HOW DOES EL NINO AFFECT DIFFERENT SPECIES OF PINNIPEDS ?
El Niño does not affect all species of pinnipeds the same. Also, the effect within a species may vary greatly depending on the geographic location of different population groups. Effects generally lessen toward the poles. Pinniped populations living in northern latitudes may actually benefit from an El Niño if winter storms are less severe, if there is an increase in fish populations, and if the water is generally warmer than in the traditionally cold winters they are used to living through.
The difficulties for pinnipeds and birds during El Niño events can be summarized in the following list. Any one of these or a combination of many of these may occur.
- food is not where it used to be. It has moved further north.
- food is no longer at the surface where it is accessible. It is there, but much deeper.
- food is of poor quality, of lower weight and density. It is not as nutritional as it is in other years.
- other unsuitable species are now in the territory and are more difficult to catch.
- invading species feed on normal prey species. New kinds of fish are competing for prey species.
- the impact on prey species is long term. It does not go away immediately when El Niño is over.
- Populations take a long time to recover.
The following four species of pinnipeds are found in offshore Southern California waters: northern fur seals, northern elephant seals, California sea lions, and harbor seals. For each of these species the impact of El Niño is different. The impact is directly related to the health of the mother and her ability to carry the fetus to term, the time for the birth of the pups and the nursing habits of the species, the ability of the mother to conceive another pup. There are distinct differences between its impact on phocids (true seals) and otariids (eared seals). The effects listed below are those based on studies or observations done during the 1982/1983 El Niño.
STUDIES ON OTARIIDS:
Northern Fur Seals: In California, there were fewer births, smaller pups, and lower survival rates among the pups that were born. There were fewer females and juvenile males left at the end of the event. A positive effect was recorded for populations in the Gulf of Alaska due to increased food supplies.
California Sea Lions:
In Mexico, there was a 50% reduction in pups with fewer females and sub-adult males found on Pacific side of the Baja peninsula. There was little or no effect on animals living in the Gulf of California
In California similarly there were fewer, smaller pups and a high pup mortality. For females, observations indicated a less frequent copulation rates, a decline in female fecundity, and a higher abortion rate. During the nursing period, longer absences were needed for feeding mothers. Pups generally began foraging earlier than normal; their intake of milk was less and the quality of milk itself was poorer. Attempts by pups to suckle non-mothers or yearlings were observed with increased frequency.
STUDIES ON PHOCIDS
Elephant Seals: The mortality was high among pups due to severe storms at peak of pupping season which swept the pups off the beaches before they had learned to swim. Surviving pups generally were not underweight at weaning. Fewer females were observed mating. Overall there was an 44% increase in pup mortality .
For a more complete discussion of these topics refer to:
Trillmich, Fritz and Ono, Kathryn, eds., Pinnipeds and El Niño, Responses to Environmental Stress, Springer-Verlag, New York, 1991
Reidman, Marianne, The Pinnipeds, Seals, Sea Lions, And Walruses, University of California Press, Berkeley, 1990. Refer to the extensive bibliography in the back of this text.
Bonner, Seals and Sea Lions of the World, Facts on File, New York, 1994.
Santa Barbara Museum of Natural History, Los Marineros Curriculum Guide, Santa Barbara, CA, 1997
To test your understanding, complete the following:
Illustrate with a cartoon the difficulties during El Niño events encountered by animals listed in this section.
Explain why the geographic location of a population alters the impact of El Niño.
El Niño has been occurring for millennia. What benefits can be derived from it?
Activities for students
What are the three groups of pinnipeds? What are the characteristics of each group? Name some species that are part of each group.
How does the hardships placed on seals also impact human fishermen? What advantages do humans have over seals in surviving an event?
How has the fish that is being sold in the market near you changed? Can you buy the same kind of fish you could a few months ago? Has the price of the fish changed?
How has El Niño affected the world. Retrieve on-line impact. Plot them on a large map of the world.
If stranding data is available on-line, plot where animals are stranding. Do the stranding patterns change with latitude?