GB2444564A

GB2444564A - A method of growing and harvesting mussels

Info

Publication number: GB2444564A
Application number: GB0706605A
Authority: GB
Inventors: Desmond Lett; James Anthony Lett; Richard Lett
Original assignee: LETT RES AND DEV Ltd
Current assignee: LETT RES AND DEV Ltd
Priority date: 2006-12-05
Filing date: 2007-04-04
Publication date: 2008-06-11
Anticipated expiration: 2027-04-04
Also published as: GB0706605D0; GB2444564B; IES20070236A2; IE20070235A1

Abstract

A process 1 for the growing and harvesting of mussels is described. The method 1 comprises the steps of: obtaining partly grown mussels 2 from the seabed; sampling the partly grown mussels; preparing a culturing bed 3 in the seabed for growing the mussels; transporting the partly grown mussels 2 to the prepared culturing bed 3. Further sampling steps of the growing mussels are also performed on a monthly basis, eventually allowing the partly grown mussels 2 to mature into fully grown mussels. The grown mussels are then harvested 4 when the meat yield is 30 to 45%. Said mussels 8 are then subjected to a washing step 5 to separate the mussels 8 from the debris, followed by processing the mussels 8 and recycling the empty shells and seedlings or seed mussel 6, 7. The recycling step further comprises; placing the seedlings or seed mussel on the culturing bed, or on a further culturing bed, wherein the seedlings or seed mussel can undergo a further cycle of growth and harvesting.

Description

"A METHOD FOR HARVESTING AND GROWING MUSSELS"

Introduction

The present invention is directed to a process for the growing and harvesting of mussels.

The common or blue mussel (Myilus edulis) is the most commonly farmed mussel.

These mussels inhabit lakes, rivers, and creeks, as well as intertidal areas along coastlines worldwide.

The external shell of a mussel is composed of two halves that protect it from predators and desiccation. Protruding from a valve is an enlarged structure called the umbo, which indicates the dorsal surface of the mussel. Like most bivalves, mussels have a large organ referred to as a foot, which is tongue like in shape with a groove on the ventral surface, which is continuous with the byssus pit. In this pit a viscous secretion is poured out which enters the groove and hardens gradually when it comes into contact with sea water. This forms an extremely tough byssus thread that secures the mussel to its substrate. These byssus threads are the means by which mussels attach themselves to surfaces.

In dense beds, the byssal threads can create a thick mat of silt, mud and dead shells held together by the collective byssal attachment of the colony. These large beds go through natural cycles, increasing in size as silt and shell build up within the bed and then breaking up during major storms.

Mussels feed by filtering plankton or bits of organic material from the water. The food that they get can depend on the density of the bed, their position within the bed, the strength of the currents just over the bed, or their position above mean low water (MLW). Mussels that live under the best combination of these conditions are the most valuable for commercial harvest. Mussel quality also varies seasonally and is a function of the spawning cycle. The Mussels are at their best in terms of weight, taste and health after feeding during the hot summer months so that in September and October they have reached their greatest market value, which continues through to early spring the following year. Normally spawning will occur in the months March through to May, depending on a number of factors including water temperature and geographic location.

Mussels follow a reproductive strategy of producing a very large number of gametes and hence planktonic larvae, of which a small proportion survive to settle and establish in the seabed. In the Irish Sea, the development of the gonad generally starts during October and November and is followed by gametogenesis during the winter months. A partial spawning occurs in spring, followed by rapid gametogenesis until early summer. Less intensive spawning takes place throughout the summer. From late August to November, the mntle tissue thickens with nutrients to prepare for winter gametogenesis. An extended period of reproduction with repeated spawning in spring and summer is sometimes seen in mussels growing under favourable nutrients conditions. I Typically when the mussel spawn reaches a size of approximately 2Omm-35mm length, they are called seed mussels and can be harvested for cultivation. After the seed mussel is formed, mussels can be grown to marketable size in 12 to 24 months depending on water temperatures, feeding and geographic location.

