Joint Mobility 
Omega 3 found in Fish4Dogs can help with joint conditions. However it is a healthy food and not a medicine and so it is important to say that if you are worried you should see a vet.
A natural part of ageing is for the joints to stiffen up. However these symptoms can be reduced by a healthy diet with plenty of Omega 3.
Omega 3 reduces Inflammation.
The EPA and DHA Omega 3 found in marine sourced Omega 3 are a natural anti-inflammatory helping alleviate the symptoms of joint pain.
Fish is a Natural Source of Glucosamine and Chondroitin.
Fish are rich in cartilage, a natural source of Chondroitin and Glucosamine which, like Omega 3, helps to reduce inflammation and reduce cartilage breakdown.
Switching off the Aggrecanases and other Collagen-Degrading Enzymes.
There is some evidence to suggest that Omega-3 fatty acids can further reduce cartilage breakdown by switching off the aggrecanases and other collagen-degrading enzymes that break down joint cartilage.
So a fish rich diet can help keep those joints nice and supple.