Thursday, 25 June 2020

It's all about mass

With all the good weather recently I have found myself often sheltering from the heat in the shade beside the water at the Saxon Mill. From the terrace there are several spots where you can sit comfortably and look out over the mill pond and into the murky water to watch the fish.

Look carefully enough and you can see a pike

Today was one such occasion. We are experiencing a mini heat wave, by 10 am it was already 24 degrees C in the shade, and out in the open heading for 30! I had gone down the mill to exchange the batteries and memory cards on my trail cams, and to leave some peanuts out for the badgers, in hot weather like this they struggle to dig up earthworms. On my way back I stopped at the mill pond and looked to see what fish I could see.

I was lucky enough in late may to catch sight of a pike hunting the shoals of roach. I had heard that they were common on this patch. They lurk on the river bed beneath the lily pads and wait for fish swept through the mill race and then strike. It was amazing to watch.

Today there were no pike hunting nor did there seem to be many roach of any significant size. Instead I was mesmerised by the mass of smaller fish, fry, shoaling beneath me. The shoal perhaps number 300 individuals and it moved with one mind, just like you see in the wildlife documentaries. Occasionally you saw a flash of silver as one fish twisted to one side and signalled a change in position. As I watched them, they seemed to be doing circuits of the lily pads a though struck me.

Here were around 300 young fish in just a very small stretch of water. Even if you discounted the deeper parts of the mill pond there were enough shallows to estimate that this particular section only represented one one hundredth of the available habitat and this would be an underestimation. This then would mean potential for 30,000 young fish striving to make it to adulthood and bear in mind that this is only a tiny fraction of the number of fertilised eggs that survived to hatch.

Where do all these little fish go? They certainly don't all become big fish, 30,000 fish appearing in the river each year would soon be noticeable and fishing as a 'sport' would be come redundant a literal shooting fish in the barrel. Predators are all over the place winnowing out the weak and unwary and of course there is disease or injury. Grey Herons and Kingfisher are common right along this stretch of the river and as I have already mentioned pike are abundant. Regular readers of this blog will know that Otters are on the increase in the river although I doubt they would bother themselves with the tiny fry I was watching.

When I talk about mass, I am referring to biomass, the net weight of biological material. During the summer this mass is evident in the number of leaves on a tree, the rapid growth of the vegetation and all the insects upon which this fry and fish will feed upon. We are mid-way through the Banded Agrion flight period at the moment. In last years flight period I recorded a total of nearly 800 individuals during my weekly 1 hour visits. Again this is only a tiny proportion of those that survived the egg and nymph stages and a tiny proportion of all those that took wing that season.

I think it speaks well of the health of the Avon in Warwick that we see such good numbers. They represent a link in all those important food chains and food webs that tangle and weave their way around the ecosystem. It also highlights the losses incurred by species. Think of all the eggs laid by some insect or fish species only for one or two to make it to adulthood. This is what we ecologists call R-strategy.

R-strategists produce masses of low cost offspring that they 'know' will struggle to survive. They take the machine gun approach to life, if you fire enough shots you're more likely to hit the target. The converse of this, something that most mammals do, is the K-strategy. Here you invest all your energy into a few offspring but you make darned sure they make it to adulthood, by feeding them, teaching them and protecting them. These a more akin to the skillful sniper. They take measured calculated shots that increase the chance of hitting the target.

Back to the fry, they are one component in a vast array of interconnections and complexity, they feed on the invertebrates in the water who in turn have fed on each other and the aquatic vegetation. They in turn are food for the predatory fish and birds, who are in turn prey for something else. Its only when you start to piece this intricate patterns together that you can begin to understand the fragility of some systems. Ecology is elaborate, intricate, delicate and profligate it is why I hold it so dear and champion it as the science of kings.

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