Wild mussels can be harvested all year, but most fishing is in the winter when the quality of the meat is best. They are taken by a mussel dredge from a boat which is essentially a framed mouth with an attached bag. Across the bottom of the mouth is either a cutting bar or a chain sweep which loosens the mussels as the dredge is pulled across the bottom. The mussels are washed and cleaned on board the boat to remove dirt and grit. The mussels are then sorted by size, graded and bagged for shipment to market or in some cases shucked and the meat sold. The mussels are then normally placed into a one tonne bag.

There are a variety of conventional techniques for growing mussels.

Intertidal growth technique or bouchot technique is carried out in France where poles, or bouchots, are planted at sea, ropes, on which the mussels grow, are tied in a spiral on the poles and some mesh netting prevents the mussels from falling away. This method needs an extended tidal zone.

The suspended culture method is the most common method in the West of Ireland whereby mussels attach to ropes which are hung off a rope back-bone supported by large plastic floats. With this suspended culture method, mussel spat is collected on short ropes hung in the water. Later they are transferred to plastic mesh tubing suspended from floats where they grow until reaching marketable size. These mussels grow very fast and are of the highest quality, but the process is labor intensive and the mussels are vulnerable to storms and wave damage.

The final technique of mussel aquaculture, is bottom culture commonly used in Europe, including Ireland and generally involves placing seed mussels (20 mm-35 mm) in an area where growing conditions are optimum and where the culturist has exclusive rights to the harvest. Essentially, the mussel seed grown in nature is harvested and transferred to beds which have more favourable conditions for growth. This technique has advantages where natural seed mussel beds are overcrowded and undesirable conditions for mussel growth. With bottom culture, seed mussels are collected from dense, wild natural beds and thinly spread over another site. This process is less labor intensive than suspended culture, but there are various disadvantages. For example, mussels are more subject to predation, in particular from starfish and crabs, and the harvest is less predictable. Thus, there is a need for improving such methods and increasing the yield.

In recent years, there has been substantial growth in the demand for good quality high meat yielding mussels and the mussel-farming industry has struggled to keep up with demand. It is also therefore highly desirable to provide techniques for increasing production volume and reducing any associated production costs.

One of the major problems with conventional mussel growing and harvesting is a loss in yield from seed mussel to mature mussel in conventional bottom culture methods.

This loss of yield is caused by poor conditions on the natural beds (predators, overcrowding, poor nutrient levels, accumulation of mud and silt) and presents a significant loss in commercial terms.

The present invention is directed to overcoming these problems.

Statement of the Invention

According to the invention, there is provided a general recycling process for growing and harvesting mussels comprising the steps of a) Obtaining partly grown mussels; b) Preparing a culturing bed in the seabed for growing the mussels; c) Adding the partly grown mussels obtained from step (a) to the prepared culturing bed; d) Allowing the partly grown mussels to mature into fully grown mussels; e) Harvesting the grown mussels, preferably with a mussel dredger, such that a layer of debris including empty shells and seed mussel or seedlings is removed together with the grown mussels; f) Subjecting the grown mussel and debris obtained from step (e) to a washing step to separate the grown mussels from the debris g) Processing the grown mussels; h) Returning the empty shells to the culturing bed of step (b) or to a further culturing bed; and i) Placing the seedlings or seed mussel on the culturing bed of step (b) or on a further culturing bed wherein the seedlings or seed mussel undergo a further cycle of growth and harvesting according to steps (a) to (h).

It will be understood that the following terms are interchangeable, "seed mussel" and "seedlings"; and "half grown mussels" and "partly grown mussels.

