Understanding the feeding relationships between different species in the Ross Sea, and how they are affected by commercial fishing, is essential for the establishment of a sustainable fishery in the region.

Overview

The Ross Sea lies 3500 km south of New Zealand next to Antarctica. It encompasses the main fishing grounds for Antarctic toothfish, a species NIWA scientists are studying so that it can be fished sustainably. In order to do this, we need to learn more about the ecosystem of which the fish are a part.

About the Ross Sea

Ross Sea ecosystem and trophic model

Why choose to study this region? The Ross Sea interests scientists for a number of reasons:

1. It supports a diverse ecosystem

• as well as covering the main fishing grounds for Antarctic toothfish, the region sustains a variety of bird and sea-life, some in unusually high concentrations
• studying the interconnected food web that nourishes the local ecosystem will allow us to understand how to manage a more sustainable toothfish fishery.

2. It is ecologically significant

• the Ross Sea is home to the main nursery grounds for juvenile and sub-adult species of toothfish
• primary production (the conversion of solar energy into chemical compounds via photosynthesis) in the region has intense peaks, linked to changes in sea-ice (including Ross Sea polynya: openings in the ice)
• the Ross Sea ecosystem is probably the least human-impacted shelf sea ecosystem in the world

3. Proximity

• it is similar to regions used in previous research, allowing us to draw on published information when other data is limited. McMurdo Sound, in the south-west of the Ross Sea, has been extensively studied, partly because it is home to the research stations of New Zealand (Scott Base) and the United States (McMurdo Station)
• it sits close to the Antarctic Slope Front, a bathymetric feature that forms a buffer between the Southern Ocean and the Ross Sea continental shelf.

The study does not encompass the ecosystem below the permanent Ross Ice Shelf, as we assume it plays a small role in the ecosystem of the larger Ross Sea region.

Toothfish: distribution and conservation

The Approach

We provide scientific advice to help manage Antarctic fisheries. This includes research on Antarctic toothfish to understand the potential effects of the fishery on other parts of the Ross Sea ecosystem.

The Ross Sea trophic modelling project is supported by the Foundation for Research, Science and Technology ‘Ross Sea Sustainability’ programme, and co-funded by Ministry of Fisheries projects on the effects of fishing on the region's ecosystem.

Effects of fishing on ecosystems

Species within an ecosystem are interconnected in many ways; one of the most significant is by one species feeding on another – the trophic relationship. Our research looks at how reducing the size of one species in the Ross Sea, by the commercial fishing of toothfish, affects trophic connections in the region’s ecosystem.

First order trophic effects

At the simplest level, reducing the abundance of a species of fish by fishing will affect its prey and predators to some degree. This is known as a first-order effect: fishing affects species that are one trophic connection away from the target species.

Effects of toothfish fishery on prey

Effects of toothfish fishery on predators

Second order trophic effects

Fishing can also bring about changes to an ecosystem by altering how it is organised; these are second-order effects. These effects include trophic cascades and the keystone predator effect.

• Trophic cascade: A trophic cascade occurs if a reduction in predator numbers leads to increases in abundances of its prey, putting pressure, in turn, on their food sources.
• Keystone predator effect: In some ecosystems, keystone predators maintain biodiversity by preferentially consuming dominant prey species. If predation by keystone predators is reduced, abundance of some prey species can increase to levels where they start to exclude subordinate competitors.

Other effects

We are engaged in research on the indirect effects of fishing, such as the impact it is having on the benthic (seafloor) habitat and on by-catch species. More information is available on the research page.

Outcome

Publications and documents

Our research in the Ross Sea is allowing us to identify possible effects of the longline Antarctic toothfish fishery on the local ecosystem, and in turn identify more efficient tools for monitoring it, and managing it sustainably, based on the following areas of study:

1. Antarctic toothfish stock modelling

The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) uses two criteria to set the annual catch limit:

• the spawning fish stock must not fall below 50% of its unfished level in the long term
• the risk of the spawning fish stock falling below 20% at any time in the next 35 years should be less than 10%.

The catch level is set according to these criteria, using a stock assessment model. This involves estimating the number of fish in the stock, and how the population will change under different levels of fishing. Research is aimed at improving the accuracy and reliability of this modelling.

We helped introduce a programme into CCAMLR that stipulates that a proportion of caught toothfish be tagged and released. The subsequent recapture rate of these tagged fish helps us estimate the size of the toothfish population in the Ross Sea.

2. Plausible early life history of Antarctic toothfish

NIWA scientists have summarised what little is known about the early life stages of Antarctic toothfish, and produced a plausible biological-hydrodynamic model.

3. Assessing and reducing by-catch impact

A number of other demersal (bottom-dwelling) fish are caught as by-catch of the fishery. These include Whitson's grenadier, blue antimora, moray cod, icefish, and skate. We know little about the size of these populations, and how they may be affected by the current level of by-catch.

CCAMLR 'move-on' rules are in place so that too much by-catch of any species means the fishing vessel must move to a different location.

Tag-release experiments for skate have been underway for a number of years. Results from this will help us estimate their numbers and determine whether by-catch is likely to adversely affect them.

4. Ecosystem effects of fishing toothfish in the Ross Sea

We are developing a food web model to consider how the fishery for Antarctic toothfish may affect other parts of the ecosystem. What we learn in this research is used to improve the management of the fishery, and this will help reduce the detrimental effects of the fishery on other organisms.

5. Managing risk: the unknown effects of fishing

NIWA developed a risk assessment of fishing for Antarctic toothfish in the Ross Sea and presented it at a CCAMLR meeting of the Working Group on Ecosystem Monitoring and Management in 2007. We identified the risks of ecosystem damage from fishing, considering the following categories:

• Target species harvest: Risks of depletion of Antarctic toothfish to below a level that ensures stable recruitment.
• By-catch species harvest: Risks of depletion of other harvested species to below a level that ensures stable recruitment.
• Ecosystem impacts: Risks of changes to the marine ecosystem relationships due to the removal of harvested and by-catch species.
• Exogenous effects: Risks of change in the marine ecosystem due to, or exacerbated by, exogenous effects (e.g. the introduction of alien species, effects of associated activities on the ecosystem, and effects of environmental change).

6. Effects of fishing on habitat

Vessels fishing for Antarctic toothfish are only allowed to use long-lines – sections of rope with hundreds of baited hooks attached lying over the sea bed. This method is less damaging to vulnerable sea bottom structures such as coral than the effect of bottom trawling, but will have some impact on sea bed organisms. CCAMLR, together with New Zealand scientists, are developing a strategy to evaluate how to assess the effects of this fishing method.

7. Supporting spatial management of Ross Sea

The fishery is managed differently across the region. For example, the permitted catch rate is lower around the northern seamounts (undersea mountains) than it is along the Ross Sea slope.

Bioregionalisation may help to design a spatial management framework for the Southern Ocean. NIWA and other New Zealand scientists are engaged in research to support bioregionalisation and in investigations on how to design spatial management of fisheries, both in the Ross Sea and across the whole Antarctic Ocean.