NO346398B1

NO346398B1 - Feed detection assembly, system and method for detecting feed pellets in an effluent pipe of a fish tank

Info

Publication number: NO346398B1
Application number: NO20210320A
Authority: NO
Inventors: Ragnvald Inge Otterlei
Original assignee: Micromar As
Priority date: 2021-03-11
Filing date: 2021-03-11
Publication date: 2022-07-11
Also published as: NO20210320A1; WO2022191713A1

Description

Technical field

The disclosure relates to the field of fish farming.

Background

[0001] How to obtain proper feed management during fish farming is considered as one of the most central issues for the fish farming industry today. Feed costs account for a substantial part of the total production cost of farmed fish, meaning that proper feed management may substantially contribute to increased profit for the farmers. Overfeeding is often a result of the fear of underfeeding, which results in reduced growth rates for the fish stock and accompanying costs. Overfeeding also results in an increased amount of organic waste, thus having a negative effect on the environment. Overfeeding is a particular issue in closed fish farming systems, such as recycling aquaculture systems, as overfeeding here will lead to increased contamination of the water as well as increased growth of algae.

[0002] Existing solutions for how obtain proper feed management in fish farming systems involve solutions based on submerging optical or acoustic cameras in the fish tank/pen. Such systems monitor the number of feed pellets that sink to the bottom of the fish tank/pen, and the rate at which feed pellets sink. In situ manual observations from these cameras are then used to adjust the rate at which feed pellets are added to the fish tank/pen.

[0003] NO 344459 describes a device that enables optimized regulation of feeding rates in aquaculture systems. The invention involves a feed detector shaped as a pipe segment that can be directly installed as a pipe socket in an outlet pipe of an aquaculture system. The feed detector comprises a pipe segment, radiation means and detection means.

[0004] CN112034759A describes an intelligent net cage fish culture monitoring system which is controlled by an upper computer and a lower computer, the upper computer is a system remote monitoring computer,a visual programming language design interface is adopted, and remote monitoring and management operation on a net cage are achieved.

[0005] A problem with existing solutions however is that they are inaccurate, require a high amount of manual labour, and/or that they require fragile equipment. It is a goal of the present invention to provide an alternative solution for how to determine the amount of uneaten feed in closed fish tanks.

Summary of the invention

[0006] A first aspect of the present invention provides a feed detection assembly for detecting feed pellets in an effluent pipe of a fish tank, the feed detection assembly comprising an ultrasound transducer, and mounting means for mounting the ultrasound transducer such that the ultrasound transducer is arranged to emit ultrasound into the effluent pipe in the longitudinal direction of the effluent pipe.

[0007] According to an embodiment of the invention, the mounting means is configured to mount the ultrasound transducer at the inlet of, at the outlet of, or inside the effluent pipe.

[0008] According to another embodiment of the invention the mounting means is configured to mount the ultrasound transducer such that the ultrasound transducer is arranged to emit ultrasound in the opposite direction of an effluent flow of the effluent pipe.

[0009] According to yet another embodiment of the invention the mounting means is configured to mount the ultrasound transducer such that the ultrasound transducer is arranged to emit ultrasound in the direction of the effluent flow of the effluent pipe.

[0010] According to yet another embodiment of the invention, the mounting means is configured to mount the ultrasound transducer within a distance d/4 from the central longitudinal axis of the effluent pipe, wherein d is the diameter of the effluent pipe, or the mounting means is configured to mount the ultrasound transducer along the central longitudinal axis of the effluent pipe.

[0011] According to yet another embodiment of the invention the ultrasound transducer is configured to emit ultrasound with a frequency of less than 1 Mhz.

[0012] According to yet another embodiment of the invention the mounting means comprises a filter or a grid configured to be arranged at the inlet of the effluent pipe from the fish tank.

[0013] A second aspect of the present invention provides a feed detection system comprising an ultrasound transducer, and a fish tank comprising an effluent pipe, where the ultrasound transducer is arranged to emit ultrasound into the effluent pipe in the longitudinal direction of the effluent pipe.

[0014] According to an embodiment of the invention the ultrasound transducer is arranged at the inlet of, at the outlet of, or inside the effluent pipe.

