Zebra mussels
As anglers we are constantly told that invasive species are a problem and that we should be concerned, but why exactly? Take the innocuous looking zebra mussel, for example. How can something so small do any harm? Is it really all that bad and, if so, why exactly is it something you should know about? Read on to find out…
Issue 13 (Nov-Dec 2016) Bill Brazier
One of the most frequently heard phrases in recent years in relation to freshwater in “invasive species”. However, one of the biggest shortcomings of science, and scientists in general, is the lack of ability in communicating scientific terminology to the general public, anglers included. I’d wager that whilst most of you reading this will have heard about various introduced species that are bad or damaging, few will understand exactly why.
Take zebra mussels (Dreissena polymorpha) as a prime example. This small, unassuming and quite attractive freshwater mollusc (with its zebra-like, striped shell) was first recorded in Irish waters in the early 1990s and in the UK far, far earlier, in the early 1800s. The species originates in the Caspian and Black Sea areas and for millennia was confined to such areas; until humans intervened, that is. The huge increase in global shipping trade throughout the 19th and 20th centuries allowed many species such as the zebra mussel to literally hitch a ride on board ships, in this case contained in ballast water, which even in many cases today is still filled and emptied at the start and end of a journey. It spread rapidly throughout North America and colonised almost all of Europe too. Irish colonisation is thought to have occurred as a direct result of the lifting of VAT (value added tax) on second-hand crafts imported from the UK for personal use from 1993 onwards, and the subsequent rise in the number of boats. Very few, if any, of these would have been checked on biosecurity grounds back then.
Why are they invasive?
Rapid spread is one of the inherent characteristics of invasive species. They are more adaptable than “normal” species and often reproduce at very high rates. Zebra mussels are no different; although they live for just 2-3 years on average, they can reproduce after 12 months and quickly multiply by releasing anything up to 1,000,000 larvae per year. Larvae drift around in the water column for anything up to a few weeks before settling on a suitable substrate. They can very quickly carpet the bottom of lakes and rivers, in incredible densities (called “druses”) of up to 50,000 individuals per square meter – sometimes more!
Like most mussels, they attach to substrates via short, strong byssal threads (known in culinary terms as “beards”). Zebra mussels need structure, like rocks, stones, reeds, weed stems, submerged branches, anything really that gives them a secure hold. Silty lake and riverbeds do not suit the species, although zebra mussels can attach to old, dead mussel shells in the lack of other surfaces. As such, zebra mussels often cause problems with water intake pipes and canal locks, and are of serious concern to the much larger native swan and duck mussels, which can become smothered and killed by this invasive species. Given the sharp edges to the shell (as many anglers will testify), zebra mussels can also cause health issues with recreational swimmers and bathers.
Did you know?
Zebra mussels can survive out of water for well over a week by sealing their shell shut. If kept moist, this can be several weeks. This ability to survive is yet another reason why they are so invasive
Not only are they capable of rapid spread in new waterbodies, zebra mussels are highly efficient at what they do – filter feeding. This is exactly what it sounds like, where mussels filter water through specially designed gills and remove all the desired microscopic plants (algae) and animals (zooplankton) to get their nutrients. One single adult mussel is easily capable of filtering up to 1 litre of water per day, which is quite astounding for such a small organism and the very reason why they are such an ecological problem. One Irish study (Lucy, 2005 ) calculated that as of 1999, the total zebra mussel population of Lough Key in Roscommon (2000 acres in size) was capable of filtering the entire contents of the lake in just 10 days! Cleverly, zebra mussels are also able to selectively feed on algae, ejecting the potentially toxic blue-green algae (cyanobacteria) which can cause harmful algal blooms in our stillwaters.
But clearer water is cleaner water, right?
Filtering this amount of algae from the water is the reason why water with zebra mussels turns clear after a relatively short period of time. Mistakenly, many people assume clear water is clean water, but, as outlined above, selective feeding often means that the only significant biomass of algal species left are the potentially harmful blue-green’s. No doubt many of you out there will have seen your local waters turn much clearer in recent years thanks to zebra mussel invasions. The Shannon system would be an obvious example.
Clearer water means many changes at a basic ecological level. Firstly, if you are an aquatic plant (macrophyte), then zebra mussels are probably the best thing to ever happen to you! They reduce algal cells which compete with higher plants for nutrients and by making the water clearer, more sunlight can penetrate and plant growth can flourish like never before. This can (and does) also see the large increase in certain species of macroinvertebrates (like certain caddis larvae, snails etc.) who live on certain plant species.
Are they good or bad for fish?
