Effect of Habitat Change: Foraging habitat use and prey resources of bottlenose dolphin in Cockburn Sound

Brief description

Field studies of dolphin foraging behaviour were carried out between 2000 and 2002 in Cockburn Sound with two objectives:
1) to quantify the foraging habitat use of dolphins within Cockburn Sound and;
2) to identify the spatial and temporal patterns of large (10+ individuals) aggregrations of foraging dolphins.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Three sampling approaches were used to obtain spatial and behaviourial data on dolphin foraging in Cockburn Sound:

(1) systematic sampling of belt transects in 2000-01;
(2) event-specific sampling of foraging aggregations of dolphins and seabirds on the Kwinana Shelf in 2002; and
(3) ad hoc surveys of foraging aggregations in 2000-01.

See Section 3.2 for further details.

Statement: - Behavioural definitions, sampling protocol, and group size -

The time, group size, GPS location, and presence/absence of mother-calf pairs were recorded in behavioural surveys conducted for all groups and individuals that were encountered during transect sampling. Behavioural sampling of animals encountered was based on a scan sampling protocol in which the predominant group activity and other behavioural data were collected for the first five minutes of an encounter (Mann 1999). Predominant group activity was defined as the activity that >50% of the group engaged in within the sampling period (Mann 1999). When a single animal was observed, predominant activity sampling was used (i.e. activity of the individual during >50% of the sample). Data outside the five minute sample were collected on an ad libitum basis.

The behavioural sampling protocol and definition of behaviours and behavioural states followed that used by researchers in Shark Bay (Smolker et al. 1992; Mann et al. 2000). Behavioural activities were defined as: Forage, Feeding, Socialise, Rest, Travel, Unknown, or as some combination of states (e.g. Forage/Travel). Foraging was defined as the "regular, consistent, and more or less exclusive search for prey items" characterised by repeated dives in the same area, rapid surfaces while not interacting with other dolphins, and or fast swims after fish (Heithaus and Dill 2002, p. 484). Feeding was limited to direct observations of a dolphin with a fish or behaviours indicating the consumption of a fish, such as jaw snap or jerk of the pectoral fins. Analysis of spatial data for foraging habitat use was limited to those surveys for which foraging or feeding was the predominant activity for the group or individual (Mann 1999). Dolphins that exhibited behaviours indicative of conditioning to human interaction by food reinforcement (i.e. provisioned dolphins) were also excluded from analyses [see Chapters 5 and 6 of thesis for a further description of provisioned dolphins].

The definition of a "group" used for behavioural surveys during transect sampling was based on a 10-m chain rule, i.e. dolphins within 10m of each other at some point during the five minute scan sample are considered to be part of the same group (Smolker et al. 1992). Calves were not included in the determination of group size. The 10m-chain concept was established as a proximity rule for evaluating associations between individuals (Smolker et al. 1992). For situations in which dolphins are co-located for foraging purposes, the 10m-chain rule is restrictive as a measure of group size. For this reason, the total number of dolphins observed during transect sampling was used as the measure of abundance (see further below).

Foraging aggregations were defined as temporary assemblages of >10 dolphins engaging in foraging or feeding within the same general area. It was difficult to determine accurately the number and identity of individuals present during foraging aggregations because aggregations were highly mobile and the surfacing patterns of feeding dolphins erratic. As such, a gradient approach was applied to estimate the size of aggregations, in which estimates of the maximum, minimum, and "best" estimate of group size were recorded. Data were collected on the time and location of aggregations, the foraging behaviour of dolphins, and the presence of seabirds.

Notes

Purpose
To assist in the ecosystem-based conservation of dolphins within Cockburn Sound