July 12, 2002
California Sea Lion family:
There is a high degree of polygyny and sexual dimorphism in the California sea lion population. Females usually grow to about 1.8 meters in length and weigh approximately 100kg. On the other hand full grown male sea lions are about 2.4 meters in length and weigh up to 300kg. Sea lions normally live between fifteen and twenty four years (Berger 2000).
Sea lions are considered to be gregarious, spending most of the year in small to large groups. Sea lions are very social and will rest in large groups, packed closely together. The sea lion is a very proficient swimmer and can swim at a rate of 10mph. A sea can dive to a depth of 375m and can stay underwater for 8 minutes. They can be seen floating together at the oceans surface, behavior known as "rafting. Sea lions will porpoise or jump out of the water to increase their speed while swimming, they have also been known to surf breaking waves (Marine Mammal Center, 2002).
Sea lions inhabit rocky and sandy beaches of coastal islands. On land they gather in group called colonies, and in the water they form smaller groups called "rafts" (Sea World, 2000).
Sea lions forging strategies are different depending on variations in type of prey, and environmental conditions such as season, location and time of day. Sea lions often feed alone or in small groups of 2-5, however, they will shift to larger cooperative efforts when there are schools of fish or squid present. Groups will be laid out on the sand resting and a certain periods during the day will begin to become vocal and agitated. This vocalizing seems to organize the group to enter the water in hunt of prey. Their prey consists of squid, octopus, herring, rockfish, mackerel and small sharks (Berger, 2001).
Galápagos Islands the California sea lions main predators are sharks an Orcas. If a mother sea lion is killed, her pups will die. It is common to see the remains of sea lion pups along the beach. These pups starved to death, because their mother was killed by a shark or whale.
Mating and Reproduction:
Breeding period in the Galápagos Islands is from May to June. Sea lions breed in specific areas known as rookeries. Mature male sea lions, over the age of five, will appear at rookeries prior at the beginning of the mating season to ensure that they have proper time to stake out their territory. Males will fight until they win dominance of their territory, and then actively protect their territories and the cows that come to that area. Sub adults and bachelor adults also come to rookies and congregate in unused peripheral areas. Female sea lions are ready to breed at the age of three, when a female sea lion is ready to bread she will appear at a rookery. She will come ashore and within a few weeks will give birth to her baby. Just prior to giving birth cows become aggressive and move away form the group to prevent a mix up of offspring. A week or two after giving birth the female comes into estrus. At this time they will mate with the dominant male of the territory. This mating often takes place in shallow tide pools. Once impregnated the cow has a year low gestation period. Female sea lions usually mate once a year and give birth to one pup (Berger 2000).
Caring for and Raising Young:
Sea lion pups are born with their eyes open and can vocalize. They are normally between 70-80cm long and weigh 6-9 kg at birth. Male pups are usually born heavier than female pup. They appear to be able to swim at birth, but are not very well coordinated. For the first few days after birth the mother will remain with her pup and continually nurse. During this initial period the mother and pup develop a complex recognition system to allow them to find each other in the future. As soon as the pup is born the mother and offspring nuzzle and sniff each other, to become accustomed to each otherís scent. The mother immediately begins to vocalize so the pup will learn her distinctive "pup attraction call." The pup will learn its mothers call and will also develop its own response call.'mso-spacerun:yes'> After about eight days the mother will leave the pup to feed at sea. This begins a cycle of the mother leaving to feed during the day and return at night to nurse her pup. When the mother returns she uses her "pup attraction call" to find her young. When the mother first begins to leave the pup will say in the area she left them in, however, once the call is perfected the pup will wonder further. As pups begin to mature they start to follow their mothers on foraging trips. This serves as a supplement to their milk diet. Sea lions begin to wean their pups between six and twelve months after they are born. The weaning is a very gradual process and some mother continues to nurse their pups for two years. Occasionally a mother will nurse two pups or a year or will nurse one pup for up to three years, as long as a new pup is not born (Berger 2002).
Male sea lion discontinue nursing around the age of two; they then join the males on their annual northern migration in February. These male sea lions that have not reached maturity are known as juveniles. They may travel with the male sea lions as long as the competition for territory is absent. Around five years of age the male sea lion reach maturity and develops a sagittal crest or bony bump on the top of their skull.'mso-spacerun:yes'> When mating season arrives the male sea lions become very territorial and fight for possession of breeding rookeries. Those male sea lions who prove to be the best and strongest will acquire breeding territory. Once this territory is accomplished they still have to defend it on a regular basis. The juvenile males or the bachelors who have not won their own territories will spend time in peripheral, unoccupied areas. They will wait there until an opportunity arises that they can mate with females. They will often attempt to mate with females who are swimming in the water or will enter another maleís territory if he is not present at the time. The male sea lions chances of acquiring a territory improve with age. Male sea lions normally live between fifteen and twenty four years.
