Jennifer Carpentier & Linden Harris
Pattern Pre-proposal: Great White Shark Mating Frequencies
Background 4/4
Females in almost all species are required to invest more energy into offspring than males, due to the fact that egg production is more costly and limiting than sperm production. This is definitely the case for species in which the female carries the offspring until she gives birth. The term tradeoff is used to describe balances between different life history traits, such as fecundity, parental care, and growth. It stems from the fact that an organism has a limited amount of energy for a given amount of time. Energetic sacrifices that a female makes during the gestation period are often thought of in terms of tradeoffs. For example, in many cases brood size results from the tradeoff between parental investment and number of offspring.Animals with exceptionally high fecundity typically have small offspring and no parental care. Those who have high parental care, like humans, have relatively few offspring. There is also a tradeoff between how often a female can carry offspring and how long her gestation period is. A short gestation period lends itself towards having offspring more frequently because the energy required for one reproductive bout is fairly low. On the other hand, having a long gestation period causes females to invest a lot in the pregnancy, not allowing them to carry offspring as frequently.
It is important to note that reproductive strategies are tightly linked with all other female activities. The energy that she uses for reproduction cannot be used for travelling, growth, or maintenance. Ultimately, all tradeoffs balance out energy uses of an animal to maximize it fitness based on its environment and genetics.
There seems to be a similar tradeoff occurring in female great white sharks, Carcharodon carcharias; specifically in the ones breeding off the coast of the Farallon Islands. Females are required to invest a large amount of energy into their offspring to ensure a healthy pup. These sharksbreed off the coasts of the Farallon Islands during the Fall(Anderson et al. 2003).The females then swim to Point Conception where they give birth to their pups in the summer months. These sharks give no parental care. Thus far, there is no data on where they are going after giving birth.
Pattern 2/2
The frequency of return to the Farallon Islands is different between the two sexes. Males are seen returning twice as often as females. The female sharks return to the Farallones the fall of the year after they give birth; that is a full 2 years after they were originally there to mate. The males however, return every year in the fall to mate with any available females. The females have been observed coming back to the Farallones every other year on a consistent basis.
Figure 1: This bar graph shows the proportion of the total Farallon breeding population that is present in a given year. 4/4
Goal 2/2
The purpose of this study is to determine why female great white sharks return to the Farallones every other year as oppose to every year. The observed pattern is most likely caused by differences in reproductive tradeoffs between males and females. We are assuming in this study that the males return every year to mate in order to maximize breeding success and therefore fitness. Our main goal is to see how a female’s strategy serves to maximize her fitness. We would like to discover what kinds of tradeoffs she is making in order to increase her reproductive fitness.
Hypotheses 6/6
General Hypothesis 1: The energy required to carry a fetus and give birth is much greater than that required to produce and deliver sperm. Therefore, females must build up more energy in order to prepare for another reproductive event. Females can only gain enough energy reserves to reproduce onceevery two years.
Specific Hypothesis 1:Studies have shown that marine mammals dive to shallower depths when their energy reserves are relatively low (Kooyman et al. 1981). Because white sharks are very large, endothermic, and have similar life history-traits to many marine mammals, we assume for this hypothesis that they will exhibit the same difference in dive depths with differing energy levels. Female energy stores will reach the lowest point when they give birth, then gradually increase over the next year and a half until they are ready to mate again. Males should show a much slighter decrease in energy stores when in the Farallones, due to small energetic costs of mating. The null hypothesis is that female energy stores will stay constant throughout the two year reproductive cycle.
General Hypothesis 2:Females only mate every other year because it’s not worth their energy to travel from breeding grounds to birthing grounds and back again every year. Females cannot allocate all of their energy toward travelling; they must use some for their own growth and maintenance, as well as reproduction. So, why don’t they just mate and birth in the same place to save energy and double their lifetime reproductive output? The fact that they don’t leads us to believe that the mating area is an unfit environment for newborns. The female actually maximizes her fitness by migrating because it allows her babies to be healthy and better fit for their environment. Some aspect of the Farallon Islands makes them a bad place for white sharks to give birth, making Point Conception a more suitable spot.
Specific Hypothesis 2: Females give birth in warmer waters because sharks are unable to properly regulate their body temperature in relatively cold areas like the Farallon Islands at such a young age.This would cause a lower body temperature in newborn sharks that are transplanted to the Farallones than those that remain in Point Conception. The null hypothesis is that body temperature will be the same for newborns in the Farallones and Point Conception.
