Fascinating new paper out reviewing the likely impacts that dredging may have on marine mammals. The paper by Dr. Victoria Todd and colleagues can be read here.
It is a great review of the effects both positive and negative that dredging can have on marine life. I like that is has done this as I would have gone all BAD, BAD, BAD! However there are some positives. The paper focuses mainly on marine dredging although it is important to remember it also occurs in rivers and lakes.
Start with the bad…
The paper delves into the direct and indirect impacts of dredging on marine mammals and the potential severity of those.
The direct impacts are interactions that cause physical injury to mortality. These include Noise Pollution, Turbidity and Collision.
Noise pollution occurs at lots of different levels and in areas that are already heavily trafficked. Our knowledge of the hearing range of marine mammals is still limited so we can not be sure of all the effects. However it is possible that the noise emitted could cause masking of marine mammal calls particularly cetaceans. I talk about this more in this blog post. The effects are likely to be over the short to medium term and occur as behavioural changes such as area avoidance and call masking. All this will occur concurrently with other industrial activities so it is hard to tease out the specific impacts of just dredging.
In regards to collision, the slow speed of active dredgers means that there is low risk of collisions taking place especially if managed well and avoids critical habitat and calving areas. The bigger risk would be when dredging vessels are in transit but if this is occurring in an already heavily transmitted area as tends to be the case for dredging this won’t significantly add to the risk already in place.
The seabed disturbance caused by dredging leads to increased turbidity and sediment suspension. However, many marine mammals are use to turbid environments and limited vision is not an issue even for species that do not use echolocation for prey detection.
Indirectly marine mammals can be affected by changes to their environment and also to prey availability due to dredging. The paper lists the possibilities as:
- Habitat degradation,
- Remobilization of contaminants,
- Sedimentation, and
- Increases in suspended sediment concentrations.
Entrainment is the removal of species from their environment and while no data is available on this impact on marine mammals it has been noted that it can affect their prey species by removing them and their eggs along with the sediment. Therefore along as dredging is restricted during important egg and larval stages of prey species then effects are unlikely to be detrimental.
The degradation of marine habitats is something that I would be really worried about as dredgers go about their process especially since “45 case studies worldwide found that 21 023 ha of seagrass beds were lost as a result of 26 dredging projects over a 50-year period”. Sirenians (e.g. manatees and dugongs) are entirely dependent on seagrass beds while other marine mammals utilise them for prey and they are an important habitat for prey species. However, from this review I am enlightened to the fact that due to mitigation measure the impact on seagrass habitats has been reduced and that as long as mitigation is followed and planning is conducted with care the effects are minimal.