[0015] According to another embodiment of the invention the ultrasound transducer is arranged to emit ultrasound in the opposite direction of an effluent flow of the effluent pipe.

[0016] According to yet another embodiment of the invention the ultrasound transducer is arranged to emit ultrasound in the direction of the effluent flow of the effluent pipe.

[0017] According to yet another embodiment of the invention the ultrasound transducer is arranged within a distance d/4 from the central longitudinal axis of the effluent pipe, where d is the diameter of the effluent pipe, or the ultrasound transducer is arranged along the central longitudinal axis of the effluent pipe.

[0018] According to yet another embodiment of the invention the ultrasound transducer is configured to emit ultrasound with a frequency of less than 1 Mhz.

[0019] According to yet another embodiment of the invention the feed detection system further comprises a grid or a filter arranged at the inlet of the effluent pipe from the fish tank, where the ultrasound transducer is mounted to the grid or the filter.

[0020] A second aspect of the present invention provides a method for detecting feed pellets in an effluent pipe of a fish tank, the method comprising the steps of emitting ultrasound into the effluent pipe in the longitudinal direction of the effluent pipe, detecting ultrasound that is reflected of feed pellets in the effluent pipe, and determining, based on the detected ultrasound, a number of feed pellets in a fluid flow passing through the effluent pipe.

[0021] In an embodiment of the invention the ultrasound emitted is emitted as one or more pulses.

[0022] In another embodiment of the invention the step of determining involves determining a velocity of the feed pellets in the effluent pipe based on a doppler shift between the emitted ultrasound and the detected ultrasound.

[0023] Other advantageous features will be apparent from the accompanying claims.

Brief description of the drawings

[0024] In order to make the invention more readily understandable, the description that follows will refer to accompanying drawings, in which:

[0025] Figure 1 is a schematic representation of a feed detection assembly and a feed detection system, where the transducer is arranged a) inside, b) outside, and c) at the inlet of the effluent pipe of a fish tank,

[0026] Figure 2 is a schematic representation of an effluent pipe of the fish tank, where the ultrasound transducer a) is arranged to emit ultrasound in the direction of the effluent flow, b) arranged at the wall of the effluent flow, and c) is arranged to emit ultrasound opposite the direction of the effluent flow,

[0027] Figure 3a-c is a schematic representation of a feed detection assembly and a feed detection system, where the ultrasound transducer is mounted using various mounting means,

[0028] Figure 4 is a schematic representation illustrating arrangement of the ultrasound transducer a) along the central longitudinal axis of the effluent pipe, b) within a distance d/4 from the central longitudinal axis of the effluent pipe,

[0029] Figure 5 illustrates a method for detecting feed pellets in an effluent pipe of a fish tank, and

[0030] Figure 6 is a schematic representation illustrating the effluent pipe of a fish tank, where the effluent pipe may be connected directly to the fish tank or indirectly to the fish tank via additional units of a system comprising a fish tank.

Detailed description of the invention

[0031] In the following, general embodiments as well as particular exemplary embodiments of the invention will be described. References will be made to the accompanying drawings. It shall be noted, however, that the drawings are exemplary embodiments only, and that other features and embodiments may well be within the scope of the invention as claimed.

[0032] Unless otherwise defined, all terms of art, notations and other scientific terms or terminology used herein are intended to have the meanings commonly understood by those of skill in the art to which this invention pertains. Certain terms of art, notations, and other scientific terms or terminology may, however, be defined specifically as indicated below.

[0033] The present invention provides a feed detection assembly and system for detecting feed pellets in an effluent pipe of a fish tank. The present invention also provides a feed detection method for detecting feed pellets in an effluent pipe of a fish tank. The assembly, system and method are based on the use of ultrasound for the detection of the feed pellets. Any embodiment herein described for only the assembly, system or method is applicable to any of the assembly, system or method.

[0034] The feed detection assembly, system and method of the present invention are preferred for detecting feed pellets in an effluent pipe of a fish tank of a

recirculating aquaculture systems (RAS). It will however be appreciated that the assembly, system and method of the present invention generally may be provided for other fish tanks connected to an effluent pipe. Examples of such fish tanks are fish tanks of closed fish farming systems, either placed at sea or on land.

