METHOD FOR ATTRACTING AND CONCENTRATING FISH

TECHNICAL FIELD

Disclosed is a method for attracting and concentrating fish in a region of an ocean.

BACKGROUND TO THE DISCLOSURE

Increased consumer demand for fish may result in the fishing industry exploiting the ocean's resources beyond its carrying capacity. Increasing the ability of a region of an ocean to sustain a greater population of fish would be advantageous .

SUMMARY OF THE DISCLOSURE In a first aspect, there is provided a method for attracting and concentrating fish in a region of an ocean. The method comprises the step of delivering a source of nitrogen to the photic zone of the ocean, whereby the number of phytoplankton is caused to increase in the region.

By delivering an exogenous source of nitrogen to a specific layer of the ocean at a specific location, the growth of the phytoplankton population in the region is caused to increase. Phytoplankton growth is limited in some 80% of the ocean by the lack of the macronutrient nitrogen. As the skilled addressee will appreciate, since phytoplankton is the base of the marine food chain, increasing numbers of phytoplankton would result in an increase in ocean fish biomass . Mot only are the fish attracted to and concentrated in the region, the increased number of phytoplankton and consequent zooplankton provide a readily available source of food for the fish, which can enhance the growth rate of the fish attracted and concentrated in the region.

Furthermore, the ability to attract and concentrate fish

in a defined region of an ocean would be of enormous commercial benefit to the fishing industry. For example, the region could be readily identified (e.g. with a GPS) and fishing trawlers need only trawl that region to catch a sufficient amount of fish, providing substantial savings of fuel and time. The concentration of fish may also make management of the "fishery" more convenient.

In some embodiments, the source of nitrogen is urea. Urea is extensively used in agriculture and occurs naturally in sea water as a result of the bacterial decay of dead phytoplankton or zooplankton excretions. It has numerous advantages over many other nitrogen containing compounds, such as ammonia, in that it can be easily stored and transported, is not caustic and is pH neutral. In contrast, ammonia (or solutions of ammonia) is caustic, toxic and classified as a dangerous chemical.

In some embodiments, the source of nitrogen can be delivered to the photic zone of an ocean from a floating vessel (e.g. a ship or a boat) . Delivering the source of nitrogen from a floating vessel may provide a number of advantages over other methods of delivery, such as via a pipeline. For example, it might not be economic to provide the source of nitrogen via a pipeline because of the extensive length of pipe required to reach an appropriate region of the ocean, as well as the ongoing costs of maintaining the pipeline. Furthermore, the outlet of a pipe is in a fixed location which may limit the size and magnitude of the region of enhanced phytoplankton concentration, and therefore limit the number of fish that could be attracted, concentrated and fattened in the region. In contrast, floating vessels provide a flexible option for delivering the source of nitrogen in various locations and in various patterns.

In some embodiments, the urea may be in the form of prills

of urea. In some embodiments, the prills of urea may be broadcast onto the surface of the ocean from the floating vessel .

In alternate embodiments, the urea is in granular form and is mixed on the floating vessel immediately before delivery with water taken from the ocean.

In some embodiments, the urea may be injected into the ocean at a predetermined depth (e.g. between about 15m and 50m) .

In some embodiments, the amount of the source of nitrogen delivered to the ocean results in the concentration of available nitrogen being raised by between about 50μg/L and SOOμg/L in the photic zone.

In some embodiments, one or more additional nutrients (e.g. a nutrient containing phosphorous) may be delivered to the photic zone with the source of nitrogen.

In some embodiments, the method further comprises the step of monitoring the increased number of phytoplankton, and adding more of the source of nitrogen to regions in which it is possible to further increase or maintain an increased number of phytoplankton.

The increased number of phytoplankton may, for example, be monitored by satellite. Alternatively, the increased number of phytoplankton may be monitored by a second boat or ship down current of the floating vessel .

In some embodiments, a dye such as hydrogen hexafluoride may be delivered to the photic zone with the source of nitrogen to aid in monitoring the increased number of phytoplankton .

The fish concentrated in the region are typically subsequently harvested, for example, by trawling.

In a second aspect, there is provided a fish which has been harvested using embodiments of the method of the first aspect in which fish that have been attracted and concentrated in the region have subsequently been harvested .

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Specific embodiments of the method set forth in the Summary will now be described.

