Time series – the backbone of providing advice

08.11.17

The most important type of data set in the management of Greenland’s living resources (mammals, birds, fish and shellfish) is the time series. A time series is a number of data points arranged in order of time - e.g. a specific study repeated at about the same temporal distance over a certain number of years. It could be a count of the number of reindeer or narwhal in a certain area, which is calculated every summer over many years.

In this article Professor Mads Peter Heide-Jørgensen from the Greenland Institute of Natural Resources explains the term time series and the importance of time series for scientific research.

"The Greenland Institute of Natural Resources builds a time series for the population of a species to be able to determine how the population develops. We know, for example, that the harp seal reaching sexual maturity at a late age indicates that the population is large and in no hurry to produce offspring. Reaching sexual maturity at an early age tells us that the population is small and can produce more pups. Therefore, if we want to find out how the harp seals are doing, we can study the development of the females’ age at reaching sexual maturity by collecting and studying female seals’ genital organs over a number of years. We use systematic and sometimes complex research methods to keep track of what is happening in animal populations."

The importance of time series

Long time series give the Greenland Institute of Natural Resources a crucial basis for providing advice on animal populations with cultural and industrial significance for the Greenlandic society.

"Lacking time series is an uncertain basis for providing advice. For example, if we only have counted walrus once, we have no idea how the population is developing. If we have two counts with e.g. a few years between, the last count may give a clue as to whether the walrus population has grown or dropped since the first count. If we continue the series of counts and get more data points, it starts to become interesting, because now we begin to see and understand what is happening in the population," says Mads Peter Heide-Jørgensen and continues:

"The more years we do the same population study in the same area, the clearer the picture we get of the development of that population. Basically it means that the more surveys of a population are done, the better basis we get for providing reliable advice on the population. Scientific advice primarily ensures that we can continue to exploit the living resources far into the future. So the better time series we have behind the advice, the more accurately we can predict what will happen in the future and how a sustainable exploitation of living resources must look."

At the same time, the time series are used to document that the catch happens sustainably. For example, the international whaling commission, IWC, would like to make sure that the catch happens sustainably before dividing quotas, which has direct impact on Greenland.

Focus on quotaed animals

Time series are not made for all animal species, as it would require large resources in terms of time, people and money. Mads Peter Heide-Jørgensen explains:

"It costs a lot of money to establish time series. This is why the Institute focuses on getting robust time series for the most important species and populations that have economical and cultural significance for the community – particularly for the species whose catch is quotaed by the Government of Greenland. Therefore, we don’t have time series for the number of Arctic hare or Arctic char, as they do not have the same importance for the society as cod and shrimp."

The oldest time series

The oldest time series originate from the time when trade was established in Greenland in the mid-1800s, and the population began to trade catch surplus, such as blubber and pelt, with the trading stations. Mads Peter Heide-Jørgensen says:

"The oldest Greenlandic time series are the catch lists, where we have numbers dating all the way back to the 1860s and, in fact, for some areas and species, even further. The then-existing Royal Greenland Trading Department kept reports of catches, especially the catch of whale, and these catch lists tell us about the sizes of the catches in the individual areas. Thus, they are time series that can point to the size of populations over a long period of time."

The oldest time series of whale counts at the Greenland Institute of Natural Resources are counts of the number of beluga. The institute has counted the West-Greenlandic beluga population since 1981.

Mads Peter Heide-Jørgensen says: "The time series means that we can give a rather certain advice on the population today. We have also build a shorter time series for the narwhal. The problem with narwhales is that there are several different populations in Greenland. As a consequence, we have not been able to build more solid time series for all the populations. For example, we have only counted the East-Greenlandic narwhales three times, and the narwhales in the Melville Bay (Northwest Greenland) three times."

The longest time series for the large whales is counts of and data on the humpback whale dating from 1982.

Time series require technical equipment

The surveys that are part of the time series usually require a lot of technical equipment, which changes all the time.

"Surveys require advanced technology and equipment. Counts of whale populations include, for example, airplanes flying at a certain height with a certain number of counters, counting in a certain way. An advanced calculation method is also used to reach a result of a count. The method is constantly developed, and in 20 years we might count in a completely different way. We also attach measuring instruments onto the animals to follow the migration routes of the animals. The instruments we used 20 years ago are much bigger than the ones we attach today. These days there is talk about using drones with special cameras for counting," says Mads Peter Heide-Jørgensen.

However, there are also time series that do not require so much technology. Data on the annual catches in Greenland is also a very important time series, which is based solely on that fishermen and hunters must share their knowledge of what they have caught. Although it is knowledge that already exists with people and do not require any technology, it is still difficult to get the precise information about the catches.

Building of time series costs

A solid time series takes many repeated surveys of a certain population, and it is costly. Mads Peter Heide-Jørgensen says:

"If the time series on narwhales in Melville Bay in Northwest Greenland is added an extra year, it costs approx. DKK 1 million. Other surveys are cheaper to perform, because the animals live in a more limited area, such as the walrus population at Thule. West-Greenlandic walrus, beluga and Greenland whale can be counted and surveyed at the same time; that saves money."

The Greenlandic time series can also be used outside Greenland, where scientists are interested in finding out, e.g., how sea temperatures, fish and marine mammals change over time in the North Atlantic. But the time series are primarily a basis for the Institute’s advice to the Government of Greenland.

"Therefore, time series are important and should be seen as the backbone of the work of the Institute," concludes Mads Peter Heide-Jørgensen.

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Scientist Mads Peter Heide-Jørgen, biologists from the Greenland Institute of Natural Resources and local hunters in East-Greenland tag a narwhale. Photo: Carsten Egevang

Diagram of the time series for beluga counts. Black diamonds = number of whales from counts with uncertainty limits (vertical lines); full, black curve = the biologists’ assessment of how the population has developed from 1970 to 2012 with uncertainty limits (stippled curves); black bars = catches of beluga (read on the y-axis to the right).