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Krill
Euphausiids
Euphasiids is
the scientific name for many species of tiny crustaceans known
colloquially as krill. These tiny planktonic animals form the
base of the ocean’s food chain throughout the world. The food
chain begins with tiny floating plants known as phytoplankton
that store the sun’s energy during photosynthesis. Euphasiids
convert the plant sugars in the phytoplankton into animal
tissue and they then are eaten by fish, birds and marine
mammals. Major predators of krill in the north Pacific are
salmon,
herring and hake and the baleen whales. The hake is so
closely allied to the krill as a food species that the two are
almost inseparable. The Cassin’s auklet and ancient murrelet
eat krill directly and many other seabirds eat herring,
sandlance and other small fish that eat krill. The red
colouration of krill is derived from carotenoid pigments. In
high density, the red krill can turn surface waters a reddish
hue. There are about 85 species of euphasiids in the world’s
oceans of which 15 species are found in all oceans. About
twenty species of euphausiids have been recorded off British
Columbia with five species being most numerous: Euphausia
pacifica, Thysanoessa spinifera, Thysanoessa inspinata,
Thysanoessa longipes and Thysanoessa rashii. About
25 species are abundant off California. Unlike most
zooplankton, euphausiids can live at depth in the ocean. They
move between the ocean depths at night and surface waters
during the day. The average krill is about 16 mm long and can
live one or two years. Spawning occurs in spring and summer to
coincide with periods of high phytoplankton abundance.
Euphausiids are harvested as a feed supplement for fish farms
to give salmon flesh its pink colour and as fish food for
aquarists, and as a fishery for human consumption.
The
predominant euphausiids in the Gulf of Alaska are
Thysanoessa inermis and Euphausia pacifica. Gravid females
of T. inermis release eggs in April and May continually
over a three day period. Gravid female E. pacifica are
numerous in July to October and release eggs only for one day.
Large females release the most eggs. Eggs hatch simultaneously
over a few hours depending on incubation temperature creating
‘blooms’ of plankton in the ocean.
We don’t
usually think of plankton as being capable of migration but
each evening, these tiny crustaceans make an extraordinary
journey of 500 or more meters between the depths of the ocean
and the surface waters. During a 24 hour period, these
animals may pass through a range of temperatures of 16°C.
References
Abraham, C.L.,
G. S. Wolfe, J. M. Hipfner and W.J. Sydeman. The seasonal
cycle of euphausiid zooplankton in the California Current
system: A predator’s perspective. PISCES
On line
McFarlane, G.A.,
and R.J. Beamish. 1985. Biology and fishery of Pacific
Whiting, Merluccius productus, in the Strait of
Georgia. Marine Fisheries Review 47:23-34
Yin, K. P. J. Harrison, R. H. Goldblatt and R.
J. Beamish. 1996. Spring bloom in the central Strait of
Georgia: interactions of river discharge, winds and grazing.
Marine Ecology Progress Series 138: 255-263.
June 2010
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