What makes sea food have similar smell? I never ate fish, I always disliked the smell and had a little bit of trauma with my father forcing me to eat it while I was a kid. I had two experiences:


Some years ago, I tried sushi. It doesn't have a strong smell of fish and I found it edible. But there is a very faint smell of fish.
Some days ago, I bought some chips made of Nori algae. They were vegan chips and yet, I felt the same soft smell of fish.


I conjectured that perhaps, part of this smell is common to all things in the sea. Lately, I decided to do indoor farming and when studying about fertilizers, I heard someone saying that "certain fertilizers with phosphorus have a smell of fish and that the sea is rich in phosphorus" so, this seems to be my first guess. I understand that my question is a bit vague.
The American Society for Nutrition explains it thus:

The answer has to do with some interesting physiology unique to sea
creatures.Water in the open ocean is about 3% salt by weight, but the
optimal levels of dissolved minerals inside an animal cell is less
than 1%.In order to maintain fluid balance, ocean creatures must fill
their cells with amino acids and amines to counter the saltiness of
seawater. Ocean fish tend to rely on trimethylamine oxide (TMAO) for
this purpose.


Now this compound does not in itself have a particularly pungent odour, however:


The problem is that when fish are killed, bacteria and fish enzymes
convert TMAO into trimethylamine (TMA), which gives off the
characteristic “fishy” odor.This smell can be reduced in two ways.TMA
on the surface of the fish can be rinsed off with tap water.Treating
the fish with acidic ingredients such as lemon, vinegar, or tomato can
also cause TMA to bind to water and become less volatile.Thus the odor
compounds do not reach the nose.

This trimethylamine is something that is produced in small quantities by our own gut microbes, and usually is no trouble when kept under control - some however are unfortunatley prone to &quot;Fish Odour Syndrome&quot;.

I am not able to find any definitive evidence for the reason that nori smells of fish, however, an uncorroborated suggestion behind the scenes on a Wikipedia wiki page states this:


Please. Fish smells of algae, not the other way around! For the same
reason DHAs are not &quot;fish oils&quot;, but &quot;algae oils&quot;.--Rfsmit

Bolding Mine

As they are so ubiquitous in nature it would seem likely that the bacteria that convert the TMAO into TMA and make the smell would be present on the algae as well as in the fish and in our own guts.

Conclusion.

I'd claim that the bacteria and what it eats and it's waste products are to blame for the similar smell: it would seem to be the case that Japanese Nori (There mis-classified as genus Porphyra) contains amino acids and nutrients not inconsistent with the culprit bacteria growing on it (As it also like fish flesh). As I say I can not offer anything definitive.