FISHING IN THE DARK: A PURSUIT-DIVING SEABIRD MODIFIES FORAGING BEHAVIOUR IN RESPONSE TO NOCTURNAL LIGHT LEVELS

Abstrac

Visual predators tend not to hunt during periods when efficiency is compromised by low light levels. Yet common murres, a species considered a diurnal visual predator, frequently dive at night. To study foraging of murres under different light conditions, we used a combination of archival tagging methods and astronomical models to assess relationships between diving behaviour and light availability. During diurnal and crepuscular periods, murres used a wide range of the water column (2-177 m), foraging across light intensities that spanned several orders of magnitude (10(3)-10(-10) Wm(-2)). Through these periods, they readily dived under conditions equivalent to ambient moonlight (∼10(-4) Wm(-2)) but rarely under conditions equivalent to starlight (∼10(-8) Wm(-2)). At night, murres readily foraged during both moonlit and starlit periods, and diving depth and efficiency increased with nocturnal light intensity, suggesting that night diving is at least partially visually guided. Whether visually guided foraging is possible during starlit periods is less clear. Given the dense prey landscape available, random-walk simulations suggest that murres could benefit from random prey encounters. We hypothesise that murres foraging through starlit periods rely either on close-range visual or possibly nonvisual cues to acquire randomly encountered prey. This research highlights the flexibility of breeding common murres and raises questions about the strategies and mechanisms birds use to find prey under very low light conditions.

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TURTLE PARTS 9: OBSTACLES FOR SUBADULTS AND ADULTS

Although the number of obstacles that sea turtles face during these stages of their lives is fewer, there are still many to be overcome. As sea turtles grow from subadults to adults the number of natural predators decreases. This is simply due to the fact that the turtles are growing larger, and the number of predators that can actually successfully prey upon them is fewer. Sharks and other fish continue to prey upon them, but by the time a sea turtle reaches full mature size these attacks seldom result in death for the turtles. Females often emerge from the sea to nest with flippers, both rear and front, mangled or completely missing, which can greatly limit her ability to nest. These injuries are most likely the result of an attack from a large shark, and can also prevent a male from successfully mating. Sea turtles will eventually outgrow the majority of their natural predators. But there are two threats they cannot outgrow: disease and man.

Diseases do afflict sea turtles, but naturally occurring diseases – meaning diseases not enhanced by human activities – do not seem to occur with any great frequency or cause a large number of deaths. There is however one disease that is currently having a very adverse affect upon sea turtles, and will be discussed in more detail later in this section. Human beings are the second natural predator that sea turtles do not outgrow and our actions are causing mortality in sea turtles with great frequency.

Humans have harvested sea turtles for food consumption and for their shells and leather for various purposes

throughout history with seemingly little or no affect on their overall survival. But, the population of humans, and thus the areas that we inhabit, have been increasing through the ages. These increases, along with the non-selective fishing practices in use today, have placed additional pressure on the ability of populations of sea turtles to survive.

Historically sea turtle meat has been a primary source (and when fishing was poor, an alternative source) of protein for many coastal communities. Some communities still partly rely on sea turtles as a source of protein. The primary sea turtle dish that is consumed by humans today is green turtle soup, and a lot of this soup is prepared to satisfy foreign market and tourist demand. It is a luxury or novelty item, just as so many of the items produced using sea turtle shells and skins are. Until 1990, when the Mexican government placed a ban on the trade of sea turtle products, up to 50,000 sea turtles a year were slaughtered in Mexico alone for their skins and shells. Today it is estimated that tens of thousands of turtles are slaughtered each year for the production of luxury items such as boots, belts, purses and eyeglasses frames made from sea turtle parts. Thousands of sea turtles are also dying as so-called bycatch in the nets and on the hooks of fishermen each year.