In the method set forth in the Summary, fish are attracted to and concentrated in a region of an ocean because the number of phytoplankton is caused to increase in that region. The phytoplankton population is caused to increase by delivering a source of nitrogen to the photic zone of the ocean.

When atmospheric carbon dioxide dissolves in the ocean it exists in an ionic form and is taken into the bodies of marine phytoplankton through the process of photosynthesis. In some regions of the ocean, however, the conversion of carbon dioxide dissolved at the surface of the ocean to organic carbon (i.e. phytoplankton) during the sunlit periods is limited by the availability of the macronutrient nitrogen. It is believed that phytoplankton growth may be limited in some 80% of the ocean by the lack of nitrogen.

The region referred to above must therefore be a region of the ocean which is deficient in nitrogen in order for the addition of the source of nitrogen to cause the number of phytoplankton to increase. It is within the ability of one skilled in the art to determine appropriate regions upon which to perform this method.

The source of nitrogen may be added to the region of the ocean itself. However, as a delay of a few days (e.g. 4 to 8 days, depending on the ocean temperature and other factors known in the art) occurs between delivery of the nitrogen and a maximum phytoplankton population (i.e. maximum biomass) , the source of nitrogen will typically be delivered to the ocean at a location remote from the region, and subsequently transported to the region by ocean currents and diluted by mixing etc.

Advantageously, in the method set forth in the Summary, fish are attracted to, concentrated in and fattened in a specific region of an ocean. As the location of the region is known, the concentrated fish population can be harvested more easily and on a more regular basis than when using traditional methods of catching fish.

Typically, the photic zone (which roughly corresponds to the mixed layer) extends from the surface of the ocean to a depth of about 50 metres. However, the photic zone may extend to a depth of 100 metres or more. The actual depth of the photic zone varies, and is dependent upon a number of factors including wind strength and the loss of heat due to the temperature difference between the oceanic surface waters and the lower atmosphere . The depth of the photic zone in a particular region can be determined using techniques known in the art .

The depth at which the source of nitrogen should be distributed in the photic zone after delivery can depend on a number of factors. Few phytoplankton exist below 100m because very little sunlight penetrates that deep into the ocean. Similarly, few phytoplankton exist in the top few meters of the ocean because the sunlight is too intense. As such, the urea needs to be distributed between these levels and at a location where it is most

suitable for phytoplankton growth (e.g. between about 15m and about 50m depth) . It is within the ability of one skilled in the art to determine the most appropriate depth or depth range for the source of nitrogen to be distributed, based on measurements taken in the seawater where the source of nitrogen is to be delivered.

The source of nitrogen used in the method is typically urea due to its ease of handling, high nitrogen content and because it can be found naturally in sea water.

However, in some embodiments, the source of nitrogen can be ammonia or one of its salts (either in solution or in the gas phase) . Ammonia can also be produced naturally in sea water as a result of the bacterial decay of dead phytoplankton or zooplankton excretions. Other sources of nitrogen such as sodium nitrate and nitric acid may also be used.

In some embodiments the source of nitrogen is urea in the form of prills of urea. A prill of urea is a roughly spherical grain of urea formed by cooling droplets of molten urea in an airstream. Although other forms of urea (and indeed, other sources of nitrogen) would be expected to be well mixed vertically within 24 hours due to convective mixing, using prills of urea may advantageously avoid high peak concentrations immediately after introduction of the urea.

The urea prills can be distributed into the mixed layer of the surface ocean by any suitable method, for example, using agricultural spreaders which broadcast the urea from the deck of a ship. Various sizes and classes of spreaders are available, most use the principal of centrifugal acceleration to propel granular material out in a fan type pattern from the source. The source generally consists of a spinning disk with vanes that are fed material by gravity from a hopper mounted above.

An adequate broadcast distance is about 20 m from the spreader and will provide a good separation between prills. As the prills fall through the photic zone they dissolve, leaving behind a plume of enriched water which immediately starts diffusing laterally. A maximum desirable depth for dissolution to be complete is less than the depth of the mixed layer. Urea which falls below this level may not contribute to increased phytoplankton growth due to a lack of sunlight.

The fall depth of the prills is a function of the fall velocity and dissolution rate. Commercially available prilled urea contains a bubble of air which lowers the bulk density of the sphere from that of solid urea. As the prill sinks and dissolves, the outside circumference reduces and the volume ratio of air to urea increases. Therefore, the bulk density of the prill decreases, as does the relative density difference between the prill and the seawater. At some point during the fall, the density of the prill becomes less than that of the seawater and the prill becomes buoyant and begins to rise back towards the surface. This is usually followed shortly afterwards by the separation of the air bubble from the remaining urea, and the latter resumes sinking.

