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January is mostly too cold to go down to the shore to explore the life in the pools, both for the rockpooler, who can always wrap up in some old winter wear, but more importantly it is also too cold for the seashore life around Britain. All of the rock pool fish and most of the crabs will be resident in the deeper, warmer water. Air temperature is the important factor.

An icy northerly wind can quickly cool the pools to near freezing point, and if the temperature of the pools falls to minus 1.87oC the pools will actually freeze. This would be very rare in the south but could conceivably occur in upper shore pools on Scottish coasts.

The average air temperature during January is about 2oC on the eastern English Channel coast and at about 1.5oC on the North Sea coast with annual fluctuations, but because of the lesser specific heat of air the temperature can vary much more quickly than the open sea. In Cornwall the mean air temperature in January has an average of 5oC, so the fish and crabs may still remain on the shore during this month. 

Common Starfish

Common Starfish, Asterias rubens. The Common Starfish is a species found in a shallow water from Arctic Norway where the temperatures may fall to 2oC, all around the British Isles, and as far south as Portugal where the temperature may reach 22oC. This is the temperature range in which this starfish can be kept.



The higher specific heat of water compared to air means that the sea will heat up and cool down much more slowly than the air, and during January the sea is likely to be warmer than the air. Although the sea temperatures at the surface vary by a few degrees each year the average sea temperatures at Plymouth in January is 9oC, at Brighton 8oC and at Newcastle only 5.5oC. However, this is higher than the average air temperature.

Northern species like the snake-like Butterfish or Gunnel, Pholis gunnellus, or on the east coast the Eelpout, Zoarces viviparous, may be found between the tides during January, There are other factors like the prevalent gales and rainfall that dilutes the pools that make the shore an inhospitable place for marine fish.


How does this effect the aquarist? Simply, every organism can only survive between certain temperature tolerances. These are quite often only a narrow range for marine organisms. Because the sea does not vary much in temperature they have not evolved to withstand great variations.

Alas, as far as the aquarist is concerned, the temperature in his aquarium will quickly equal the ambient air temperature unless a heater or cooler is installed.
Bullhead, Taurulus bubalis. This Arctic species reaches its most southerly point of the distribution on the coasts of the English Channel. Adult fish cannot be kept in temperatures exceeding 22oC



Tropical aquarists rarely have any problem with temperature because the retailer should know the optimum temperature in which the fish can live. The variations in the water temperature in the tropics are much smaller.

Aquarists keeping British marine life have to resort to research. This is undertaken in two stages. The first is to find out the distribution range of the fish or invertebrate in a high quality identification guide. The second stage is to find out the minimum and maximum sea temperatures in the sea at the two edges of its distribution range, and then calculate the range in which the fish can live.


Even within its natural tolerance fish and other invertebrates are unable to survive rapid and large variations, over 2oC, in a short period of time. This is likely to occur when an aquarist is transporting fish home and putting them in a tank which is significantly warmer or colder than the bag or bucket in which the fish is being carried. There are just one or two exceptions like the rock pool fish called blennies which can withstand a 5oC sudden fall. This may occur in the wild when the tide comes in and refreshes the pools in which they were living.


Two variables are important to the aquarist:


Sea water becomes saturated with dissolved oxygen and as the temperature increases the amount it can hold decreases according to the following table: 

Salinity 3.4 per cent (North Sea) Maximum levels of dissolved oxygen at different temperatures.
oC oF 100% 70% O2 (dissolved oxygen mg/l)
28 82 6.4 4.5% = saturation level
26 79 6.6 4.6
24 75 6.9 4.8
22 72 7.1 5.0
20 68 7.4 5.1
18 64 7.6 5.3
16 61 7.9 5.5
14 57 8.3 5.8
12 54 8.6 6.0
10 50 9.0 6.3
8 46 9.4 6.6

Higher amounts of dissolved oxygen are found in freshwater.

If the gas is not renewed from the atmosphere as quickly as it used up the fish and other animals will suffocate. Stocking levels need to be lower at higher temperatures.


The specific gravity of the water decreases with increasing temperature when the salinity remains constant. The denser water will show a higher reading on the hydrometer according to the following scale: 

Specific gravity of seawater with a salinity of 3.4 per cent
6o - 8oC = 1.027
9o - 13oC = 1.026
14o - 18oC = 1.025
19o - 21oC = 1.024
22o - 25oC = 1.023
26o - 28oC = 1.022
29o - 30oC = 1.021

The ability of fish to tolerate changes in salinity varies with different species. Invertebrates, especially sea anemones, will not thrive in reduced salinities. As a very rough guide a specific gravity reading of 0.001 corresponds approximately to a 0.1 per cent alteration in salinity, and this variation is the tolerance limit. Water with a salinity of less than 3.0 per cent is defined as brackish; and in excess of 3.7 per cent as metahaline, and likely to be found only in enclosed lagoons.

Beadlet Anemone, Actinia equina. This anemone is found on the eastern coast of the Atlantic Ocean from the polar circle to the equator. It has been suggested that there are different ecotypes based on temerature amplitude, but all anemones collected in the English Channel will survive between freezing point up to 28oC.



Surface seawater temperatures:
Temperature range oC
Arctic Ocean -2 to 5
North Scotland 5 to l3
Newcastle 6 to 14
English Channel 7 to l7
North Spain  11 to 20
Gibraltar 14 to 21
Cyprus 18 to 27
Hawaii 20 to 22
Caribbean Sea 23 to 25
Great Barrier Reef 23 to 26
Indian Ocean 24 to 28
Philippines 26 to 28
Persian Gulf 20 to 30
Shallow Persian Gulf to 35

These temperatures are taken at the surface and cover and in some cases cover a wide range of ocean. To ascertain the precise requirements of a particular fish more accurate measurements may be necessary. The sun only warms the oceans down to depth of 550 metres. As the average depth of the sea is 3,729 metres, it follows that the great depths are only kept warm by water circulation and mixing. The abyssal depths are above the vast ocean plains that cover the ocean floor and vary in temperature between 1oC and 5oC. It is the warmer, shallow waters close to land over the Continental Shelf that contain most of the marine life.

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