Doubling up the Avon smolt trap

After the catch of almost 300 salmon in the Avon trap on Monday morning things slowed down as the river dropped. There were 19 new salmon on Tuesday with one recapture, 9 new fish on Wednesday and no recaptures and today (Thu) 16 new fish and 2 recaptures. From 204 salmon marked on Monday we have recaptured 3 so far giving an efficiency of 1.5%. We may still get more recaptures although our extensive experience in several trapping location across the Spey tells us that most of the recaptures occur the next day with only a few stragglers in the following days. Of course this is a new location, it is still early in the smolt migration season and the river had dropped so the smolts might be travelling slower than anticipated.

However we were concerned by the apparent low recapture rate yesterday so we discussed options for increasing the trap efficiency including relocating the trap to a narrower part of the river or moving it further towards the channel centre. After discussion with Steve Brand we decided to deploy our other spare trap alongside the existing thereby almost doubling our trapping effort.

The 4' diameter trap morred alongside the 6'. The 4' trap obviuosly requiures less water depth and rotates much quicker than the larger trap.

The 4′ diameter trap moored alongside the 6′. The 4′ trap obviously requires less water depth and rotates much quicker than the larger trap.

The channel depth shallows towards the left bank but the current speed is slightly faster closer to the channel centre. It will be interesting to see which trap catches the most fish.

The double trap set up from downstream. We still haven't measured the chanel width but Google Earth tells me it is about 22m. The combined width of the two traps is 3m so we are still sampling less than 15% of the total width and even then only the arc of the submerged drum, not the full depth.

The double trap set up from downstream. We still haven’t measured the channel width but Google Earth says it is about 22m. The combined width of the two traps is 3m so we are still sampling less than 15% of the total width and even then only in the arc of the submerged drum, not the full depth below the traps.

Any further Monday marked fish which are caught in the outer trap won’t be included in our efficiency calculations but all marked and released fish from now on will be treated as being from one unit. By almost doubling our trapping effort we hope to raise our trapping efficiency to a higher level. The guidelines recommend that a minimum of five fish are recaptured from each marking event in order to provide robust data. At the same time we want to be marking reasonable quantities of fish when we do carry out a mark/recapture trial. Given the apparent low efficiency of the single trap we were facing a situation where there would have been few days with high enough catches for marking followed by low numbers of recaptures. Hopefully by increasing our trapping capability we will be able to collect good data from the Avon this spring.

Our main concern following the first mark/recapture trial was that we caught almost 300 fish one night when the recapture rate to date suggests that we caught only 1.5% of the smolts passing downstream. I’ll let you do the maths but you can see why we need good recapture data from many trials so that we can all have confidence in the results.

 

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