Tag Archives: Fish

What have ‘The Sea Trout’ ever done for us?

Ok, they haven’t built any aqueducts, roads or made any wine… However, recent research has demonstrated how important sea trout are in maintaining freshwater resident brown trout populations. This gives greater incentives for freshwater conservationists to improve fish passage so migratory sea trout can return to their natal areas to spawn.

Same species, different life-style

Sea trout and brown trout are the same species (Salmo trutta L.), but have different life history. Sea trout are ‘anadromous’, meaning they migrate from freshwater to the marine environment to feed and grow. Whereas brown trout stay resident within the freshwater river their whole life. Both anadromous (sea) trout and freshwater-resident (brown) trout freely interbreed to produce fertile offspring. Like salmon, sea trout return to their natal river to spawn, this has made each rivers salmon and brown trout populations genetically distinct. The homing behaviour of the fish effectively isolate each population from each other, although some gene exchange can take place when fish ‘stray’ into another river and spawn there. Each river catchment presents slightly different conditions; this has led to local adaptations through natural selection to the local environment, meaning the genetics of each rivers population are slightly different. This genetic diversity and local adaptations makes each rivers population unique and best adapted to life in that river, this could be lost forever if populations suffer chronic events e.g. pollution incidents.

Sea Trout, Dave Brown

A proportion of any brown trout population migrates to sea, and thus become Sea Trout. This is known as ‘Partial Migration’, where a fraction of the population exhibits migratory behaviour while another does not. Different populations will exhibit different tendencies to migrate, some populations being dominated by freshwater-resident trout while others may be near wholly migratory; this may have a genetic link, but it is also known to be influenced by the environment. For example, low food availability is known to influence migratory behaviour. Trout populations from nutrient poor streams draining the acidic New Forest, for example, have a high proportion of individuals exhibiting migratory behaviour. While populations from more productive chalk streams, due to their stable temperature regime, tend to have a lower proportion of individuals migrate to sea. An increasing body of evidence is suggesting that stress plays a key role in the ‘decision’ to migrate to the sea or to remain in the river. Stress can be triggered by low growth, accounting for the differences discussed above, or be caused by specific events such as drought-associated low-flows. Therefore, the proportion of any population migrating to sea will differ from year to year, according to in-river conditions.

Benefits of seaward migration.

After 1-3 years in the freshwater environment, trout may undergo a process called ‘smoltification’, which is a period of great morphological, behavioural and physiological change when juvenile salmonids develop various adaptations that enable them to survive at sea. These changes include, a change to a silver coloration, to aid camouflage in the open water marine environment, and the ability to osmoregulate (regulation of an organism’s body fluids, in relation to internal and external salt concentrations) at sea.

Migration to sea is risky. The sea contains many predators that make for a risky environment for a young trout, and the journey down river is in itself associated with an increased mortality risk. However, the sea also contains numerous large prey items, like small fish and sand eel,  that trout grow quickly on. With much higher food resources available at sea, those trout that do enter the marine environment exhibit significantly increased growth. This can be observed when reading scales from trout that have spent time at sea. Fish scales, like tree trunks, have growth rings that can be read, to give the age of the individual, and the distance between growth rings gives an indication of the growth rate during that year / growing season.

Rapid growth at sea makes for large individuals returning from the marine environment. Freshwater wild brown trout will grow up to about 2 lbs in southern England, with fish up to about 4 lb being exceptional individuals. However, after a couple of years at sea, sea trout can grow to about 6 – 8 lbs, with some older individuals growing to well over 10lb. These large individuals are often females, and as a result of this increased growth rate, these individuals can produce many more eggs than a freshwater-resident trout. The eggs of a large female sea trout will also be much larger in size, and therefore contain more yolk which will fuel egg development and early growth. As a result, the offspring of sea trout emerge from spawning gravels earlier and at a larger size, giving them an advantage over the progeny of freshwater-resident trout.

