What attracts sharks? Which are most dangerous?

Considerable research has been devoted to finding out what stimuli attract sharks and incite them to attack. Results are mostly inconclusive, but some general principles have been advanced: Certain types of irregular sounds - like those made by a swimmer in trouble or a damaged fish - seem to attract sharks from great distances. Sound, rather than sight or smell, seems to be a shark's primary cue for moving into an area. Some scientific experiments indicate that sharks can distinguish light colors from dark, and that they may even be able to distinguish colors.
hammerhead shark
Copyright See & Sea Travel
Service, Inc. 1996

Yellow, white, and silver seem to attract sharks. Many divers maintain that clothing, fins, and tanks should be painted in dull colors to avoid shark attacks.

white sharks
Great White (and not so great white)

Though blood itself may not attract sharks, its presence in combination with other unusual factors will excite the animals and make them more prone to attack. The most dangerous species in order of documented attack records are: the great white shark, bull shark, tiger shark, grey nurse shark, lemon shark, blue shark, sand tiger, several species of hammerheads, and the mako. Some species such as the nurse shark are extremely sluggish and have poorly developed teeth, but even these have been known to attack man when excited or disturbed.

What sea creatures other than sharks may be dangerous to swimmers?

The barracuda (though divers claim its ferocious reputation is undeserved), moray eels, octopuses, and sharp-spined sea urchins can be dangerous to swimmers. The Portuguese man-of-war has tentacles up to 50 feet long with specialized cells that produce painful stings and welts on contact by swimmers. caution jellyfish sign Sting rays, toadfish, catfish, and jellyfish can inflict damage on swimmers and waders. Certain coral-reef organisms are to be avoided by divers. Visit Lifeguards' guide to O'ahu's popular guarded beaches for more information on dangerous sea creatures.

barracuda
Barracuda


How many species of Pacific salmon are there?

Pacific salmon is a generic term used to describe those members of the genus Oncorhynchus that die after spawning. At present, there are seven species commonly referred to as Pacific salmon. There are five species that occur on both sides of the Pacific Ocean:

Chinook salmon (Oncorhynchus tshawytscha) a.k.a. king salmon,
chum salmon (Oncorhynchus keta) a.k.a. dog salmon,
coho salmon (Oncorhynchus kisutch) a.k.a. silver salmon,
pink salmon (Oncorhynchus gorbuscha) a.k.a. humpback salmon, and
sockeye salmon (Oncorhynchus nerka) a.k.a. red salmon.

Two species occur only in Asia:
masu salmon (Oncorhynchus masou) a.k.a. yamame, and
amago salmon (Oncorhynchus rhodurus) a.k.a. biwamasu.

An excellent discussion of the varied life histories of these seven species can be found in C. Groot and L. Margolis [editors] 1991. Pacific Salmon Life Histories, University of British Columbia Press, 564 pp.

coho salmon
Coho Salmon

pink salmon
Pink Salmon


Is it true that salmon return to spawn in freshwater areas where they were born?

Almost always. Some straying has been documented, but it is minor. Most spawning salmon return to the precise stream of their birth, sometimes overcoming great distances and hazardous river conditions to reach home.

What is the difference between the Atlantic salmon and the Pacific salmon?

The Atlantic Salmon (Salmo salar) is actually one species within the genus Salmo. Pacific salmon are represented by seven different species, see question above, and belong to the genus Oncorhynchus. The seven Pacific salmon species have life histories that are extremely complex and vary widely within and between species. However, all the Pacific salmon die shortly after spawning. Atlantic salmon have a much less variable range of life history strategies across the species and have high post spawning mortality but are capable of surviving and spawning again.

atlantic salmonAtlantic Salmon
Are steelhead (rainbow trout) trout or salmon?

Until 1988, steelhead (the anadromous form of rainbow trout) was classified in the genus Salmo along with Atlantic salmon, brown trout, and several western trout species. With additional osteology and biochemistry data, biologists have now reclassified steelhead as members of the genus Oncorhynchus. The reason for this is that new information suggested that steelhead are more closely related to Pacific salmon than to brown trout and Atlantic salmon. As such, the American Fisheries Society - American Society of Ichthyologists Committee on Names of Fishes voted unanimously to accept Oncorhynchus as the proper generic name. For full scientific details, see Smith, G. R., and R. F. Stearley. 1989. The classification and scientific names of rainbow and cutthroat trouts. Fisheries 14 (1): 4-10. As such, the scientific name of steelhead was changed from Salmo gairdneri to Oncorhynchus mykiss. The generic names of the golden, Mexican golden, Gila, and Apache trouts were also changed to Oncorhynchus. Since all of these western trouts including steelhead are biologically capable of repeat spawning and do not die after spawning, it has been suggested this group be called the Pacific trout.


The image of the man-of-war warning sign is Copyright c 1986, Hawaiian Lifeguard Association.


Barracuda and Salmon images courtesy of "Regulatory Fish Encyclopedia, Office of Seafood and Office of Regulatory Affairs, Food and Drug Administration, 1993-1996."

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