Sea turtles breathe air and must surface regularly to survive. When they get caught in fishing nets or on fishing hooks, they drown. Modern fishing methods such as shrimp trawls, large nets drug behind boats, drift nets, and long lines, baited hooks stretching out great distances, inadvertently capture sea turtles. These turtles are wastefully discarded as by- (unwanted) catch, since they are not the intended quarry of the fishermen. An estimated 150,000 sea turtles were captured worldwide in fishing nets last year, with one-third of those being

caught in U.S waters. Numerous other non-targeted species suffer the same fate. Shrimp trawling has been identified as having the largest bycatch rate of all fisheries, representing 35 percent of the worldwide total for all fisheries combined. Shrimping in U.S. waters is responsible for close to 1.5 million metric tons of bycatch per year. Up to fourteen pounds of fish are destroyed and discarded for each pound of shrimp harvested in some shrimp fisheries. Therefore not only do these fishing practices place sea turtles at great risk; they threaten the biodiversity of the entire ocean. However, there is a simple and inexpensive, device that can be installed in the nets of shrimp trawlers that can greatly reduce these negative affects of the industry.

These devices are known as TEDs, or Turtle Excluder Devices. TEDs are metal grids that are sewn into shrimp nets and guide sea turtles and other unwanted bycatch out an escape hatch. U.S. government studies show that the proper design, installation, and use of TEDs can reduce the number of turtles killed by shrimping by 97% or more. Furthermore, TEDs reduce the bycatch of other marine organisms by up to 60%. TEDs are required on U.S. shrimp fishing vessels by way of a provision of the U.S. Endangered Species Act. The problem is in getting some fishermen to properly install, or use, a TED at all.

Like other provisions of the endangered species act, the TED provision is one that is very difficult to enforce. Many fishermen see the value of TEDs. TEDs were actually developed by a fisherman from Georgia in the 1970s to reduce bycatch and increase the time he could pull his net for shrimp. Still other fishermen contend that TEDs cause them to lose shrimp, and thus alter the TED in a manner that makes it ineffective in releasing sea turtles and other bycatch.

Pollution is the final threat from man that subadult and adult sea turtles must contend with. Unfortunately many humans view the ocean as an unlimited resource, and they exploit it by dumping pollution and trash into the sea. Materials such as plastic bags or congealed oil are often mistaken by sea turtles to be jellyfish, a common food source. When these materials are ingested by the turtles they either choke on them or in rare instances die of starvation because of their inability to digest the foreign object that is now obstructing their digestive tract. Pollutants invisible to the turtles are a very real threat to their survival as well.

A disease known as fibropapilloma is now affecting large numbers of sea turtles and as alluded to earlier it is not a naturally occurring disease. This disease is believed to be caused by a toxin, okadaic acid, which is produced by a microscopic bottom dwelling alga known as a dinoflagellate. These dinoflagellates are always present and producing this toxin, but runoff – primarily nitrogen and phosphorus – from land activities such as farming acts as fertilizer for the algae. Fibropapilloma does not directly cause death, but is instead responsible for the formation of tumors on the turtles. These tumors often form around the eyes and mouth of sea turtles

severely limiting their ability to locate and ingest food. Other fertilizing and toxic pollutants introduced into the ocean by man undoubtedly have an adverse affect on food sources of sea turtles. The dying off of turtle grass off the south coast of Florida is only one such example.

The End of  Turtle Education



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TURTLE PARTS 8: SUBADULT AND ADULT

The subadult stage of a sea turtle’s life lasts from the time they reach one year of age until they are capable of reproduction (adulthood) which may take 12 to 25 years depending on the species. Adult sea turtles are believed by some to live to be 90 years or more. Subadulthood appears to begin with the young turtles leaving the relative safety of the floating sea grasses.

At exactly what stage in their development the turtles abandon the grass drifts altogether is not certain, but their

increased rate of growth would suggest that it is during this time period. Food sources located within the grasses are not believed to be substantial enough to support the increased rate of growth exhibited during subadulthood. As the turtles begin this stage of their life they are about the size of a coffee cup saucer and grow to the size of a large Frisbee within 5 years. The turtles continue to grow at a rate that has some species reaching a mature size of over 125 centimeters in length and over 90 centimeters wide in just 7 additional years. To support this rapid rate of growth, the turtles must now begin migrating to feeding grounds located closer to shore.

