Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
The present invention relates to a
method and apparatus for the field rearing of
planktonic larval forms of marine animals. This
method and apparatus has particular application to
fish and decapod crustaceans.
Over the past century, considerable
effort has been expended in attempts to rear
marine larvae in the laboratory, but such methods
have had very little success in rearing such
larvae past the metamorphosis stage. The greatest
difficulties with laboratory rearing have been in
the collection and maintenance of wild plankton,
or the laboratory culture of microplankton, used
as food, and in maintenance of proper sanitation `
conditions within culture chambers. Such labora-
tory methods, moreover, have required unacceptably
large amounts of time and labour making them
impracticable for commercial use~ Other problems
in laboratory rearing of larvae include the difficulty
in establishing proper lighting conditions, destruction
of food plankton by pump impellors used to circulate
unfiltered seawater systems and establishment of a
proper seawater flow pattern to maintain the
plankton in proximity to the larvae.
As a consequence of the aforementioned
difficulties in the laboratory rearing of marine
larvae, some attempts have been made to develop
field rearinq methods. Geoffrey Lawrence, at the
1978 Conference of the American Society of Limnology
and Oceanography held in Victoria, British Columbia,
Canada, described a screened cylindrical chamber
employed in Oceanic tidal water, whereby the water
was allowed to simply flow straight through the
chamber which contained flat fish larvae. No
feature was present in the latter method to accumulate
food plankton or to maintain the food plankton in -
close proximity to the larvae. The latter method
also failed to provide a self-operating mechanism
for withdrawing waste material from the tank.
Consequently, the Lawrence method was limited to
rearing relatively small concentrations of larvae
in numbers insufficient for a commercial operation.
SUMMARY OF THE INVENTION
According to the present invention,
there is provided a method and apparatus for the
field rearing of planktonic larval forms of marine
animals involving the suspension of a field culture
chamber in oceanic tidal waters. The chamber
itself is provided with an entry port in its side
to adjustably admit tidal flow water, a means for
deflecting water around the chamber periphery, and
an exit port in the chamber bottom to permit the
outflow of water from the chamber. A curtain is
suspended over the interior openings of the entry
vanes to prevent the exit of larvae through the entry
port. The interior of the chamber is stocked with
larvae of a preselected species of marine animal.
The chamber is oriented in response to the tidal flow
of water so that the water flows into the chamber
through its entry port. Prior to entering the
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chamber, the water is screened to prevent admittance
of plankton of greater than a predetermined size,
and to permit admittance of plankton small enough to
be utilized as food by the larvae of the preselected
species~ Water flowing out of the chamber through
the exit port is filtered to prevent the escape of
larvae and food plankton.
Advantageously, the flow of water into ;~
the chamber may be adjusted on the basis of lunar
10 tidal flow cycles to establish an optimum average
tidal flow therein. Preferably, moribund larvae,
excretions and waste material are withdrawn from
the interior of the chamber to prevent formation
of a bacteriaI scum on the bottom surface thereof.
One means of withdrawal of such material is by
utilization of a filter bed on the bottom of the
chamber covering the exit port. Such a bed serves
not only to impede the escape from the chamber of
larvae and food plankton, but also to allow the
20 moribund larvae, excretion and waste material to
penetrate into the interior of the filter bed and
to undergo bio-degradation, thereby avoiding the
development of a bacterial scum on the surface.
Since it has been observed that larvae which sink
are more likely than not to swim near the bottom
of the chamber, bacterial scum present on the
uppermost surface of the bottom can infect and
thereby kill such larvae. r
By deflecting incoming water around the
30 chamber periphery and permitting its outflow from
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the exit port atthebottom, a substantially
circular flow of water is set up in the chamber,
with a central down-welling. In such a current,
food plankton tend to accumulate at the centre of
the water surface in the chamber where minimum
flow velocities exist, and the larvae locate in
and around this food plankton. Optimally, the
interior walls of the chamber are a dark colour in
order that the larvae are not phototactically
attracted to the chamber periphery. The method of
establishing current flow in the chamber and the
darkened walls of the chamber interior thus avoid
or reduce potentially damaging contact by the
larvae with the periphery of the container.