Ideally, the process is a recycling process for growing and harvesting mussels and comprises the steps of: a) Obtaining partly grown mussels from the seabed, in size range of approximately 10mm to approximately 40mm in length,; b) Taking a sample of partly grown mussels from step (a) and measuring the average size, the average meat yield and the average count of partly grown mussels per 2.5kgs; c) Preparing a culturing bed in the seabed for growing the mussels comprising the steps of raking the seabed, adding extra stones from approximately 30 grams to 150 grams in weight, adding a layer of empty mussel shells to provide the mussel with an anchoring point and clearing the seabed of excess mud and/or silt; d) Transporting the partly grown mussels obtained from step (a) under chill conditions which are as close to the temperature of the receiving water of the prepared culturing bed of step (C); e) Placing the partly grown mussels obtained from step (a) onto the prepared culturing bed of step (C), using sea water, through a dispersement system to spread the partly grown mussels evenly on the prepared culturing bed; 1) Carrying out a sampling step of the growing mussels at least every month after step (e) to measure the average size, the average meat yield and the average count per 2.5kgs of the growing mussels; g) Allowing the partly grown mussels to mature into fully grown mussels for a period of approximately 4 to 12 months, preferably for a period of approximately 6 to 9 months; h) Harvesting the grown mussels when the meat yield is approximately 25 to 50% by weight, preferably with a mussel dredger, such that a layer of debris including empty shells and seed mussel or seedlings is removed together with the grown mussels; I) Subjecting the grown mussel and debris obtained from step (h) to a washing step to separate the grown mussels from the debris j) Processing the grown mussels from step (i); k) Recycling the empty shells obtained in step (h) comprising the steps of placing the empty shells on the culturing bed of step (c) or on a further culturing bed located in the seabed; and I) Recycling the seedlings or seed mussel obtained in step (h) comprising the steps of placing the seedlings or seed mussel on the culturing bed of step (c) or on a further culturing bed wherein the seedlings or seed mussel can undergo a further cycle of growth and harvesting according to steps (a) to (h).

Advantageously, this recycling process provides for a greater yield of mature mussel from the seed mussel cultured than conventional techniques outlined above.

The present invention deals with the problems associated with the location of the seabed. Such locations cannot guarantee an ideal environment for mussel growth as factors such as temperature fluctuations, tidal flow or lack of plankton food supply are difficult to control using conventional mussel culturing methods. The preparation of the culturing bed provides for the increased yield as it deals with these problem of undesirable conditions on the seabed.

Ideally, the location chosen for the culturing bed is a seabed area of known good growing quality with high yielding, culturing attributes and with good food availability for the growing mussels.

A further major advantage of the invention is that the process provides for the recycling of seed mussel/seedlings and the recycling of materials during the production of the culturing bed. This recycling step increases the yield of the process and ensures that the culturing bed provides the desired growing properties.

According to one embodiment of this invention, the half grown mussels are harvested from the seabed by conventional means such as a mussel drag or dredger.

Alternatively, the half grown or partly grown mussels may be obtained from a rope culture system. Any method suitable for obtaining half grown mussels may also be used according to the present invention.

The half grown mussels are then placed on a prepared culturing bed which is prepared as follows.

In the method of the present invention the seabed area is dredged extensively in order to ensure a reasonably smooth and even bottom before shells and small stones are placed thereon. The shells and stones allow for the immediate attachment of small mussels thus anchoring them and providing immediate stability on the prepared seabed. The shells not only safeguard the attachment of small mussels for survival purposes but also guards against predation and the possibility of seabed destruction where increased tidal flow may be experienced, particularly in times of heavy rain fall when a "fresh" is experienced within the estuary system.

Furthermore, the placing of shells on the prepared seabed allows for the even distribution of small mussel attachment and access to unrestricted supplies of plankton when temperatures are correct for feeding thereby ensuring a stable and regular growth cycle to production size.

The size of stones added to the seabed varies anywhere from 30g to 1 50g and the main point is that stones, together with shells, provide a safe anchoring point for the seed mussel to attach itself to the seabed using its bysus.

The transporting of mussel seed is undertaken in temperatures similar or as close as possible to those of the receiving waters, e g. 000 tolO C. This is very important aspect of the present invention. Ideally, the mussels are transported in sea water.

