Ocean Currents Determine the Travel Path of
Seaquake-injured Whales
by Captain David Williams
Deafwhale Society, Inc
PO Box 319, Dumaguete City
6200 Oriental Negros
Philippines
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DIRECTION OF THE CURRENT FLOW COUPLED WITH THE shape of
the land MASS DETERMINES the STRANDING SITE
Pods determine their location by generating powerful clicking sounds within their head and directing these loud clicks toward the bottom. The returning echoes produce a picture of the bottom terrain. The whales read this picture as man uses eyesight to visualize his landscape. They know their location because they have been there before and recognize the area.
Oceanic whales also orientate themselves along the axis of the mid-oceanic ridge system by listening to the constant seismic rumbling coming from the rift valley below. They could also use the position of the sun in relation to the line of undersea rumblings going on along the ridge axis.
Functional echo-navigating clicks returning useful information from the bottom over the background noise need to be at least 220 db (re: one micro Pa), which are well within the capacity of the whales. The problem is that 220 decibels at the surface in above the threshold of cavitation--the clicks would waste all their energy forming bubbles in the water. There is also another problem: the first 10 meters of surface waters usually contain a lot of bubbles put there by breaking waves. Thus, echo-navigating signals need to be emitted below 10 meters, which would be difficult for whales with ruptured sinuses.
The exact role the sinuses play in echo-navigation is unknown. However, common sense dictates that turning echoes into a useful road map also requires a balance in reception between the left and right ears as any weakness in one side would throw the entire system out of kilter.
The loss of of acoustic navigation is though to occur early on in seaquake-injured pods.
The fearful pod, unable to dive to the depth of their natural prey due to decompression sickness, barotrauma, and/or cavitation trauma, likely huddles together in a tight pod for protection against shark attack. At this point, the pod is being driven by fear and moving in any and every direction to avoid being eaten alive.
If they do not recover the ability to dive within a few weeks, malnutrition, dehydration, and parasitic infection would begin to dull their sensory abilities making it even more difficult for the pod to find food.
Anything and everything moving or swimming on the surface of the ocean, without a strong sense of direction, travels in the path of least resistance---downstream. The speed of the current matters little--the only thing that is important is the "path of least resistance." The situation is similar to a wind vane in that even a slight current will quickly point the non-navigating pod in the direction of the flow.
In time, the doomed pod would either fall pray to sharks and killer whales or eventually be carried into coastal waters by filaments of current that break off from the major oceanic streams.
Their path near-shore would be controlled by such things as wind, ice, sharks, ship traffic, local currents, and tidal flow. Surface chop might also influence the direction taken by the pods. Do whales prefer to swim into the chop or do they prefer to quarter the chop?
A stranding might not be imminent when they first near shore if they are still visually and acoustically able to avoid shallow water. Even so, a stranding could occur if the starving pod was driven ashore by predators, exceptional currents, panicked reactions, overaggressive pursuit of prey on the surface, or, as often happens, when the tide recedes and leaves the whales stranded.
That strandings occur where beaches are building confirms the concept that current, the force that carries each grain of sand to the beach in the first place, is the same force directing the whales. The observation that most rescue attempts are successful when attempted with the outflow of the tide and fail when attempted against the inflow further confirms current as the factor controlling their travel direction.
These whales are deep water animals and spend their entire lives far away from shallow water. They have no experience navigating in waters only a few feet deep. Thus, in the model developed for this work, the pod either (a) eventually losses the ability to sense depth acoustically, or (b) simply gets lost in shallow water and cannot find its way back to deep water. They might also be suffering auditory failure due to: (a) parasitic injury of the acoustic nerve, and (b) diminishing blood flow through the minuscule vessels of the cochlea brought on by the effect of dehydration on blood viscosity.
The truth is that mass stranded whales are seldom "rescued" by pushing them back out to sea because the odds are high that they will eventually die somewhere else downstream.
Even without auditory clues, the pod is postulated to be able to avoid stranding as long as they can see the bottom. However, at some point, vitamin depletion will reduce their visual ability, especially their night vision. When this happens, a stranding at night is likely to occur, explaining why visitors to the beach in the early morning are usually the ones to first spot the whales.
Seaquake-injured pods in this late stage of their odyssey, might be able to avoid shallow water during daylight hours, but would have difficulty at night. They may find themselves on the flats or inside a cove or bay the next morning without any concept of how they got there or how to get back to deep water. Pods, washed in by the current through inlets into backwaters at night by an incoming tide, stand an excellent chance of stranding during the daylight hours on the falling tide, especially if they panic for any reason, or encounter exceptional currents.
