Question

1. How do they measure the cormorant population size?

1. How do they calculate the cormorant productivity (Fledging rate)?

1. How can cormorant pellets be used to estimate the abundance of forage fish species? Population size? Fish size?

Answer 1

Ans 1- To measure the cormorant population size a particular method was used which was as follows -

The study was carried out during the breeding season . The selected inland colonies constituted about 85% tree nesting population while rest were coastal colonies.

Each colony was visited prior to the expected egg laying period to assess the timing of breeding in each year.

After this , second visits were made to coincide with the earliest pairs being midway through incubation.

Further visits were then made at least once every two weeks until the last chicks fledged .

On each visit , nest distribution was mapped to identify individual nests, and nest content were recorded for all nests.

Ans 2- To calculate the cormorant productivity ( Fledging rate ) a study was conducted which is as follows-

Study was conducted on cormorants which nested on dead trees 10-20 m high surrounded by water in a basin called ' Bassarone ' .

Breeding tress are mainly located at the centre of the basin but several birds nest on trees close to its banks .

Firstly number of chicks at fledging were recorded in successfull broods . Unsuccessful broods were not considered.

Nests were checked few days before the young started to leave the nest temporarily , after 30-40 days after hatching.

Surveys of colony were made out every three weeks.

Each brood was assigned to one of the three laying periods fixed within the breeding season , samples refer only to the total number of broods started in each period.

Chick loss was only verified for broods regularly monitored for the entire rearing period .

Finally data was entered on a statgraphics worksheet .The mean fledging rate was given with the standard error.

Ans 3- Cormorant pellets were used to estimate the abundance of forage fish species by following process-

An estuary in Denmark was selected as a study area .

Firstly , coded wire tagging is done which is a non invasive tagging method that can be applied to a wide range of fish species and sizes.

The tagged fish were distributed evenly around the main fjord area.

The rate of tag loss from fish was estimated by scanning a sub sample of tagged individuals.

Secondly , pellet sampling was done for cormorant's main colony during dry weather.

All visible pellets at the side of the nest and directly on the shore were collected .

Pellets were collected in , wrapped in a sheet of ordinary paper , placed in plastic bag , taken to laboratory , then frozen .

Frozen pellets were scanned individually using a V - scanner .

All pellets that evoked a response were defrosted and analysed under microscope and the tags were identified.

The probability of recovering the tag varied widely.

The resulting calculations of the weekly proportion of tags per pellet were associated with large intervals often.

Ans 4- Population size- It is the actual number of individuals in a population. Population size was of fish was measured as above process .

Ans 5- Fish size- Erosion criteria was used to determine the fish size . Otolith , yellow perch, rock bass etc which were recovered from cormorant's pellets were used to estimate the length of the fish.

Annual variation in the size of the fish consumed by cormorants was more pronounced than seasonal variation for most species.