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Tom Baird

What’s in your Lake

Evening All, Hope you are all safe and well and hope in a few weeks we can travel to fish. Until then I will make my brain hurt and pass on some of my skills and knowledge. If you can call it that lol. A few weeks ago, I done a blog about stock management in the way I see to get that perfect fishery. After I done that blog, I have spoken to several keen anglers and lake owners asking what’s the best way to find out what stock you have in a lake. This is my recommendations and how I would go about it. Obviously, it all depends on budget and time and what resources you have available from a small club to a rich vibrant owner of a lake complex.

However, undertaking a new fishery can be a frightening prospect. Seeking advice from professionals within the field is a brilliant first step. Clear communication is required in these early stages as to what the fishery owner hopes to achieve from carrying out an assessment of the lake. The methods for obtaining actual density or abundance of fish, can be vastly different from a ‘look and see’ method which will enable a rough species guide and composition. Planning at this early stage can help prevent costly or unnecessary expenses.

Within the early discussions any known history of the site should be shared with the fishery expert including geographical reference. It is noted that the lake in question, “knows it contains fish” and that there is, “circumstantial evidence of people catching fish.” This information, however patchy or second hand, can help gauge future management and sampling techniques. In an extreme example, if anecdotal accounts of Wels catfish, were present in the lake, then sampling methods could be tailored to capture said species. It may also be worth asking some of the proposed members or an experienced angler to rod and line the lake for a few days to see what/if anything, is caught.

Some of the survey methods highlighted later in the report are only appropriate at certain times of the year, due to high temperatures, low dissolved oxygen/water levels and spawning which causes additional stress to the resident population. These environmental stress factors can be magnified with some survey techniques, which may suggest any assessment is postponed until more favourable conditions are available, such as November to February.

Pre survey information

Important lake contour information should, if possible, be gathered prior to any survey. Simple depth analysis can be carried out with measuring staffs and the use of a boat or more advanced bathymetric surveys used to further inform which sampling method will be best served.

Competency

Before undertaking fishery management or stock assessment, owners should ensure the team undertaking the work has appropriately trained staff and expertise to carry out the work in a safe and professional manner. Generic risk assessments, method statements and relevant certification should be produced before the commencement of any fisheries activity.

Permitted activity

Prior to any surveying the following equipment permit may be required from the Environment Agency should methods other than rod and line be employed to remove/sample fish. Make sure your legal when doing this otherwise you could be in a heap of trouble and it could have been prevented.

Survey methods available

Seine netting

 Probably the most widely used and accessible fishery survey technique used within still waters. This a method which can be used for surveying and stock thinning. The operation requires a knotless mesh net with floats across the top section and a lead line along the bottom. This allows the net to be pulled through the lake creating a barrier to escaping fish. In the case of a large lake, a boat would be required to set the net. The lake can be portioned off using stop nets to ensure maximum coverage minimising effort to factor in the length of seine net available.

The following diagram demonstrates the method of capture.

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As both lines are drawn into the bank, fish are captured and processed using hand nets.

PositivesNegatives
Relatively inexpensive compared to other methodsIn large lakes, can be labour intensive
Repeated and correct use can collect large quantities of fish for accurate stock assessmentMay disturb lake silt and macrophytes
Can be used to collect all size ranges of fishIf lake is deep of comprises hollows, fish can escape underneath net
Use of fuels/engines/batteries restricted to boat use only, reducing health and safety riskCan cause abrasive damage to fish
Although skilled operatives required for operation, good method to allow fishing club members to participateNet can get snagged on branches, macrophytes or structures, making it difficult to pull in, possibly causing damage to net and allowing fish to escape

Electric fishing

Electric fishing uses the physiological effect of an electric field in water to attract and immobilise fish. Electrodes, immersed in the water, stimulate a fish’s nervous system so that it swims towards the operator, or is unable to swim away, and can be caught.  (Environment Agency Operational Instruction). Fish can then be safely netted and removed for processing. It requires a highly skilled team of operatives to ensure the safety of the users and the welfare of the fish are not compromised throughout the survey.

This method is widely used in rivers and shallow, smaller lakes or in combination with other techniques such as seine netting.

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Demonstration survey using electric fishing.

PositivesNegatives
Effective at sampling margins and shallow lakesOnly skilled and trained operatives to use equipment
Excellent tool to quickly assess species presentAs a standalone sampling method, not suitable for deep or large lakes.
Relatively non-intrusive. Does not disturb great quantities of siltCan be expensive
Reduced risk of damage to fish through nettingTargets certain sizes of fish more effectively than others
 Temperature and Conductivity dependant

            Drain-downs  

Provide the opportunity to assess fish stocks with a high degree of accuracy. The steady drawing down of a lake gives greater ability to remove fish via netting as the water level drops. Repeat seine and hand nettings can be carried out to ensure the maximum number of fish are captured, providing a comprehensive study of all lake species, especially the benthic dwellers such as Tench,which are often difficult to capture in deep water. Due to the potentially large number of fish captured during this method, careful consideration needs to be taken as to where the fish will be retained during the draw down/processing and whilst the lake is backfilled.

