Population Viability of the Lemon Shark

This topic submitted by Matt Nelson ( nelsonmh@miamioh.edu) at 9:40 PM on 6/9/05.

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Population Viability of the Lemon Shark

The Lemon Shark, Negaprion brevirostris, is one of thirty-nine shark species protected by the US Government through the Department of Commerce. Although it is one of the most studied sharks, very little is known about its breeding habits and the adult phase of its life. The Lemon shark is a part of an aggregate of sharks maintained by the National Marine Fisheries Service and is rated by the World Conservation Union/Species Survival Commission (IUCN/SSC) as “Near Threatened”. It is close to being classified as vulnerable, but does not quite meet the criteria. The Classification is based on the current population and effects of current fishing practices on the future population. Despite being protected by the US Government, the Lemon Shark is being hunted both commercially and recreationally for its fins, skin, and meat. The Lemon shark makes its home on the coastal inshore waters from New Jersey to Southern Brazil, Gulf of Mexico and is most common in the Caribbean. It is also found on the Ivory Coast of Africa and the western Coast of South and Central America. There are many studies currently being conducted to learn more about the Lemon Shark before it is too late. Its population viability is a concern among many ecologists. Its location being mainly in the Caribbean along with its existence being in danger make the Lemon shark a relevant topic for Tropical Marine Ecology.

The Lemon shark was first described in 1868 and named Hypoprion brevirostris. It was later turned to Negaprion brevirostris. It gets its name from its pale yellow-brown coloring found on the dorsal surface and a lighter yellowish color on the undersides. The lemon shark is one of the larger species of sharks. Its sizes vary according to the source. Most say their large, stocky body grows between 8 and 13 feet in length, 13 being the largest ever recorded. According to www.flmng.ufl.edu, Lemon sharks have an average growth rate of .21 inches/year. It is a blunt nosed shark with two dorsal fins of similar size. The snout is round and shorter than the width of the mouth. Its upper teeth are narrow and broad with triangular smooth-edged cusps and finely serrated bases. The lower teeth are narrow and triangular with smooth-edged cusps. Lemon sharks have between 3 and 5 ridges, or rows, of teeth on both the upper and lower jaw. Their long thin sharp teeth are designed to catch slippery fish, the mainstay of the shark’s diet. A young lemon shark loses an entire set of teeth, one at a time, every 7-8 days. They are grown rapidly in the back of the jaw and as the shark loses teeth they are rotated forward.

The lemon shark is primarily a subtropical, shallow water shark and inhabits coral reefs, mangroves, enclosed bays, sounds and river mouths. It is commonly found over sandy or muddy bottoms at a depth of at least 90 meters. The Lemon shark is a nocturnal shark. It retreats to deeper waters during the day, and returns to hunt among the shallow corals at night. One of the shark’s advantages is that it can lay motionless at the bottom in order to draw in prey. It feeds mainly on fish, including smaller sharks. It also feeds on crustaceans and mollusks and occasionally sea birds.

The lemon shark has been known to attack humans on occasion. According to the international shark attack files, the lemon shark has been attributed with 10 recorded unprovoked bites, all of which were recorded in Florida and the Caribbean. None of the attacks resulted in death. Even though this is the only area to experience attacks, the chances of humans being attacked are slim to none. The lemon shark is a generally shy shark and will not approach humans. The shark is aggressive, however, when its space is invaded and it is provoked. Nevertheless, the shark is nocturnal and will usually not be seen in the corals during the day. It typically only comes out at night to feed.

Lemon sharks have an average life span of 25 years and usually reach sexual maturity between 12 to 15 years of age. Female Lemon sharks give birth every other year. Reproduction with the Lemon shark is viviparous, meaning they give birth to live, free swimming offspring. Tests have revealed that Lemon sharks demonstrate polygamous mating and multiple paternity. In other words, many males can impregnate a single female at one time, producing offspring with differing genetic structures. Their reproductive patterns are still largely unknown.

General observations of the lemon shark’s reproductive habits have given us indication of certain behaviors. Male Lemon Sharks are thought to be nomadic with no permanent home. They roam around hunting and searching for females when breeding time comes. Conversely, it is hypothesized that female sharks from the same lineage return to the same spot every two years to give birth. In fact, they tend to stay generally in the same area their whole life. Mature Lemon sharks have been observed aggregating in certain locations at times. It is thought that when it comes time to breed, in the spring, females locate near their nursery areas and give off pheromones that attract the nomadic males who come and mate with the females. There is then a 10-12 month incubation period until the female gives birth to litters ranging from 4-18 young. The young are then abandoned in shallow water nurseries where they remain for a couple years until they are big enough to survive against predators.

Little is known about these nurseries. What we do know is that, with the absence of parental care, they must provide protection and adequate food to allow survival for up to two years. Research done on baby lemon sharks by Dr. Samuel Gruber from the University of Miami is taking large steps to learn more about the nurseries. His research is primarily done at the North Sound in Bimini in the Bahamas. This is one of the few known nursery grounds for Lemon Sharks. Dr. Gruber is tagging and collecting data on the survival rates of these baby sharks and the DNA structure, since the same mothers are thought to return to these grounds every other year. Dr Gruber hopes to learn a lot about their lineage and influences on survival rates.

