The Mysterious Sunfish Strandings

stranded sunfish

Stranded Sunfish: photo credit Rob Lucking

(Apologies for the depressing start, cheery sunfish cartoon waiting at the end…)

Recent photos on twitter have shown a group of small immature ocean sunfish dead on a beach in Norfolk and many people have been asking the same question:

-What causes sunfish and other marine creatures to strand themselves on beaches?!

Well… stranding is where marine animals including dolphins, turtles and fish swim or float close to land and become beached or marooned on the shoreline.  Interestingly there seems to be no primary conclusion as to why such a wide variety of species would become beached, however several key theories have been suggested:

-Accidental stranding. Where dead, injured or dying animals are unable to prevent themselves from being stranded by onshore wind and wave action.

-Single stranding. Where one animal becomes beached, perhaps due to disorientation, inexperience or illness.

-Mass stranding. Only highly social cetaceans appear to do this, where one animal perhaps to due illness or any of the above reasons, becomes beached and other members of the group (who may be perfectly healthy) will remain with the stranded individual and often become beached with them. It appears that this unusual behaviour may also be related to predator evasion, sonar disturbances and geomagnetic anomalies.

When it comes to ocean sunfish, recorded strandings in many areas appear to increase over autumn and winter months and this has been attributed to cooling water temperatures. Like most fish, ocean sunfish are classed as cold-blooded or ectotherms, which means that their internal body temperature changes with that of their local environment.

This means that most ectotherms must live within environments that suit their optimal temperature range to avoid hypothermia and potential death. The optimal temperatures for ocean sunfish has been suggested to be between 12-25°C (although of course this is not a definitive range!)

So when apparently healthy ocean sunfish strand, as occurred in Norfolk in December 2014, it is often suggested that they had been swimming within warmer water patches, perhaps eddies that detached from the Gulf Stream, and once they came into contact with colder waters, such as the North Sea, this lead to ‘cold-shock’, stranding and unfortunately death.

It is interesting to note that some fish species are actually endothermic or warm-blooded! For example, many shark species including the Great White (Carcharodon carcharias), Porbeagle (Lamna nasus) and Common Thresher (Alopias vulpinus) have evolved the ability to keep their internal body temperature as much as 13°C warmer than the surrounding water! These species have evolved the ability to capture and retain much of heat generated by muscular activity by having enlarged blood vessels alongside the thick flank muscles. These then enable warmed, deoxygenated blood to run through dense vein systems travelling in parallel to newly oxygenated but cold blood travelling from the gills in arteries. This system enables the heat to transfer from the veins to the arteries and so spreading warmer blood around the animal. Clever stuff!

Sadly not all fish have evolved this ingenious system and so to be safe the ectothermic ocean sunfish must remain within its’ temperature boundaries!

sunfish holiday (1)
As always if you happen to be on the beach and spot a stranded sunfish, please get in touch!

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