[0035] The present invention is based on the principle of employing ultrasound in order to detect feed pellets in an effluent pipe 102 of a fish tank 104. More precisely, as schematically illustrated in figure 1, the present invention is based on a particular approach where said detection of feed pellets is performed in the longitudinal direction 110 of an effluent pipe 102 of a fish tank 104. Ultrasound may here be emitted into an effluent flow of the effluent pipe 102 along the longitudinal direction 110 of the effluent pipe 102, before being reflected of feed pellets and eventually detected. The detected signal may subsequently be processed, e.g. by a multipurpose computer, in order to determine the number of feed pellets in the effluent pipe 102. Detecting feed pellets along the longitudinal direction 110 of the effluent pipe 102 using ultrasound has been found to be highly advantageous over detecting feed pellets in the transverse direction. The former allows inter alia for detection of feed pellets over a larger portion of the cross section of the effluent pipe 102 and allows for improved ease of retrofitting a feed pellet detection assembly. Using ultrasound for detection of feed pellets has also been found to be advantageous over using optical methods for detecting feed pellets. Using ultrasound is here less prone to being affected by contamination of the effluent flow water. An ultrasound reflection from a feed pellet may also have a different characteristic than a reflection of a agglomeration of fish manure, meaning that post processing of reflected ultrasound may filter out the signal from the feed pellets from the signal from fish manure agglomerates.

[0036] Figure 1 schematically illustrates a feed detection assembly 100 and a feed detection system 200 comprising an ultrasound transducer 106 that is arranged to emit ultrasound into the effluent pipe 102 in the longitudinal direction 110 of the effluent pipe 102. The ultrasound transducer 106 may here be positioned outside, inside 114 or at the inlet 124 of the effluent pipe 102 as seen in figure 1a,1b and 1c respectively. The ultrasound transducer 106 may alternatively be positioned at the outlet 126 of the effluent pipe as schematically illustrated in figure 6. The ultrasound transducer 106 is preferably positioned, inside 114, at the inlet 124 of, or at the outlet 126 of the effluent pipe 102. The direction and divergence which the ultrasound transducer 106 may emit ultrasound are determined by the arrangement of the ultrasound transducer 106 and the beam shape characteristic of the ultrasound transducer 106. The beam shape characteristic of an ultrasound transducer 106 determines the shape of the beam emitted by an ultrasound transducer 106, which may for example be conical, or linear etc. Beam shape characteristic may in the art be known under different terms, e.g. beam directivity, radiation pattern, beam directionality, directional characteristics etc., and the exact beam shape of a given ultrasound transducer 106 may further depend on whether the beam is focused or not focused.

Examples of ultrasound emitting transducers include convex, linear and sector transducers. An ultrasound beam may be symmetrical about a symmetry axis, or have an average direction determined for example by a direction of average sound energy transfer.

[0037] The effluent pipe 102 of a fish tank 104 may in any embodiment of the invention be connected directly or indirectly to the fish tank 104. Figure 6 schematically illustrates a fish tank 104 connected with a series of fish farming system units 105. The fish farming system units 105 may for example be filtration units, filtration tanks, additional water tanks or other fish farming system units 105 that may be present in a closed fish farming system. The effluent pipe 102 of the fish tank 104 may thus be connected indirectly to the fish tank via such a fish farming system unit 105, and act as an outflow pipe of the fish farming system unit 105. The ultrasound transducer 106 may generally be positioned outside, inside, at the outlet 126 of, or at the inlet 124 of the effluent pipe 102. The outlet 126 of the effluent pipe 102 may here be considered as the interface between the effluent pipe 102 and a fish farming system unit 105 into which the effluent flow from the effluent pipe 102 flows. Alternatively, the outlet 126 of the effluent pipe 102 may generally be considered as the mouth of the effluent pipe 102 downstream from the fish tank 104.

