Sudden changes as we all know need to be avoided where fish are concerned. Rapid changes of pH, salinity and temperature are all bad for fish aren't they? Well no! or at least not for all fish. Let me explain.
What could be more ideal for a small fish than your own little almost private pool away from large predatory fish, an ample supply of food and twice daily 100% water changes? Rock pools offer exactly this, but are they really such an idyllic place to live? The answer is no not really, in fact rock pools are a very harsh place to eke out a living. They are subjected to large swings of temperature and salinity due to evaporation, rain and the action of the sun not to mention sea birds often look there for small fish which are relatively exposed with little chance of escape.
Rockpool fish are exposed to some very large swings in conditions, whilst on holiday recently we realised that some rock pools near the top end of the tidal zone were left baking under the sun for many hours, some of these pools were quite small and shallow and the rock was quite dark in colour which magnified the effect of the sun. We set about making a few measurements in pools which contained a number of fish and the results were quite surprising.
Even more surprising when compared to the nearby sea which due to it's size didn't really alter much what ever the weather.
The fish which inhabit the area between low and high tide must have become adapted to be able to tolerate these extreme sudden changes which would kill some other fish, particularly marine fish which aren't noted for their ability to withstand sudden change, almost instantly yet these little fish have to put up with them twice daily and remain fit enough to be able to cope with all the other pressures of staying alive such as predation, defence of territory and feeding, in other words they have to be able to carry on as normal despite these sudden and dramatic changes. To be left stunned or disabled for even a few moments could spell disaster for an individual and even for a species.
Some of the larger pools closest to the sea under went the least amount of change, just one or two degrees Fahrenheit and this is where the majority of the fish were found. but further up the tidal zone and still containing some fish we noted some pools were 70F yet the sea was a constant 58F this means that when the tide returns to fill that pool the fish in there will experience a 12F drop in temp - instantly, but remain unharmed. And this was in April so the change will be even greater in July and August.
As well as the action of the sun there is also rain. A sudden summer downpour will not only have a cooling effect but in small pools particularly it could have an effect on salinity. We found one area of a beach where there was a land drain emptying constantly which made a small stream run between some rocks. The water from the drain was obviously fresh water and it was running into some of the pools where we found quite a few species of fish such as - Sand Gobies, Common Gobies, Elvers, Shanny's, Worm Pipefish and Scorpion fish. When we measured the salinity using a refractometer and found that the sea was 1.027 and the water where we found the fish was an astounding 1.008 yet the fish were all well and full of vigour just as you would expect them to be. Again when the tide returns to cover this area there will be an instant change back to the conditions found in the ocean which apparently does no harm to the fish. We always found this area to be quite rich in numbers and diversity of fish and we made an assumption that if these fish were harmed by the changes then the area would have far fewer fish.
We also tested the pH in a number of locations and found it to be quite constantly within 0.1 of the ocean so perhaps these fish may not cope quite so well with sudden pH changes as they do with SG and temp, we would have no way to test this without risking harming some fish so we didn't bother.