Authors: Kelly J. Benoit-Bird and Whitlow W. L. Au
Stable URL: http://www.jstor.org/stable/4602229
One can easily agree with a concept that in order to loose as little energy as possible on foraging, a predator has to keep track of changing patterns of its prey. This concept manifest itself in coherence between predator and its prey both in small and large spatial and temporal scale. However, most of our studies did not find one o f that coherence in marine pelagic systems. Almost no positive correlation has been proven in small-scale predator-prey overlap which is quite surprising
Some theories were put forward but they usually speak about specific animals. Generally, scientist agree that our traditional models do not fit with pelagic predator-prey systems and therefore, that those systems are fundamentally different from others.
One of such cases is pan-tropical species of spinner dolphin. To be specific, Hawaiian population of spinner dolphins. First studies by Norris et al. (1994) came with a hypothesis that these animals forage at nigh, far offshore on small fishes, shrimps and squid from mesopelagic boundary community. Those, small creatures are found over many km over the slope of each Hawaiian island. Moreover they migrate diurnally both horizontally (approaching shore in the night and retreating before morning) and vertically (approaching surface up to 400 meters below surface and going dawn as deep as 700 meters in the day).
In this light, simple idea of foraging offshore proposed by Norris et al. seems extremely simplified and not likely to be describing actual foraging behaviour.
In order to study underwater, in the night You need to rely on techniques enabling You to “see” in the water. To that you need to use sound as sound propagates in the water much better than light. Moreover sound travels through water much farther than light and if You look at animals and their adaptations to find their way in the water, majority of them rely in some way on sound instead of vision. Following that, in this study an active acoustic surveys were conducted using echo sounder. The goals of the scientists were:
- to determine minimum and maximum depth of the prey layer,
- to determine relative abundance of prey and of spinner dolphins (percent of sampling time that dolphin were observed,
- to point the depth at which the density of prey was near its maximum,
- to point the depth of spinner dolphins,
- to describe the geometric and density characteristics of prey patches.
But, having results from an echo sounder is not enough to describe all above, one also need to test those results statistically in variety of ways.
In this study, there were two site observed. One, “inshore” approx. 1.0 – 1.3 km from the shoreline and “offshore” site 2.8 – 3.0 km from shoreline. Sampling were conducted between 1800 and 0400 hours by 5-10 km long transect.
The echo sounder emitted sound of 200 kHz which is far above the hearing range of dolphins. Echo sounder emits sound strait down and “listen” to echo. This echo is created when sound bounces of an object or group of object and comes back to echo sounder. Bouncing is caused by differences in density of the medium, just like our voice is bounced of a wall while travelling through air. The device then, pick that echo and transform it into a map of what is below. By assigning different colours to different densities, you can conclude whether you have below a rock, just water or a school of fish. Of course this is not that easy, but software deals with that for us.
How You distinguish a dolphin and how its prey? Dolphins have lungs which have some air in it even though animal exhale when diving. Sound therefore, travelling through water, meets those sacks of air and gets reflected. Software in the echo sounder shows that sacks of air as white areas so it is very easy to spot.
As for patches of prey, they cause sound to bounce as well but this time due to having higher density than water. Again echo sounder transforms this data into a colourful image
As you might expect from highly social animals (dolphins are considered to be highly social), foraging show very good cooperation. Usually, dolphins were detected in pairs. Pair/ group foraging occurred even when the prey showed large differences in its distribution. This suggests that pairing is either obligatory or it involves some social component.
As You might expect, dolphins also proved to follow their prey both vertically and horizontally. So, Norris’s hypothesis, that spinner dolphins in Hawaii spent the whole night foraging offshore is not correct as these animals forage were their prey is. And their prey migrate as close as 1 km from a shoreline as so are dolphins.
Why, then micronektonic animals (dolphins’ prey) migrate to from deep layers shallow waters and why do they do this at night? As other studies showed, those little organisms also track their prey just as dolphins track them. And prey of mesopelagic boundary community are even smaller organism like crustaceans. Those can be found during night in shallow waters where they feed on microplanctonic algae. The other factor that is very important is darkness that reduces visual exposure of micronektonic animal (dolphins’ prey) to visual predators like tuna and other fish.
This chain is a typical example of bottom up effect were algae that thrived during daylight in the shallow waters, attract microplanktonic organism that feed on them. They, attract micronekctoni animals that spend day in depths hiding from visual predators and come up to shallow waters during night when visual predation risk is minimized. And those, attract dolphins.
There is a question why spinner dolphins stays inshore during day? They usually spent their day resting in shallow bays where the risk of being attacked by a predator is much less than in open waters. Also, resting factor is important. It is much easier to rest in calm, shallow bay than it is in open ocean.
Photo Credit: A Day in the Life of a Hawaiian Spinner Dolphin, can be downloaded from http://www.fpir.noaa.gov/Library/PRD/Spinner%20Dolphin/Swim%20With%20page/Spinner%20Dolphin%20poster%20revsk.pdf