Sea Trout on Redd New Forest Stream

Sea Trout research

A recent research paper from scientists from Exeter University, Queen Mary University and Game and Conservancy Wildlife Trust highlight the importance of a small number of large female sea trout in   maintaining freshwater-resident brown trout populations. The paper is: Goodwin et al. (2016) A small number of anadromous females drive reproduction in a brown trout (Salmo trutta) population in an English chalk stream. Freshwater Biology 61, 1075–1089.

Using a novel combination of stable isotope analysis and genetic analysis, the study aimed to;

  1. Quantify parentage by sea trout (ST) and freshwater-resident (FR) brown trout of newly emerged juvenile trout (fry) in a lowland English chalk stream (River Frome, Dorset)
  2. Assess relative parental fitness, in terms of number, size and time of emergence, of offspring.

The results demonstrated:

  1. An overwhelming contribution of anadromous (sea trout) parentage (both female and male) to fry production, with 76% of the juveniles sampled being parented by sea trout females and 63% parented by sea trout males. Despite the numerical dominance of freshwater-resident adults (6F & 12M [ST] vs 22F & 56M [FR]).
  2. Offspring of anadromous females emerged earlier and at a larger body size than offspring of resident females. Overall, this study suggests that sea trout offspring have an adaptive advantage and greater fitness in early development, and that a small number of female sea trout (six of 96 adults sampled) are the main drivers of trout production in this river.

Restoration downstream can bring benefits upstream

Given this evidence, restoration of fish passage at downstream barriers will allow sea trout to return to their spawning areas, often in the very upper reaches of river systems where preferred trout habitat exists. Allowing sea trout access to these upstream areas will see these fish to once more contribute to the spawning stock and allow their offspring, with their greater fitness thanks to their parent’s migration to the sea, to increase local populations of trout.

Great Stour (Kent) Your Fisheries Plan and Workshop

The South East Rivers Trust has created a pilot ‘Your Fisheries’ plan for the Great Stour in Kent, to drive rehabilitation of the sea/brown trout population in this catchment. The plan is available here: https://www.yourfisheries.org/catchments/42a55480-d5f9-4b44-8901-86370b7f104a.

A workshop will be held on from 7pm – 9:30 pm on Thursday 15th March 2018 in the Committee Room at the CDAA headquarters: 14 Mill Road, Sturry, Canterbury, CT2 0AF. The workshop will aim to feed back on the consultation carried out to create the plan and, specifically, to identify local opportunities to deliver projects to address the issues raised. Your local knowledge is crucial to this aspect of the workshop and we would like to draw on this, in an interactive session, so potential projects are aimed at addressing your priorities.

Please RSVP to confirm attendance:  chris@southeastriverstrust.org



Knowing your rudd from your roach

Our lucky Pollution Patrol volunteers were treated to a FIN-tastic day with our local Environment Agency team, learning all about fish.

It may come as a surprise to some people, but the Hogsmill, Wandle and Beverley Brook all contain a variety of different fish species. Common species across all three rivers include chub, dace, roach, barbel, stickleback and European eels.

But how do you tell the different between these species? For some, it is easier than others. The European eel is quite distinctive compared to the others for example. But as to the rest, it’s a bit more difficult.

Tom Cousins, a local EA Fisheries Officer, started the day for us with a presentation on the different fish species and the key identifying features.

The diagram below shows the external morphology of an average fish, and the features that help us distinguish one species from another.

For example: Roach and Rudd

These two fish are quite similar in appearance, both large-bodied with reddish fins. So how can we tell them apart? The answer is by looking at the mouth.

The roach is a bottom feeder, and its mouth points downwards, with the upper lip over-hanging the bottom lip – whereas a rudd feeds from the surface, and therefore the bottom lip overhangs the top lip.

The presentation from the Environment Agency is available below for you to download, with many more tricks and tips for ID.

Fish ID Presentation

As part of the training day, we also got to witness the Environment Agency’s electrofishing survey at Morden Hall Park on the River Wandle in South London.