This all seems good for fish, right? More food, more weed to hide in… and hide is something a lot of fish will seemingly have to do more of in clearer water. Predation is far more of a threat if a predator is better able to see you, and if they have more places to wait in ambush. Although not fully understood, increased predator-prey rates as a direct result of zebra mussels have been suggested across many countries. This may well increase lure fishing prospects for predators such as pike.
However, let’s not sing the praises of this invasive species just yet. Filter feeding removes algae, which is the food source for zooplankton. In turn zooplankton are the major food source for all species of fish fry – trout, pike, bream, roach, you name it. All fish need to eat zooplankton when they are very small. I’m sure you can see where this is going… Zebra mussels can directly and indirectly dramatically reduce the zooplankton communities of a given water body, or at the very least change them considerably. This means that there will be less food for fish fry, which means that there will be reduced survival of those fish, which means less will grow into adults, less will be able to spawn and create the next generation and so on. Despite the quite astounding lack of study into the impacts zebra mussels have on fish survival and recruitment; this is the number one concern if you are an angler. In the long term, a zebra mussel infestation may drastically reduce a fish population. Clearly more study is needed on exactly how large an impact they can have, and on what species and under what conditions.
Did you know?
One of the major fears of zebra mussels is that they reduce zooplankton – food for every species of fish when they are young
There are some indications that some species may benefit, at least in certain ways, from the presence of zebra mussels. Lack of fry food aside, the adults of numerous fish species are known to actively eat zebra mussels. In Irish waters roach, bream, roach x bream hybrids and perch have all been documented to consume fairly large quantities. Lough Corrib is a good example of this. This shouldn’t come as any surprise to most of you. Not only are fish naturally programmed to eat whatever is most abundant but our coarse fish species are all equipped with powerful pharyngeal teeth in their throats which are used for crushing up food – zebra mussels are clearly not a problem for them. Interestingly, perch have also been found to eat zebra mussels, as well as brown trout and pike, although this consumption may be unintentional. So, this trend might mean that whilst there may be less numbers of fish coming through, the ones that get through may grow better and possibly larger due to the addition of zebra mussel to their diets. Food for thought…
Increased water clarity as a result of zebra mussels has definitely had an impact on angling returns. Many anglers agree that the feeding habits of the stalwarts of Irish coarse angling – bream – have noticeably changed, with their feeding times switching far more to darkness. The same can be said of most silver fish species in clearer-than-before venues, as these fish are acutely aware of the increased risk of being eaten clear water brings. Some say that the national downturn in bream catches in the last 10-15 years is largely to do with zebra mussel impacts… On the other hand, many trout anglers consider their fishing improved since the invasion of zebra mussels due to clearer water and more fly life. Some, however, feel that increased weed affects boating and casting) and that an increased abundance of fly life makes catching more difficult…
They’re here to stay…
So, the answer to whether zebra mussels are good or bad is firmly both yes and no. For sure, there are some benefits but also some significant drawbacks. Looking at the bigger picture, one would have to say, objectively, that our waters would be better off and more balanced without the zebra mussel invasion. But, they are here now and they are here to stay. They are far too widespread and far too numerous in Irish waters to ever remove them, not that there have been any real attempts to do so, even at an early stage. Some research has shown promising results, with certain chemical treatments harmful only to zebra mussels proving effective in the lab, but whether such control methods ever get rolled out on a national or even local scale is doubtful. Zebra mussels have become well established in a great many water bodies and there are signs that some sort of new natural balance is being met – whilst populations explode initially in a new waterbody, the rate of expansion does slow significantly. To try and remove zebra mussels from such venues at this late stage would likely create another whole host of problems.
Nature is quite simply amazing – whatever we humans do to mess it up, it always adapts, always survives, although perhaps in different ways to what we’d like. Still, I think it would be irresponsible for any of us anglers to sit by and watch a potentially harmful, largely misunderstood species invade a new waterbody or system. So, make sure to dry your nets and general equipment in direct sunlight after fishing a zebra-mussel containing water, and make sure no pools of water remain in your boats, unhooking mats etc.
Remember, a zebra mussel population can theoretically start from just two single individuals, and they can survive out of the water for a week or more in wet conditions. The hulls of boats should also be power-washed and dry-docked in sunlight to make sure no zebra mussels remain attached to the hull. These are all common sense actions that should be done automatically anyway to stop the spread of infections and diseases, let alone invasives. Perhaps if more had been done to raise awareness of this species in the beginning then they wouldn’t have spread to the extent they have? For now though we will just have to wait and see exactly what the long term outcomes are… There is a lot more work to be done to even begin to fully understand the impact of zebra mussels.
Bill Brazier