Management strategy for Darlow in 2021

Fish catch and release. The clubs “raison d’être” is to provide fly fishing at the lowest practical cost. To this end we encourage catch and release, while continuing to allow members to take fish for “the pot”. Stocking is our biggest annual cost and risk. Catch and release fishing reduces the number of fish we need stock annually to maintain our stock levels. To improve fish welfare a new rule will be introduced. From the start of the 2021 season fishing will be with barbless or debarbed hooks only.

Fishing season. The 2021 season will start on 14 March and finish on 31 December.

Water quality. This is the key to good fishing, if the water quality is good, the fish will be stress free and good fishing will follow. The high summer water temperatures remain a problem and our focus is attempting to reduce them. The “long” summer of 2020 led to substantial fish mortality. But not a total loss. Fish were being caught in July and were seen through August and September. 2020 was a long summer because April and May were warm and very sunny, with the water temperature getting to 20oC+ in May. That sort of water temperature stresses trout, and stressed fish lose condition which leads to increased susceptibility to disease, parasites etc. We cannot control the weather, or the climate but we can take actions that will moderate the effects.

Water turbidity. Turbid water more efficiently absorbs solar energy resulting in the heating of the water. To reduce turbidity in recent winters (2017 and 2018) we have netted out approximately 2500lb of bottom feeding coarse fish (carp, bream and tench). We are now monitoring the bottom feeding coarse fish population. There is still a good number of tench. However, tench have proved difficult to net and remove. Crayfish also contribute to water turbidity through their foraging and borrowing. We will continue to trap and remove crayfish. Targeting egg carrying female crayfish in the winter. Turbid water also results in poor weed growth.

Weed growth. Good weed growth has many beneficial aspects. It absorbs and reflects solar energy. When weed absorbs solar energy, the energy is converted to biomass not heating the water with the bonus release of dissolved oxygen. When weed reflects solar energy it does not heat the water. Another potential beneficial effect of weed growth is reduced water movement (currents) and create a more stable water column that may stratify with respect to temperature, as cooler water is denser and sinks. This may provide a refuge of cooler water for the trout. One way to encourage weed growth is to add chalk (calcium carbonate) powder which we are likely to do again in December 2021. The chalk improves the composition of the sediment for both weed (and invertebrates) by introducing oxygen for aerobic microbes and de-acidifying historical decomposition of sediments created by anaerobic microbes. This historical anaerobic sediment decomposition may account for the dark, almost black appearance of the sediment in some areas of Darlow.  One product of anaerobic decomposition is hydrogen sulphide, this can interact with pyrites in the clay bed to form ferrous sulphides, which are black. Dark sediment will undermine our strategy to reduce water turbidity, as the solar energy will be absorbed by the dark sediment. The addition of chalk should encourage weed growth to cover the sediment and lighten any exposed dark sediment by reducing the ferrous sulphides. Hydrogen sulphide is also toxic to weed roots, invertebrates and fish. Fish will avoid it, forcing them higher in the water column, where the temperature may be more elevated than at depth, exasperating the heat stress.

Ground water. We suspect the ground water is another part of the equation. Above Darlow is the Gill Mill quarrying complex, where the process of “working-dry” means large areas are “dewatered”. This can impact local groundwater levels. The dewatering process involves pumping groundwater to surface water (river water) via sediment lagoons. This transfer of groundwater to surface water can reduce groundwater levels locally. Anecdotally we have seen a reduction in groundwater since the Gill Mill quarrying extension, for instance, the drying up of the Darlow outflow early in the year and the pond on the north bank. The reduction in groundwater “flow” might partly explain the elevated temperatures in Darlow if cooler groundwater is no longer displacing the warm surface water at Darlow. However, this is something we cannot control right now, just try and understand.

Shade. Another way of reducing solar energy gain is shade. Darlow has a large surface area with little natural shade. In the summer of 2019, we introduced native lilies in 2 new areas of Darlow to create a little more natural shade. We will allow these to develop. In 2020 we planted up the reed rafts provided by the EA. And although the surface area shaded by the rafts is relatively small, by careful positioning over deep water, we can maximise the volume of water shaded. We plan to build our own reed rafts in 2021 to increase the area and volume of water shaded. The lilies and reed rafts also create new habitat.

