Reflections on Marine Debris - Professor Steve Smith

Professor Steve SmithOne of the questions I am most commonly asked about my diving experiences is: “What do you see underwater?” Sadly, if I was to be truthful, the most obvious items, at an increasing number of dive sites, are litter! A burgeoning human population coupled with the increasingly disposable nature of so many of our everyday items means that litter-free dives are a comparative rarity.

As a collective, divers have been amazingly pro-active in attempting to combat the problem of marine debris, with many organisations and dive groups around the world promoting clean-up activities on nominated days (e.g. Project AWARE’s World Clean Up Day). These activities remove vast amounts of debris and are critical for maintaining the ecology and aesthetics of our underwater environment. With this rising tide of voluntary involvement in underwater clean-up activities, it is timely to review some key questions about the impacts of debris on benthic (bottom-dwelling) marine organisms and on appropriate clean-up measures.

The impacts are often quite stark. Take, for example, observations at a popular fishing spot in Hawaii revealing that the majority of corals had been killed by entanglement in monofilament fishing line. Surprisingly, despite the alarm bells that such observations should trigger, there is remarkably little global information on broader impacts of debris on the marine benthos. The few studies that have been conducted, however, echo the same trends – areas with high debris loads support lower biodiversity. The processes leading to this may be many and varied including: smothering; interference with feeding and foraging patterns; and loss or modification of important habitats.



Fishing line and soft coral


Divers gathering data on debris

In New South Wales (NSW) in Australia, groups of divers associated with voluntary underwater research groups (see are currently being encouraged to find out more about the distribution of marine debris and to clean up their own “back yards”. Using standardised sampling methods, these volunteers are progressively documenting and removing underwater litter from various sites along the vast (> 1000 km) NSW coastline.. These activities not only enable divers to look after their local sites, but also contribute to a broad-scale snapshot of debris loads along the entire NSW coast. This type of information provides invaluable guidance for long-term strategies to combat the problem.

Can marine debris ever be good for wildlife?

While documentation of debris loads is quite straightforward, its removal is not always quite so simple. For example, I have encountered Dive-masters at destinations such as Lembeh Strait (where litter is particularly prevalent) who were concerned that, in the process of assessing debris, I was also going remove it, depriving a range of weird and wonderful marine critters of critical habitat. While it is highly unlikely that the removal of plastic bags and a range of other litter items will affect these animals in any way other than positively, they certainly have a point with respect to items that are inert and can rapidly become habitat. Thus, a key question that needs to addressed before removing debris items during clean-up activities is - will removal be more detrimental to the habitat than leaving it in place?

Removing debris from reef habitats – some tips

Blenny in a bottlePlastics should always be removed. While plastic items do not biodegrade, most will progressively fragment over time. As particles get smaller, they may resemble food items and be ingested by a variety of species. Examples of the fatal consequences of ingestion are all too frequent for large animals such as birds, reptiles and mammals, but data are lacking for many other groups of marine animals. It is true that plastic bottles may provide temporary refuge for a range of organisms (pictured) but the long-term costs to the environment outweigh those short-term benefits.

Fishing line should always be removed but this needs to be done very carefully. Fishing line is usually found on the seafloor because it is entangled around the reef structure or on larger benthic organisms (e.g. corals, kelp, sponges, ascidians - picture). Unless removal is conducted with care, this process can often lead to injury or total detachment of organisms. The best way to approach this is by applying as little tension as possible to the line, and preferably cutting it into sections (blunt-ended diving shears are perfect for this) to work around entangled organisms.

Inert items such as glass bottles can either be left in place or removed depending on the site and the length of time the bottles have been there. Some sites are special because they are pristine – I usually remove bottles at these sites. Other sites, for example, those close to centres of urbanisation, maybe less pristine and so their aesthetic appeal may be relatively unaffected by the presence of bottles. As glass is inert, intact bottles pose little threat to marine organisms and may even enhance the abundance of some species.

Sabretooth BlennyFor example, on a recent dive, I encountered an unusually large abundance of Sabretooth Blennies (pictured below). On closer inspection, it became apparent that this was simply because there was a large number of old, discarded beer bottles in the area which were being used as de facto burrows. As a rule-of-thumb, I generally remove bottles that have been there for only a short time. However, once their surface becomes encrusted with marine growth, and they blend more readily into the natural habitat, I tend to leave them in place.

The ultimate solution for marine debris is to stop it at source - we can all help to spread this message and do our bit to reduce, recycle and reuse. We can also help to educate the wider community that an “out of sight out of mind” attitude to litter on the ocean floor doesn’t cut it for our important marine biodiversity, or for the long-term sustainability of our marine resources.