Upcoming Shark Week + Science!
In-depth shark facts (long post)
The Discovery Channel is kicking off Shark Week soon with the National Aquarium shark tank live cam and a Google+ Hangout on Air this Sunday, August 4th.
You can submit questions using #SharkWeekHangout .
I was investigating my own questions and thought I’d share some findings for #ScienceEveryday / ScienceSunday:
1. What do white sharks eat in the wild? Do they primarily hunt seals?
2. How (in)significant of a role does shark eyesight have on hunting ability?
3. How (un)common are shark attacks on humans?
What’s the Deal with Seal Meals?
White sharks (Carcharodon carcharias) have varying dietary preferences that likely change with their age, sex, size, and habitat. Although they were once considered a nearshore species that preyed primarily on seals, this perception of shark foraging behavior in seeking out pinnipeds was probably due to studies focusing on more easily observed coastal areas, as well as biased reports of astounding coastal shark attacks.
Direct observations of white sharks consuming prey other than seals have been more infrequent, although prior analysis of their stomach contents has revealed fish, invertebrates, turtles, and other smaller sharks too.
Population level information about offshore feeding on lower level prey has improved with the help of satellite tagging and is corroborated by sample data from individual sharks.
One very interesting way of estimating individual diet specialization is by analyzing the change in composition of stable isotopes of carbon and nitrogen within animal tissues.
Use of isotopic information to study relationships of organisms with their environment has provided additional ecological insights. Because shark vertebrae grow by accretion — that is, by gradual accumulation of layers — values of isotopes from concentric rings of growth bands provide more contextual detail on lifetime variations in diet and habitat characteristics.
Although limited to identifying distinct qualitative patterns and not exact species, measuring these natural tracers help match chemical signatures to further reconstruct predatory preferences. Depending on the individual subtype, data from white sharks in the northeastern Pacific , for example, demonstrate feeding preferences for both lower level and higher level prey potentially consistent with the harbor seal, harbor porpoise, northern elephant seal, California sea lion, dolphins, tuna, shallow coastal (neritic) fish, and cephalopods like octopus and squid.
Overall, most northeastern Pacific white sharks appear to be generalists among a full spectrum of consumers, including some high-degree seal specialists.
Shark-o-Vision (sans lasers)
Different species of sharks have senses refined for different purposes, including mechanical receptors to orient to large tidal currents or small-scale currents to detect nearby predators and prey, electroreceptors for geomagnetic navigation and detecting low-frequency bioelectric fields, low sensitivity hearing, taste likely comparable to other vertebrates, chemoreception, and olfaction — the last of which has been identified as perhaps the most important way sharks find food .
In terms of vision, oceanic and deep-sea sharks tend to have larger eyes than coastal sharks. Opposing eyes allow for a nearly 360° visual field.
Studies as early as the 1960s have shown certain sharks could be trained to visually discriminate between different high-contrast shapes and patterns like squares, diamonds, and stripes. Evidence from the late ’60s indicate that lemon, bull, and nurse sharks can learn visual tasks as quickly as mice.
One estimate of the spatial resolving power of a lemon shark’s eyes based on the topographic distribution of its retinal cells is about 1/9th that of human retina, but the lemon shark’s lens has an optical power 7 times that of a human lens. In other words, distinguishing details are low, but its ability to see far is high. In order to focus, sharks do not have intraocular muscles that change the shape of their lens like humans do, but rather move the lens closer or farther away from their retina.
A white shark’s visual discrimination may be 6 times more than that of a lemon shark, although more thorough assessment is required. Before the discovery of cone receptors in some of their retina in 1963, people thought most sharks had extremely poor visual ability and no color vision, which we now know is untrue.
Lemon sharks can dark-adapt with 1 million times more sensitivity than under bright light conditions, which is better than humans. However, rod-dominated retinas do not automatically mean their vision is adapted primarily for low-light conditions and crude visual acuity; the human retina also has many more rods than cones, and our day vision and acuity are among the best in the animal kingdom.
Although many sharks rely upon their sense of olfaction, touch, and electroreception for navigation, it is common they use vision during their final approach to prey. Studies in the late 1990s indicate white sharks visually orient to surface targets of 1/2 a foot in diameter from a depth greater than 50 feet below.
Sharks could potentially mistake the movement of bright reflections off wet bare human skin as prey, although some scientists believe they approach and investigate mainly out of curiosity. Visual “attractiveness” depends on shape, brightness, contrast, size, color, and distance.
When Sharks Attack!
An internationally recognized source of accurate scientific information on shark attack data appears to be maintained by the University of Florida and the Florida Museum of Natural History .
The total number of confirmed attacks on humans from any kind of shark specifically occurring along the Pacific coast of North America over the past half century was 107 persons . Going back over a century, there were 180 documented cases, 13 of which were fatalities recorded near California. A majority of these injuries were due to white sharks .
The greatest number of confirmed white shark attacks worldwide has occurred along the western US, but Australia has the highest proportion of white shark attack deaths.
Although shark attacks may be underreported, media coverage and improved scientific documentation likely account for the increase in worldwide death rate from about 0.2% to 0.7% over the past several decades .
#Sharks #NotShaqAttack #NotSharknado
 Ontogenetic and Among-Individual Variation in Foraging Strategies of Northeast Pacific White Sharks Based on Stable Isotope Analysis (2012): http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0045068
 The Sensory Biology of Elasmobranchs (2004): http://www.kmaruska.biology.lsu.edu/Hueter%20et%20al%202004.pdf
 International Shark Attack File: http://www.flmnh.ufl.edu/fish/sharks/statistics/statistics.htm
 Shark Research Committee: http://www.sharkresearchcommittee.com/statistics.htm
 White Shark Attacks and Fatalities Worldwide from 1876-2011: http://www.flmnh.ufl.edu/fish/sharks/White/World.htm
 Worldwide Unprovoked Shark Attack Fatality Rate: http://www.flmnh.ufl.edu/fish/sharks/statistics/worldwidefatalityrate.htm
Photo by Jester Fawls