Aeration. One potential consequence of encouraging more weed growth is the risk of oxygen depletion at night (darkness), particularly in the summer when warm water contains less dissolved oxygen. This is dangerous to fish stocks, especially trout. In 2020 we doubled our paddlewheel aerator capacity and turned them around to push water out into Darlow. We will run these at night when the water temperature is greater than 15oC as a safety net against oxygen depletion. A secondary benefit of the paddlewheel aerators is a small cooling effect on the water by throwing it into the cooler night air. Also, last summer we introduced a different aerator system, the air diffuser, this bubbles air through the water column. It uses less power than the paddlewheels and we intend to run it 24/7, 365 days a year.

Fish health. The fish louse, argulus is an on-going threat to the trout. To limit the impact of argulus on our trout stock we do two things. First we buy SLICE treated trout. SLICE is a drug administered to the fish at the suppliers. It offers time limited protection against the argulus parasite. And second we place black plastic water pipes around Darlow in approximately 1m of water. The pipes are good sites for the argulus to lay their eggs. We then regularly harvest the eggs and destroy them. The combination of SLICE treatment and egg harvesting should prevent extensive fish infestation with the parasite and healthier fish.

Winter feeding. While Darlow has an abundance of natural food appropriate for trout, it does become scarce and or difficult for the trout to access in the winter. Supplementary feeding in the winter will help the trout stay in condition and overwinter well. We will start feeding in December and continue feeding through to the end of February.

Fish predation. Cormorants are an on-going issue. They tend to visit more when the rivers are in flood in the winter. The actions we are undertaking include shooting. We have a licence (issued by Natural England) to shoot cormorants between 01 Sept 2020 and 15 April 2021. And will apply for a further licence covering next winter (2021/2022). Other actions are the placement of a mannequin to act as a “scarecrow”, using a blank firing pistol and rockets to scare birds, shooting to miss outside of the licenced ‘shooting to kill’ period, stock larger fish and do not stock highly visible blue trout. We now have evidence of otter activity at Darlow. The only option we have to mitigate this directly is a fence. We have no plans for a fence. At the time of writing, the otter visits appear to be infrequent judging from the camera traps and they are probably more interested in the crayfish.

Poaching. This is an on-going threat. Actions we take to reduce this is signage, set camera traps to gather evidence, stay in touch with TVP Wildlife Crime Officer, employ a security firm to provide night-time patrols to deter poachers and continue to defend the boundaries with defensive planting to create a dense prickly barrier and barbed wire where necessary.

Stocking. We are targeting between 30 and 40 fish per acre as a stocking density in prime fishing periods. We are planning to stock fewer fish per batch, with more frequent batches. We will stock more rainbows than browns as they are better value for money. Rainbows get caught more frequently than browns and are cheaper. We will split the browns between spring and autumn, so if summer mortality is high, there will be some browns to catch in the autumn.

We will use historic data to guide the timings of stocking relative to the water temperature, avoiding periods of predictably warm water which from mid May to the end of September.

Landscape and boundaries. Using the mower, we will manage the grassland ourselves this year. Access to swims will be mown regularly. However, there is an opportunity to leave a significant portion of the north bank uncut in the spring and early summer to create a “wild-flower meadow” and encourage terrestrial insects. There are other areas where we can leave the grass long to encourage wildlife to prosper and we will do this. Also, we will continue to develop natural barriers along our boundaries using native planting that is prickly/thorny.

In summary, the actions are:

  • Fish predominantly catch and release with mandatory use of barbless/debarbed hooks
  • Start the season 14 March 2021 and close 31 December 2021.
  • Continue to trap crayfish
  • Treat the sediment and encourage weed growth with chalk
  • Create more shade with additional reed rafts
  • Use the paddlewheel aerators during the night-time when the water temperature exceeds 15oC
  • Use the air diffuser system 24/7, 365 days a year
  • Stock SLICE treated fish, and capture and destroy argulus eggs
  • Feed the fish in December, January and February
  • Continue to deter cormorant predation, employ night-time security patrols and improve boundary security
  • Stock larger fish
  • Aim for a stock density of 30 to 40 fish per acre at prime fishing times
  • Avoid stocking into warm water
  • Encourage more wild-flowers on the banks and native shrubs and trees on the boundaries