|
Shark Conference 2000
Online Documents Honolulu, Hawaii February 21-24
|
||
|
Sponsored By:
|
Presented By:
|
||
| LIFE STYLE OF SHARKS | |||
|---|---|---|---|
|
Samuel H. Gruber Abstract In the nearly half BILLION years since the direct ancestors of sharks came into existence, they have gone through several episodes of adaptive radiation, becoming increasingly sophisticated until today they represent one of the most perfectly adapted, successful life forms in the sea. Now the 450 million year legacy of the ocean's master predator has been called into question by the Earth's supreme predator, MAN. With his mechanized fishing fleets, satellite imagery, electronic fish finders and GPS-based pinpoint accuracy, the bounty of the sea simply cannot withstand mans depredations. While many finfish stocks have declined over the decades, sharks have proven to be especially vulnerable. Fishing records show that some intensely targeted shark stocks can be decimated in only half a dozen years. Why is it that sharks are so vulnerable to over exploitation and why should we care? Sharks and their allies the skates and rays belong to an ancient independently evolved class of vertebrates called the Chondricthyes, (cartilaginous fishes). While most everyone knows what a shark is, in truth, marine biologists have found them to be difficult to study and so, of all the classes of vertebrates, sharks and their relatives the skates and rays are the most poorly known. What we do know, suggests that sharks are both misunderstood and much maligned. Today, as many shark species suffer drastic declines and some even face extinction, we may rightfully ask whether they will be allowed to survive long enough for us to understand the role they play in the marine environment. Through hundreds of millions of years of evolutionary experiments, sharks evolved a life style that is vastly different from their bony cousins. The life style of sharks is characterized by slow growth, late maturity and low fecundity. Lemon sharks can live to 50 or more years. Sharks are thus on a slow track. In contrast, bony fish are on a fast track. Compare, for example the life history strategies of a salmon and a lemon shark: Two foot long lemon sharks are born alive with 6-8 siblings in the spring of the year and take up to 13 years to reach maturity for 1st reproduction. Salmon are born from thousands of eggs as tiny, fragile larvae. In a period of months, they transform through several stages called smolt and parr, swim down to the sea and disappear. A year later, they reenter the river as adults, lay and fertilize thousands of eggs and promptly die, completing this life cycle in only 12 months! This is a fast track! Based on these fundamental differences in life history strategies, Dr. Gruber will explain, with slides and video, just why sharks are at such risk of extinction today. He will show you the diversity and marvelous adaptations of sharks and will suggest ways that sharks and man can share our great blue planet. A diver descends into the crystal clear waters off Bimini, Bahamas. The living reef comes into sharp focus revealing colors, movement, and a myriad of creatures. Suddenly the diver is aware of a presence - a large shark glides into the area, serene, graceful, and apparently unconcerned with the alien human. As quickly as it arrived, it disappears into the warm blue haze. This first close encounter with a shark leaves a permanent impression on the diver. The initial, intense pangs of cold fear give way to awe and admiration. The creature's size, form, movement, and easy power set it apart from everything else on the reef. Yet overlying the reality of the great fish is a residue of western mythology at odds with the truth. Later, the diver excitedly tells and retells the story of this encounter. Then the questions come. What kind of shark was it? Are they common, dangerous? How big do they get? Do they live in schools? How old are they? These kinds of questions define the life-style of all shark species. The systematic study of these very general questions is subsumed under the Theorv of Life History and can teach us much about the ecological role and especially the biological evolution of sharks. In a more practical vein, understanding the life style of sharks can provide important insights into protecting them from over harvesting by fishermen. Throughout the following pages, I will acquaint you with life history theory, what it can tell us, how it is studied, and especially how the life style of most sharks differs from the other great group of fish-like vertebrates, the teleosts or bony fishes. Sharks are an ancient group of vertebrates (backboned animals). They arose in the Paleozoic