Why we need a healthy seabed

Dr Sally Campbell is particularly interested in the sustainability of complex ecosystems and discusses why a healthy seabed is vital for the long term productivity of the marine environment.

All over the Clyde estuary there has been a collapse in white fish stocks and a great reduction in sea trout populations. As this has happened so the sport of sea angling and the commercial tourism industry associated with it have been hard hit. The bottom trawling for Nephrops and dredging for scallops have been destructive to the bottom sediments and the habitats which they support. These sediments of many types support the creatures at the bottom of the food chain which are often the bait for fish.

Infaunal species are those animals that live in the sediment, whilst epifaunal species are those that live on the surface of the seabed. Collectively this system is known as the benthos. Many epifaunal species are active and may move considerable distances. For this reason environmental monitoring often targets the infauna, as their presence at a site means that conditions are suitable for them over long periods. Most environmental monitoring uses benthic invertebrates, which include a wide range of polychaete worms, gastropod molluscs, bivalve molluscs, and various crustaceans.

Dredged Seabed showing destruction of Benthos.

COAST (Community of Arran Seabed Trust) was founded 14 years ago. The collapse of the annual sea angling festival, due to collapse in fish stocks, was a starting point. The vision for sustainable bioproductivity in a protected Lamlash Bay has never wavered in all that time, despite the difficulties of persuading politicians and others that short-termism will not work when we think of the complexity and long term productivity of the marine environment.

Benthos in Lamlash Bay©

A No Take Zone, the first in Scotland, came to fruition with the legal declaration of a No Take Zone in part of Lamlash Bay under Scottish law in 2008. The aim is to protect the benthos as nursery areas for fish and many marine invertebrates. Lamlash Bay also has maerl beds, a small red calcareous seaweed which is an ideal habitat as a nursery bed.


Lamlash Bay, Isle of Arran©

Some of the most common epifauna are hydroids. They do not survive with dredging and bottom trawling as they are quite delicate in construction. Colonies of hydroids are typically 5 to 500 mm (0.2 to 20 inches) or more high and are branched; the branches bear the individuals, or zooids (hydroid polyps). Most hydroids inhabit marine environments, although many of you may remember Hydra a solitary rather than colonial polyp, from biology sessions in school. There are about 2000 species of hydroids.

It had already been realised that the benthos was important in the development of productive ecosystems. Many years ago, I had the good fortune to spend 3 weeks at Biologische Anstalt, a superb marine research station on Helgoland. This small island lies at the mouth of the Elbe in north Germany.

The research project was to look at different marine deposits, their characteristics, epifauna and infauna and so in Helgoland we looked at such seemingly mundane things as sand, gravel and mud grain size, water storing capacity, flushing and oxygen transfer from samples taken in and around the island and Elbe estuary. In addition the size and quantity of the infauna was measured by means of different mesh size of sieves, and the specimens preserved for identification. It was my first experience with infauna and I was amazed at the number of species, variety in size and the sheer density of these to be found in these microenvironments and the complexity of ecological systems. Many juvenile fish were found in these sediments.                                                                                                                                                         Hydroids©

It is hardly surprising therefore that dredging and deep water trawling, by damaging and destroying epifauna and infauna, strike at the heart of ecological sustainability. The establishment of a dense epifaunal community depends on the habitat not being subject to excessive disturbance. Scallop dredging (for both Pecten maximus and Aequipecten opercularis) and other types of bottom fishing reduce habitat complexity by impacting sessile epifauna species, and by extension their associated organisms, and is now well documented (e.g. Bradshaw et al. 2000, 2001).

Feather Starfish and Tunicates©               Maerl Beds in Lamlash Bay©

During the spring and early summer there is the peak growing / breeding season for many marine species. The warmer waters and increased phytoplankton create ideal conditions for reproduction and growth. Given that epifaunal hydroids are an important settlement substrate for scallop spat, no take zones (NTZs) create environments where spat settle and grow undisturbed especially in the early stages of their development. In NTZs, such as in Lamlash Bay (closed areas to dredging and bottom trawling), spat will have a greater density of hydroids on which to settle, and will be able to grow undisturbed for the whole of their lives. This is undoubtedly one reason why, 11 years after the closure of an area during certain seasons in the Isle of Man, scallops in the closed area are found in greater densities and are on average larger than their counterparts off Bradda Head, also the Isle of Man, which had no closure (Bradshaw et al. 2001).