It was amazing to learn about the fish in the classroom, and then come outside and see some in the flesh. Each fish caught was measured, and scale samples from some were taken in order to age the fish.

Once they had been recorded and had recovered, they were returned safely back to the Wandle. 

Many thanks to Morden Hall Park for hosting us and to the Environment Agency for running the event!

Hogsmill reconnected to the Thames with new fish pass in Kingston

The Hogsmill gauging station is an Environment Agency flow monitoring structure, essential for water resources planning and regulation.  It is the furthermost downstream weir in the catchment and poses a significant barrier to fish passage, preventing the recolonisation of fish to the river from the Thames below. Addressing passage at this key site has been discussed for several years but due to the sensitivity and importance in the recorded flow-gauging data, in combination with the unconventional structure it has resulted in a complicated and extensive process to identify and develop a suitable solution. JBA have helped inform the positioning of the upper baffle and through a programme of spot gaugings will update the rating of the gauging station.


The impassable Hogsmill Kingston gauging station

The selected approach is a variation on the Low Cost Baffle (LCB) Solution as we have used elsewhere, whereby rigid baffles are put on the downstream weir face in a specific geometry and spacing to slow the water, deepen the flow and provide a distinct passage route on the weir face. However, due to the significantly steeper gradient at the site in comparison to that the design was developed for, the Trust and EA have modified the arrangement to help promote the correct hydraulics to enable fish to pass.  The maths aside, in essence this meant that each line of baffles is incrementally taller than the last, starting at 120mm and ending at 288mm at the downstream end.

With this novel approach the Trust and the Environment Agency are keen to establish how efficiently the pass/easement operates, as the principle could be adopted at other similar challenging sites. In order to get a comprehensive assessment, an exciting opportunity arose to work with Durham University who will use the project as part of a wider study looking at fish passage past human made barriers. That’s the good news. The bad news being that in order to be ready for the dace spawning season I was committed to delivering for three weeks through the bleak days of January and February. The summer would have been far too pleasant.

Due to the non-standard design the baffles were manufactured as a bespoke commission carried out by Northwood Forestry & Sawmills, made of oak as opposed to the standard recycled plastic. Once complete, myself and Norm spent the first week of the year huddled in his workshop drilling and bolting the stainless steel angle to each baffle which would enable them to be securely fixed to the concrete weir. All 36 pieces of the jigsaw were complete, coded and stacked in the lockup ready for installation.


In the workshop with Norm fixing the stainless steel angle to the baffles


Half of the baffles with angle attached and coded ready to go

The baffles would have to wait, as first we needed to help fish of the anguilliform variety, eels, to also be able to successfully navigate past the weir. With the eels having already made the 6000+km epic journey from the Sargasso Sea we were keen to ensure this weir would not be an abrupt dead-end. Although there was an eel pass on the weir, it has been largely ineffective for some time. A new and improved pass was called for, so out with the old and in with the new.


Out with the old…


and in with the new

With the eel pass installed, preparation to fix the baffles was underway. This primarily revolved around creating a dry working area, not necessarily an easy task when working in a river but with the temporary coffer dam supplied from RN Inspection Services this was achieved with unbelievable effectiveness.


The coffer dam installed providing a manageable working area behind

For the next two weeks, Roo and I lugged baffles, drilled holes, spaced, chocked, clamped, injected resin, removed then re-erected the dam and battled the elements. As the jigsaw slotted together the theoretical schematic drawing I was now well familiar with became a reality.


The baffles being clamped into position before the stud secured to the concrete with resin


Getting into the flow of things


Half way!


Time to move the dam and install the second half of the baffles


Finished! The notch bisecting the baffles to allow a deeper channel for fish to pass

Once complete, the time came to remove the dam and allow the water to flow over and through the baffles. With this action the Hogsmill became re-connected to the Thames once again.

