Counting goosanders on the Cumbrian Derwent has led Dave Calvert to calculate their affect on the river
The recurring concerns over damage by sawbill ducks to fish stocks in our local catchment had produced little action other than adding records to the Angling Trust’s Cormorant Watch website. However, that changed back in the spring of 2012 when, at our local fishery owners' meeting, a photograph of a local mining flash (lake) was circulated which showed that the water surface had a lot of tiny black dots which, with the aid of a magnifying glass, were revealed as goosanders. Over 300 of them in all, sitting on a very few acres.
Now (summer 2012), not even the best run stock-pond could support that number, so where were they all feeding? No one had the answer, but a few weeks later Cocker-mouth angler, Adrian Mills had the great initiative to organise local anglers to each visit a separate part of the catchment to try to get some idea of how many of these predators our river was supporting, and where they were feeding. At the appointed date and time (a two-hour slot on a given Sunday morning) the team turned out to count our individual locations and then submit the results to Adrian. The totals at a conservative estimate (from around 70% coverage) showed that our river was supporting at least 100 breeding pairs. Apart from an initial shock reaction, what was to be done? A previous application to Natural England for a licence to cull by one local association had produced a licence to cull just three as part of a programme of scaring.
Gentle cogitation suggested a spreadsheet might shed light on the impact of this population on fish stocks, which indeed it did.
Calculation showed that that 100 pairs would, by August produce a hungry horde totalling some 1200 adults and maturing juveniles. Seasonal movements in the population were added together with figures for daily fish consumption from the RSPB to show a likely total fish consumption over a year of around 52 tons of fish. Of course, that is for fish of all species and what we game angling licence holders wanted to know was what damage was being done to our salmonid populations?
So, to estimate the damage to each species some assumptions were made. If 30% of the diet is salmonid fishes, and 70% of those are salmon, then approximately 195,000 juveniles would be consumed, resulting in a projected 1,300 adult salmon being lost to our annual salmon run. If 70% of the diet is salmonid then, on the same basis, approximately 3,000 adult salmon are being lost to our annual salmon run (from 455,109 juveniles consumed) – this from a total run at present of around 6-7,000 salmon and grilse (Environment Agency estimates).
I don’t pretend that the detail of the derivation of these figures comes from peer-reviewed published science. I’m not a scientist. However, the detail of all is given in my spreadsheet below, and if there is someone out there who is qualified in this field I’d be delighted to contribute what I can to some further more authoritative work.
Cut it any way you like, the impact is a staggering loss which seems to be between 20% & 45% of our annual salmon run – and I’ve yet to add in the estimates for cormorants and mergansers.
Many millions of pounds of public money have in recent years been pumped into habitat restoration schemes on our rivers under a plethora of government initiatives. One of the key targeted outcomes from these large scale investments of public funds is to improve fish stocks and this has particular relevance to the Government meeting its obligations under the Water Framework Directive (WFD).
‘Fish’ is one of the key WFD criteria which fails to meet its required standard of ‘Good’ (as defined within the WFD) in a large numbers of WFD water bodies.
The annual electro-fishing of juvenile salmonid fishes by the EA locally regularly reports generally good numbers of recently hatched fry across much of our catchment but, in comparison, the numbers of returning fish are significantly less than would be expected. Habitat and water quality in these areas are by and large of good quality.
One of the main reasons for that under-achievement in adult numbers and the subsequent failure under the WFD of the fish criterion is likely to be predation from fish-eating birds. So, high numbers of sawbilled predators is highly likely to be negating much of the investment of our taxes in our rivers.
Can you do something similar to produce figures to show the damage to your catchment? A little organisation and a modicum of number-crunching could produce some countrywide results. In the meantime, please do all you can to support the Angling Trust campaign to loosen the restrictions on controlling these predators.
• Download Dave's table as a pdf file by clicking HERE.
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