Specific Hypothesis 3: Females give birth where prey availability is ideal for newborns and juveniles due to size and ease of catch. Newborns transplanted to the Farallones will have insufficient prey availability, and will thus grow at a slower rate than those in Point Conception. Our null prediction is that growth rate will be identical between the two groups of sharks.
Specific Hypothesis 4: Predation rate of newborn and juvenile sharks is greater in northern areas like the Farallones, but reduced or absent in the southern areas where females give birth. Our null hypothesis is that mortality rate due to predation is zero for both groups of sharks.
Species Description 4/4
Thus far, Great White sharks have been incredibly difficult to study due to their elusiveness and huge size. For this reason there is a lot of missing information on this unique and powerful species that is often referred to as one of the oceans top predators. There is little if anything that preys on this species, and even as newborns these individuals have no natural predators other than large marine mammals like Orca whales. Great whites tend to have low fecundity, a long lifespan, and late age at maturity, indicating that the life history traits and lifestyle of the shark may be more similar to that of marine mammals (Pardini et al. 2001).
Male white sharks mature between the ages of seven and nine, while females mature between the ages of twelve and seventeen. During each reproductive event a female can have between two and seventeen babies (Last et al.). The sharks are known to have ovoviparous births, meaning that a baby develops within an egg inside the mother’s body, but the egg hatches inside of her when she is about to give birth, and she then gives birth to live young. There is no documented parental care in this species. Individuals on average live 30-40 years,
Great whites spend most of their time at the oceans surface, usually less than 5 meters deep. However, a small portion of their time is spent at great depths, usually averaging between 400- 600 meters (Bonfil et al. 2005). Some sharks have even been recorded to dive down to 1000 meters. These sharks are one of only a few endothermic sharks. The term endothermic refers to warm-blooded animals that maintain a fairly constant body temperature by regulating their metabolic rate. Most sharks however, are ectothermic, meaning their body temperature fluctuates because they rely on environmental heat sources to control it. Many people are unsure as to why the white shark and less than a handful of other sharks are endothermic, but some think it may have to do with having a more active lifestyle. Many endothermic species have a higher level of hemoglobin and hematocrit in their blood than do ectothermic species. Great whites tend to have hemoglobin and hematrocrit levels that are similar or even higher than many mammals, suggesting again they do have similar traits to many marine mammals (Emery 1986). Mean stomach temperature, which is how body temperature is measured for this species, is 26.6 C (Goldman 1997).
Site Description
2/2
Great white sharks inhabit many of the world’s oceans, however, we are focusing on a group of white sharks that mate in the Farallon Islands and give birth around Point Conception. The Farallon Islands are a group of islands located in the Gulf of the Farallones, off the coast of San Fransisco, California. They are just 27 miles outside the Golden Gate and are officially part of the city and county of San Fransisco. The islands are uninhabited by humans, but do have several research stations that scientists visit. The ocean water surrounding the islands ranges from 13.3 to 16.4 C (Goldman 1997). The islands are often inhabited by up to five species of pinnipeds, making them a great destination for great whites who prey upon seals.
After mating in the Farallones the females swim down to Point Conception, located in southwestern Santa Barbara County, California. This is the point where the Santa Barbara Channel meets the Pacific Ocean. This point is an area of temperature discontinuity and there is a drastic difference between the species living above and below this point. Many studies of distributional patterns of fishes are conducted here. This area is highly productive and there is strong upwelling.Most species inhabiting the waters above this point prefer the cold water that is coming from the north, while species inhabiting the waters below the point prefer the warmer water that is coming from further south. For this reason, Point Conception is a very unique area. The female sharks from the Farallones come here to give birth.
