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Scouring the Seafloor

What sort of effects is commercial fishing having on our coastal seafloor and how can these be handled ecologically?

Greig Funnell

Since very early on in the history of trawl fisheries -- about 1376 -- there has been discussion as to what impact trawling may have on the seafloor. However because of the difficulties associated with the study of marine seafloor habitats, potential impacts received very little attention and have, in the past, been largely ignored.

Fisheries management science tends to concentrate almost entirely on the target commercial species and, to a lesser extent, on other species that are brought up by the trawl as bycatch.

Due to the history of managing fish stocks in isolation from the ecosystem that sustains them, there are currently large gaps in the knowledge needed to manage exploited marine resources within an ecosystem context.

Recognition of the importance of broader scale issues has led to increased research in recent years on what effects trawl gear and dredges may have on the seafloor communities that they are dragged across.

The ecological consequences of trawling and dredging have largely been out of sight for fishers, managers, scientists and the public alike up until relatively recently.

With the advance of technology such as remote-sensing techniques (such as side scan sonar) and video systems that can cover large distances, and with the increased awareness of this potential problem, far more work is being dedicated to the subject.

At NIWA (the National Institute for Water and Atmospheric Research), we are working on:

  • how we can link the way the seafloor environment affects the animal communities around them (i.e. how species utilise different habitats)
  • how fishing alters the habitat
  • the potential effect this has on the seafloor community

The understanding of how human impacts affect the marine environment will enable us to more effectively manage how we utilise resources and protect valuable ecosystems.

Complex Ecosystems

Complexity is a key word in this research. A complex habitat in the marine environment is one that is highly structured, either above or below the sediment surface. That is, it may have areas where there are complex hole and burrow structures or extensive beds of animals or plants projecting above the sediment surface.

Three-dimensional complexity in the marine environment can be provided by epifauna such as sponges, bryozoans, hydroids, sea fans and anemones; by feeding pits from rays and starfish, by burrowing animals (such as bivalves, polychaetes, holothurians etc.) and by shellfish protruding above the sediment surface (e.g. horse mussels).

Not only can complexity be provided by the biology of the area, but physical structure, such as sand ripples and larger bedforms (or sand waves), can be created by water currents and wave action.

Having this complexity in the environment is essential, as these features play important roles in the structural framework of the habitat, providing settlement surfaces for recruiting animals and plants, and refuge for prey.

Research overseas has shown that these refuges can provide hiding places for early life history stages of commercial fish species, as well as providing food sources for both adults and juveniles. Complex habitats also offer a greater biodiversity and density of species compared to simple or less complex habitats in the same vicinity.

But what about New Zealand species and habitats? Is it the same for our commercial fish? These are questions that we are attempting to answer with our current research.

Three-dimensional complexity can have important effects on the physical environment as well. In shallow waters where storms may disturb seafloor sediments, bottom-dwelling organisms can help to stabilise the substrate, minimising the effect of sand movement on other organisms.

Epifauna may also play an important role in influencing the flow of water close to the seabed, which may further influence sediment characteristics and deposition of larvae (i.e. reduced flow may increase the settlement of larvae out of the water column).

Scouring the Seafloor Figure A (30KB)
Complex habitats full of sponges and other marine life, such as this one in Foveaux Strait, are a sign of an environment rich in biodiversity.
Scouring the Seafloor Figure B (32KB)
The Mercury Bay seafloor shows signs of having been scraped bare by scallop dredges. Parellel tracks along the seafloor indicate where the dredges have been dragged.

Effects of Fishing Disturbance

Trawls and dredges potentially have an impact on many species living on, in, or near the seafloor. Results from previous disturbance studies have shown that in habitats dominated by small crustaceans, scallop dredging decreases species richness and diversity. These areas can be an important food source for many fish species. Predicted changes in marine benthic communities with an increase in fishing pressure are decreasing densities of large epifauna, long-lived near-surface-dwellers, echinoderms and total number of individuals.

Disturbance also increases the density of deposit feeders and small opportunist species. The disturbance caused by fishing can also reduce the natural variability in the region, causing a decrease in biodiversity.

Previous research at NIWA has shown that there are significant changes in the benthic habitat as a result of fishing pressure.

Our research conducted in the Hauraki Gulf demonstrated that, across the Gulf, 20% of the variability in the composition of the seafloor communities could be attributed to habitat disturbance by fishing. No other direct human induced change in community composition is likely to result in this much change over such a large area.

