Skip to main content

Life in between the gravel grains


So, at this time of year, trout streams across the UK will play host to some genuine - and almost entirely hidden - miracles. Much of this will be played out in the microcosmos found in the tiny breathing spaces between irregularly-shaped gravel chips. It doesn't matter whether the trout stream is in the middle of a busy city or in pristine countryside - new life is currently finding a way. In fact, I can think of a bus stop only a few hundred meters from where I currently sit that the queues of passengers will be standing almost within touching distance of a new generation of tiny trout. Each occupying parallel but completely separate universes.

Our (largely) warm and wet winter of 2012 will mean that lots of streams would have seen spawning efforts starting perhaps in November. The males chasing rivals away from prime spawning sites and the females fluttering their bodies sideways to thrash and scrape small depressions in the gravel bed.

The eggs shed and fertilised by the most persistent (or sneaky!) males in these depressions have then been buried by further thrashing. The resultant mounds of gravel covering the scooped out nests containing the fertilised eggs are called "redds" and come with a remarkable ventilation system. The raised "bump" profile (that can initially be seen as a much brighter patch - until the regrowth of algae camouflages it again) forces water to flow between the gravel particles. This keeps the eggs supplied with vital oxygen. However, especially at the early stages of egg development, the redds are very sensitive to disturbance. Anglers wading on top of or through such redds can easily kill the majority of eggs inside. Also, if the gravels become smothered with fine sediment at any point when either the eggs or the baby fish (alevins) are sheltering inside - they will suffocate and die.

As the baby trout develop within the spherical egg membranes, the eyes, spinal columns and yolk sacs of individual fish become visible. The longer this development goes on, the more mobile the little trout become inside their protective shells.

An arduous hatching procedure ensues in which the tiny trout (just over a centimeter long) wriggle, gasp and thrash their way out of a split in the egg membrane. These newly-emerged baby trout (complete with large yolk sac for nutrition) are programmed to STAY WITHIN THE GRAVEL until the large yolk sac is entirely absorbed (which will take them through to spring).

Emerging from the gravel in spring is a huge step into the big wide world - and with no yolk sac to rely on - independent feeding must take place to avoid starvation. For now though let us consider that from November through to somewhere roughly around April the gravelly beds (especially at the tails of pools) of our trout streams will hold the future generation of trout that can live up to the wildest of fly fishing dreams.

Even for a totally selfish angler, stomping on these areas of refuge is a bad idea for the prospects of future encounters. The fact that, even after the eggs have hatched, the tiny trout are hiding in the cramped spaces between gravel chips make them extremely vulnerable to being trodden on. Probably more insidious though is the threat of suffocation caused by siltation. This could be the result of channel modification/presence of weirs that promotes silt accumulation, rampant bank erosion due to one of two main causes (invasive plant species that die back in winter or unrestricted heavy grazing and trampling by livestock), badly implemented drainage of roads, construction sites, buildings, forestry plantations or agricultural fields - the list goes on.

The video is a short insight into what goes on in the gravel (and why people should think twice about disturbing or smothering it).

Troutearlylifeblog from Paul Gaskell on Vimeo.



Comments

Popular posts from this blog

Why Presume to Remove Weirs? (with River Dove Case Study)

Weirs and the Backwards Ways that Rivers Work One of my favourite sayings on river restoration is a mangled quote from a movie

"...boxing is an unnatural act. Everything in boxing is backwards: sometimes the best way to deliver a punch is to step backwards...but step back too far and you ain't fighting at all".

So my mangled version starts out "Everything in rivers is backwards...". Basically, I never seem to run out of new examples of "what SEEMS to happen in a river is actually the complete opposite of what really happens".



The rest of this article looks at many of the "backwards" things about weirs and rivers - and finishes off with a real-world case-study that is playing out right now on the River Dove.

One spoiler alert is that, from an ecological point of view, it is almost always safe to assume that:

The best biological outcome for a river is the removal of some or all of an artificial weir. 
Now, I don't expect you to believe that…

First Survey Record of Wild Trout Returning to Lyme Brook Habitat Works Site!

You may have seen the first three phases of works on the middle reaches of the Lyme Brook (shown in previous blogs Here and Here) from project works that began in 2015...

Well although the first surveys after that work found some nice coarse fish populations - there was no cold hard evidence that any trout had found the newly-improved habitat...Until now!
I received a phone call today from Matt Lawrence who is the EA's Catchment Host for the Trent Valley Catchment Partnership (with key partners Groundwork West Midlands and the Wild Trout Trust who conceived and delivered the habitat works). Matt told me that he'd had some exciting preliminary reports from a EA Midlands fisheries surveys team. Their survey on 7th September had caught several wild trout as part of their sample on the habitat works site.
These are the first modern records of trout in the brook and is also the exciting news that we have been waiting for on these first phases of work to create spawning, juvenile an…

CATCH in Wincanton and News of the First Recorded Wild Brown Trout Following Their Hard Work

Blog posts are like London Buses it seems!

This one is just a very short "Congratulations" to the Folks at CATCH (Community Action to Transform the Cale Habitat) and the video put out by Wincanton Window (embedded below).



All of the folks in the partnership mentioned in the video have done HUGE amounts of work (from classroom education projects to habitat working parties and endless enthusiasm for engaging more people in their local river and much more besides).

A big disclaimer from me is that, although this project is supported by/affiliated with our Trout in the Town project - it has been Mike Blackmore who has fulfilled that role for the WTT rather than myself.

So massive well done to all involved (especially you Gary Hunt!)- it is wonderful to see all of the fish and wildlife coming back to the Cale. Of course, it is absolutely delightful to see that wild brown trout put in an appearance as well!

It seems to be all the rage for recovering urban stream projects in the &q…