One fact about sharks in general and especially lemon sharks is that their genetic structures have gone millions of years with minimal change. The breeding ground and mating habits may have a lot to do with this fact. Dr. Gruber is trying to prove that females locate to certain breeding grounds depending on where their mothers before them did. This would be a possible explanation of the limited cross breeding. Polygamous mating, then, would prevent inbreeding.

According to www.fisheries.vims.edu, Lemon sharks in their first year have a survival rate of only 39%, depending on conditions. After their first year they have an approximate 88% annual survival rate. These numbers are suggestive that the baby Lemon shark being abandoned by its mother in the first year may render it defenseless and an easy victim. On the other hand, research suggests that these nurseries are predator-safe areas. Consequently, the low rate of first year survival is the leading factor inspiring most research done on nurseries and lemon sharks in their first year.

My hope for this project was to perform my own population viability analysis on the Lemon shark because of the well documented questions posed by this shark species. While there are many studies being done currently, little is known about the population viability of the Lemon shark. This is because little is known about breeding habits and population numbers. I was able to locate one journal that performed an analysis on the Lemon shark along with several other coastal sharks. The biological data used to perform is lacking at best, but the most accurate available due to our lack of knowledge about Lemon shark populations. The results of the study returned that the Lemon shark and the Sandbar shark are the two most endangered sharks.

If the input life history parameters used in this study represent reasonably well the demography of the large coastal shark species examined, it is unlikely that their populations can sustain even moderate levels of harvesting. This appears to be particularly true of the lemon and sandbar sharks, two species which exhibit very low values of population growth(Adobe).

The lemon shark, in two trials, returned values suggesting the species was declining at a rate of 85% per year and only with 50% survivorship in year 0 would the population increase at a rate of 1.2% per year. It is apparent that the species is not decreasing at 85% per year. Because we know that the first year survival rate for lemon sharks is around 30 or 40%, and not the required 50% to produce a 1.2% population growth, we can reasonably guesstimate that the species is at most in equilibrium. These statistics may not appear very alarming, which could be why the lemon shark is only listed as “near threatened”, but they may be in more danger than we know. It is the lack of knowledge that is most worrisome about the Lemon Shark.

Fortunately, the enhanced warning signals have spurred many ecologists to research the breeding habits and population viability of the lemon shark. High infant mortality is the main problem in question. The reasons are not clear yet, but since we can reasonably rule out predation, over-fishing is the next most likely candidate. “The populations in the western north Atlantic and eastern Pacific Ocean are declining due to Over-fishing”(Lemon Shark). The shark is highly revered for its shark fin soup, shark meat, and leather bags, which make it a commercial and game target. That coupled with its dependence on smaller fish to survive might be the key. However, there may be many reasons for the infant shark’s low rate of survival. It is up to the research being done now to figure out for sure. When we do obtain the knowledge it may already be too late and the factors causing the population to decline may be irreversible. Our hope is that we can eventually change human behavior to be more enlightened of the consequences of our actions because, sadly, in the grand scheme of things, the lemon shark is not nearly the most threatened species in the sea. It is one of many and it is our responsibility to learn why.

Works Cited

Bimini Biological Field Station Retrieved Jun. 09, 2005, from

Cortes, E. (). Fisheries research. Demographic analysis as an aid in shark stock assessment and management, 39(199-208), 1-9.

Gruber, S. (n.d.). Retrieved Jun. 09, 2005, from Multiple Paternity of a Lemon Shark Litter Web site: http://www.bioone.org/bioone/?request=get-document&issn=0045-8511&volume=001&issue=03&page=0781.

Learning, E. (n.d.). Lemon shark. Retrieved Jun. 09, 2005, from Zoom Sharks Web site: http://www.enchantedlearning.com/subjects/sharks/species/Lemonshark.shtml.

Morgon, A. (n.d.). Biological profiles: lemon shark. Retrieved Jun. 09, 2005, from Ichthyology Web site: http://www.flmnh.ufl.edu/fish/Gallery/Descript/LemonShark/LemonShark.html.

Negaprion brevirostris: Lemon shark Retrieved Jun. 09, 2005, from

Saving the Lemon Shark Retrieved Jun. 09, 2005, from http://www.ecoworld.org/Projects/LemonSharks/LemonSharks.cfm
Shark Nursery Yields Secrets of Breeding Retrieved Jun. 09, 2005, from

Survival of Juvenile Lemon Sharks Retrieved Jun. 09, 2005, from

Tiburon, R. (n.d.). Lemon shark study. Retrieved Jun. 09, 2005, from Lemon Shark Study Web site: http://www.rvtiburon.com/LemonSharkAggregation.htm.

Vinent, B. (n.d.). Lemon shark fact sheet. Retrieved Jun. 09, 2005, from WorldShark.com Web site: http://www.worldshark.com/species/lemon/lemon.html.

Lemon Shark - Negaprion brevirostris - MarineBio.org. Retrieved Thursday, June 9, 2005, from http://marinebio.org/species.asp?id=490.

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