Friday, 24 May 2013

Build It and They Will Come — Nature's Finest (Pt5)

Four years ago when I first took an interest in the Sowe I couldn't understand why it contained such a small head of fish but those few stretches of river I managed to find where the river had never been interfered with contained good mixed shoals with such large individuals in them. It seemed a puzzle because at that time I had no idea that fish living in rivers, and especially very small ones, have exacting standards when it comes to habitat. 

The infant river three miles upstream of the park, diminutive but pristine in every respect...


I thought fish simply required water, and in a large river with plenty of depth that might be the case, but as I walked miles upstream and downstream of the short natural stretch where I'd first found them and in search of more, it became abundantly clear that water alone was not enough here because they were absent if it was too shallow or lacked cover and that was the case pretty much everywhere else.

The mature & ancient natural stretch two miles below with its perfect unspoiled habitat

You may have become accustomed to viewing the river where these experiments of mine 
have been conducted as being far larger than it is in this picture. You'd be right in terms 
of width but not in depth. The river in the picture above has gained the water from one of 
it's main tributaries between here and there and yet it's much narrower, but exactly as it 
should be for the volume it carries — where I'm working is twice as wide but very shallow

I had to walk a full five miles downstream before I discovered another shoal. Between was a river that looked clean and healthy but hadn't any fish in it that I could see apart from one deep pool (the only deep pool!) bursting with minnows but without attendant perch. Upstream things looked far better but the only fish I witnessed there were sticklebacks and an isolated group of small fish I thought to be stoneloach living beneath a piece of roofing tile who vacated when boys took it away and used it in play, never to be seen again.

Sampling the menu
That's about when I started taking a look at what it contained in the way of available food wanting to know if it was a lack of that causing their absence or if it was actually lack of suitable habitat as I thought. So, I ran my first kick samples, getting into the river and throwing gravel up off the toe of my boot collecting what was dislodged in a small net.

I really didn't expect to find much and thought I'd go a long way before getting any result but in the first netful it was clear that not only was there food in the river and plenty of it, but when I got home, identified the species I'd found in an hour or so and checked them against a table divided into three or four classes indicating water purity — the bottom ranking able to live in sewers, the middling classes tolerant to degrees but the top ranking species highly intolerant of pollution — I found the food supply to be high class stuff of the finest quality...

Freshwater pea mussel I believe, and case-less caddis grub
I knew the little green caddis grubs for what they were without diagrams to check against but hadn't a clue about the abundant tiny clams because it was the first time in my life I'd ever seen anything like them living in a river. 

Besides them, all kinds of strange little creatures came up in the samples and when I took a closer look at the stones, limpet-like things I thought to be molluscs were attached to them... 

They were very common indeed.

Water penny — an aquatic beetle larvae



Freshwater shrimps and a mayfly nymph
There were lots of small shrimp-like creatures that looked like the sandhoppers I knew from my sea fishing days but alongside them another lifeform I hoped was a nymph...

Though not as common as the 'hoppers' they were present in good numbers and I was quite excited by them because I thought they might be mayflys, and if so one of the best indicators of water purity there is because they just can't stand any level of pollution...

As it turned out, my instinct was correct because they were indeed swimming mayfly nymphs, and better still, the little 'limpets' turned out to be water penny, an aquatic  beetle larvae and another top ranking species as were the tiny clams, a now quite rare species of freshwater pea mussel due to their need for very clean water.

All together it seemed the river was in sparkling good heath.


Swimming mayfly nymphs.





Then along came the first fish...

A bullhead! 

Only a very small one, but again, one of those species who don't like to live in unclean water. I've only ever caught a very small handful of these curious little fish with heads as big as their bodies but always in very clean rivers that support populations of wild brown trout. 

What I didn't expect to find was an earthworm below the gravel because worms live in soil, right? Well it seems they don't mind living in riverbeds either!



And then to top things off came one of the weirdest looking forms of life I have ever met. A large creature with long legs but who looked as ancient as the hills, as if he was actually a fossil rather than a living thing. Moving very slowly he crawled about clumsily on my hand unable to support his own weight out of water on his spindly limbs... 



I think he's a damselfly nymph who never emerged into the air and never would but missing most of his antennae and all his tail I'm not at all certain about that... 

I only wished I'd taken more photos, because I'm sure I found more that day than I realised.

I'm not much of a scientist when it comes to these things so I didn't count them all and come up with tables and the like, but was satisfied that the river held good food for fish life and plenty of it and even though you'd be forgiven for thinking it was anything but in certain ugly places, the river was actually very clean and its water pure. That was a revelation —I'd expected fair results, not excellent ones!

It was now clear to me that the fish grew just as large as such a small stream could support precisely because of its abundant high quality food supply in combination with good habitat wherever it was found — they couldn't make use of it where habitat was poor — but they could occupy the rest of river, breed and flourish, should what they need be created where for one reason and another it had ceased to exist.

To be concluded...

2 comments:

  1. Same here. What makes it even more appealing is the fact it is on your doorstep. Looking forward to the next one!

    Dave B

    ReplyDelete