[0038] An ultrasound transducer arranged to emit ultrasound into the effluent pipe in the longitudinal direction of the effluent pipe may according to any embodiment of the invention be considered as arranged such that the bulk of the ultrasound emitted by the ultrasound transducer moves in the longitudinal direction of the effluent pipe. The ultrasound transducer may thus be arranged such that the bulk of the acoustic power emitted by the ultrasound transducer moves along the longitudinal direction of the effluent pipe. The ultrasound transducer may in other words be arranged to emit ultrasound substantially directed along the

longitudinal direction of the effluent pipe. As a way of example an ultrasound transducer with a conical beam shape characteristic, where the beam is symmetrical about a symmetry axis, may be arranged such that said symmetry axis is aligned with the longitudinal axis of the effluent pipe. In said example, the ultrasound transducer may be arranged such that the symmetry axis is aligned within 10 or 20 degrees with the longitudinal axis of the effluent pipe. A person skilled in the art with knowledge of the present invention will appreciate that the ultrasound transducer may simultaneously emit smaller amounts of ultrasound that moves in other directions as well, e.g. due to divergence of the ultrasound beam, but where the substantial acoustic transfer is along the longitudinal direction of the effluent pipe.

[0039] The ultrasound transducer 106 according to the invention is arrangeable or arranged to emit ultrasound into the effluent pipe 102 in the longitudinal direction of the effluent pipe 102. The exact arrangement of the ultrasound transducer 106 may according to any embodiment of the invention be made using suitable mounting means 108. The mounting means 108 are preferably mounting means 108 that allows for retrofit of the ultrasound transducer 106 outside, inside, at the outlet 126 of, or at the inlet 124 of an effluent pipe 102. The mounting means 108 are more preferably mounting means 108 that allow for retrofit of the ultrasound transducer 106 inside, at the outlet of, or at the inlet 124 of an effluent pipe 102. The mounting means 108 may here be mounting means 108 for mounting the ultrasound transducer 106 in a fixed position. A first example of suitable mounting means 108 is schematically illustrated in figure 3c, which shows mounting means 108 that comprises a filter 122 or grid 123 for being mounted at the inlet 124 of the effluent pipe 102 of a fish tank 104, i.e. the inlet 124 from the fish tank 104 to the effluent pipe 102. The filter 122 or grid 123 may for example be mounted to cover the inlet 124 of the effluent pipe 102, for example such that any effluent flow 118 from the fish tank 104 has to pass through the filter 122 or grid 123. The ultrasound transducer 106 may here be attached directly or indirectly to the filter 122 or grid 123. Another example of suitable mounting means 108 is schematically illustrated in figure 3b, which shows mounting means 108 that comprises a rack for being mounted at the inlet 124 of the effluent pipe 102 of a fish tank 104. A rack may alternatively or additionally be fastened in the fish tank 104 outside or inside the effluent pipe 102 and be configured such that the ultrasound transducer 106 is arranged inside, outside, at the outlet 126 of, or at the inlet 124 to the effluent pipe 102.

Figure 3a schematically illustrates a configuration where the mounting means 108 arranges the ultrasound transducer 106 inside the effluent pipe 102. The mounting means 108 may here be a grid 123, filter 122 or rack that may be fastened to the walls of the effluent pipe 102 by suitable fastening means.

Alternatively, said rack may be configured such that the ultrasound transducer 106 is arranged in the fish tank 104, i.e. outside the effluent pipe 102. The latter is schematically illustrated in figure 3b. Yet another example of suitable mounting means 108 is mounting means 108 that comprises an adhesive for mounting the ultrasound transducer 106 to the internal wall surface of the effluent pipe 102. The latter is schematically illustrated in figure 2 reference b. Yet another example of suitable mounting means 108 is mounting means 108 that comprises a bolt or similar for mounting the ultrasound transducer 106 to the internal wall surface of the effluent pipe 102. The inlet 124 of an effluent pipe 102 of a fish tank 104 may here be considered as the interface between the fish tank 104 and the effluent pipe 102.