some 400,000,000 years ago during a period we call the Devonian. At that time, most of the recognizable invertebrates such as starfish, sea worms, jelly fish, clams, and crabs existed, but very few fishes could yet be found. Those that swam were like small jawless vacuum cleaners - primitive, armored things. There were, however, some larger, jawed fishes with articulated bones covering their bodies like medieval, aquatic knights. The fossil record shows that these first, archaic fishes had skeletons composed of true bone. But a few million years later, a new, very different, and much more recognizable fish-like vertebrate began to show up. Unlike their bony cousins, the skeletons of these fishes were completely cartilaginous and flexible. If you caught one of these eight-pound Devonian fishes, you would have no trouble recognizing it as a kind of small primitive shark. Thus arose a new group of fishes, the Chondricthyes, which today comprise the sharks, skates, rays, sawfishes, and a strange group, the chimaeras. Over the next hundred million years, the Chondricthyes underwent extreme and often bizarre evolutionary experimentation with a variety of body-types, odd mating structures, and feeding specializations. In the Carboniferous, some 320-250 million years ago, sharks and their relatives passed through what Dick Lund calls their "Golden Age." Swimming in the warm shallow seas of Montana along with another survivor from that dim age, the coelacanth, sharks actually outnumbered the bony fishes by a ratio of 6 to 4.  A Mesozoic littoral shark, Hybodus sp. Drawing by M. Gruber. In addition to structural differences, studies have suggested that the cartilaginous fishes already possessed a lifestyle radically different from their less than numerous bony counterparts. Analyses of the fossil record yielded strong evidence that the basic pattern of shark life in the Carboniferous era was much the same as today. To biologists, the key to this insight is provided by a fin structure more characteristic of sharks than their jaws and teeth. Even the most ancient sharks possessed finger-like extensions of their pelvic (rearward paired) fins called "claspers." We know from modern sharks that these claspers are sex organs used by males during mating to impregnate females through copulation and internal fertilization. Thus, fully 350,000,000 years ago, sharks had evolved a reproductive strategy which favors the production of a small number of offspring, retained, protected, and nourished within the body of the mother, and requiring a strong investment of the female's time and resources. This fact alone, as interpreted within life history theory, was to have profound implications on the evolution of sharks and set both biological - and for man - economic constraints on their abundance. So what is it that we are talking about when we deal with life history? According to ecologists, an organism's life history is composed of patterns or episodes of growth, where the individual reaches maturity through a process known as differentiation, obtains and stores resources, and produces offspring. From a research viewpoint, the scientist collects data on growth, behavior, reproduction, and inheritance (genetics) to produce theories about how the course of evolution has adapted a particular species to its niche. The manifestation of this evolutionary course is called the "life history strategy" of a species. Here, strategy may be thought of as a set of inherited traits, molded by natural selection, which allows an animal (or plant) to solve an ecological problem. Since all evolution depends upon "survival of the fittest," reproductive success plays a central role in life history theory. Simply put, in the game of life, an animal stakes its babies on a changeable and often unreliable world. The animal wins the game if its offspring survive to play another round of the game of life. The appropriate tactics for winning the game make up the successful life history strategy. The crucial and practical question for us is whether the 400,000,000-year-old life history strategy of sharks will permit them to play another round of life in an ocean increasingly populated by superpredators plying the seas with mechanized fleets, freezer ships, 100-mile long lines and 30,000 miles of drifting gill nets. But more about that later. Obviously, any life history is composed of a large number of biological variables, so the potential number of life history patterns is enormous. But this must be so. No two creatures can have exactly the same pattern or one would eventually out compete and destroy the other. Thus, when we speak of the "life history of sharks," it must be clear that we are talking about 