExtentDurationIntensityProbabilityStatus (+ of -)Significance(no mitigation)MitigationSignificance(with mitigation)Confidence levelNature of theimpactExtentDurationIntensityProbabilityStatus (+ of -)SignificanceThe mining operation is located seaward of the mud belt where high levels ofhydrogen sulphide are known to be associated with soft sediments. Hydrogensulphide releases from the sediments in the Mining Licence Area (which is adjacentto, but not in the mud belt) are thus envisaged to be significantly less frequent andintense.Dredge EventExtremely short term: The duration is short (hours), related to the pulsed release ofhydrogen sulphide.Minor effects.Probable. In the event that the combination of adverse factors comes together atany one time, jellyfish moralities will occur.Negative to individual jellyfish, possibly positive for fisheriesLowNo mitigation is presentedLowHigh: Although there is no information on the tolerance of jellyfish to hydrogensulphide, they are unlikely to have special adaptations thereto. More research onthis is needed.Lean water overflow from the vessel generates a tailings plume of fine sedimentswhich settle out through and are dispersed in the water column. These finesediments if present in sufficient quantities may cause mortalities to jellyfish,though this is considered unlikelyMine site: < 25 km. It is understood that whilst dredging a sediment plume of~1500 m long and 800 m wide will be generated over the cut length of up to 22 km.This plume is determined to sink to the seabed over a distance of 500-1500m fromthe point of discharge. The maximum concentrations of sediments in the sedimentplume are envisaged to be
E N V I R O N M E N T A L I M P A C T A S S E S S M E N T R E P O R TD r e d g i n g o f m a r i n e p h o s p h a t e s f r o m M L 1 7 0J E L L Y F I S H(no mitigation)MitigationSignificance(with mitigation)Confidence levelNature of theimpactExtentDurationIntensityProbabilityStatus (+ of -)Significance(no mitigation)MitigationSignificance(with mitigation)Confidence levelNo mitigation is presentedVery LowLow – research on this is neededRemoval of seabed sediments will change the nature of the sediment surface.<strong>Jellyfish</strong> populations are known to increase in areas where there is an increase ofhard substrate. Typically this occurs where rock, concrete or iron structures areerected. The removal of the upper relative soft layers of sediment, leaving arelative hard clay footwall surface may provide such a hard surface.Annual Mining Area.Very Short term:Minor effectsRarePositive for jellyfish, negative for fisheriesLowNone: If between 10 - 15 % of the original thickness of the sediment is notrecovered, there will sufficient soft-substrata to preclude polyp settlement.Very LowHigh6 Mitigation and Monitoring Recommendations6.1 MITIGATIONMitigation in this context refers to the mitigation of risks to the operational activities, and not tojellyfish per se. Although a number of techniques exist to “repel” jellyfish, the exact techniquesemployed vary with the type of operational activity and none is 100% effective. It is important tonote that the techniques described briefly below have been deployed in shallow, near-shore areasand may not be appropriate at the operational depths considered here. And it should also berealized that almost all have been tailor-made to the particular situation by engineers: there is nooff-the-shelf method that fits all situations. It should also be mentioned that publishedinformation on this topic is hard to find, and that most sources consulted are from the internet(Internet (1) and (2)).Vertical nets, in conjunction with booms may be deployed around resort beaches, or partsthereof, to exclude jellyfish from entering bathing areas. Nets and booms are also used at theentrance to power-plant ponds and the offshore intakes for desalination plants (Azis et al. 2000),as well as around naval vessels (internet 1) to prevent jellyfish from clogging pumps etc. Typically,two sets of nets (an outer coarse mesh net system and an inner fine mesh system: internet 2) areeither suspended from surface floats and extend to close to the seafloor, or they are fastened topilings sunk into the seafloor (e.g. Marks and Cargo 1974). The problem with nets, as highlightedby the latter authors, is that they tend to clog and are subject to bio-fouling, which means thatwithout regular, routine maintenance (cleaning and repair) they will eventually either sink, or thehydrodynamic load on them will become so great that they will be ripped away. Regardless, inDraft Report<strong>Namibia</strong>n Marine Phosphate (Pty) Ltd.Page 20
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Appendix 1d - Jellyfish Study.pdf - Enviro Dynamics Namibia

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