Sea lions undergo a full molt of their pelage during the year. The molt process is gradual throughout the year, similar to the way in which a dog molts. The whole process is undetectable. A baby sea lion is born with a layer of soft black hair called lanugo. The lanugo helps to keep the baby warmth until their thick layer of blubber forms. Once the baby is about 2-3 months old, it will gradually begin to replace its lanugo with an adult coat (Berger, 2001).
After the breeding season male seal lions migrate to the northern parts of the islands, but the female sea lion stay in the breeding area. The female will congregate with the other female sea lions and their pups. Often one female sea lion will watch a group of pups, while the other mothers go off and search for food.
Past and Present Human Threats to the Sea Lions:
The California sea lion is faced with many threats beyond the chance of attack by a predator. Humans also serve as a predator to the sea lion population. Humans fish off the coast of the Galápagos Islandsand each year hooks and fishing equipment is responsible for the death pf numerous sea lions. The largest percentage of sea lions killed this way are between the ages of one to three years, because their youthful curiosity lures them close to the hooks. Sea lions often get to close to boats and are injured by propellers, the sea lions will also swallow plastic or other man made objects that are thrown in the water.
The Jessica oil spill of January 2001 was also a man made disaster that caused injury and death to the sea lion population of the Galápagos. The oil tanker ran a ground on the island of San Cristobal and dumped an estimated 600,000 liters of oil into the water. Twenty six oiled sea lion pups were collected and washed, fifty had eye infections and one sea lion had a diesel burn from the spill. In March 2001 a sea lion covered in oil was found dead on the island of Floreana, a casualty of the Jessica spill (Seal Conservation Society, 2001).
In 1990 the Ecuadorian government extended the no fishing zone around the Galápagos Islands with the goal of protecting the sea lions from poachers. This increased fishing zone has decreased the number of sea lions killed, but sea lions are still illegally killed in the Galápagos Islands . In July 2001, thirty five sea lions were found clubbed over the head along the beach of San Cristobal. The sea lions teeth had been removed and the maleís genitals were also gone. The genitals are presumably sold in the Asian markets as an aphrodisiac. Authorities were outraged at the murder of these animals, particularly in the Galápagos Islands, where hunting and fishing is prohibited. Unfortunately these restrictions do not always prevent the murder of the sea lions, cases such as the one described above are fairly common in the islands (BCC News, 2001).
El Niño and the Sea Lion Population:
The sea lion population of the Galápagos Islands was hit very hard by El Niño in 1997-1998. The El Niño event brought a rush of warm water to the Galápagos Islands, causing the sea lion population to drop by 48%. Many of the sea lions attempted to migrate during the event, but ultimately died from starvation. 90% of the sea lion pups born in 1997 and 67% of the dominant males died due to El Niño. Once El Niño passed recuperation of the marine conditions and food supply allowed the sea lion population to begin its slow recovery (The Charles Darwin Foundation for the Galápagos Islands , 2000).
The following graph depicts that change in sea lion population due to the El Niño event:
Obtained from the Charles Darwin Foundation
Charles Darwin Research Station and the California Sea Lions Conservation Project:
The Charles Darwin Station has begun a program of monitoring and conservation of the California sea lion population in the Galápagos Islands. This effort is fueled by increasing numbers of sea lions that are becoming trapped in hooks and fishing nets. The oceans off the coast of the Galápagos Islands are quickly growing into a popular area for deep water tuna and billfish fishing. Most of the sea lions caught in these hook were between the ages of 1-3 years. The main goals of the conservation project are to determine the size, structure and distribution of the population. The program are also plans to set up a sighting log to keep track of how many sea lions become entangled in fishing hooks and equipment. The research station is developing simple rescue methods to help sea lions that may be found in an entanglement. The conservation program is scheduled to last two years. The first year will be dedicated to establishing and implementing a monitoring program to understand the status of the sea lions. The next year will focus on understanding the connection between human activities and the drop in the sea lion population, and determine how to protect these animals. The success of the program is requires comprehensive data to be taken throughout the islands and require a great deal of funding (The Charles Darwin Foundation for the Galápagos Islands, 2001).
BBC News. July 19, 2001. "Galápagos Sea Lions Slaughtered." <http://news.bbc.co.uk/hi/english/world/americas/newsid_1446000/1446865.stm>July 12, 2002
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