Rakali Observations 2003-2004

Tiana Preston © Earthcare St Kilda
 
 
Introduction
Rakali (Hydromys chrysogaster) are native water rats common to most coastal areas in Australia (Menkhorst 2001). A group of volunteers from Earthcare St Kilda have observed the local rakali population since June 2003, to extend upon previous research conducted by Earthcare. The broad aim of this research was to determine their distribution and density in the area, as well as making behavioural observations.

Sites
Three sites have been surveyed in the St Kilda area. These represent three very distinct set of environmental conditions. The St Kilda breakwater (the site of previous monitoring activities) is a marine habitat, sheltered to the north, but exposed to sea conditions to the south. Rakali burrow between large rocks that make up the breakwater (also a site of penguin inhabitation).
Albert Park Lake is sheltered away from Port Philip Bay, and harbours several ecosystems including reed beds (pollution control ponds), small vegetated islands and concreted walls and jetties. Rakali bed amongst the vegetation and any infrastructure in the reed beds and islands, and burrow in pipes and cracks that are common through cemented areas. The water is typically brackish, varying in depth over the seasons.
The third site of Elwood canal passes through St Kilda, from a golf course to the bay. The concreted area of the canal amongst the suburban area has been the foc
us of observations, as the vegetated part of the canal proved very difficult to spot rakali in. The concreted canal is littered with cracks and drain pipes which the rakali reside in. The water in the canal experiences extreme fluctuations, from being flooded to completely empty. Water varies from very salty to fresh.
Methods  
A small but committed group of volunteers from Earthcare has participated in the observation nights, which have run fortnightly. Numbers of volunteers have fluctuated between 4 and 14, with an average of 6 volunteers per session. There is no relationship between the number of volunteers present and the number of sightings made (statistically tested).
Most observation sessions run for just over one hour and are held just after sunset, using torches to follow rakali movements. Volunteers record sightings of rakali, noting the time, location, distinguishing features, activity, food and interactions with other animals (intraspecific and extraspecific).
As the rakali have not been captured and marked in any way, it was not possible to determine the sex or age of individuals. Also, a lack of distinguishing features amongst the populations made it difficult to differentiate between individuals.
Details of the sightings of rakali have been used to provide an estimate of the total number of individuals viewed on each given night. Some level of error in determining the exact number of individuals observed is apparent, due to the difficulty in differentiating individuals. The average number of sightings made on a given night was 11, and the average number of individuals per observation night five.
Results
Distribution and Density
Rakali have been observed at all three sites, although they appear to be absent from particular locations within some of these sites.
The majority of observations at the St Kilda breakwater have been made around the sheltered north facing section. Whether this is due to a higher density of animals in this area, or just due to better visibility in this section (generally calmer water, better lighting), is impossible to determine. Individuals have been observed on the southern and eastern sides of the breakwater, the two most common localities being where the rocks meet the wooden section, and the point at which the breakwater is fenced off. This side has fewer feeding platforms. Rakali seen at the wood/rock intersection have been observed to swim under the pier (as well as climb over it), and move from one side to the other. It is not known however if rakali further up the rock section of the breakwater make the journey from one side to the other. Less observations of rakali were made beyond the fenced off area, as observation efforts were rarely concentrated there (although several rakali have been spotted).
Further away from the main pier and breakwater, rakali have been observed offshore at St Kilda beach, near the yacht club slipway. The jetty for the Williamstown ferry (approximately a quarter of the way up the pier) has also been a site that rakali have been observed at. The small boats harboured near the pier are often boarded by rakali, as evidenced by their feeding middens left on board.
Albert Park Lake appears to have several areas where rakali congregate, and other areas which are completely uninhabited by water rats. The two reed beds and areas in front of the rowing/boating clubs (near The Point restaurant and near YMCA rowing club) have had several sightings of rakali. Rakali have also been spotted boarding the boats that are moored in front of the boat clubs. No observations of rakali have been made on the small islands themselves, although they have been seen swimming in the general direction of Gunn Island. The area from just east of the YMCA rowing club, around to the Carousel restaurant has not yielded any rakali sightings.
Rakali have been observed the length of the Elwood canal. It is probable that some individuals may frequent the coastal area just outside of the canal, in addition to roaming the canal itself. Observations of rakali have been made up to Point Ormond, and it has also been reported that the rakali inhabit the area around the marina (not confirmed by Earthcare volunteers). Most sightings along the Elwood canal have been near the Broadway bridge (where a large drain area is present), and near Marine Pde.
Population
Given the difficulty in identifying individual rakali, only an approximate estimate of their population can be given. The following table lists how many individuals were thought to have been seen on each rakali watch. As it is not likely that every rakali present is likely to be observed at a given observation night, it is probable that these numbers underestimate the rakali population at each site. Hence, the population of rakali at the St Kilda breakwater is at least 9, Albert Park Lake 5, and Elwood canal 5.