PositivesNegatives
Only viable method available to precisely assess fish stocksVery labour intensive
Gives fishery owners opportunity to redesign lake, creating shelving/lake features to enhance biodiversity/habitatLarge impact on fish, habitat and aquatic invertebrates
Desilting can be completed and silt traps dug to ease future managementVery expensive, use of personnel and equipment such as nets and holding tanks
Assessment of bank/lake structure can be carried outRequires area to dewater and source to refill
Opportunity to remove unwanted fish speciesDischarge consent/abstraction licence may need to be granted from EA
 Unsuitable during high temperatures

Hydro acoustic Surveys

Hydro acoustic surveys use an echo sounder, which transmits short pulses of sound through a transducer into the lake. The sound waves reflect off of objects such as the lake bed or fish swim bladders and a computer-generated image is returned back via the transducer. This method of sampling is commonly used in large expansive lakes and reservoirs and other deep waters, where other sampling methods are ineffective. Surveys are carried out via boat or fixed-point locations and can be used to map fish abundance and provide topographic information of the lake. Acoustic cameras such as DIDSON, Dual-frequency Identification Sonar and ARIS, Adaptive Resolution Imaging Sonar, (www.soundmetrics.com), produce high resolution images which can be captured and even relayed to the bankside for anglers to marvel at. These are becoming an important tool in monitoring our large water bodies.

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PositivesNegatives
Cover large areas of deep water in relative short timeOnly estimates of fish sizes. Equipment struggles to detect fish smaller than 70-80mm
Non-invasive method, preventing damage or stress to fish and impact on lake through silt disturbanceAlthough speciation is possible in some cases, this is labour intensive and can provide delays whilst analysis is carried out
Minimal person power requiredProhibitively expensive
Reduces the need for additional bathymetric surveysHighly trained staff required to operate equipment and process data
Reduces risk of physical injury to operatives, as may be found when seine nettingUnable to estimate population due to free movement of fish
Can work in areas with low visibility and high turbidity 
No environmental restrictions, such as temperature 

Fyke netting

Fyke nets (with leaders or wings) are conical nets with inscales and a circular or D-shaped opening held open by metal rings. There is a series of interconnecting nets with one-way entry to trap fish. EA Operational Instruction.

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Fyke netImportant to note that these static traps hold fish until they are collected by the operative. This poses the fish health problems due to overcrowding, predation from crayfish and mammals. It is a legal requirement to ensure an otter guard is fitted to the entrance, as they are understandably attracted to the held fish, especially eels, which this type of trap is often used to catch.

Fyke net set up, with otter guard

PositivesNegatives
Easy to deploy, can be single personNot suitable for determining fish populations
Relatively cheap and can be readily reusedSelective for species and size of fish
Good for targeting Perch, Crucian Carpand EelsRequires checking at least every 24 hours
Useful as one element of a population surveyCaptured fish at risk from predation from crayfish

eDNA

Organisms continuously release DNA into their environments in the form of shed cells,

waste matter, blood, gametes and decaying material. Analysis of this ‘environmental

DNA’ (eDNA) is revolutionising the way biodiversity is monitored. This exceptional new

technology has been rapidly adopted for targeted monitoring of single species and

studies have consistently shown that it outperforms traditional survey methods in terms of ease of sampling, sensitivity and cost. (EA publication).

This method of sampling could be used to ascertain whether a particularly desirable/undesirable species is present, which could aid the fishery manager with decisions on suitable monitoring techniques, stock management and complying with site permits.

PositivesNegatives
Inexpensive method of presence/absence samplingDoes not allow fish biometrics to be recorded
Non-invasive, no fish removed or lake disturbanceNot suitable for determining fish populations
Quick method to obtain evidence of desirable or undesirable species reducing survey costsTechnology still in its infancy

Written reports

Upon completion of the survey and analysis of the results, a report should be compiled and supplied to the fishery, complete with recommendations for future fishery and stock management. For the first time fishery owner this will then represent the baseline for any future management decisions and the data, combined with any habitat, biological or chemical surveys should encourage future monitoring as the business evolves.

As habitat changes, fish stocks and species dynamics will also change. Helping the fishery understand that one initial survey is unlikely to represent the state of the fishery 10 years in the future, will be important when considering stock management within a commercial fishery.

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