350 different patterns corresponding to the 350 or so living species. Within the sharks, there will be a range of variables which have evolved and represents a trade-off between the cost of developing that variable and the benefit of possessing it. For example, sheer size is one of the most obvious and readily appreciated characteristics of a species. The weight and length of living sharks varies through several orders of magnitude from a few ounces to many tons. On the one hand, the Pacific dwarf shark grows to a maximum length of less than 12 inches. On the other, the gigantic, filter-feeding whale shark certainly reaches 45 feet and is the largest fish in the sea. Benefits of large size include better offensive and defensive capacities. Elephants and large sharks have no natural enemies other than man. But it costs a lot to maintain a large body. This is what I mean by trade-offs. On the overall scale of animals, sharks may be considered large. Growth and development are related to size and are very important as they affect the onset of maturity and ultimately reproduction. All kinds of growth patterns are found in nature. Many fishes go through a tiny larval stage where the flimsy individual looks nothing like its parent. But most sharks follow a rather conventional pattern of growth and development. After a long period of fetal development from the egg - up to 22 months in the dogfish, even longer than an elephant - the shark pup is born in a large and advanced state. Most newborn sharks are miniatures of their parents and recognizable from the start. After birth, the process of growth to maturity assumes an important role in life history theory. One strategy is to pour effortt into rapid maturation so that the species can reproduce as fast as possible. Some insects require only hours after hatching to reach maturity, and the male neither feeds nor sleeps but functions only in sexual reproduction. Such animals are on a fast track. But this is definitely not the case for sharks. They are on a slow track. My studies of the lemon shark, a large, tropical predator, illustrate the general strategy followed by most of the familiar species. Lemon sharks are born in the spring on shallow mangrove flats, fully formed and ready to go. The pups average two feet and a little over two pounds at birth. The mothers on the other hand are at least eight and a half feet long and around 350 pounds at first maturity. Thus a lemon shark pup must double its length and double it again, undergoing a simultaneous increase in mass of over 150-fold before it can reproduce. From our tagging studies we were very surprised to see how slowly lemon sharks grow. Most biologists believe that the pattern of growth in sharks follows the so-called VonBertalanffy model. According to this model, the most rapid growth occurs after birth, gradually slowing down as the animal ages until it just about stops after maturity. Thus the newborn shark is growing at the fastest rate just as is the newborn human infant. What was so surprising to us was that a two-foot lemon pup grows only 3-4 inches in its first year of life. Even if it grew a steady 4 inches per year, it would take over a decade to reach sexual maturity. In fact, lemon sharks require 13-15 years to become sexually active. Compare this strategy to that of the salmon which passes through several larval stages, migrates to the sea, undergoes long oceanic migrations, returns to its home stream, mates, lays 2000 fertile eggs, and dies all in the space of two years. Sharks are indeed on a slowtrack. Reproduction is really the central issue in life history studies and is the key to many of the evolutionary aspects of life history theory. Reproduction is a highly variable process ranging from asexual development where species such as corals simply form buds as one of the colonial animals, called polyps, divides in two, to sexual reproduction requiring the union of egg and sperm. All reproduction in sharks is sexual, but because they are a very ancient group, they display some of the most variable reproductive strategies of any vertebrate class. As you will read, there are sharks that lay eggs like chickens, sharks that retain eggs within their bodies like rattlesnakes, and sharks that develop from materials passed via the mother's blood through a placenta and umbilicus in a way perfectly analogous to the mammals. But even with all this variation, one thread unites the reproductive strategies of all sharks: fertilization is always internal and the shark mother produces a small number of well formed babies after a long period of development usually requiring a strong investment of her food resources.  Nurse sharks mating. Photo by N. Rotise.