Bays, channels, harbors, inlets, rivers, tides, and bottom topography influence the local currents that are directing the pods near the shoreline. These near shore current patterns are complex, and would alter the pod's direction of travel in numerous ways. Incoming tides flowing rapidly through inlets would be difficult to avoid for these whales, increasing the chances for a backwater stranding.
To understand how current deposits the non-navigating pod on the beach, one simply needs to understand how current builds beaches in the first place. Pods generally strand where sand is accumulating, and do not strand where sand is in the process of eroding unless trapped by a falling tide (Williams 1987).
The stranding records indicate sloping beaches receive far more whales than beaches with acute angles. Naturally, a sloping beach would act more effectively to trap pods, but current is the force that built the sloping beaches in the first place, and current is the same force that carries the pods to these beaches.
Beaches with acute angles were eroded in that fashion by alongshore currents. These currents would direct non-navigating whales parallel to the shoreline and a stranding would be less likely to occur.
On the other hand, small landmasses, rocks, and/or jetties that extend themselves out to sea in an opposing fashion to these alongshore currents show up often on close examination of the stranding sites.
The picture above, taken in Worcester County, Maryland, show structures built out to sea in order to slow the alongshore current and trap the sand. These types of structure also trap marine mammals. Beaches with acute angles that don't normally present a potential stranding site during normal tides, make excellent entrapment areas for whales during extraordinarily high tides, especially when the wind blows strong toward shore, setting the surface currents shoreward.
Whales seldom stand in heavy seas simply because large breaking waves do not produce inshore currents. They produce strong alongshore currents that would direct the pod parallel to the beach. Rip tides during these periods serve to carry pods away from the shoreline.
Rescue attempts made against an incoming tide or in calm waters usually fail, while rescue attempts made with a relatively good outflow usually succeed. Rescued whales, when they re-strand, always strand down current (Williams 1987).
Close examination of the stranding file reveals that the major standing sites are all located about 65 days downstream from a major habitat of the species in question. Sandy areas closer to the point of injury seldom receive strandings because the whales are still able to avoid the beach, unless chased ashore by sharks. Sandy areas further downstream seldom receive strandings because the pod members become too weak to fend off the sharks.
The key to understanding major stranding sites is their distance down stream and the weakened state of an injured pod when they arrive at these sites. Closer catching arm systems are avoided unless the pod is chased ashore by predators, exceptional currents, panicked reactions or overaggressive pursuit of prey.
Strandings occur in Cape Cod usually in November, December, and January and rarely happen outside this period. Shorelines in New Zealand where strandings are frequent show this same type of seasonal variations. This pattern is easily explained by the migration of the pods as they follow the squid up and down the oceanic ridge system and in and out of areas where the current flows in different directions. A pod injured during September or October might strand in November or December.
Comparing the oceanic current on the chart at the top of this page with the mid-oceanic ridge system in the chart below shows how the direction of the currents would change when the pods change locations along the ridge system. Both the squid and the pods also move in a seasonal pattern in and out of seismically active waters and thus in and out of harms way. In this fashion, the movements of the pods will account for the seasonal changes noted in the stranding patterns around the globe. Of course, ice in the northern latitudes (especially near Newfoundland), and the directional changes in the flow of predominate winds that occur during the change of the season are also critical factors affecting the stranding patterns.
More pelagic pods live in the Eastern Pacific than in the Atlantic, yet more pods strand on the Atlantic Coast. Upwelling and wind driven current patterns that would direct injured whales away from shore along the US Pacific Coast explains the unbalance. On the other hand, the changes in current patterns that occur during El Nino tends to directed wounded whales to the beach, accounting for the increases in strandings during this period. Although the availability of prey species has been noted to demise during El Nino, the change in current and wind patterns is thought to be responsible for the increase in strandings.
The same thing can be said about stranding in New Zealand during certain cyclic wind patterns. Whenever the winds blow consistently in a westerly direction, the circumpolar currents is shifted more toward New Zealand so stranding are naturally expected to increase.
References:
Brabyn, Mark W., Ian G. McLean (1992) Oceanography and Coastal Topography of Herd-Stranding Sites for Whales in New Zealand, J. Mammalogy 73(3): 469-476
Klinowska, M. (1985) Interpretation of the U.K. Cetacean Stranding Records. Rep. Int. Whaling Commission 35:459-467
Klinowska, M. (1985) Cetacean Live Stranding Sites related to Geomagnetic Topography. Aquatic Mammals 11(1):27-32
Lein, J. (1990) Personal communications, Whale Research Group, Memorial University of Newfoundland, Newfoundland Canada
Prasad, A.S. (1992) Personal Communications, Professor of Medicine and Director of Research, Wayne State University
Williams, David W. (1988) Auditory Trauma as the Major Factor in Whale Strandings, Report Submitted to the US Marine Mammal Commission 17 October, 1988