There is still time to save the mussels

Last Sunday I spent the afternoon at an Otago Regional Council presentation on the health and vitality of Lake Tuakitoto.

Now hang in in there because, like many of you, I didn’t think I was about to be spellbound, but that was before I was introduced to a gnarly wee sucker with the official title of Hyridella menziesii.

They are freshwater mussels that have most likely inhabited the lake for thousands of years, quietly going about their business of filtering the water.

In 1991 a survey was conducted that estimated their numbers were so prolific they could filter all the water in the lake every 32 hours. That’s an amazing effort which was obviously fundamental to water quality and the overall health of the ecosystem.

Unfortunately, several detrimental factors have combined in recent years to really upset the balance, and the most recent survey shows a catastrophic decline in numbers. It now takes about two weeks for the mussels to filter the same amount of water.

When you consider the significance of Lake Tuakitoto’s contribution to the wider ecosystem of the lower Clutha Delta we should all be concerned at the extent and pace of change, and how nature’s fragile interactions and dependencies are being disturbed and possibly permanently changed.

There is an extensive list of challenges the mussels now face. Strangely, in order to procreate, mussels spray their eggs on to any unsuspecting whitebait that passes by too closely. Not your standard way of treating guests, but a critical step in the breeding cycle. With a sharp drop-off in whitebait numbers found in the lake, there is also less likelihood of chance encounters. Add to that the propensity of Canadian geese and black swans to favour native plant species over exotic plants. This contributes to the lake now having a stark change in the mosaic of vegetation types, increasingly dominated by introduced species that are clogging up the lake and contributing to the rapid changes.

This process has been accentuated in recent years by a rapid increase in nitrogen load and silting from run off. There has also been a steady increase of introduced fish such as perch and trout that, as we know, are quite found of dining on whitebait. And to round it all off, there has been an increase in the water temperature in recent years which increases plant growth, reduces oxygen levels and generally changes the overall environment.

All in all, things are looking rather bleak for the mussels, they are battling the odds against so many formidable opponents — or are they? Because there is one obvious common denominator in this story and that is the unquestionable part humans have had in introducing exotic species, creating the unfavourable dynamics and generally messing up the environment to the point that the mussels must hardly recognise their own home.

The sad thing is the most likely saviour is now the No 1 contributor to the impending demise, but I’m picking, par for the course, we will procrastinate and invent any number of counter arguments that will enable us to justify virtual inaction.

Sorry Hyridella menziesii, nice knowing you.