River Dulnain juvenile salmon population dynamics

The Dulnain was surveyed by electrofishing intensively in 2012 with some sites revisited in 2013 to monitor whether the better salmon fry densities noted in 2012 had resulted in improved parr densities this year. Four sites were revisited in 2013, the sites geographically spread in varying habitat along the Dulnain mainstem.

The River Dulnain is one of the larger tributaries of the Spey and it contains about 6% of the juvenile habitat available in the Spey catchment. The habitat is generally of good quality, although downstream of Carrbridge the gradient is lower and the channel has been subject to quite a lot of management with flood banks etc. Conductivity in the upper reaches is lowish; it is therefore not the most naturally productive but it appears to be capable of support a good juvenile population.

Photos of the sites are shown below for reference.

Site D15 Balnaan Bridge

Site D15 Balnaan Bridge site is in the lower river located a few miles upstream of the confluence with the Spey. The gradient here is relatively low and the river wide and shallow. This site normally holds a lot of fry.

 

D21 Inverlaidnan

D21 Inverlaidnan Bridge. Faster flowing here the substrate is dominated by boulder with deep runs between.  This site has only been surveyed twice.

 

Dalnahaitnach electrofishing site D22

Dalnahaitnach electrofishing site D22 is at the upper end of a big wide floodplain. Substrate sizes here are smaller, mostly cobbles and small boulders. The red lines indicate the 2013 survey limits.

 

Suspension Bridge site in the upper Dulnain

Suspension Bridge site D9 in the upper Dulnain at 1500′ altitude. Typical of the upper reaches it is ideal mixed juvenile habitat with a variety of flows and depths. Few if any trees get a chance to grow this far upstream.

The salmon size distribution graphs from the four sites are shown below, earliest survey for each site first. The graphs show the abundance in 1mm size intervals. The colours indicate the ages of the fish based on scale readings collected during each survey.  The actual breakpoint may not be as clear cut as indicated as a large 1+ parr could be bigger than a small 2+ parr but based on many years scale reading experience within the Spey catchment we have reasonable confidence in the breakpoints. Note that the vertical scales on each graph varies!

The actual area surveyed each year varied slightly due to water levels etc but the sites were effectively the same.

Site D15 has a long survey history and usually supports a lot of fry. The parr density in 2013 was the best recorded at this site.

River Dulnain Balnaan Bridge site D15 2012

River Dulnain Balnaan Bridge site D15 2012. 77 fry and 9 parr were captured.

Balnaan Bridge site D15 2013

Balnaan Bridge site D15 2013. 197 fry and 24 parr were captured.

Site D21 has only been surveyed twice, both fry and parr were more numerous in 2013. There were more larger parr, fish that are more likely to smolt the following year.

River Dulnain Inverlaidnan Bridge 2012

River Dulnain Inverlaidnan Bridge 2012. 94 fry and 31 parr were captured.

River Dulnain Inverlaidnan Bridge site D21 2013

River Dulnain Inverlaidnan Bridge site D21 2013. 157 fry and 75 parr were captured.

The Dalnahaitnach site was also only been surveyed twice but again there were many more fry and parr recorded in 2012 than in the previous year.

River Dulnain Dalnahaitnach site D22 2012

River Dulnain Dalnahaitnach site D22 2012. 87 fry and 24 parr were captured.

Dalnahaitnach site D22 2013

Dalnahaitnach site D22 2013. 158 fry and 70 parr were captured.

The Suspension Bridge site has a long survey history. The  results from here have been highly variable. The 2013 fry density was below average but the parr density was the best recorded at the site.

Suspension Bridge site D9 2011.

Suspension Bridge site D9 2011. 19 fry and 12 parr were captured during the first run through the site. Note the graph shows the fish captured during all three runs.

Suspension Bridge site D9 2012

Suspension Bridge site D9 2012. 39 salmon fry and 4 parr were captured.

Suspension Bridge site D9 2013

Suspension Bridge site D9 2013 7 salmon fry and 34 parr were captured. Note the strong 1+ yearclass.

These sites were resurveyed in 2013 for a specific reason: to try and understand the juvenile salmon population dynamics in the Dulnain. So what can we deduce? Well as anticpiated the population of 1+ parr increased dramatically following the good 2012 fry yearclass. But the 2013 1+ parr population were able to develop with little competition from older yearclasses. It will be interesting to see how the current, even better, crop of fry matures with what will be strong competition from the 2+ parr year class in 2014? To that end we will endeavour to resurvey these sites again next year.

The presence of a strong yearclass can have a significant influence on smolt age. Competition could result in more parr having to remain in the river for another year, i.e. smolting as three year olds rather than two. Conversely a weak yearclass could be partly compensated for by earlier smoltification; the net effect being to smooth out smolt production between years. Salmon population dynamics are quite interesting!

 

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