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Ross Sea Ecosystem and Trophic Model

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.

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.

Find out more about the effects of toothfish fishery on prey.

Find out more about the 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. 

Ross Sea Trophic Model

Below is a simplified food-web model of the Ross Sea (a more detailed model was developed in this research project). Detailed information on each of the groups in the model is available on the publications and documents page.

Ecosystem

Environmental setting: The Ross Sea is covered by sea ice for most of the year. The ice expands from late February onwards, and retreats from late October. The advance and retreat processes start near the front of the Ross Ice Shelf. Areas of open water surrounded by sea ice (polynyas) form in the inner Ross Sea.

Phytoplankton and Primary Production: Growth of phytoplankton makes up the largest contribution to primary production in the Ross Sea. Sea-ice algae also contribute to primary production. Bursts of growth occur in the spring and summer along the coast, in polynyas, and in the waters left as the sea ice melts.

Zooplankton: Primary production is channelled through copepods, the most common type of zooplankton, and a number of smaller species of zooplankton such as flagellates and ciliates. Larger zooplankton include krill, amphipods and salps. Two species of krill are found in the Ross Sea: the larger Antarctic krill in the north, and smaller crystal krill in waters over the continental shelf to the south.

Detritus and Bacteria: Algae, dead organisms and excreted products can fall to the sea-floor, often in large amounts. Detritus can be decomposed in the water column and on the sea bed by bacterial action.

Benthos: Falling detritus, and the growth of coralline macroalgae in coastal regions, provide nutrition for the benthic (bottom-dwelling) invertebrates. These benthic organisms have varied feeding strategies and include grazers, such as urchins, sea cucumbers, and snails; predators, such as the Antarctic whelk and seastars; filter-feeders, such as Antarctic scallops, bivalves, anemones, soft corals, and sponges; and scavengers, such as large worms.

Fish: Antarctic toothfish, ‘cryopelagic’ (freezing, open water) fish that live on the underside of sea ice, grow up to 2 m long. Silverfish, found through the Ross Sea, grow to a length of about 30 cm and comprise the diet of almost all large predators in the area. Other fish include grenadiers (or rat-tails), skates, deep-sea and moray cods, dragonfish and ice-fish.

Cephalopods: A number of species of squid and octopus live in the Ross Sea, including the colossal squid that can grow to more than 4 m in length.

Birds: There are large numbers of breeding emperor penguins and Adélie penguins in the Ross Sea. Several other species of birds breed in the region, including Antarctic petrels, snow petrels, and the south-polar skua. Many other birds visit in summer, including two species of albatross.

Seals: Seals are the most common marine mammals in the Ross Sea, with crabeater seals, Weddell seals, leopard seals and Ross seals.

Cetaceans: Baleen whales in the region include dwarf minke whales, Antarctic minke whales and smaller numbers of fin, humpback, sei and blue whales. Toothed whales sighted in the Ross Sea include orca, sperm whales, southern bottlenose whales and Arnoux’s beaked whale.

Minimum Realistic Model

To explore the potential effects of the toothfish fishery on the population dynamics of Antarctic toothfish and its main prey, grenadiers (Macrouridae) and icefish (Channichthyidae), a spatially explicit model has been developed using a predator-prey suitability model for the Ross Sea Region. The age-based population dynamics of toothfish, grenadier, and icefish, including natural mortality and predation mortality, in addition to fishing mortality, was modelled for all three species. The model suggests that the predation release caused by the fishery effect on toothfish abundance is greater than the direct fishing mortality on both prey species and that icefish is expected to show a larger increase in biomass through time than grenadiers. It also suggests that a prey-suitability function is more likely than a Holling type II function to describe the predatory relationships in the model. The model was used to compare the predicted population changes with available abundance data for each species to develop hypotheses of the nature of the interaction. Whilst this model is in a development stage, it provides a useful tool for evaluating potential impacts of the fishery on key prey species, and for assessing and designing monitoring tools for fish species associated with the toothfish fishery. It is recommend that targeted sampling of toothfish for diet analysis be carried out, and icefish and grenadier populations in SSRUs 88.1H and 88.1K be monitored through the development of age frequencies (length measurements and aging).

 

 

Modelled percentage change in biomass of grenadiers and icefish between 1995 and 2013.

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For further details see CCAMLR document WG-EMM-14/51 in publications and documents.