Differences and similarities of a fjord and an estuary?

Both firth and fjord are now generally defined as an arm of the sea. The difference is crucial - a firth (or fjord) is made up of undiluted sea water, but an estuary has a gradation of salt water dilution from fully salt water at its mouth to fresh water at its head, and experiences tidal fluctuations

the answer to your question is tooth decay.

it has to be understood that the sea water contains a huge amount of sodium and chlorine ions and if they enter the cell of the atlantic cod fish, the cell will shrivel and the fish will die. to avoid such a condition, the fish uses active transport for the purpose of removing the ions against the gradient.

explanation:

the atlantic cod is not a maritime osmoconformer. its native application of salts is less than that of the seawater it lives in. it is continually falling water from the gills to the ocean via osmosis.

countering that, the fish is going to be swallowing seawater. but since its internal collection of salts is lower than seawater, the fish will want to go free of excess salt, and since that's going upon a gradient, then active transport, requiring energy, must be used to pump out the salt. much marine fish use the organs to filter out the salt and these fish discharge very straight urine.