Screening of the water entering the tank
prevents entry of predators and large plankton
which can damage or kill the preselected larvae
and food plankton, while permitting continuous
entry of food plankton.
The aforesaid filter bed is composed of
filtration medium that is of a specific gravity
soemwhat greater than that of seawater but is
generally light in weight. The chamber must be
ballasted so that in combination with flotation
means attached to the top of the chamber, it main- -~
tains a stable upright position. By selecting the
chamber in the form of a cylinder, and the flotation
means in the form of a buoyant collar surrounding
the chamber near its top, the chamber can rotate
within the collar.
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The weight of the rudder and struts must
be buoyed or counter-ballasted to prevent binding
in the vertically oriented flotation collar, that
is, the chamber must be balanced in its ballasting,
with total buoyancy only slightly negative, to
lessen friction with the flotation collar. The
collar can be fixed to a floating pier or by
a plurality of anchor lines connected between the
collar and anchors resting on the seabed, so that
the chamber rotates within the collar. Alterna-
tively, the chamber can simply be connected by a
tether line to a fixed object in or over the
water and be allowed to swing in order to become
oriented properly with respect to the tidal flow
of water. The chamber orients in response to the
tide, by means of a rudder supported from one side
of the chamber opposite the entry port at a distance
from the side of the chamber equal to at least its
diameter. The chamber assembly is ideally located
in a protected inlet of oceanic tidal water,
having high plankton densities and a temperature
and salinity closely matching that where spawning
naturally occurs in the species being reared.
The invention avoids the labour intensive - ~
requirements common in laboratory rearing methods ,
by utilizing tidal currents for both the accumulation
and retention of food plankton, and for providing
a self-operating sanitation mechanism.
Energy costs are also avoided by utilizing
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tidal currents for creating water flows within the
chamber and for proper orientation of the chamber,
and also by having available natural oceanic
lighting and temperature conditions favourable to
larval development.
SUMMARY OF THE DRAWINGS
FIGURE 1 is an elevation view of the
chamber assembly according to the invention.
FIGURE 2 is a plan view of the chamber
assembly partially in section to show the entry
port and deflection vanes.
FIGURE 3 illustrates the structure of
the porous filter bed and chamber floor.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
In the following description, words such
as horizontal, vertical, top and bottom are used
in a relative rather than absolute sense with
reference to the chamber assembly ln situ in
oceanic tidal waters.
Illustrated in FIGURE 1 is a chamber
assembly 11, having a cylindrical chamber 10
surrounded near its top, and rotatable within a ;
buoyant collar 12. A flange 13 at the top of the
chamber 10 rests on a buoyant collar 12, and a lid
14 covers an opening in the chamber 10 within the
flange 13. Affixed to the outer surface of the
collar and circumferentially spaced therearound
are a plurality of eyelets 16. Below the collar
12, a plurality of support members 20 are each
rigidly affixed at one end 21 to the outer surface
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of the chamber 10, and extend outwardly therefrom
to a distal end 19, which pivotally supports a `
vertically oriented drop-rudder 18. On the side
of the chamber 10 opposite the drop-rudder 18 is
an arcuate plate 22 affixed at its ends 23 to the
exterior surface of the chamber 10 and extending
horizontally outwardly therefrom. -Attached to the
margins of the plate 22, covering an opening --
formed therein is a screen 24. Adjacent to and
horizontally spaced from the screen 24 as illustrated
in FIGURE 2, is an entry port 27, having spaced
thereover a plurality of deflection vanes 28
continuously movable through a limited range from
closed positions at which the space between ad~acent
vanes 28 is a minimum to an open position at which
the space between adjacent vanes 28 is a maximum.
