|Ph.D Student||Tamir Strod|
|Subject||Visual Capacities and Prey Preference in the Great|
Cormorant Phalacrocorax carbo sinesis
|Department||Department of Biology||Supervisors||Professor Emeritus Arad Zeev|
|Dr. Katzir Gadi|
Underwater activities in diving animals impose physiological, mechanical and sensory constrains, which are conflicting with the flying adaptations of birds. Diving birds are thus needed to be simultaneously adapted for activities both in air and underwater. The Great Cormorant (Phalacrocorax carbo sinensis) is a diving bird, which feeds on fish while diving.
An aggravating ecological and economical conflict arises due to the dramatic increase of the cormorants’ world-wide population in the last decades and the competition with humans on the same food source. I hypothesized that underwater visibility conditions and existing prey preferences might play important roles in the foraging site selection of Great Cormorants.
The goals of the present study were to test in the Great Cormorant: 1) the underwater visual capacity in different levels of water turbidities and 2) the prey preferences. For this purpose I tested five hand-reared individuals in a controlled experimental set-up.
The experimental system consisted of a roofed water tank divided into 2 sections and a large aviary, constructed adjacent to it. The underwater experimental set-up consisted of an underwater Y-maze and two prey boxes, one in front of each of the maze arms. The prey (or artificial stimulus) was presented in a pseudo-random position order and the visual capacities were determined by the proportion of correct choices of the prey/stimulus side. The tests were conducted under outdoor light conditions, with no direct radiance.
The results of the present study show that in clear water, the Great Cormorant is able to detect a fish, as small as 4cm in length, from a distance of at least 3.1m, independent of fish body length (range 4-10cm), or of detection distance (range 0.8-3.1m). The cormorants were unable to detect a fish behind a thin black fabric, indicating that prey detection is visually-guided. The prey detection distance decreased exponentially with increasing turbidity (range 0.3-4.0 NTU): Y(m) = 3.79e-0.455X (r=0.9808, P<0.001) where Y is the maximal prey detection distance and X is the turbidity (NTU). This means that in a water turbidity of 1.0-1.5 NTU, the maximal prey detection distance is shortened to half of the maximal distance in clear water. The maximal underwater visual acuity was between 6.31±0.79 and 8.66±1.01 minutes of arc in clear water and the maximal aerial visual acuity was between 3.08±0.40 and 3.77±0.48 (N=3) minutes of arc. Increasing water turbidity significantly affected underwater visual acuity: MAR = 3.71X+7.6 (r=0.991, P<0.001) where MAR is the minimal angle of resolution and X is water turbidity (NTU). The Great Cormorant was found to significantly prefer Tilapia fish over carp and a dead fish over a live one. It is concluded that the Great Cormorant will prefer to forage in fairly clear waters, where the preferred prey is available. The distribution of the Great Cormorants at the foraging sites in the Hula Valley fishponds and in the Sea of Galilee is in accordance with this conclusion.
The present study provides the first ever measurements in a diving bird of: 1) the underwater and aerial visual acuity, as well as the capacity of underwater prey detection 2) the effect of low-level water turbidity on the visual acuity and on the underwater prey detection capacity 3) prey preferences.