[0040] The assembly and system of the present invention comprises at least one ultrasound transducer. In an exemplary embodiment of the invention, said ultrasound transducer may be an ultrasound transceiver, i.e. the ultrasound transducer being both an ultrasound transmitter and an ultrasound receiver. It will, however, be appreciated that the assembly and system of the present invention may comprise more than one ultrasound transducer. The assembly and system may in one example comprise one ultrasound transceiver. In another example, the assembly and system may comprise two ultrasound transducers, where one ultrasound transducer is an ultrasound emitter, and where the other ultrasound transducer is an ultrasound receiver. In yet another more general example, the assembly and system may comprise a plurality of ultrasound transducers comprising an arbitrary number of ultrasound emitters and an arbitrary number of ultrasound receivers. A person skilled in the art with knowledge of the present invention will appreciate that many different configurations of ultrasound emitters and ultrasound receivers are possible.

[0041] In order to detect feed pellets in an effluent pipe of a fish tank, ultrasound may initially be emitted into the effluent flow in an effluent pipe in the longitudinal direction of the effluent pipe. The ultrasound will upon encountering a feed pellet subsequently be reflected, and the reflected ultrasound may be detected by the same ultrasound transducer, or a separate ultrasound transducer. The detection of such reflected ultrasound signals may then be used in order to determine a number of feed pellets in a fluid flow passing through the effluent pipe, i.e. effluent flow. A method for detecting feed pellet in an effluent pipe of a fish tank is illustrated in figure 5.

[0042] The ultrasound may in an embodiment of the invention be emitted as a single pulse. The ultrasound pulse may propagate from the ultrasound transducer until it impedes on and reflects of a feed pellet. The reflected pulse may subsequently be detected by the same ultrasound transducer that emitted the ultrasound, or by a separate ultrasound transducer. The distance between the ultrasound transducer and the feed pellet results in a time delay between the time of emitting the pulse until the time of detecting the reflected pulse. In the event of the single pulse impeding on multiple feed pellets at various distances from the ultrasound transducer, the reflections from the various feed pellets will thus be detected at various times. The number of feed pellets in the field of view of the ultrasound transducer may thus be determined directly from the detected signal as a function of time from the time of emitting the original pulse. The latter may thus be termed a time-of-flight (TOF) measurement. In an example where the detected signal of the receiving transducer is a voltage as function of time, the presence of a feed pellet in the field of view of the ultrasound transducer will appear as a voltage peak. The peak will here appear with a time shift from the time of emitting the pulse based on how far away from the ultrasound transducer that the pellet was when it reflected the ultrasound pulse, i.e. a TOF-shift. The number of feed pellets in the field of view of the ultrasound transducer may thus be determined by counting the number of voltage peaks in the detected signal. In the event of a high density of feed pellets in the effluent flow, it may be necessary to integrate the size of each peak, as multiple feed pellets may be present at the same distance from the ultrasound transducer and thus result in reflections being detected simultaneously.

[0043] The ultrasound may in an embodiment of the invention be emitted as multiple pulses. In this embodiment the detection of feed pellets may be similar to what described above for one pulse, but multiple pulses may alternatively be used in order to generate an image showing the feed pellets in the effluent flow. Multiple pulses may generally be employed in order to sample the number of feed pellets in the effluent pipe at various times. The number of feed pellets detected at each time, i.e. each sample, may then be used to determine the average number of feed pellets in the effluent pipe over a given time interval. The time between two consecutive pulses may here be determined based on the flow rate in the effluent pipe, for example if it is desirable or not that two consecutive pulses measure the same pellet.

[0044] In order to determine the number of feed pellets in a fluid flow passing through the effluent pipe per time, it is perfered to measure, or at least estimate or assume, a flowrate in the effluent pipe. The velocity that a feed pellet moves through the effluent pipe may for example be determined based on a doppler shift between an emitted ultrasound pulse and a detected ultrasound signal. A person skilled in the art with knowledge of the present invention will appreciate that the flow velocity in the effluent flow may be measured by a wide range of methods. Examples are the use of a flow probe, vortex meter, pygmy meter, current meter or a pitot meter.

