Gill Score Guide

Amoebic gill disease (AGD) has been the main disease affecting farmed Atlantic salmon in Tasmania (Australia) since the mid-1980’s. The disease occurs year round and adds significantly to the cost of production because fish need to be regularly treated with fresh water throughout the marine growing cycle.

AGD is proactively managed by regularly sub-sampling each caged population and inspecting the gills of anaesthetised fish for gross AGD signs (characteristic white mucoid spots and patches on the gill surface). This ‘Gill Score’ is a 0 (clear) to 5 (extensive lesions) scale and is assessed across all 16 hemibranch surfaces (i.e both sides of all 8 gill arches), it is a conservative scale that is designed to pick up and manage AGD from the onset of the disease. As gill score progresses, the risk to the fish increases from reduced feed intake, to increased stress and eventual death if not treated. Monitoring of the frequency of gill scores in a population and the average gill score (‘Gill Index’) over time allows the farmer to make informed stock management decisions to minimise adverse impact to the animals and to ensure that farm resources can be balanced to treatment requirements in a cost effective manner. A central concept of the gill score is that it is simple and consistent so that producers can compare AGD development trends between farms.

Since 2011, AGD has become a major health concern in Northern Europe (particularly Ireland, Norway and Scotland) and has been reported in Chile and Canada. Without prior experience in managing the disease, European
producers were quick to adopt the Tasmanian gill score method as described in Taylor et al. (2009). However, the interpretation of the score has varied between European users because the initial visual guides were based upon single hemibranch photographs. Some farmers now use a ‘worst arch’ score, while others score each arch and then average the score for each fish, thus complicating essential communication of disease management between farmers. These different interpretations risk understating the gill score in the early stages of development and represent increased risk to the fish. It is in the interests of all producers to
standardise AGD monitoring and communication.

With a need to standardise gill score training across the Marine Harvest organisation, the company requested that a simple AGD training guide be developed by CSIRO and Tassal to document the gill score method. The
central element of this guide is that fish are first externally photographed showing the gill arches being turned over as they would be during gill checking. Because it is not possible to show the entire gill surface this way, the gills are thereafter laid out to enable both surfaces to be photographed in a fresh state.

This booklet is released for reference to the Atlantic salmon industry and fish health professionals.

Richard Taylor
CSIRO
November 2016