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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.
Krill Behaviour
Krill Breeding Biology
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.
Krill Migration and
Distribution
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.
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