Other work being carried out in Foveaux Strait is assessing the interaction between the oyster and blue cod fisheries and the epifaunal reefs. Records show that these once-extensive reefs were the original oyster habitat and have been heavily modified by dredging for oysters. The reef environment favoured the development of oyster populations and, with their disappearance, oyster recruitment and survival has probably changed. These reefs provided a complex habitat for adult and juvenile blue cod to feed and shelter in; the loss of these reefs have contributed to the reduced size and distribution of the blue cod fishery.

Scouring the Seafloor Figure D (43KB)
In New Zealand, seafloor disturbance from fishing gear is caused by bottom trawling and dredging for shellfish such as scallops and oysters. Commercial trawls use doors on the sides of their nets to hold the net open and guide fish in. Scallop dredges are towed across the seafloor where large metal teeth at the front of the dredge dig into the sediment and rake up scallops in its path. These dredges, which can be up to 2m wide, leave obvious tracks and effectively flatten the seafloor features.

Filling Knowledge Gaps

These issues raise many important queries about the management of the marine environment, as well as giving rise to challenging scientific questions that need to be addressed.

The aims of our current research at NIWA under the PGSF-funded "Fishing: Ecosystems and Resource Sustainability" programme, are to fill our knowledge gaps by:

  • defining the importance of seafloor habitat structure to both the sustainable exploitation of near-shore fisheries and the maintenance of coastal biodiversity and ecosystem functioning
  • identifying relevant environmental indicators reflecting ecosystem productive capacity and changes in structure and function
  • developing rapid assessment techniques to enable cost-effective environmental reporting

We know that fishing methods cause a change in habitat, as shown by previous studies conducted at NIWA and overseas. The next step is to determine the effect removing seafloor structure has on the ecosystem's functioning. To do this, we need to be able to understand the link between the complexity of the habitat and habitat utilisation by different species.

Currently we are undertaking field studies of scallop spat settlement and survival in the Hauraki Gulf. By comparing numbers of spat that have settled on the seafloor with those found in settlement bags suspended above the seabed, we hope to be able to link different levels of habitat complexity with scallop recruitment.

It is expected that an increase in three-dimensional structure will provide more settlement surfaces and protection for juvenile scallops, increasing the recruitment to the fishery. The settlement, survival and growth of oysters in habitats of differing complexity will be studied in Foveaux Strait.

Similarly, we expect that juvenile snapper will prefer an area of high complexity, as such habitats would provide both refuge from predators and a more plentiful food source. A greater degree of habitat complexity tends to have greater diversity and abundance of fauna, thus increasing food availability.

Our studies will also involve cage experiments where we introduce predators and assess juvenile snapper responses and utilisation of refugia. Similar studies in Foveaux Strait will explore the impact of habitat complexity on the population dynamics of blue cod.

Rapid Assessment Techniques

Typically, benthic sampling surveys are time-consuming, especially when you are interested in large-scale problems. Therefore it is important to develop methods in which you can survey large areas and assess their health and vulnerability to disturbance in a cost-effective and accurate way.

One method of doing this is by integrating fine-scale sampling such as benthic coring, with coarse-scale sampling such as side-scan sonar and video. In this way, you can cover large areas of seafloor in relatively little time.

Adding to the ability to conduct diver operated video surveys NIWA has invested in a Benthic Ecology Video Acquisition System (BEVAS), a sled that is towed along the seafloor collecting high-resolution video footage. From this footage, we can obtain estimates of epifaunal density and diversity, and assess habitat complexity.

Scouring the Seafloor Figure C (39KB)
The Benthic Ecology Video Acquisition System has made it quicker and cheaper to check out what is happening on the seafloor.

Once we have linked the interaction of species to habitat complexity we will be able to identify from video footage areas that will be sensitive to the effects of fishing. From this information it will be possible to identify essential fish habitats, and improve fisheries management by taking into account not just the commercial species themselves, but the whole ecosystem that supports the fishery.

Compared to traditional methods of surveying the seafloor by coring or scientific dredging, the use of video to survey large areas is potentially a very cost-effective method. With this reduced cost it is now possible to incorporate ecosystem-wide surveys into the development of codes of practice to minimise damage from fishing; criteria to identify sensitive areas that are unlikely to maintain their current ecological values under disturbance; and environmental monitoring.

Most continental shelf and coastal areas around the world are now impacted on by fisheries. The most sensitive areas are those that are already rare and act as nurseries for commercial and non-commercial species.

As fisheries are sustained by the natural productivity of the habitats fished over, any shift in ecological functioning may influence the sustainability of the fisheries resources. Including the broader implications of the ecological effects of fishing into the management system is an important step toward both sustaining the industry and conserving marine resources and biodiversity.

Greig Funnell is a Marine Benthic Ecology Technician with NIWA in Hamilton.