 
DATE
SITE
SIGHTINGS
INDIVIDUALS
25/06/03
Breakwater
14
9
9/07/03
Breakwater
17
6
23/07/03
Breakwater
23
8
6/08/03
Breakwater
9
4
10/09/03
Breakwater
14
4
24/09/03
Park
8
4
8/10/03
Park
4
5
22/10/03
Park
5
2
5/11/03
Breakwater
14
7
19/11/03
Breakwater
9
3
3/12/03
Canal
5
5
17/12/04
Park
11
4
7/01/04
Canal
11
4
21/01/04
Canal
6
3
4/02/04
Canal*
4
3
18/02/04
Canal
12

5
3/03/04
Breakwater
14
6
       
Table 1: Number of rakali observed at each site.
*Marina also surveyed.

These numbers indicate a relatively high density of rakali in the St Kilda area, compared with studies from rural Victoria (Smales 1984; Gardner and Serena 1993), where study sites were larger.
Population and Social Structure
Varying sizes of rakali were observed at all sites. Some rakali were clearly juveniles, indicating that successful breeding is occurring in the St Kilda area (most rakali reach adult size in a year) (McNally 1960). Quite often large rakali were seen engaging in what appeared to be ‘playful’ interactions with smaller rakali. Whether this was between a male and a female, two rakali of the same sex, or an adult and a juvenile (or cases of both) is unclear. Intrasexual aggression between rakali has been reported, with other studies showing that although home ranges often overlap with rakali of all other age and sex classes, overlap with other adults of the same sex is less (Gardner and Serena 1993).
Due to the difficulty in differentiating individuals, it is not known whether the rakali in St Kilda tend to be quite territorial or not. Some locations had more rakali observations than others. Whether this was because the same rakali was always present, or a number of different individuals frequented the same location on different nights, is unknown.
More ‘positive’ (non-aggressive) intraspecific interactions were observed than ‘negative’ (aggressive) interactions. Most interactions observed occurred in the water. Some involved the sharing of food, however during most interactions no food was seen. The nature of interactions between individuals may be courtship, fighting over mates, play, teaching young, territorial aggression or sharing/fighting over food.

Food
The rakali were quite often observed eating food. As observations were quite often made from a distance, exactly what the rakali were eating was not always possible to determine. Common foods identified include worms (such as tube worms), crabs (identifiable from feeding middens left on platforms including rocks and boats), small fish and yabbies. The rakali had also been seen eating pieces of bread (at Elwood canal and the St Kilda pier) discarded by humans.
Other Observations
Rakali are very adept at swimming. The majority of observations made were of rakali swimming. The distinctive V shape that they produce in the water as they swim allowed volunteers to identify animals as rakali (as opposed to penguins or fish). The speed at which rakali swim is surprisingly quick, observers sometimes needing to walk very briskly to keep up with swimming animals.
The great swimming that rakali possess allows them to swim quite large distances for considerable periods of time. In Albert Park Lake rakali have been observed to swim out around 50 m before heading back into shore, whilst they have been followed for lengths of greater than 100 m at both the St Kilda breakwater and Elwood canal. Unfortunately at this stage, the range of individual rakali has not been determined. This is a point that Earthcare wishes to follow up with the use of tracking equipment.
The exceptional ability of rakali to dive has also been recorded through this study. Dive times have been observed up to 16 seconds. Several dives (varying time lengths) during the one swim are not uncommon. Swimming in tight circles has also been observed, thought to be made during the rakali’s attempts to capture prey.
Very few observations have been made of the rakali interacting with other species. One notable observation was a rakali that approached a duck with ducklings in tow. The adult duck displayed aggression towards the rakali, which resulted in the rakali changing direction. The co-existence of rakali with penguins on the pier has not been observed in depth. No interactions between the two species have ever been observed.
Most rakali do not seem to be perturbed by the presence of humans. Some have scared by torch light, but most rakali do not seem to mind terribly (although they do not go out of their way to approach humans). This indicates that the rakali are not targeted in any direct way by people in the St Kilda area. It has been reported that rakali are thought to have benefited from human activities, resulting in quite high urban populations (Smales 1984).
References Gardner JL, Serena M (1993) Observations on activity patterns, population and den characteristics of the water rat Hydromys chrysogaster along Badger creek, Victoria. Australian Mammalogy 18, 71-75.
McNally J (1960) The biology of the water rat Hydromys chrysogaster geoffrey (Muridae: Hydromyinae) in Victoria. Australian Journal of Zoology 8, 170-180
Menkhorst P (2001) 'A Field Guide to the Mammals of Australia.' (Oxford University Press: Melbourne, Australia)
Smales LR (1984) A survey of Hydromys chrysogaster, the Australia Water Rat in Central Gippsland. Victorian Naturalist 101, 115-118.

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