Furthermore, the placing of seed mussel onto the prepared seabed involves the use of sea water to transfer the seed mussel from the boat to the seabed. This effectively limits as much stress and strain as possible on the young mussel seed and increasing greatly their survival ratio, thereby provided a much improved process.

Ideally transfer to the prepared culturing bed takes place within 24 hours after collecting the partly grown mussels.

The growth of mussels on this prepared seabed allow for mussel growth in natural conditions and the final product will be much stronger in terms of survival because for almost two years the mussels have been open to all the stresses that a natural estuary environment can provide.

After placing the half grown mussel on the culturing bed, the mussel is allowed to grow for approximately 6 months to reach maturity, achieving maximum meat yield in excess of 25%.

According to another embodiment of the invention, the mussel development is monitored continually. Ideally, the half grown mussels are sampled, at least each month or every three months, to determine average size, average meat yield and average mussel count per 2 5 kilo, prior to the being placed on the prepared bed.

Once the half grown mussels are placed on the culturing seabed, each month thereafter during the growing period the mussels are resampled and the growth rates charted. This provides the advantage of quality control wherein the mussels can be monitored to determine when they are ready for the market to achieve the highest possible price.

The present invention provides the additional advantage that it may be possible to obtain two harvests of mature mussels when using conventional techniques only one harvest would be obtained. Half grown mussels seeded in the period March to May can be harvested September to December. Half grown mussels seeded in August to October can be harvested in January to March. This method gives the unique possibility of achieving an unprecedented two harvests per twelve month cycle, thus optimising the growing area attributes to the maximum.

After harvesting the grown mussels, they are then subjected to a gentle washing step wherein the harvested mixture (which comprises dirt/grit/debris and also seed mussel) is placed in a washing system to allow the gradual filtering and washing away of small grit and seed mussel followed by the harvested mussel.

The grown mussels are processed and packaged in the usual way and the debris is recycled to obtain the advantages outlined above. The seedlings, empty shells and stones are returned to the same or a further culturing bed. This provides for culturing bed which provide good growth conditions and the recycling of the seedlings increases the yield of grown mussels at the end of the process.

Detailed DescriDtion of the Invention The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a diagrammatic layout of the process of harvesting and growing mussels according to the invention.

Referring to Fig. 1, where there is illustrated a general process according to the invention, there is now described in general outline a process for harvesting and growing mussels indicated generally by reference numeral 1.

Partly grown mussels 2 are obtained from the seabed. These partly grown mussels are generally of a size of approximately 10mm to 50mm, preferably in a range from 20mm to 40 or 35mm. Mussels of this size are termed "half grown seed mussels" or "partly grown seed mussels" and it will be understood that these terms are interchangeable. Half grown mussels are seed mussels which have been planted on a suitable sea bed for on-growing purposes over a period of approximately 2 to 4 months. This size has the advantage that it gives the possibility of growing the -10-mussels to maturity En a good culturing site within approximately 6 to 9 months.

These partly grown mussels may be obtained from the seabed having been grown from mussel spawn using the conventional bottom culture method or alternatively from ropes or suspended culture methods.

Generally, it takes two to four months to achieve the ideal size of approximately 20mm to 35mm from the mussel spawning. This timeframe depends on the location and the type of culturing bed and process used for the seed mussel.

Sometimes half or partly grown mussels are used from cultured rope grown mussels.

They are removed from the rope by hand and transferred to a mussel sack and transported immediately in chill conditions to the culturing bed of the invention. At this stage the half grown mussels may be washed and/or screened.

Half grown mussels are best transferred to a culturing bed in a period outside of the high summer temperatures to ensure low mortality risk.

Once the half grown mussels are harvested they are immediately placed on the culturing bed and this total operation should be concluded within 24 hours to ensure maximum yield and minimise mortality.

A culturing bed 3 for growing the mussels is prepared. This generally involves the steps of cleaning the bed. Additionally, further mussel shells are added to the bed.

This ensures that the half grown mussels do not sink to the bottom of the bed and have adequate surface to attach themselves to.

