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Tom Baird

The Perfect Lake

Running a fishery or club can be difficult, and anglers complaining of low catches and often blanking. Demanding the owners/committee do something to rectify this.

The typical answer is to add more stock to allow anglers to catch. Unfortunately this in most cases makes the situation worse and a very quick downward spiral.

Many fishery traits which are often the basis of poor fishing returns. Fish become stressed if the environment they are situated in, that does not support their needs for healthy growth.

Common stress factors:

  • Poor habitat
  • Lack of suitable spawning substrates
  • Inadequate natural food sources
  • Unsuitable water quality
  • Over stocking
  • Disease and parasites, often as a secondary result of other stress related issues
  • Minimal habitat for predator avoidance
  • Algae dominance

In some cases, if one or more of these elements are not in the right balance, fish will reduce feeding and consequently angler’s catches will reduce. Often the reaction is to stock greater numbers to increase catches, but this is not often effective in a water.

If we look at the stress factors above, a conclusion can be drawn as to what the underlying problems are within the lake and what approach is best to return it to a fishery which is agreeable with anglers.

As primary producers, the plant community are the basis to lake ecosystems, without which, a lake cannot support a healthy fish population. Macrophytes and algae use nutrients entering the lake such as nitrogen and phosphate, to support growth. This growth provides oxygen through photosynthesis, shelter from predators, food and habitat for zooplankton and macroinvertebrates – which in turn are essential food source for fish, bank stabilisation and spawning substrate for certain fish such as Roach. One element not relating to fish, but important to anglers is these plants help to improve the aesthetics of the lake and consequently the angler’s enjoyment. Habitat surveys are a quick and ideal assessment of the lakes ability to support a suitable ecosystem and a plan of this can be drawn up and stark comparisons made with other more diverse lakes.

The picture below shows the various zones and lake depths which plants and subsequently macroinvertebrates and fish can thrive and support healthy growth rates. The various zones will support different communities, all reliant on each other. For example, Tench, will be found in deeper water, utilising the productivity of the lake silt. However, Rudd are surface feeders and will benefit from invertebrates fallen from trees and water lily pad growth such as Nymphaea alba to provide cover for predator avoidance.

Algae

If lack of macrophyte growth is evident, an algal dominance could potentially cause unwanted problems within the lake. Algal blooms can cause dramatic changes in the chemistry of the lake affecting parameters such as pH, dissolved oxygen, (DO) and ammonia. Without plants to buffer these chemical extremes, fish can become stressed and mortalities are regular occurrences in poorly managed still waters. Water quality sampling is the basis of good fishery management with a fishery owner being better placed to make informed decisions based on known water quality history. Sampling for pH, DO, temperature and ammonia should form the basis of any daily checks, preferably two or three times a day to understand the diurnal changes that occur following increased photosynthesis in the day and respiration of plants and algae at night.

Predator avoidance

The presence of a variety of habitats and substrates provide fish with the necessary hiding places to avoid being eaten by predators. Like all creatures, fish will become stressed if they are unable to seek sanctuary from those further up the food chain. Within any aquatic environment fish will hide from predators under rocks, within macrophytes or in amongst tree roots to avoid being eaten. If they do not have this protection, not only will the likelihood of being eaten by piscivorous fish, birds and mammals increase, but the stress levels within the fish will be heightened, causing the fish to be easily spooked and be less likely to take an angler’s bait.

Lack of spawning substrate

All animals have a desire to breed and foster the next generation. If conditions within a lake are not suitable for spawning, i.e. no gravels or suitable macrophytes, not only does the fishery not increase its stock but stress can also be induced. In the case of Carp,the fish can become egg bound, a condition known as dystocia. Within a stable environment the eggs will be absorbed by the females, if there are extremes in temperature or oxygen levels the eggs can build up within the ovaries leading to possible infection. In extreme cases the fish can die. (www.koi4u.co.za)

Insufficient natural food sources

If the habitat is not there, the invertebrates will at best be sparse and species poor. Within a diverse plant and habitat community the lake will benefit from spring emergence of species such as Common Hawker and Banded Demoiselle, the lake bed will be crawling with the likes of freshwater shrimp and the larva of the marbled sedge. In lakes, one would expect to find blood wormsand water hoglouse, which are more tolerant of poorer water quality and lack of habitat.

This will have a detrimental effect on the fish as the lack of natural food and diversity will impact on fish health. In extreme cases where little or no food is available, fish within the lake rely solely on the input of anglers, which often does not contain all the necessary ingredients for a healthy diet and can be intermittent depending on angler numbers. Biological sampling using fine meshed nets is a useful tool in assessing the taxa within a lake, with the data collected helping to inform the fishery whether natural food stocks are scarce and a possible reason for poor fish weights and catches. A sample of scales from various species could be analysed to ascertain growth rates which would inform whether the fish were putting on suitable growth or whether the fish were stunted due to insufficient dietary requirements.

Stock levels

Whilst there is no information provided as to stock levels, it is an important consideration when assessing the reasons for the faltering catch numbers. A lake overstocked with small Roach and Rudd can have a detrimental effect on Carp weights and numbers. If there are too many mouths to feed within the lake, larger non piscivorous fish may be unable to forage for sufficient food and see a reduction in the number of desirable fish. At the other end of the scale, too few fish may be present due to uncontrolled loss to predation and/or insufficient fecundity of the water. Stock assessments using, netting and/or electrofishing techniques are probably best placed to give an indication of fish numbers. Which would inform on species composition and aid direction of future management.

Proposals

Following habitat, biological and fisheries surveys if it is found there is insufficient natural food due to the lack of habitat and uniform depth, the first course of action may be supplementary feeding. This will allow a short-term fix whilst the following measures are instigated.

Habitat creation should be the primary aim. A number of options are available which will benefit fish productivity, welfare and angler’s enjoyment.

  1. Reprofiling banks and lake bed to increase variation of depth, margins and shelving. This will allow marginal, emergent and submerged plants such as water plantain, to take hold and will provide habitat and shelter for fish and invertebrates. Depth variation is also an important factor when attempting to regulate temperature. A shallow mono-depth lake will warm up quickly over a period of dry, warm weather and in a feature less lake may provide the perfect conditions for an algal bloom.
  2. Stabilise banks using materials such as geotextiles and revegetation. These provide the support to reduce erosion whilst providing more natural habitats. Brushwood mattresses are also useful for stabilisation and can act to capture sediments entering the lake. Tree/shrub planting around the lake is also essential in reducing wind erosion and will further support bank stability. Trees also provide shade, habitat and a food source when shedding invertebrates into the lake.
  3. Add interest within the lake. This can be provided with island creation or floating islands and predator avoidance structure. This will add further diversity and interest to the lake and provide refuge for fish to aid predator avoidance.
  • Monitoring should continue to be a vital tool in fisheries management. Continued chemical, biological and habitat assessment will gauge the success of fishery management and guide further improvements.

  • Once all or some of these measures have been carried out it is essential to give the fishery time to adjust. This may take longer than anglers are prepared to wait, but the message should go out to anglers that the long-term aim of supporting a sustainable fishery far out ways a short-term fix. Following a settling period, if catches still remain low, further surveys can be carried out to ascertain stock density and at this point small scale stockings may be sensible fisheries management.

Whenever carrying out stock assessments/stocking it is essential the correct permits are in place for introductions, use of equipment other than rod and line and when supplying fish.

I hope this helps with your fishery, tight lines…….

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