As such, broadcasting prills of urea from the deck of a ship can provide a good vertical distribution of urea throughout the photic zone and the majority of the nitrogen will ideally be distributed at a depth between 15 and 50m. Further, when broadcast in this manner, a good horizontal distribution prills throughout the photic zone can also be achieved. As will also be appreciated, prills of urea are relatively easily handled.

In alternate embodiments, the source of nitrogen is a granular form of urea, which can be distributed in solid form from a floating vessel, or mixed with water taken

from the ocean on a floating vessel immediately before delivery.

As one skilled in the art will appreciate, the properties (e.g. temperature, density and salinity) of a solution of urea dissolved in seawater taken from the ocean will be similar to those of the seawater into which the urea is to be delivered. Indeed, if the solution of urea is a relatively dilute solution, its properties will be very . similar to that of the surrounding seawater. As such, the seawater containing the urea will not appreciably rise or fall in the water column before the phytoplankton are able to access the nitrogen in the urea.

The urea is typically injected into the region at a predetermined depth (e.g. between 15 and 50m) in order to form a concentrated solution at the depth most suitable for phytoplankton growth.

Regardless of the method by which the source of nitrogen is delivered to the photic zone of the ocean, the amount of the source of nitrogen delivered to the ocean would typically result in the concentration of available nitrogen being initially raised by between about 50/ig/L and 500μg/L in the ocean to which it is added.

The concentration of available nitrogen may, for example, be raised by between about lOOμg/L and 300μg/L, or by between about 200μg/L and 400μg/L, of seawater in conditions where the mixed layer is unusually deep or the shear diffusion unusually high.

Once the source of nitrogen has been delivered to the photic zone, the prevailing ocean current will carry away the added nitrogen to either form a patch or, if continuous injection is employed, a plume of enriched water. Ideally, a sufficient amount of the source of

nitrogen (and optionally other nutrients) is added whereby the concentrations of added nutrients are near zero at the region of maximum phytoplankton concentration (i.e. all of the nitrogen added is consumed by the extra phytoplankton in the region) .

In determining where to deliver the source of nitrogen to the ocean in order to cause the maximum possible phytoplankton population increase in the region, it may be necessary to conduct culture bottle enrichments of natural phytoplankton stock from seawater taken from the ocean using nitrogen (and optionally other nutrients such as phosphorous) , because the time until maximum growth is dependent on the temperature, etc. of the seawater. For example, the time until maximum growth has been found to vary from 4 days in the SuIu Sea to 8 days in colder latitudes such as the Canary Current in the Atlantic Ocean. Longer incubation periods may increase the susceptibility of the enriched plume to change because of currents and greater diffusion variability. This would make some locations for attracting and concentrating fish more challenging than others, for example, at a current of 40 cms ^"1 , the area of maximum biomass 8 days after injection may occur some 280 km down-current from the injection location.

Typically, the amount of the source of nitrogen delivered to the ocean is calculated empirically, working backwards from the target concentrations of phytoplankton in the region and considering dilution of the patch and the time to maximum biomass. For example, in some embodiments, sufficient urea is delivered to the ocean such that a total of 2μM of additional nitrogen is available for phytoplankton uptake at the region of maximum biomass. When there is both diffusion and uptake by phytoplankton there should ideally be no introduced nitrogen left at this location, as it will all have been consumed.

It may therefore be necessary to plan the introduction of the source of nitrogen at a location some days up-.current of the desired region of maximum growth, in order to optimise the amount of the source of nitrogen added. Whilst some key parameters affecting the movement of the nitrogen-enriched water include the current velocity, depth of the surface mixed layer, diffusion rate, and time until maximum phytoplankton biomass concentration is achieved, these variables are all environmental factors that are difficult to manipulate. The primary variables which can be controlled are the area of the ocean into which nitrogen is delivered, the initial concentration of the source of nitrogen (and optionally other nutrients) , as well as the release location.

If the source of nitrogen is delivered from a floating vessel, diffusion can be used to advantage by adding urea to the ocean in strips perpendicular to the current direction, then allowing diffusion and convective mixing over the mixed layer depth to even the concentration of nitrogen after a number of days. The process can be designed so that near uniform concentration is achieved as the phytoplankton patch reaches maximum growth.