Ideally, it is also essential that the bed is cleaned of any excess mud, otherwise high mortality would occur with the mussels smothering.

The bed preparation is an essential step needed in order to obtaining high meat yield from the culturing bed, and if it is carried out properly gross yields of 1.5:1 up to 2:1 yield of mature harvested mussel to seed mussel can be achieved. -11 -

Ideally, it is also essential to ensure that the bed has the correct nutrients for mussel growth and also provides a suitable surface for adhesion of the mussels. The culturing bed is generally located in a known natural mussel growing area. This means that where natural mussels have existed in the past the area provides nutrients which allows for the intensive culturing of half grown mussels.

The preparation of the culturing bed 3 is an important step and results in an approximately 1.5:1 to 2:1 yield of mature harvested mussel:seed mussel.

Conventional mussel harvesting does not involve this step of preparing the seabed and one would expect only to achieve yields of between 0.25:1 to 0.8:1 end yield of mature harvested mussel:seed mussel in sea beds which have not been prepared in accordance with the present invention. In some cases where the seed mussel has been placed on a sea bed which contains high quantities of mud total mortality of all the mussels has been recorded.

Thus, the process provides for a greatly improved yield of mature mussels.

Furthermore, as shown in steps 6 and 7 below, the recycling aspect of the invention also has great advantages.

The partly grown mussels 2 may be added to the prepared culturing bed 3 by washing the mussels through a dispersement system on board the dredger and spreading them as evenly as possible on to the culturing bed 3. This process of washing the mussels from the dredger using sea water allows for less damage to the half grown mussel and again is a vital step in obtaining high yields. The mussel is placed almost stress free on to the bed with no shell breakages.

Prior to the half grown mussel being placed on the bed the mussels are sampled from size, meat yield and count per 2.5 kg.

Meat yield is measured as follows: A 2.5 kg. sample of mussels is weighed and counted to determine the count per kilo.

The 2.5 kg. sample is then placed in a saucepan with 10 mm of water and brought to -12 -boiling point until all of the mussels are open, this usually takes approximately two minutes. The mussels are immediately cooled using cold water. The meat is then extracted by hand and all water is allowed to drain from the meats for five minutes.

The meat weight is now determined and the percentage is directly proportional between the cooked meat weight and the fresh mussel weight i.e. 750 grams of cooked meat from an initial 2.5 kg sample of fresh mussels gives you 30% meat yield.

Meat yield is determined as above. Each sample step is carried out at three monthly intervals to ensure (1) the size and growth of the mussel and (2) the meat yield. Both size and meat yield determine when the mussels are harvested and the mussels are taken from the water when the count per 2.5 kg falls below a normal threshold of approximately 145 mussels per 2.5 kg sample. Also, the meat yield determines when harvesting should occur, it must be at its best percentage value, i.e.: between 30-45%. This normally occurs in September/October/November each year and also indicates that there has not been a second spawning (when mussels spawn the meat yield within the shell would drop almost overnight from 25/30% down to 10/15% which makes the mussel unsuitable to sell at that point and time).

Each month thereafter during the growing period the mussels are resampled and the growth rates charted. With this control of the quality of culturing the mussels can be monitored to determine when they are ready for the market to achieve the highest possible price. Densities are surveyed and monitored to ensure return is in line with normal production expectation.

The culturing bed with the half grown mussels is then left for a period of 4 to 6, 9 or 12 months. At the end of this time period, the mussels are fully mature and ready to be harvested 4.

In step 4 the grown mussels are harvested with a conventional mussel dredger such that a quantity of mature mussels, together with natural seed mussel and dirt is taken from the bed. Thus, the next step is to separate the mature mussels from the seed mussels and the dirt. -13-

Because of the method of harvesting shells and seedlings are often found attached to the grown mussel.

In step 5 the harvested mussels, shell/dirt and seedling mixture are subjected to washing in order to separate the harvested mussels 8 from the dirt/debris.

