Population monitoring week 6

Question 1

Intended learning outcomes

Answer 1

Describe why estimating the abundance of marine mammals is important, and describe the different methods and discuss their pros and cons

Question 2

Population dynamics

Answer 2

Population dynamics refer to the population size (abundance) and trends of a population

• The abundance of marine mammals vary enormously!
  – Millions of harp seals vs. hundreds of Saimaa seals
  – 100.000s of minke whales vs. tens of Vaquitas
• Population dynamics influenced by life-history
  – Age at physical and sexual maturity
  – Fertility and birthrate
  – Pup, juvenile and adult mortality
  – Longevity(life-span) and generation time
  – Age distribution
• Knowing the abundance and trends of a
  population is crucial for assessing impacts of past, present and future climate change and human activities (e.g. whaling, bycatch, noise, contaminants, etc).

Question 3

Estimating the size of the entire population of a marine mammal is often (very) difficult

Answer 3

• Estimating a part of the population (followed by extrapolation) is “much” easier

1. Mark-recapture: aggregated populations/species
2. Distance sampling: dispersed populations/species
3. Colony counts: most otarids and land-breeding phocids
4. Back-calculations from hunting statistics to estimate past population size and trends

Question 4

Mark-recapture

Answer 4

Often used for aggregated populations
* First, “mark” a number of animals in a population (n1)
* Then, capture a second sample of animals (n2) and count the ones that have already been marked, i.e. those that are “recaptured” (m2)
* Population size N1 = n1 * (n2/m2)
* Methods to “mark” and “recapture” require that the animal can be identified to individual
– Photo-ID
– Genetic profiles
– Freeze-branding, or tags in flippers or skin

Question 5

Photo-ID

Answer 5

Often used for cetaceans, but also seals and manatees
* Individuals recognised by colour, shape, scars, etc
* Build an image catalogue and make a system for (automatic) processing of all photos

Question 6

Genetic profile

Answer 6

Often used for animals where photo-ID is difficult
* Hard to get sufficient sample size for rare or offshore species
* Provides much additional info (relatedness, migrationrates, sex, population structure, evolutionary adaptations, etc)!!

Question 7

Genetic profile

Answer 7

Often used for animals where photo-ID is difficult
* Hard to get sufficient sample size for rare or offshore species
* Provides much additional info (relatedness, migrationrates, sex, population structure, evolutionary adaptations, etc)!!

Question 8

Plastic tags or freeze branding

Answer 8

Plastic tags in hind-flippers
* Freeze branding using liquid nitrogen
* Typically used for stationary animals (often pinnipeds)
* Can require large effort to catch animals and put a tag on

Question 9

2. Distance sampling

Answer 9

Often used for dispersed populations
* Individual counts (direct) or cue counts (indirect)
– Counting number of observed animals
– Counting cues, such as cetacean blows or acoustic signals

• Method 1: Line counts
  – Counting observed animals along a specified line
  – Most used method for cetaceans and some seals
• Method 2: Strip counts
  – Counting ALL animals along a line within and within an area
  – Sometimes used for ice-breeding seals
• Method 3:Point counts
  – Count animals within a specified area

Question 10

Method 1: Line counts

Answer 10

Counting observed animals along a specified line

Most used method for cetaceans and some seals

Example:
When estimating the abundance and density of cetaceans in the European Atlantic and North sea, they used line transect, using visual and acoustic observations. Also using both shipboard and aircraft surveys.
There were many uncertainties due to weather, observer bias, diving or silent animals and non-random distribution of animals.

Question 11

Method 2: Strip counts

Answer 11

Counting ALL animals along a line within an area

Sometimes used for ice-breeding seals

Example: When they wanted to estimate the current population size and distribution of Caspian seals bc of severe decline in population size due to hunting, they did an aerial strip count during winter pupping season. They assume that all animals within the strip is observed, and used hunting statistics to calculate historic population size

Question 12

Method 3: Point counts

Answer 12

Count animals within a specified area

Example: The SAMBAH project, which was static acoustic monitoring of the baltic harbour porpoises, was used to get a sense of population size, distribution and structure of baltic harbour porpoises, as it’s currently unknown. They did 2 years of passive acoustic monitoring at 304 locations with C-PODS across the baltic sea, which had a 400 m detection range of porpoise clicks (“cues”).

Question 13

Acoustic

Answer 13

When counting cues and using acoustics and a hydrophone, there can be some uncertainty, as animals that don’t make sound or aren’t facing the right way won’t be detected.

Question 14

3. Colony counts

Answer 14

• Often used for land-associated pinnipeds
• Count number animals on land during breeding or molting, and correct for animals at sea during survey
• Typically by aircraft, but can be ship- or land-based
• Digital photography or infrared thermal sensing