[0045] Figure 2a schematically illustrates an embodiment of the invention where the ultrasound transducer 106 is arranged to emit ultrasound in the direction of the effluent flow 118 of the effluent pipe 102. Such an arrangement may be preferred in systems with high effluent water velocities that causes feed pellets and other particles or waste to impact the ultrasound transducer 106 at high speeds. The latter may for example damage the ultrasound transducer 106. An ultrasound transducer 106 being arranged to emit ultrasound in the direction of the effluent flow 118 of the effluent pipe 102 will be shielded from such impacting particles or waste, as the sensitive elements of the ultrasound transducer 106, such as face plates, acoustic lenses and piezoelectric materials, will be facing away from incoming particles or waste. The ultrasound transducer 106 may further be provided with a protective casing or protecting element on the side facing the incoming effluent flow 118 for shielding the ultrasound transducer 106 from incoming particles or waste.

[0046] Figure 2 c schematically illustrates an embodiment of the invention where the ultrasound transducer 106 is arranged to emit ultrasound in the opposite direction 116 of the effluent flow 118 of the effluent pipe 102. Such an arrangement has been found to be advantageous for reducing the effect of turbulence in the wake of, i.e. behind, the ultrasound transducer 106 caused by the effluent flow 118 flowing past the ultrasound transducer 106. Dependent on the effluent flow 118 rate and velocity, turbulent water flow may form behind the ultrasound transducer 106 and cause artefact reflections that may be interpreted as feed pellets. Air bubbles may additionally or alternatively also form or gather behind the ultrasound transducer 106, and consequently give rise to artefact reflections that may be interpreted as feed pellets.

[0047] Turbulence behind the ultrasound transducer may in addition to the issues mentioned above also cause wear and/or instability of the ultrasound transducer. Turbulent water flow behind the ultrasound transducer will cause a vibration on the ultrasound transducer that in worst case may damage the ultrasound transducer or even cause the ultrasound transducer to detach from any mounting means holding it in place. In order to reduce the negative effects of turbulence, the ultrasound transducer may in any embodiment of the invention have a streamlined body or streamlined half-body. The latter shape is preferred when the ultrasound transducer is arranged adjacent to a wall of the effluent pipe. As an alternative, the feed detection assembly and/or the feed detection system of the present invention may further comprise a casing in which the ultrasound transducer is provided. The casing may have a streamlined body or streamlined half-body.

[0048] Figure 4b schematically illustrates an embodiment of the invention where the ultrasound transducer 106 arranged within a distance d/4 from the central longitudinal axis 120 of the effluent pipe 102. Figure 4a schematically illustrates an embodiment of the invention where the ultrasound transducer 106 is arranged along the central longitudinal axis 120 of the effluent pipe 102. d is here the diameter of the effluent pipe 102. Arrangement of the ultrasound transducer 106 within a distance d/4 from the central longitudinal axis 120 of the effluent pipe 102 may be preferred as such an arrangement allows for the ultrasound transducer 106 to emit ultrasound more homogeneously across the cross section of the effluent pipe 102. An ultrasound transducer 106 arranged along the central longitudinal axis 120 of the effluent pipe 102 having a conical beam shape 112 characteristic may for example emit the ultrasound such that the conical ultrasound beam covers the whole cross section of the effluent pipe 102. The latter arrangement will make any post processing of detected ultrasounds easier than what would have been the case for a less symmetric combination of ultrasound transducer 106 arrangement and ultrasound transducer 106 beam shape 112 characteristics. An ultrasound transducer 106 having a conical beam shape 112 that is symmetric about a symmetry axis may here be arranged such that the symmetry axis is aligned with the longitudinal direction of the effluent pipe 102, or at least within 20 or 10 degrees from the longitudinal direction of the effluent pipe 102.

[0049] The frequency/frequencies of the ultrasound transducer may generally be determined based on the desired signal to noise ratio. Frequencies of less than 5 Mhz have been found to achieve a satisfactory signal to noise ratio. Frequencies of less than 2.5 Mhz have been found to be more preferred, while frequencies of less than 1 Mhz have been found to be most preferred. Frequencies of less than 1 Mhz is most preferred as use of such frequencies reduces the effect of thermal noise. Frequencies of less than 1 Mhz is also most preferred ultrasounds configured to emit such frequencies are typically adequately robust while at the same time giving a high performance per cost.