This washing stage extracts the dirt and seed size of mussel and allows the mature mussel to continue in the process of being washed, graded 9 and bagged for the market.

Finally as shown in 6 the small stones, grit and empty shells are returned to the culturing bed. This recycling aspect of the invention is important to prepare the bed for the next season of culturing.

In step 7 seed mussel is recycled and is placed back on the original culturing bed.

This has the additional advantage that the seed mussels are returned to the harvesting bed and will then mature into fully grown mussels. The continuous recycling of natural seed mussels on to the culturing bed helps to increase the yield of mussels from the next harvest and is a vital step in this invention.

The washing/grading machine can either take place on board the boat or on land where the fishing boat lands. This machine is designed to have the flexibility for both onshore and/or fishing boat operation.

It will be appreciated that the present invention is not limited to the embodiments described herein and may be varied in detail. -14-

Claims

CL6JMS: 1. A recycling process for growing and harvesting mussels

comprising the steps of: a) Obtaining partly grown mussels from the seabed, in size range from approximately 10mm to approximately 40mm in length,; b)Taking a sample of partly grown mussels from step (a) and measuring the average size, the average meat yield and the average count of partly grown mussels per 2.5kgs; C) Preparing a culturing bed in the seabed for growing the mussels comprising the steps of raking the seabed, adding extra stones of approximately 30 grams to 150 grams in weight, adding a layer of empty mussel shells to provide the mussel with an anchoring point and clearing the seabed of excess mud and/or silt; d) Transporting the partly grown mussels obtained from step (a) under chill conditions which are as close to the temperature of the receiving water of the prepared culturing bed of step (C); e) Placing the partly grown mussels obtained from step (d) onto the prepared culturing bed of step (c), using a dispersement system with sea water, to spread the partly grown mussels evenly on the prepared culturing bed; f) Carrying out a sampling step of the growing mussels at least every month after step (e) to measure the average size, the average meat yield and the average count per 2.Skgs of the growing mussels; g)Allowing the partly grown mussels to mature into fully grown mussels for a period of approximately 4 to 12 months, preferably for a period of approximately 6 to 9 months; -15-h) Harvesting the grown mussels when the meat yield is approximately 25 to 50% by weight, preferably with a mussel dredger, such that a layer of debris including empty shells and seed mussel or seedlings is removed together with the grown mussels; I) Subjecting the grown mussel and debris obtained from step (h) to a washing step to separate the grown mussels from the debris j) Processing the grown mussels from step (i): k)Reycling the empty shells obtained in step (h) comprising the steps of placing the empty shells on the culturing bed of step (c) or on a further culturing bed located in the seabed; and I) Recycling the seedlings or seed mussel obtained in step (h) comprising the steps of placing the seedlings or seed mussel on the culturing bed of step (c) or on a further culturing bed wherein the seedlings or seed mussel can undergo a further cycle of growth and harvesting according to steps (a) to (k).
2. The process according to claim 1 wherein the partly grown mussels are seed mussels or seedlings which have grown for a period of 2 to 4 months.
3. The process according to any of the preceding claims wherein the partly grown mussels are harvested when they range in size from approximately 20mmto35mm.
4. The process according to any of the preceding claims wherein the partly grown mussels are washed and/or screened prior to step (d).
5. The process according to any of the preceding claims wherein the partly grown mussels are harvested from the seabed by conventional means or are obtained from a rope culture system.
6. The process according to any of the preceding claims wherein step (d) -16 -occurs under chill conditions from 0 C to 10 C.
7. The process according to any of the preceding claims wherein step (d) occurs within approximately 24 hours of harvesting the partly grown mussels.
8. The process according to any of the preceding claims wherein the partly grown mussels of step (a) are obtained from the seabed by a dredger and are washed, using sea water, through the dispersement system located on the dredger prior to spreading the partly grown mussels on the prepared culturing bed.
9. The process according to any of the preceding claims wherein the grown mussels are processed by a subsequent cleaning step followed by a packaging step or a shucking step.