Feature

Spotting Fish From Space

Satellites keeping an eye on sea surface temperatures can spot the best places to go fishing.

Dr J. B. Jones

Fishers have long recognised that there is a relationship between the seasons and the presence of fish. They have also known that good fishing grounds occur in areas where water masses mix -- from large areas such as the Grand Banks in the North Atlantic to small-scale areas, such as "tide lines" and river plumes where estuarine water meets the open sea.

Because of its importance to fishers, especially those fishing on the high seas outside the 200-mile jurisdiction of coastal states, the relationship between fishing success and water temperature has been studied for over 50 years. During the 1960s, there were crude attempts to provide tuna fishers along the US West Coast with sea surface temperature (SST) charts based on ship recordings and aerial infrared radiometry.

However, it has been the advent of the polar orbiting weather satellites that has revolutionised both the scientific study of fish-oceanography interactions, and the fisheries themselves. Now the temperature of whole areas of ocean are available in real time, and suitable current boundaries can be identified and specifically targeted by vessels.

It is not necessary for the vessels themselves to receive the signal from the satellites, though this is now an option. Most vessels rely on a shore station to receive the satellite data and then provide an interpretation of the information which can be used to target fishing activities.

In Japan, such a service has been available to the fishing fleets for over 10 years, and similar services are available in Europe, the United States, and Australia.

In New Zealand, Advanced Very High Resolution Radiometer (AVHRR) high-resolution data from the US National Oceanic and Atmospheric Administration series of polar-orbiting weather satellites are received by the MetService in Wellington four times daily. This data is bought by the National Institute of Water and Atmospheric Research (NIWAR) and used to calculate sea surface temperatures and produce SST imagery.

Sea temperatures are currently retrieved from the measurements of two infrared channels. Depending on the amount of water vapour present, the radiation from the sea surface is altered by the atmosphere, and the satellite-observed surface temperatures may be one to four times lower than the true surface temperature. This "contamination" is removed analytically, producing estimates of the temperature at one metre's depth.

The satellites can provide a resolution that ranges from a three-kilometre square for the waters surrounding New Zealand to a four-kilometre square for retrievals over our entire reception area. Data obviously affected by cloud is removed and the results are overlain on standard map projections.

The temperatures generated at NIWAR from the raw satellite data have been validated against ship temperature measurements, showing that they have an accuracy of near 1^oC. However, an additional error in satellite SSTs arises from errors in knowing the exact location of the satellites relative to the earth receiving station. As a consequence, the exact location of a feature may vary by about three kilometres, although the shape of the feature will be accurate.

Because much of the New Zealand sea area is often fully or partially covered with cloud, the four daily SST images are combined, generally on Tuesdays and Fridays. In order to replace as much cloud contamination with clear sky areas, the warmest temperature recorded over the previous five days is used for each location, or pixel, on the SST image. This temperature is the least likely to be cloud contaminated and provide the best estimates of the true SST.

The resulting time-composited, high-resolution SST data is then computer analyzed onto a lower resolution grid, and isotherm and gradient charts are generated. In this process some of the detail within the SST imagery is lost, but the resulting charts can be transmitted by conventional high-speed communication methods.

The temperature and gradient charts are sent by fax to MAF Fisheries for further processing. Tuna and other predatory fish, such as billfish and sharks, are associated with temperature fronts and boundary layers between water masses. The reasons for this association are not well understood and are under research. A computer-generated gradient chart is used to detect the areas of high temperature gradient. Where these gradients occur within the known temperature range of the target fish species, the location is marked on the temperature map.

The annotated temperature charts are then sent to client fishers either by fax, or they are transmitted to vessels at sea on the "weatherfax" frequencies. The service has been operating now for four years, and fishers report that the maps accurately predict areas of high fish concentration about 70-90% of the time. The maps save them fuel through reducing the search time and provide them with a better chance of catching the fish they want.

Dr. J. B. Jones is with MAF Fisheries Research.