Added to this, many sharks require decades to reach maturity, then spend up to two years of pregnancy, and may mate only every other year. Lemon shark pups develop over a long 12 months of pregnancy, and the mothers require an additional 12 months to regenerate their bodies before mating again. Thus a mating pair of lemon sharks barely reproduce themselves over the 24-month reproductive cycle. Typically 8-12 pups are born every other year with a first year mortality approaching 50%. So from two mature sharks only 1-3 babies survive every third year. Clearly rebuilding a population under these circumstances would take many decades. And so it goes for the majority of sharks. There are some small species which are on a somewhat faster track such as the hound, tope, and sharpnose sharks. These mature relatively quickly in perhaps only two years and some species even reproduce annually through two seasons. But they still have a very low reproductive potential, giving birth in some cases to only 3-4 pups each season. Many other factors such as longevity, migration and dispersal, energy storage, behavior as it relates to obtaining resources (food, space), and parental care all are sources of life history variability and provide the raw materials for evolution to mold a particular species to its niche. In addition to these individual life history variables, there are a number of composite factors such as age at maturity, frequency of reproduction and litter size, age at last reproduction, and rate of survival. Such factors can be combined into a so-called life table. Life tables are the basic tool of actuaries, the statisticians who determine how much your insurance premiums will cost. The life table gives actuarians information on whether the segment of the population you belong to is stable, shrinking, or growing; what the age distribution in your segment of the population is; and, most important, what the average chances of your survival are. Fishery biologists do the same thing with life tables. While they do not bill the fish for insurance premiums, they nevertheless can have a profound effect on the fish's quality of life since they make recommendations to government managers which can keep a stock from being overfished. I will take up the question of overexploitation and management for sharks shortly. But first I would like to outline some of the theoretical aspects of life history theory as they relate to evolution and ecology. In general the life pattern of sharks may be characterized by slow growth, delayed maturity, low fecundity combined with production of a few advanced offspring, longevity, multiple breeding, and large size. This pattern can be found in a number of terrestrial and marine creatures and represents one end of a scale of life history patterns. The other end might be illustrated by certain bony fish. As already mentioned the main features of a salmon's life history pattern include rapid growth, early maturity, high fecundity combined with the production of thousands of tiny, poorly developed, flimsy offspring, single breeding followed by death: in other words, a short, fast life cycle. Biologists studying various life histories recognized this apparent dichotomy many years ago. In 1958 after years of study, the famous ecologist, Robert McArthur, was able to explain the significance of these two ongoing patterns. McArthur knew that the ground work for describing how a particular population grows or declines was formulated two centuries ago when Malthus predicted that humans would eventually overpopulate the world. Sixty years ago the mathematician Lotka refined the original work and produced a mathematical model of population growth which is used today by most ecologists. Lotka realized that for animals and humans living under constant and favorable conditions the population will tend to stabilize. Increases due to new births are exactly offset by deaths. Such a population has a stable age distribution and is said to be in equilibrium. Now imagine that a sudden catastrophe kills off a large number of individuals. Clearly, the population will rebound if conditions after the disaster are restored. If this were not the case, each catastrophic event would push the population ever closer to extinction. With fewer mouths to feed after the event, a survivor might have an easier time fmding food which would provide more time for successful breeding. So as conditions improve over time, more and more parents will be having more and more babies. Thus in a favorable and uncrowded environment with unlimited resources, the numbers would gather speed then explode. The analytical expression for such an explosion is called an exponential curve. However, a time will come when the habitat again gets crowded, resources will begin to limit the behavior of the individual, and the environment will be carrying about as many animals as possible. As this happens, population growth declines until once again the number of births and deaths will stabilize. This situation is analogous to placing a fund in a savings bank and allowing it to grow with compound interest. At some point the depositor begins to withdraw the interest but leaves the principal intact. The overall mathematical expression describing both the population and bank account is an S-shaped graph of numbers (dollars) over time. This is called a logistical curve. Now consider a species that lives in an unstable and unpredictable environment such as a rocky shoreline where the vagaries of both terrestrial and aquatic conditions conspire to cause frequent catastrophes. Such a species must have a life style adapted to these disasters and be able to quickly take advantage of new and uncrowded conditions. Consider on the other hand a different species living under constant conditions in a very stable environment such as the deep sea where the sun never shines and the temperature doesn't vary more than a few degrees.  Relationship between the theoretical Von Bertalanffy growth model (smooth curve) and estimated age-in-length of lemon sharks (solid circles).
