Cropping & Destocking

Although cropping and destocking both indicate the removal of fish this is often undertaken for very different reasons. Cropping is commonly carried out for financial gain. A successfully run fishery is able to crop a small percentage of its standing stock on an annual basis, and species dependent, this can be a lucrative way of generating extra income; certain species have a value of 8.00/lb and upwards.

In contrast, destocking is a process usually unplanned and one that has the intention of a short-term reduction of pressure on a fishery. Contrary to belief lakes are susceptible to overstocking which can become detrimental to the health of the fish; reduced angler catches and outbreaks of disease can result. Throughout this page I hope to give a brief insight into the reasons behind the use of these management tools; images will enhance the discussion and provide valuable descriptive aids.

The first series of pictures show the process of a destocking exercise at a well known English match fishery. This lake, one of two feeders for their larger lake, suffers badly from a deoxygenation problem in the summer months. To resolve this problem the annual netting and removal of the excess juveniles relieves enough pressure on the fishery to allow excellent angling throughout the year.

The exercise was relatively simple. Net, grade, and then remove a quantity of <100mm roach, perch and common bream.
  When on site the most commonly asked question is how can 'more fish' reduce catches? So what problems can overstocking cause? A good example would be to picture a simple fish farm stock pond. This will contain very high stock densities with the addition of aeration and artificial feeding. There is an increase risk of disease and reduction in dissolved oxygen that require daily monitoring. If you couple these with the stress of being caught, all putting pressure on the immune system, then there is an increase risk of fish mortality. More commonly large scale fish losses are being reported in the press. Although some may be the result of illegally stocked fish it is just as lightly that angling pressure on fish living in cramped conditions is the reason.

  Standard seine netting techniques were applied resulting in the catch pictured on the top left. First impressions show a net of quality fish, however, after using 12mm graders (above) the smaller fish were sieved, leaving the valuable commodity (right) increased space within the lake. Over 800lbs were removed from each lake (picture below) with cohorts 0-2+ in abundance. The oxygen requirements for this quantity of small fish, when under pressure, would be substantial enough to cause fish kills at high temperatures.



Whereas destocking is undertaken to relieve some pressure on a fishery cropping is more calculated and is targeted and certain species and cohorts. So what fish can be cropped and why? Although any cohort of any species can be cropped certain implications have to be considered. Fish are sold by weight so those with the highest value would be mature and of an age exceeding 3+. These, however, are also the target for anglers thus large scale removal can only be detrimental to the quality of fishing. Furthermore, these cohorts are the broodstock, essential for natural regeneration. The cropping of broodstock does require careful planning as the removal of too many can cause the collapse of the fishery; large scale examples of this instance are those of the cod and haddock fisheries of the North Sea. Obviously these are extreme examples , but theoretically they are the same and as with any management a balance needs to be met then adhered too.

Certain species, such as roach and bream, reach sexual maturity at an early age. This allows non-commercial fisheries the chance to crop these on a yearly basis whilst leaving the juveniles to grow on for a later date. 2+ year classes are ideal to crop as they will always be in demand hence they will hold their value. Fast growing species, such as roach, rudd, perch, bream and carp, are suited to yearly cropping. Higher value species, namely tench and crucian carp, will rarely accept annual cropping as their growth rates are much slower; often they are cropped every three years.
The following series of pictures show the cropping process of 2+ year classes. Again an annual contract, this series of ponds have the juvenile roach, rudd and bream left to grow on. The larger fish, 2-4+ bream (right) and 2-3+ roach (left), are moved to the owners fishery enhancing the quality of angling. If this type of stock pond is available it makes an excellent source of cheap stock
         

Financially, the final area that could be cropped are the juveniles; cohorts 0-2+ constitute this important group. Obviously more numbers are required to generate the same income as selling 2+ cohorts, and with the demand not being as high, this crop is not as lucrative. However, commercial fisheries will tend to lean towards the cropping of juveniles as they are the most common subject of complaint about by paying anglers. Furthermore, those fisheries that have their own stock ponds can utilise their newly gained crop to grow for restocking.

Conversely, predators are cropped to reduce natural mortality. Unfortunately, predators are more commonly the targets of people who do not understand the benefits of such species. Predators, in moderation, rarely do much harm and are required for a good ecological balance. The removal of smaller predators can benefit individual fisheries, but total eradication is usually unnecessary.

In reality, just leaving the fish in, what ever the size, is of no benefit to the fishery. Active management is rewarding in many areas. Cropping will often increase growth rates, the general health of all fish and provide increased financial income.

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