The time lapse of the build conveniently summarises the full 3 weeks into 1 minute video can be seen here:


About to ‘pull’ the dam


Flow and fish passage returned to the channel

Over the coming year JBA (hydrology experts) will undertake a spot flow-gauging program, to ensure that the weir continues to gauge accurately with the modifications. Meanwhile Angus at Durham University and ourselves will monitor the LCB fish pass/easement aiming to understand its performance for coarse fishes such as chub, dace and roach.


One aspect of the monitoring is a camera looking across the upper notch

A BIG thanks to: All those involved in the various teams at the Environment Agency, partners in the project especially those in Hydrometry and Telemetry and Fisheries.  Angus, Martyn and Jeroen at Durham University for bringing their wealth of monitoring experience and expertise to the project. JBA for carrying out the hydraulic assessments associated with the project. Bedelsford School who kindly agreed to us hooking up to their electricity supply to power the monitoring equipment. Rob Waite at the Royal Borough of Kingston for helping us to secure access and parking at the site. The Thames Anglers Conservancy, especially Will, for getting involved and helping to install the eel pass. Norm Fairey for your continued help with all things fishy and manufacturing. And my good mate Roo for the long hours, hard graft, permanently cold hands and near permanent good humour.


Working on the Eden

In August 2016 we were contracted by Kent Wildlife Trust to install a backwater on the River Eden near Penshurst in Kent.

The river Eden is in the upper reaches of the Medway catchment and joins with the Upper Medway approximately 1 km downstream from Penshurst.

What is a backwater and why is it needed?

A backwater is an aquatic habitat connected to the main channel, sometimes only during higher water levels. Backwaters can be formed naturally as a river migrates across its floodplain, cutting off meanders.

Backwater habitats can be used on rivers that have been straightened or resectioned to increase the diversity of flow, habitat and ecology.

In this case we were asked to install a backwater that would provide two important functions for the river depending on the flow. In high flows, the backwater would become a refuge of slack water for adult fish to rest in until the normal flows return. In normal flows, the shallower water left in the backwater will warm up quicker than the main channel and act as a nursery area promoting the growth of young fish fry.


Photo provided by Kent Wildlife Trust of project area last Winter 2015

What we did

The project got off to a quick start with local tree surgeon Ivan Carson, from Penshurst Tree Surgery, making short work of the 3 cricket bat willows.

These willows were coppiced to near ground level to allow more sunlight to reach the backwater – helping to raise water temperature during normal flows. A bonus benefit of this tree removal is a reduction in the amount of leaf litter entering the backwater, reducing future maintenance.

With a tight schedule, work on the backwater commenced. First the old fence was removed and the topsoil was stripped back.

Our expert excavator, Jimmy, started shaping the backwater entrance. All the spoil we removed was tracked to the far end of the landowner’s orchard to improve their existing track.

Everything seemed to be going smoothly until Jimmy’s bucket hit something solid! This turned out to be a huge tree trunk of approx 1 m diameter running parallel to the river, across the backwater entrance.



With no idea how long the trunk might be, we decided to remove the central section. Most of it was actually below water level, which meant our saw wasn’t quite up to the job. Fortunately, Ivan kindly lent us one of his and the obstacle was overcome by our very own Toby.

The silver lining of this small delay was that we didn’t need to install any large woody as cover for fish.  We even spotted a Perch of approx. 1.5 lbs sunning itself in the shallows until it was spooked, and darted underneath one of the remaining stumps – woody cover in action!

Whilst Jimmy was finishing off the backwater, Toby and Alex started to install the new fencing around the backwater. This fencing will exclude cattle from the backwater, preventing over-grazing and reducing bank erosion.


With the backwater construction complete, it was time to add the plants. Marginal plant plugs were added around the backwater, and wild flower seed was scattered on the ground to provide some ground cover before winter.


The landowner seemed really happy with the final result – especially once he saw all the marginal plants.

The project took 6 days to complete and we look forward to seeing how the backwater matures overtime.  Lots of fish fry were spotted on our most recent site visit which suggests we did a good job!