Claims

Claims:

1. A feed detection assembly (100) for detecting feed pellets in an effluent pipe (102) of a fish tank (104), the feed detection assembly (100) comprising:

an ultrasound transducer (106), and

mounting means (108) for mounting the ultrasound transducer (106) such that the ultrasound transducer (106) is arranged to emit ultrasound into the effluent pipe (102) in the longitudinal direction (110) of the effluent pipe (102).

2. The feed detection assembly (100) according to claim 1, where the mounting means (108) is configured to mount the ultrasound transducer (106) at the inlet (124) of, at the outlet (126) of, or inside (114) the effluent pipe (102).

3. The feed detection assembly (100) according to claim 1 or 2, where the mounting means (108) is configured to mount the ultrasound transducer (106) such that the ultrasound transducer (106) is arranged to emit ultrasound in the opposite direction (116) of an effluent flow (118) of the effluent pipe (102).

4. The feed detection assembly (100) according to claim 1 or 2, where the mounting means (108) is configured to mount the ultrasound transducer (106) such that the ultrasound transducer (106) is arranged to emit ultrasound in the direction (116) of the effluent flow (118) of the effluent pipe (102).

5. The feed detection assembly (100) according to any one of the preceding claims, where

the mounting means (108) is configured to mount the ultrasound transducer (106) within a distance d/4 from the central longitudinal axis (120) of the effluent pipe, wherein d is the diameter of the effluent pipe,

or where

the mounting means (108) is configured to mount the ultrasound transducer (106) along the central longitudinal axis (120) of the effluent pipe (102).

6. The feed detection assembly (100) according to any one of the previous claims, where the ultrasound transducer (106) is configured to emit ultrasound with a frequency of less than 1 Mhz.

7. The feed detection assembly (100) according to any one of the preceding claims, where the mounting means (108) comprises a filter (122) or a grid (123) configured to be arranged at the inlet (124) to the effluent pipe (102) from the fish tank (104).

8. A feed detection system (200) comprising:

an ultrasound transducer (106), and

a fish tank (104) comprising an effluent pipe

where the ultrasound transducer (106) is arranged to emit ultrasound into the effluent pipe (102) in the longitudinal direction (110) of the effluent pipe (102).

9. The feed detection system (200) according to claim 8, where the ultrasound transducer (106) is arranged at the inlet (124) of at the outlet (126) of, or inside (114) the effluent pipe (102).

10. The feed detection system (200) according to any one of the claims 8 or 9, where the ultrasound transducer (106) is arranged to emit ultrasound in the opposite direction (116) of an effluent flow (118) of the effluent pipe (102).

11. The feed detection system (200) according to any one of the claims 8 or 9, where the ultrasound transducer (106) is arranged to emit ultrasound in the direction (116) of the effluent flow (118) of the effluent pipe (102).

12. The feed detection system (200) according to any one of the claims 8 - 11, where

the ultrasound transducer (106) is arranged within a distance d/4 from the central longitudinal axis (120) of the effluent pipe, where d is the diameter of the effluent pipe,

or where

the ultrasound transducer (106) is arranged along the central longitudinal axis (120) of the effluent pipe (102).

13. The feed detection system (200) according to any one of the claims 8 - 12, where the ultrasound transducer (106) is configured to emit ultrasound with a frequency of less than 1 Mhz.

14. The feed detection system (200) according to any one of the claims 8 - 13, further comprising a grid or a filter arranged at the inlet (124) to the effluent pipe (102) from the fish tank (104), where the ultrasound transducer (106) is mounted to the grid or the filter.

15. A method for detecting feed pellets in an effluent pipe (102) of a fish tank (104), the method comprising the steps of:

a) emitting ultrasound into the effluent pipe (102) in the longitudinal direction (110) of the effluent pipe,

b) detecting ultrasound that is reflected of feed pellets in the effluent pipe (102), and

c) determining, based on the detected ultrasound, a number of feed pellets in a fluid flow passing through the effluent pipe (102).

16. The method according to claim 15, where the ultrasound emitted is emitted as one or more pulses.

17. The method according to any one of the claims 15-16, wherein the step c) involves determining a velocity of the feed pellets in the effluent pipe (102) based on a doppler shift between the emitted ultrasound and the detected ultrasound.