Geraldton Fishermen's Co-operative Rock Lobster Exporters

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Dissolved oxygen – the unseen necessity! -March 2019

By now you’ve probably heard the words Dissolved Oxygen used around the Co-op in regard to water quality, but what is it and why is it important? Dissolved Oxygen, or DO as it’s commonly referred to, is the level of free oxygen dissolved in water.  In the same way that we need oxygen in the air we breathe, many forms of aquatic life, including lobsters, need oxygen dissolved in the water they breathe to survive.  Dissolved oxygen is probably the single most important water quality factor when holding live lobsters on board your boat.

Oxygen enters water in three main ways: 1) by diffusion from the surrounding air, 2) via photosynthesis by aquatic plants and 3) via aeration, either natural (waves, waterfalls, etc.) or man-made, using blowers, etc.  Water with an oxygen content in equilibrium with air is said to be 100% saturated.  However, even at 100% saturation, the actual quantity of oxygen in water can change due to variations in water temperature and salinity.  As both water temperature and salinity increase, the amount of oxygen in each litre of water drops, and therefore its availability to your lobsters decreases.

Like us, a lobster’s oxygen demand changes depending on external factors.  Activity and exertion, like tail-flapping, has a large effect, increasing oxygen consumption greatly.  Exposure to air, handling (e.g. during gauging), feeding (e.g. on bait) and increased water temperature also increase lobster oxygen consumption rates.  When lobsters experience one, or a combination of these events, they require more oxygen than they do when at rest.  What’s more, depending on the severity of the event, lobsters can take several hours to fully recover.  You can think of it in the same way you feel after playing a game of footy, compared to sitting on the couch.  If your lobsters are using oxygen at a rate faster than it’s being replaced by aeration or pumping water through the tanks, dissolved oxygen levels in the tanks will start to fall and can reach dangerously low levels.

At GFC, we use vigorous aeration in all tanks at all sites to ensure our lobsters have access to plenty of oxygen.  Dissolved oxygen levels are maintained above 95% saturation across all of our live lobster holding sites.  DO levels are checked and recorded multiple times each day to guarantee this is maintained.  You should be aiming for the same DO level in your on board tanks and should be checking frequently to ensure this is maintained.  Maintaining high DO ensures lobsters will recover from capture and handling as quickly as possible and will be ready to face the next challenge in the supply chain, like truck transport or grading.

So how can you tell if your lobsters are getting enough oxygen?  The best way to accurately measure oxygen is by using a portable dissolved oxygen meter.  These days, a reliable meter for use on the boat can be picked up for a pretty reasonable price.  DO meters are great for finding “dead spots” in tanks or for identifying problem tanks that are not operating to the required standard.  If used on a regular basis, DO meters can also highlight the need for maintenance and repairs (e.g. diffuser hoses starting to block, etc.) before a serious problem occurs, saving you money by avoiding those dreaded spikes in second grade.

If you would like information on where to buy your own dissolved oxygen meter for the boat, GFC staff can advise on the best types of meters and help with some basic training.  Proper use of DO meters does require some knowledge about their operation and maintenance, so it’s important to get that training to start with.

Please contact GFC Research and Development Officer, Joel Durell (0407 135 712) or GFC Continuous Improvement Manager, Brad Armstrong (0437 513 837) to take the next step in your quality journey.


Figure: A dissolved oxygen meter will tell you if your lobsters are getting enough oxygen and will give you early warning of any problems.


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