Wandle Restoration Part XII

trout eggs

Trout tank trilogy: part 1

17 December

The tank arrived this afternoon. The door-bell rings, and there’s Gideon with trout tank and beer chiller number 20, the last spares from the “Trout in the Classroom” project this year. After all, it’s better to have them working hard for the river, and for all our knowledge, than sitting disassembled in the corner of his spare room…

Two more trips out to the van, and then we’re up to our elbows in bleach and long, sinister-looking black protective gloves, decontaminating everything in sight. The tank’s easy enough: it’s the pea-gravel from the aquarium suppliers that really takes the time and effort, continuously stirring and washing till there’s not a trace of silt that could suffocate the precious eggs when they arrive.

At last, assembly time in the corner of my dining room: the tank set on a rather fetching tubular-steel-and-metal-grille console table built from leftover bits of bathroom shelves (thanks, Ikea!), the beer chiller on the floor beside it, everything linked up with insulated pipes and plugged into a circuit-breaker.

Bucketfuls of tap water come out of the pipe at a cool 11 degrees. That’s the same as the Wandle springs half a mile upstream in Grove Park, so the chiller shouldn’t take much calibration to hit the magic 10 degrees where the eggs will be happiest and the biofilter will work most efficiently.

With due ceremony, I hit the switch on the chiller and the oxygenating powerhead pump. A bass hum fills the air. Trout in my dining room 2007: it’s a go.

18 December

That beer chiller is bloomin’ noisy.

Maybe you don’t notice it in the pub when the pints and the craic are flowing, but here in the back right hand corner of a Victorian cottage, where the mid-renovation floorboards are vibratingly bare and you’re trying to talk over diner a deux, it becomes almost impossible to hear yourself think.

The project’s unimpeachable – but are we already counting the months before we’re rid of this conversation-stopping rumpus?

At least the dog seems unfazed. After a single sniff at the strange red LED’s on the chiller, she turns her back and ignores it as just one more inexplicable fact in the life of a young cocker spaniel…

20 December

5am: We’re driving to Scotland for a long Christmas en famille. After some deliberation, we determine to turn the chiller off while we’re away – what if it overheats, and how much electricity is it using anyway, juggling the chiller’s target temperature up and down by half degrees?

Fire and carbon-footprint risks prevail: after all, the natural ambient temperature of the house is a definitely non-tropical 13 degrees, and we’ll leave the powerhead pump on in any case, to keep the water circulating in the tank.

Blessed peace descends before we grab bags, presents and dog, drop the latch, and hit the long road north.

30 December

We’re back in Carshalton – and turning off the chiller hasn’t produced any obvious ill effects. The gravel is clean, there’s no smell of stagnation, and the water is still clear as crystal.

Sure enough, the thermometer inside the tank reads a level 13 degrees, and now it’s actually easier to work out balancing the temperature: just set the chiller to the opposite side of 10 degrees. Within a couple of hours, we’re back in business. Temperature is stable, eggs are due in less than a week, and even the thrum of the chiller is somehow less noticeable.

4 January

The eggs are here!

Balancing Styrofoam ice-boxes, Gideon and Erica appear at tea-time, looking frazzled after a full day’s egg-laying in trout tanks the length of the Wandle Valley. Mine are what’s left of the rearguard: about 200 little orange globules, invisibly eyed ova that float out into the tank’s gentle currents, just slightly heavier than water, spreading and spiralling down to the dark pea-gravel.

Younger and less developed than those we got from Sparsholt last year – some of the alevins were actually hatching even as we slid them out into the tanks at Sutton Grammar and St Philomena’s – the eyes in these eggs are still so small as to be functionally invisible. Erica describes seeing their brood-mother at the fish farm: “a monster… this huge fish in a dark tank!”. A great spade-tailed wild-strain Itchen brownie, probably as close a match as we’ll ever now get to the right genetic stock for the poor little ruined Wandle.

According to some of my Christmas reading, our eggs should take about 40 days to develop and hatch at a temperature of 10 degrees – a very vital statistic in the days when trout were being transported all over the world on steamships, packed in moss over ice.

The lower the temperature, the longer the eggs take to hatch, with a viable range from 20 days at 16 degrees to a whopping 150 days when the temperature hovers just above freezing:

Temperature in degrees Celsius Days of incubation
2 150+
3 128
4 108
5 90
6 75
7 63
10 40
12 27
16 20

(Source: Rupert Watson: “The Trout: a Fisherman’s Natural History”)

The eggs in my dining room also been gently “shocked” to help them survive the trauma of transport from Sparsholt to South London. So, barring a few early fatalities, most of them should be viable for hatching into little fish.

Settled on the gravel, they’re remarkably well camouflaged: maybe half a centimetre’s diameter to match the gravel, coloured in a range of soft orange-pinks that immediately make me rethink longstanding doubts about the egg-imitation flies used for luring steelhead in the churning winter rivers of America’s Pacific Northwest. Even to me, these eggs look appetising, so a little ball of yarn could look downright realistic to a steelhead in any shade.

It’s the first fishing-specific insight I’ve taken from this experiment, but I’m sure it won’t be the last. We close the insulating curtains round the tank’s glass walls, check the temperature again, and kick back with well-earned tea and leftover Christmas mince pies.

5 January

Rubber-gloved up to my elbows again, brandishing a glass bowl and a sawn-off turkey baster, I spend a careful quarter of an hour chasing dead ova round the tank. 8 or 10 haven’t survived the journey, turning a deathly, delicate shade of alabaster pink, distinctly different from their healthier brothers and sisters. It’s a procedure I’ll follow every few days till the survivors hatch – sucking up and decanting dead eggs that could otherwise overload the biofilter and cause catastrophe.

Other than that… we wait… while the little bit of river in my dining room works its magic, and life is born in those little round globes resting in the cold, still dark.

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