Diadromy

Sweet or salty? It matters for popcorn. It matters for water.

Freshwater and saltwater differ in their salt content (duh!). And with this come a range of physical and chemical changes that affect (fish) life dramatically. A very straight forward demonstration of this is one many of us have experienced in the past: opening your eyes in the ocean is way more painful than opening your eyes in a lake. The reason for this is that the mucus and tears that cover and moisturise our eyes are adapted to life in air and thus have a salt content (~1g salt per kg tears) much lower than that of sea water (~3.5g salt per kg Atlantic). When our eyes are exposed to the ocean, they would thus dehydrate and the stinging sensation warns us of this (most lakes have salinity close to that of our tears, making a pain-warning less necessary). That is why children (and some grown men…) are afraid to ‘get water in their eyes’. The eyes of sea fish (and maritime mammals) have a mucus cover that protects them in this salty environment (so yes, whale tears are saltier than human tears; but they are also sweeter – they contain more sugars. My next pitch for a sweet-and-salty popcorn treat will leverage the power of ‘Dolphin Tears’ as brand name!). Freshwater fish on the other hand have mucus that works well in low salinity. Basically, fish ‘do not get water in their eyes’, irrespective of which water they are in.

Now that we have established that the difference in salinity between ocean and stream/lake systems is painfully obvious once you open your eyes to it, let’s consider the general problems that come with this. The key issue is that fish life is only possible with bodies and bodily fluids within a fairly narrow range of salinities (again <1g salt per kg body/blood/tears/etc.) – that is the range in which nerve cells work, metabolism functions, and fish (and human!) cells are generally happy. If such a body sits in water with a much higher salt content (ocean!), it will dehydrate. If the same body sits in water with a somewhat lower salt content (lakes/streams!), it will soak up water. In other words, sea fish have to constantly drink to make sure they stay hydrated while freshwater fish have to constantly pee to not swell up.

This wouldn’t be all that spectacular if there weren’t fish that migrated between the oceans and rivers and lakes. Some of the most famous fish do, in fact, including salmon, trout, eels. There are some 500 species that frequently switch from ocean to stream or vice versa. When migrating from one type of water to the other, these fish have to change their whole physiology! Imagine going from a state of constant thirst (hangover, anyone?) to a state of constant strangury (yes, this is an English word. And yes, it is the medical term for the need to pass water. Which is the polite term for needing a piss. And yes, I had to look all of this up). I guess it is not so uncommon for humans to change within a few hours from a state of constantly needing to wee to having unquenchable thirst. Especially on Saturdays. Only that thirst and strangury (my new favourite word!) are the least of a fish’s problems – they ‘breathe’ the water they swim in. And they have to change their eye protection to ‘not get water in the eyes’! So really, the ability to migrate between oceans and streams is rather remarkable and there really isn’t a human equivalent; oxygen changes with altitude are probably the closest we can get. Basically, when a fish goes from one type of water to the other, it is the same as climbing Mount Everest with a hangover (or strangury. What a fantastic word!). Because of this, fish typically do what mountaineers do as well: they acclimate in zones of intermediate conditions so as to give their bodies time to make the necessary physiological changes.

For some fish, this is just part of their life – they are born in one eco-system, spend their adolescence in the other, and return to their birthplace for retirement (and reproduction). Like going to school in Cambridge (21% Oxygen), then joining Colorado Mountain College (14% Oxygen), and then moving to Florida (21% Oxygen – where else would one retire?). There are a myriad ways in which a fish’s body has to change in order to make these migrations – just look up ‘diadromy’ and you will be amazed! That is why scientists long thought that for these species ocean/stream migrations are a necessary part of development; like various plants that need to experience frost (or fire) in order to germinate. Basically, you have to climb Mount Everest on a hangover in order to call yourself an adult. However, there is increasing evidence that this might not necessarily be true: many species are flexible (biologists would say: plastic) with regard to their transitions between oceans and lakes/streams. Especially in New Zealand, many species that have previously been described as making obligatory migrations to/from oceans and streams have now been found to sustain landlocked populations. In other words, there are areas in New Zealand where the coming of age rite no longer involves climbing Mount Everest (or Aoraki/Mount Cook) with a (probably Sauvignon Blanc induced) hangover. How very progressive of these fish! It appears that these migrations really rather serve to be in the best possible environment at any given stage of life. And that some species prefer to send their kids to Cambridge and others favour Colorado Mountain College. Just as some people (and peoples) favour sweet popcorn over salted popcorn. So there you have it – next time you argue over which type of popcorn to get, think of fish. And then do the only clever thing: buy Riva & Arne’s Dolphin Tears!