Four sand and gravel extraction areas in the North Sea and English Channel were studied over a three-year period to determine the effect of differing levels of historic dredging activity on the nature of epifaunal recolonisation. (Smith et al. 2006). Diversity and abundance of epifaunal assemblages were generally lower at intensively dredged areas in comparison to those observed at nearby reference locations.

Sheppard (2006) reports that in many cases of dredging the destroyed, dominant colonies are centuries rather than mere decades old, and grew on antecedents which may be thousands of years old. Vast expanses of these have already been destroyed. The loss of the three-dimensional habitat that they provide to countless other species is likely to be as important as the loss of those framework species themselves, which means also that the unrecorded ‘by-catch’ and unseen ‘discard’ must be orders of magnitude greater than the targeted catch in terms of both biomass and diversity.

Kelp Beds and Juvenile Fish©

The maerl beds in Lamlash Bay are the remnants of vast tracks of this fragile calcareous seaweed in the Clyde, and are thought to have been hundreds of years old in development.

Within the NTZ in Lamlash Bay, the research being carried out by Scottish Natural Heritage may begin to show that there is epifauna improvement in density and variety over time, and this in turn may provide an ideal environment for spat to settle and juvenile fish to grow.. Although only a small area, lack of disturbance is the key to regeneration. Not just for hydroids and through them scallops, but also as nursery beds for white fish and many other marine species.


Tube-dwelling Fan Worms©                Maerl and Cushion Starfish©                   

Work being done in California (Wethey et al. 2008) shows that marine sedimentary infauna alter the substrate, creating voids and air bubbles, manipulating grain and shell distributions, moving interstitial fluid and creating surface roughness elements. Worms and other burrowers continually change the characteristic of the sediments. They create permability and oxygen flow. The effect of pumping by Arenicola marina, Abarenicola pacifica, and Macoma nasuta affects the nutrient flux in sediments differing in permeability. So even though out of sight the infauna are an important part of the complex ecosystem that sustains epifauna on the sediment surface and create desirable habitats for juvenile growth..

These results are often at the microlevel yet of extreme importance to the productivity of the inshore waters. It seems surprising therefore in the face of such well researched science that there is little control over dredging for scallops and bottom trawling for Nephrops in the Clyde. It is clear that sea angling has taken the hit. Many groups are calling for the return of the three mile limit for these forms of fishing in the Clyde. What is required is political will since the science is in place. As Poul Degnbol Head of the Advisory Programme at the International Council for the Exploration of the Sea (ICES) said in this year’s excellent Annual Lecture at the Scottish Association for Marine Science UHI, there are five elements in an Ecosystem Approach to Fisheries (EAF): these are to reduce fishing pressure to sustainable levels X 5.

The will to change has to come politically, first and foremost.

Dr Sally Campbell is Vice Chair of COAST

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Bradshaw C, Veale L. O, Hill A. S, Brand A. R (2000) The effects of scallop dredging on gravelly sea-bed communities. In: Kaiser MJ, de Groot SJ (eds) Effects of fishing on non-target species and habitats. Blackwell Science, Oxford, pp 83–104

Bradshaw C, Veale L. O, Hill A. S, Brand A. R (2001) The effect of scallop dredging on Irish Sea benthos: experiments using a closed area. Hydrobiologia 465:129–138

Kaiser M J, Clarke K. R, Hinz H, Austen M. C. V, Somerfield P.J, Karakassis I (2006) Mar Ecol Prog Ser. 311, 1–14

Sheppard C (2006) Editorial: Trawling the sea bed. Marine Pollution Bulletin 52, 831–835

Smith R, Boyd S.E, Rees H.L, Dearnaley M.P, and Stevenson J. (2006)  Effects of dredging activity on epifaunal communities: surveys following cessation of dredging. Estuarine Coastal and Shelf Science, 70(1-2): 207-223

Wethey D. S, Woodin S. A, Volkenborn N, Reise K. (2008) Porewater advection by hydraulic activities of lugworms, Arenicola marina: A field, laboratory and modeling study. Journal of Marine Research 66: 255-273.

Photos copyright of Howard Wood, chair of COAST