What constitutes a suitable feeding ground depends on what species of sea turtle you are concerned with and what stage of development the turtle is at. Sea turtles feed on a wide variety of both pelagic (open ocean) and coastal water organisms as subadults and adults. Almost every type of crab imaginable and other crustaceans, tunacates, mollusks, jellyfishes and fish make their way into the stomach of a sea turtle. It seems that most species, except the green, have the same diet both as subadults and adults. The proportional amount of each type of foodstuff does change as the turtles grow older, but with greens this proportional change is drastic. They go from an omnivorous (both animal and plant matter) diet as subadults to a strictly herbivorous (plant matter) diet as adults, feeding on sea grasses and algae that grow in the shallow coastal flats of warm water regions. A great majority of the sea turtle’s life is spent in search of and consuming food. The remainder of their time is spent resting and migrating from feeding grounds to mating and nesting sites.

Subadult and adult sea turtles will generally rest at the surface of the water. They are also known to rest underwater, by lodging themselves under some sort of structure or digging themselves into the ocean floor. Exactly how long the turtles can remain under water resting is not known and most likely varies from species to species. However, it is known that most species can spend approximately 40 minutes underwater while swimming. With body functions slowed, it seems logical to presume that turtles can remain submerged longer than 40 minutes when they are resting. The amount of time spent resting can not be too great, because sea turtles migrate such great distances.

Distances covered by sea turtles migrating between feeding grounds and mating and nesting sites can be greater than 6,000 miles. They often traverse entire oceans. Swimming such distances shows incredible stamina and an even greater navigational ability. The turtle’s ability to locate these sites for mating and nesting is truly incredible and not yet fully understood. It is presently believed that the turtles use a combination of physical markers, mineral imprinting and some sort of biological directing device to find these areas again. Further research into this subject will hopefully solve this mystery. Now that the turtles have migrated this far, it is time for mating to commence.

Just prior to mating the turtles begin congregating en masse in waters adjacent to their nesting beach. This three or four week period, before the female first emerges to nest, is believed to be the only time that she is receptive to mating. Females generally mate with several males during this period of time. This ensures that the eggs will be fertilized by several males and likely assists in keeping the genetic diversity high within the population.

Before mating can actually occur the male must first court the female. Courtship in sea turtles entails the nuzzling of heads and the playful biting of the back of the females' neck and rear flippers by the male. If the love bites and nuzzles do not chase the female away, the male will attach himself to the carapace (top shell) of the female using claws that protrude from his front flippers. Copulation (sexual intercourse) occurs once the male wraps his long tail, which only the males have, under the female pressing it against her plastron (bottom shell). This sperm is then stored by the female to be used to fertilize the eggs that she shall lay 2 to 3 years later. Copulation can occur either underwater or at the surface.

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TURTLE PARTS 7: OBSTACLES FOR HATCHLINGS

Artificial lights, predators, and pollution can all be fatal to turtle tots. When people place lights on or near a sea

turtle nesting beach they can create a situation in which the horizon of the ocean is no longer the most brightly lit area, and this causes the hatchlings to become disoriented. This disorientation can lead to the demise of baby turtles.

Crawling in the wrong direction into nearby roads and parking lots, where cars then inadvertently crush them, or into dense vegetation, where they become entangled or completely lost in the vegetation, are just a few of the problems that lights may cause for the hatchlings. Lights may also cause a hatchling to crawl around aimlessly on the beach all night until they succumb to exhaustion or the heat of the following day's sun, which dries them out.

Natural lights such as star, moon and sunlight can cause disorientation problems as well, but this occurs very infrequently and even less frequently causes death. Natural light, primarily moonlight reflecting from a tide pool, may cause a hatchling to crawl along the beach instead of directly down the beach. But, it does not attract the hatchling to crawl landward instead of seaward as artificial light so often does. Any unnecessary crawling around, whether caused by artificial or  natural light, makes the tiny turtles more vulnerable to predators. There are many predators lurking in the dark waiting to pounce on them and even more if the emergence occurs during daylight hours.

