Salmonids undergo a developmental transition from parr to smolt that involves a number of physiological and morphological changes.
After two years or sometimes more of stream life, steelhead parr begin their down river journey to the sea.
Over the months, their development is monitored by ‘babysitters’ as the fish grow from fry into young fish called parr and pre-smolt.
From what I can ascertain after a quick search on the net, a parr is a juvenile form of salmon and/or trout.
There is significant shift in the MHC composition of trout red muscle between parr and smolts.
Within minutes of the accident, fish began to leap furiously over all its quiet little pools, and within an hour the river became a floating graveyard of brown trout and salmon parr , carrying its cargo of poisoned corpses out to sea.
To examine expression levels, muscle samples were taken from three body positions (anterior, middle, and posterior) of red and white muscle of five parr and five smolt rainbow trout.
The fish killed were salmon parr (one year old) and trout which would have been the future stock of the river.
As they gain in size, they become parr , a stage which lasts until they make their way down to the sea, undergo various changes designed to fit them for life in salt water, and acquire a new name, smolts.
Quantitative analysis of duty cycle shows a significant increase in duty cycle at each position between parr and smolts.