The vanes 28 which open inwardly are each formed
by a three-sided cut in the chamber material
itself. The resulting vanes may then be propped
open along one side a desired amount by insertion
of a nub between the side of the vane and its
adjacent edge in the chamber so that each vane is
curved inwardly toward a common side of the chamber
periphery. A curtain 29 of plastic, or some other
suitable material is suspended on the interior side
of the vanes 28 to prevent exit of larvae in the
chamber.
As illustrated in FIGURE 3, the floor 40
of the chamber 10 is perforated with a plurality
of small holes 38 which are covered by a mesh 36.
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A bed of filtration medium 32, for example, plastic
or gravel, is located on top of the mesh 36. The
filtration medium 32 should be thick enough to ;
allow penetration and bio-degradation of moribund
larvae and plankton, excretions and other waste
in the interior of the bed.
The filtration medium 32 should be only
slightly negative in buoyancy and of coarse grade,
permitting debris to pass out of the chamber, yet
providing dark spaces into which viable organisms
will not usually enter. A plastic ring material ~ -
available under the trade mark BIO-RING 15, from
Norton Co., has been employed successfully as the
filtration medium. This material has a high surface ;
area/volume ratio. A thickness of 2 inches has
been found to be satisfactory. ;
The filtration medium 32 is divided by
a grid of vertically oriented partitions 34 into
a plurality of sections within respective grid
20 elements of the grid of vertically oriented partitions ~-
34. The spacing between individual partitions 34
is selected to prevent shifting of the filtration
medium 32 over the bottom of the chamber. A suitable
spacing for this purpose has been found to be 10
inches between ad]acent partitions. The dimensions
of chamber 10 are selected to be large enough so
that its surface to volume ratio is low enough to
reduce the possibility of larval contact with the
peripheral walls of chamber 10 to acceptable levels.
Suitab1e dimensions for the chamber 10 have been
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found to be a diameter of about 5 feet and a
height of about 5 feet. ~ -
The method of the invention involves
lowering the chamber assembly 11 into suitable
tidal flow waters, and suspending it therein by
flotation means, such as a floating collar 12, ~:
which is secured to a floating pier, or
anchors by lines attached to the collar eyelets 16.
The chamber 10 is then stocked with larvae of a
preselected species of marine animal. To facilitate
rotation of chamber 10 within the collar 12, a layer :~:
of low friction polyethylene, or the like, may be
placed on the interior surface of the collar 12.
During tidal flows, the-drop-rudder 18 orients the
chamber 10 so that water flows into screen 24. Any :
larger plankton or predators are stopped by the
screen 24, leaving smaller plankton to be carried
by the current through the inlet port 27, and against :
the deflection vanes 28. The amount of tidal flow
water.entering chamber 10 is adjusted depending on
the average tidal flow rate over a given lunar
tidal flow cycle by the propping open of deflection
vanes 28. The incoming water is also deflected by
these deflection vanes 28 toward a common side of
the chamber 10, thereby establishing a circular
flow of water in the interior of the chamber 10. :
The current gradient thus set up in chamber 10
causes food plankton to accumulate near the centre
thereof. By using a light-transmitting lid 14,
not only are chops and swells in the water prevented
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from entering the chamber interior, but the plankton
being attracted by the light, tend to stay near
the top of the chamber and provide a concentration
of larval food in the lower current region within
the chamber interior. The interior walls of the
chamber are black in colour to reduce their tendency
to attract the larvae. Moreover, being encouraged
to stay near the centre and top of the chamber 10,
the larvae have a reduced probability of having
their feeding disrupted by contact with the chamber
periphery.
Other obvious variations, modifications
and departures from the specific method and apparatus
described above, will readily occur to those skilled
in the art, but are intended to form part of the ;
overall invention. The scope of the present in- ,
vention is set forth in the accompanying claims.
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