Such an animal must be adapted to withstand competition and predation, find and process food efficiently, and generally be prepared to live under conditions approaching the ability of the environment to support or "carry" its kind. McArthur called the former species "r-Selected" and the latter "K-Selected," after the constants in Lotka's formula for population growth. The theory of r/K selection has been with us for three decades and has its adherents and opponents. Its appeal lies in its ease of understanding, and especially in the framework it provides for judging the life history pattern of a species and how that pattern relates to the ecology and evolution of an animal. The framework allows us to list the traits of a species life history and predict more or less how its life history strategy evolved. So, what does the theory or r/K selection predict, and how do sharks fit in? First, where adult mortality is high compared to that of juveniles, single breeding is favored ... as with the salmon. In contrast, low fecundity combined with high juvenile mortality would tend to favor repeated breeding as is found for sharks. The rationale here is literally why put all your eggs in one basket. Another prediction involves age at first reproduction: Expanding populations will have a lower age at maturity; stable or declining populations will have delayed maturity especially when increased size, age, or social status favors reproductive success. Such is the case for sharks. There is some evidence that dogfish sharks making a comeback from overfishing in the North Sea are maturing at a somewhat earlier age than virgin dogfish stocks. But, by and large, sharks mature very slowly. We can predict that increased predation will favor large offspring while increased resources favor smaller ones. For the lemon shark the most critical time is the first year when competition and predation is high. Only about half the lemon shark pups survive the first year because competition and predation are high. But as they grow, their chances for survival increase to a point where there would be very few predators, other than man, that could kill a six-foot lemon shark. So, growing populations are characterized by early maturity, early and strong reproductive effort, and large brood size. Animals adapted to a fluctuating or r-selected environment should be small, mature quickly, mate early, and produce a herd of tiny offspring with little or no parental care but with a major reproductive effort and low survivorship as an adult. In contrast, animals such as sharks, adapted to a stable or K-selecting environment, can be expected to grow slowly to a large size, mature late in life, reproduce seasonally (year after year), producing a few large babies which receive parental care (in the case of sharks, during pregnancy only), and have a good survival rate as adults. Thus in all respects, sharks may be considered the architypal K-selected species. Considering the geological age of the sharks, they may have been the first vertebrate group to take up (evolve) the K-selected lifestyle. But these are theoretical concepts. From a more practical viewpoint how does the shark's K-selected life strategy serve them today? Two factors that are characteristic of animals with K-selected life-history strategies are (1) that the habitat they occupy must be relatively stable with respect to resources and conditions, and (2) that competition and predation, especially on the adults, is relatively low. Both of these requirements have been severely disturbed in recent years due to the activity of man. No more are many of the habitats of sharks stable, benign places. For instance, sharks often give birth in special inshore areas of high productivity known as nursery grounds where the pups spend up to several years of their most sensitive period. But beachfront development and wetland industrialization has seriously impacted this critical environment with oil spills, chemical pollution, and physical degradation. Added to this, man, the superpredator, is not only out-competing sharks for food with mechanized killer fleets and drifting death nets, but taking sharks themselves at ever increasing rates. Clearly the requirements of stable conditions, ample resources, and low predation in many cases no longer hold. This, combined with the low fecundity and slow growth of the (K-selected) sharks has put them in the trouble we see today. The hammering that shark stocks are taking simply cannot be sustained. So, the 400,000,000-year-old K-selected life history strategy, so successful for sharks - as well as sea turtles and whales - has today placed them in jeopardy. An ever-increasing awareness of the importance of sharks to the overall health of the oceans and a more enlightened understanding of sharks in general has led conservation groups to take up the plight of the cartilaginous fishes. Now the U.S. Government recognizes the role of sharks and is moving, albeit slowly, to protect them. We can only hope that you, our concerned readers, will support the growing movement to stop the killing and restore the shark to its previously successful, K-selected life style. Additional Reading: Honig, John and S. H. Gruber. 1990. In Press. Life history patterns in the elasmobranchs. U. S. Dept. of Commerce, NOAA Tech. Rpt. NMFS. Horn, Henry. 1984. Optimal tactics of reproduction and life history, Ch. 14, pp. 412-429. In: Krebs, E. and N. Davies (eds.). Behavioral Ecology: An Evolutionary Approach. Blackwell Scientific Publ., Oxford. McArthur, Robert and E.O.Wilson. 1967. The Theory of Island Biogeography. Princeton University Press, Princeton, NJ. 203 pp. Stearns, Stephen. 1976. Life history tactics: A review of the ideas. Quart. Rev. Biol. 51(1):3-47.
Sharpnose shark. Photo by D. Perrine. * Reprinted from Gruber, Samuel H., ed. 1990. Discovering Sharks. Highlands, NJ: American Littoral Society. |
|||