Methods
In order to test the first hypothesis that average female dive depths vary during the two-year period between each mating event, we will tag six female sharks and six male sharks with satellite tags and time depth recorders when they arrive in the Farallones to mate. We will lure them to the ocean’s surface using plywood cutouts of seals like UCSC graduate student Scott Davis did in a study on great white migration (Shwartz, 2002). When the sharks surface, trying to mouth the decoy, we will use a long pole to stick a pop-up satellite archival tag (PSAT) on each shark with an anchoring device. These tags are electronic storage devices that can record ambient light levels, swimming depth, and internal and external temperatures of the shark. We will then let the sharks go as they please for the next two years; for females this will be for the duration of a single reproductive event and for males this will be for the duration of two reproductive events and thus two mating events. At the end of the two-year period when all twelve sharks return to the Farallones, we will collect the tags and download the archived data using the Advanced Research and Global Observation Satellite (ARGOS) system. We can then graph the average dive depths of females along with average dive depths of males against the two-year time scale. We will be focusing on collecting the average dive depths for the sharks’ deeper dives because we expect their shallower more common dives of less than five meters to remain constant. If the females make their shallowest dives in the spring when they give birth at Point Conception and then gradually increase in depth during the next sixteen to eighteen months until they mate again in the Farallones, and the males make dives to fairly constant depths during the entire twenty four month period, then we will assume that female sharks have to build up more energy before their next breeding season. 4/5 no stats
In order to test the second general hypothesis that the Farallones are an unfit breeding ground for this species, we will tag eighteen newborn sharks in and around Point Conception. These tags will allow us to locate the newborn pups and measure body temperature, growth rate, and predation rate for specific hypothesis two, three, and four respectively. The eighteen sharks will be split into three groups, allowing six of the sharks to be used in each of the three experiments. We will tag these eighteen baby sharks by luring them to the ocean’s surface using chum. Once they are at the surface they will be small enough that we can catch them with a netted gurneymade of pvc pipes. The four pipes will be in a rectangular shape, 2x1 meters, with a net stretched over it. There will be a rope from each corner of the gurney that connects in the center of the gurney so that once we have lured the shark onto the gurney we can haul it onto our boat. Once on board we will continuously run seawater over its gills and cover its eyes with a cloth so that it will remain calm and feel less stressed. We will then surgically implant a satellite tag into the abdomen of each shark. Implanting the tag will help us obtain more accurate information on internal temperatures of the sharks and also help us re-locate the sharks. 4/5
To test the second specific hypothesis that the females give birth in warmer waters like Point conception because newborn sharks are unable to regulate their body temperature well enough to survive in colder water, we will leave three of the tagged sharks in Point Conception as a control, and take three of the tagged sharks to the Farallones by boat. We will recapture the sharks in Point Conception and the Farallon Islands one year later, luring them to ocean’s surface using the same method as previously stated. The sharks at this point will still be small enough that we will be able to bring them on board with the gurney again and then surgically remove the tag. After downloading the stomach temperatures of the sharks, we can graph the data; averaging and comparing the internal temperatures of sharks in the manipulated group in the Farallones and the control group in Point Conception. If the average stomach temperature for the group of newborn sharks in Point Conception is higher than the average stomach temperature for the group of newborn sharks in the Farallones, then we will conclude that it is more difficult for the sharks to regulate their body temperature in the colder waters.
The third specific hypothesis isthat the female sharks give birth off the Santa Barbara Coast because there is greater prey availability for the newborns. For this experiment we will be measuring rate of growth in terms of length and relating the growth rates to prey consumption. Upon tagging each of the sharks we will also measure and record the length of shark from the tip of the snout to the beginning of the split in the tail. Once again we will leave three of the sharks in Point Conception as our control group, and take three back to the islands off of San Fransisco’s coast. We will locate and recapture these six sharks once every thirty days by luring them to the boat with chum. After twelve captures or about one year, we will capture them for the last time and surgically remove the satellite tags. We will not need to download any information for this experiment since the tags were only used to locate the sharks because we were taking the physical measurements. We will graph and compare average growth rates of the two groups. If the average growth rate of the group of sharks in Point Conception is greater than the average growth rate of the group of sharks in the Farallones during the one-year period, we will 4/5
To test our fourth and final specific hypothesis that female sharks give birth at Point Conception because there are fewer or no predatory species compared with the waters in northern areas like the Farallon Islands. We will again tag six of the previously mentioned eighteen babies, taking three to the Farallones and leaving three at Point Conception. We will also be locating these sharks once every thirty days for a year and lure it to the surface using chum. However, for this experiment we will merely be locating them to make sure they are still alive, and therefore no consecutive recaptures or measuring will need to be made until the end of the year when we surgically remove the tags. If we can locate and physically see a shark, we will therefore assume that it is still alive and that nothing has preyed upon it. If we cannot locate a shark or lure it to the surface, we will assume that the shark was preyed upon. If another large animal or possible predator surfaces when we are trying to lure a shark, we will also assume that the shark was eaten and that the tag is now in the stomach of the predatory animal. If the tag stops working we will again assume that the shark was attacked and killed because once the shark dies, unless eaten completely, it will sink to the ocean floor. The tags cannot withstand depths great than 1200 meters (which a newborn shark would not be able to dive to), and thus if it stops working, it means the dead body of the shark is sinking. At the end of the first year we will compare percent mortality due to predation between the two groups of sharks. If there is a higher percent mortality due to predation on the sharks in the Farallones than the sharks at Point Conception, we will assume that there is a higher risk of predation in the Farallones. 4/5