The 2 to 7 day period during which hatchlings are still in the nest completing absorption of their yolk is a very

dangerous time for them. Their movement within the nest brings the hatchlings existence to the attention of many predators such as ants, ghost crabs, raccoons, opossums, coyotes and dogs. If the hatchlings are not discovered inside of the nest, all of these predators as well as night herons are waiting to grab them on their journey to the ocean. Should their emergence from the nest occur during the day, the hatchlings are safe from the nocturnal (night) predators, but they still must contend with ants, ghost crabs, the sun’s heat, which can quickly dry them out, and the swarming hordes of shorebirds that can now easily pick them off the beach. Making it to the sea is no easy task for the hatchlings and life does not get any easier once they reach their aquatic home.

Just about any fish that can open its mouth large enough to swallow a hatchling will do so. Grouper, snapper and sea bass are notorious for gobbling down the bite-size turtles. Sea bass have even been known to congregate offshore from a nesting beach waiting for the hatchlings to come into the water. Marauders from the sky are after the little guys as well. Hatchlings are buoyant and do not yet possess the strength to dive more than a few feet below the surface of the water. This limitation makes them easy targets for sea birds as they swoop down to prey upon the hatchlings. Avoiding all of these fish and birds will land a hatchling in sea grasses floating in warm offshore currents, where they are less susceptible to predators. But, predators are not the only obstacles that the hatchlings must avoid in the water. Pollution must also be avoided.

Plastics and congealed oil floating in the water can be mistaken for jellyfish or comb jellies, which are common food sources for the hatchlings. If these items are ingested by a hatchling they can result in death. Various kinds of marine debris, such as monofilament fishing line, can entangle hatchlings. This entanglement can greatly limit their ability to collect food, avoid predators, and may even cause injury, as the hatchling grows larger.

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TURTLE PARTS 6: HATCHLINGS

For the first twelve months following a sea turtle’s emergence from its shell, it is known as a hatchling. Life is a struggle for hatchlings from the very beginning. The first turtles in a nest to emerge from their shells find themselves buried in the beach with a varying number of other turtles and the remaining eggs that have yet to hatch. These little turtles spend the next 2 to 7 days completing the absorption of their yolk and probably waiting on some of the remaining turtles to break out of their shells, before they can emerge from the nest. In order for a hatchling to successfully emerge from the nest, they must have the assistance of some of their nest mates.

Through a cooperative effort the turtles slowly fill with sand the additional space they created by emerging from their eggshells. By flinging their small flippers about they cause the sand above and along the sides of the nest to shift towards the bottom of the nest cavity. Moving the sand about in such a fashion raises the tiny turtles ever closer to the surface until they finally breach the surface of the sand and emerge from the nest.

On rare occasions all of the turtles emerge from the nest simultaneously. But, usually they emerge from the nest on consecutive nights in groups of varying sizes until all the hatchlings – except a few stragglers too weak to make it – have emerged from the nest. Emergence from the nest occurs mostly at night, generally after midnight. This is most likely due to the cooling of the surface sand that occurs during the non-daylight hours and is an indicator to the hatchlings of a preferable time to emerge from their nest.

Once the 6-centimeter long hatchlings have emerged from the nest, they now have to crawl down the beach to the sea. Depending on the level of the tide, this can either be a short crawl or a long exhausting crawl. Hatchlings orient themselves to the sea by visual brightness clues. Under natural conditions the ocean horizon is the brightest area that the hatchling can see following emergence from the nest and will subsequently start crawling towards the sea. Making their way to the ocean successfully appears to be made easier if there are other hatchlings present.

The occurrence of disorientation and the period of time spent resting by a hatchling on their way to the sea decreases when there is a group of them headed for the ocean. They assist one another in maintaining the correct direction and stimulate others to keep moving by bumping into each other. As the hatchlings crawl they use their front flippers in an alternating motion to pull themselves down the beach. This practice immediately changes to a simultaneous motion of the flippers when the hatchlings come in contact with the surf.

Swimming is what the hatchlings are now prepared to do and must do. Waves are the first water they must navigate on their way to offshore currents. It could be as far as 40 or more miles until these currents are reached, but refuge from the open sea and food awaits the hatchlings here. Sargasso grass, also known as turtles grass, and other sea weeds floating in these warm currents provide cover from predators and house various food sources that will nourish the hatchlings during their first year of existence. Little to nothing else is known for certain about the behavior of sea turtles during this first, or so called lost, year of their life.

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