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Trends in Abundance

Populations of animals my rise or fall in abundance for many reasons. For example, if many animals are taken by hunters, and they are not replaced quickly enough by natural reproduction, a population will fall. A population might rise if it had previously been reduced to a low level but hunting had since ceased. The detection of trends in abundance is of great importance to management, because we need to know if the management measures taken are having the desired effect on the population.

Many species of whales live a long time and don’t have calves very frequently. As a result, whale populations tend to change slowly, neither rising nor falling very quickly. This makes the detection of changes in whale populations difficult, because only small changes can be expected from year to year. In addition, abundance estimates are approximations, with variance expressed as a 95% confidence interval. A change in abundance between two or more surveys can only be detected if the difference in abundance is falls outside of the overlap between the confidence intervals.

The detection of trends in abundance requires two or more surveys, usually separated by several years so that some change can be expected. The surveys must cover the same area which ideally must include most of the range of the population being surveyed, because animal distribution can change from year to year. Finally, the surveys should be conducted at the same time of year, because many whales migrate seasonally.

Five NASS have been carried out over a 20 year period (1987 to 2007). These surveys therefore provide a good opportunity for detecting trends in abundance over that period. However, the survey area has changed over that period, and one survey (1989) was conducted later in the year than the others. Therefore not all the survey data can be used to look for trends.

 Common area all NASS cropped for web

T-NASS 2007, showing the Common Area that has been covered in all surveys since 1987 in red.
The Common Area is divided into E and W subregions.

Detecting trends: Pilot whales as an example

 

Pilot whales are taken every year in a drive hunt in the Faroe Islands. Therefore trends in abundance are of great interest to management: we need to know if the population is rising or falling over time. Although five NASS have been carried out between 1987 and 2007, only a relatively small proportion of each survey area, the common area, has been covered every time. To look for trends, the abundance of pilot whales was estimated in the E and W common areas for each survey. While there appears to have been some decline in abundance estimates over the period, the high degree of uncertainty within survey estimates means that this apparent decline is not statistically significant. In contrast, highly significant trends have been detected for fin and humpback whales.

 

 Pilot trends E W ALL

Trend in pilot whale abundance in the E, W and Total common areas of the NASS, 1987-2007.
95% confidence intervals are shown.