In order to perform the method set forth in the Summary, the hold of a floating vessel in the form of a ship could be loaded with urea, preferably in the form of prilled or granular urea. Urea is not classified as a dangerous chemical and has suitable storage stability and flow properties. The ship could then sail to any region of the ocean in which the phytoplankton population would increase upon addition of urea. For example, the ship could be anchored near the edge of a continental shelf or could be steamed in a predetermined position using a GPS.

Once the ship was located at the desired location of the ocean, the urea could be delivered by blowing into a

venturi, mixing with seawater to provide a solution of about 5%w/v urea, and the resultant solution injected via a pipe from the ship to a depth of the photic zone that is determined to be most suitable for phytoplankton growth.

Alternatively, a dense solution of urea and seawater could be sprayed out into the water, where it would subsequently be diluted as it sinks in the photic zone. Alternatively, prills of urea (or granules of urea) could be broadcast from the deck of the ship as described above.

The ship could deliver the urea as it steams along a predetermined path, chosen to produce an extensive phytoplankton bloom in a desirable region. Alternatively, the ship could steam in a grid pattern throughout the region, As discussed above, in some embodiments, the urea is delivered in strips which are perpendicular to the direction of the current and the region is located a few days down-current .

As discussed above, about a week after the source of nitrogen has been delivered into the ocean, a phytoplankton patch of maximum biomass will occur in the region. The existence of this patch can be monitored down current of the delivery point (s), for example by satellite or by a second boat (or by the ship which originally injected the urea) and the concentration of the phytoplankton can be measured if desired.

If desirable, the ship (or another ship) containing urea could return to that region of the ocean and add additional amounts of urea to further increase or maintain the increased number of phytoplankton in the desired region.

The released source of nitrogen (e.g. urea) forms a nutrient plume in the photic zone, which is transported by

the ocean currents throughout the photic zone. Ocean currents and diffusion assist in the dispersion of nitrogen through the region. The presence of the added nitrogen together with sunlight enables the phytoplankton in the photic zone to multiply as the source of nitrogen (e.g. urea) and other added nutrients or naturally occurring nutrients are consumed. In this manner, phytoplankton patches of substantial size could be maintained for as long as desired.

The increased number of phytoplankton will cause an increase in zooplankton and attract marine species which feed on the phytoplankton and/or zooplankton. This in turn will attract marine species which feed on other marine species. Thus, the region will attract a relatively high concentration of fish and, as the precise location of the region having the enhanced phytoplankton population is known (e.g. by monitoring with satellite or by a knowledge of local ocean currents and delivery points) , the location of the attracted and concentrated fish will also be known. Accordingly, the attracted and concentrated fish can be harvested (e.g. by trawling) more easily and economically (e.g. because less fuel and time is used) than would be the case if the fish were not so concentrated. Furthermore, the fish attracted to and concentrated in that region have an abundant source of food and their rate of growth will be greatly enhanced.

It will be appreciated that delivering a source of nitrogen (e.g. urea) from a floating vessel may offer numerous advantages over alternate methods of delivering nutrients (e.g. if fixed pipelines were used) . For example, as described above, a floating vessel could be used to cause a long-lasting phytoplankton patch over a large region of the ocean. Thus, a particular region could be harvested periodically, but would be less likely to be over-harvested because the activity can be

supervised more easily than if the fish were distributed over a more widespread region or separate regions .

Whilst urea is a preferred source of nitrogen, other compounds which are sources of nitrogen (e.g. ammonia) could also be used (optionally in combination with urea) . As noted above, ammonia is less preferred because it is not as easy to handle as urea and is, in fact, classified as a dangerous chemical. However, in some embodiments, such problems could be overcome and ammonia could be added to the photic layer by bubbling gaseous ammonia from an outlet located beneath the ship. Alternatively, ammonia in solution could be sprinkled either onto the surface of the ocean, where it sinks into the photic zone, or directly into the photic zone at the predetermined depth from the ship .

Furthermore, additional helpful nutrients (e.g. phosphates) could also be added to the source of nitrogen and delivered to the photic zone (or delivered separately) if it were determined that the presence of such nutrients may further increase the number of phytoplankton in the region.

It will be appreciated by those skilled in the art that the methods set forth in the Summary are not intended to be limited by the specific embodiments described above.

In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments.