Science

Who Killed Jack Mackerel?

31 October 2012

James Whitmore

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Something strange is happening off the coast of Tasmania. In August, the Commonwealth Scientific and Industrial Research Organisation (CSIRO) reported that tropical fish were turning up in cold seas usually home to temperate marine life. In nearby waters, the Dutch trawler Margiris was granted permission to fish in Australian waters by the Australian Fisheries Management Authority. Back in Canberra, Environment Minister Tony Burke is in the final stages of the largest revision of marine reserves yet. At first these three stories may seem unrelated, but they all have two things in common: the common jack mackerel and a fish known as redbait.

A tale of two fish

These two fish are no Rainbow Fish. They are the archetypal fish: long, slender bodies with streamlined fins and silver scales. They might be the fish a child would draw. On the other hand these are sophisticated marine predators, live knives cutting through water. No drawing in a fishing guide can possibly capture the astonishing iridescence of their scales. Those scales might help them disappear in full view by reflecting almost exactly the light of the surrounding water. Jack mackerel can grow to 60 centimetres long and live perhaps 25 years. Redbait are slightly smaller. They form vast, glittering schools that are constantly on the move along the east coast of Tasmania, hunting small crustaceans, their favourite prey. They are themselves food for seals, tuna and albatross. These are oceanic fish, never coming into contact with land or the seabed. Despite their similarities they are unrelated, but their fates are bound together.

Usually the common jack mackerel doesn’t even receive the distinguishing ‘common’. It is lumped together with two other species of jack mackerel, which join redbait and blue mackerel as part of the Small Pelagic Fisheries industry. Most of them go to fish farms where they feed tuna and other fish that humans love to eat. For the purpose of the fishing industry they are indeed almost identical. They can often be found altogether in the same feeding schools, making them an easy target for trawlers.

The Small Pelagic Fishery came under scrutiny when the Dutch-Tasmanian fishing company Seafish brought the Abel Tasman aka Margiris into Australian waters this year. It became known in the media as a super-trawler, a name that inevitably evoked ecological apocalypse.

Midwater trawl, as the process is known technically, is the technique of dragging enormous nets through the water, sweeping up whole schools of fish in their paths.
Jack mackerel used to be fished in the 1980s using purse seines, a different method. The ship motors around the school dragging a net until a complete circle is formed. Then the net is closed and tightened, and the fish hauled in. By 2001 this had all changed. The jack mackerel fishery had collapsed and the target fish was redbait. It sounds like a good mystery. Who killed jack mackerel?

Hotspot

According to the University of Tasmania (UTAS), the fingerprints of the fishery were all over the scene. They noted that the jack mackerel had downsized. The large, old fish had been removed, leaving smaller fish to continue breeding. It is a pattern that is seen all over the world and it is the smoking gun of intense fishing. When the size of fish and harvest continued to decline the fishery switched to midwater trawl and began to target redbait.
The Australian Fisheries Management Authority (AFMA) regulates fisheries by providing figures for Total Allowed Catch (TAC). For many years the TACfor jack mackerel has been around 5,000 tonnes. Next year, it will be increased to 10,100 tonnes—more than twice the usual allowance. The AFMA, backed by the CSIRO, says that surveys of fish spawn show that this is still a tiny fraction of the sustainable limit. However, the UTAS research suggests that the fishery is not the only culprit.

The sea off the east coast of Tasmania has become known as a ‘hotspot’. The water there is warming faster than almost anywhere else in the world. Flowing down the east coast of Australian is the East Australian Current (EAC), the highway of warm water of Finding Nemo fame, and yes, green turtles do ride its current. It originates in the Coral Sea off the coast of Queensland, generated by the vast circulation of the entire southern Pacific Ocean.

In recent years the current has strengthened, and the UTAS team have made the link between warming waters and the jack mackerel. Why is the current strengthening? It is hard to say because the current varies from year to year under the influence of El Nino and La Nina. However such a long-term trend is linked to the gradual warming of the atmosphere, and that points the finger at climate change. Warm waters are poorer in nutrients than colder water and nutrients feed microscopic plants that in turn feed tiny animals. Together these tiny plants and animals form plankton, and they are the base of the oceanic food chain. They are like grass in the African savannah. Krill, the favourite food of jack mackerel, prefer the cold, nutrient-rich waters of the Southern Ocean. Warm water favours tiny crustaceans called copepods. These are the favourite food of redbait.

The UTAS team says a switch is underway from jack mackerel to redbait. It seems a simple equation: cold water equals jack mackerel, warm waters equal redbait. Even if it is not quite as simple as that, what is certain is that changes in temperature lead to a change in an ecosystem. It is a pattern being repeated over and over again across the world.

Reservations

The East Australian Current doesn’t just provide lunch and dinner, it is the place where these fish breed. Every December, jack mackerel gather at the edge of the continental shelf. At the continental break off Tasmania, the meeting of cold deep waters and warm surface waters can create a rich ground for krill. Exactly on the full moon, in an act of extraordinary synchronisation, the jack mackerel release eggs and sperm into the water. They appear to be quite choosy about where they spawn, in temperatures between 15-17 degrees Celsius. This is known as a ‘thermocline’, and it appears that the jack mackerel follow it up and down the coast, perhaps as far as New South Wales.

In short, these are exactly the sort of animals that are difficult to protect in reserves. Reserves are good at helping animals and plants that don’t move about, that breed in one place and are tied to the sea floor like the species that depend on coral reefs. One of the biggest, and most controversial, changes in the Tony Burke’s revision is a new marine national park in the Coral Sea that protects an area half the size of Queensland full of nesting seabirds, coral reefs and migrating whales. Reserves like these can improve marine ecosystems by protecting the breeding grounds of fish. If they close off large areas to fishing, they can actually improve fisheries as fish recover and they spill over into nearby fishing areas.

Fourteen such reserves are proposed in the South East reserves network off Tasmania and Victoria. Most of the individual reserves have actually been around since the early 2000s, so the 2012 revisions change little. The Freycinet and Flinders protected areas are the ones that cross the east Tasmanian shelf break where jack mackerel breed. They exclude fishing from a large part of the reserves that extends past the continental shelf 200 kilometres to the edge of Australia’s economic exclusion zone. Unfortunately most of the animals in this area like whales, sharks, seabirds and jack mackerel don’t realise they are in a national park and move in and out at their whim. As climate change takes hold it will not just be animals on the move, but whole ecosystems.

The other problem is lack of data. One of the other jack mackerel species that is part of the Small Pelagic Fishery has been assessed by the International Union for Conservation of Nature as “data deficient”. That means next to nothing is known about its population—and without data there is no way the fish can be managed or protected.

There is a sort of happy ending to this though. What is known about the common jack mackerel and redbait is largely thanks to the assistance of the fishing industry. Scientists and fisheries work in tandem to update data and respond with better management strategies. It is for the most part a happy partnership. In recent weeks, this resulted in the federal government putting a stop to the super-trawler fishing in Australia. Now fisheries, scientists and governments need to continue this relationship in the face of warming oceans.