dv said:
The Rev Dodgson said:
I’m reading the new version of Attenborough’s Life on Earth, and have just been reminded of the breeding cycle of salmon.
This raised the following questions:
1. What is the evolutionary advantage of swimming up difficult streams to breed, then dying?
2. What is the evolutionary advantage of returning to the place where you were conceived to breed?
3. How do people know that salmon use a combination of smell and magnetic fields to find their original breeding place?
4. Many other species breed in a different location to where they spend most of their lives, but are there any others that return to a specific location, and only breed once?
Firstly, I don’t know.
Secondly, most traits don’t have an advantage. Species just rittle into a local minimum which, viewed absolutely, can often be a terrible way of doing things.
But I can suggest some ways in which some of these things could be advantageous.
1/ There’s more food in the ocean, but high altitude streams might be a safer place for eggs and the young parr.
2/ Well at least you know that was suitable place for breeding once: seems like fair odds that it will be again.
3/ Here’s an article discussing the evidence that salmon use magnetism.
https://www.sciencedaily.com/releases/2013/02/130207131713.htm
4/ Dunno.
Yes. That makes sense. Then you also have to consider the opposite. What is the evolutionary advantage for all those fish species that don’t do what the salmon does?
One thing which has bogged me down in the past is the plethora of freshwater fish. Rivers are eventually going to dry up (eg. during an ice age or through desertification) and the drying up of even one river system is going to kill all the species that rely on that system. Further, river systems are extremely sensitive to pollution both natural (eg. tree and algal poisons) and artificial. And suppose a new river system forms, then almost all freshwater fish aren’t going to be able to migrate into that new river system.
From the point of view of all freshwater fish (and other freshwater species), it would be advantageous in the long term to be able to swim totally out of the mouth of that river system and up the coast to find a new river system, but few do. Because the environments are just too different. I know more about cetaceans that do than about fish that do. Among the cetaceans, literally the only two that manage this are the finless porpoise and the tucuxi. Those cetaceans confined to one river system, such as the baiji, are close to extinction. Among the fish, I only know of the bull shark, sawfish, European eel, and salmon, though there are probably many more.
OK, so from a fresh water fish perspective it makes evolutionary sense to be able to travel from river to river via the sea, though very few species manage it. It also makes evolutionary sense to aim for the least polluted part of the river, right up at the headwaters.
But why enter rivers at all, why not stay all your life at sea? There’s a very good reason – insects. There are a negligible number of insect species in the sea. So this major source of food is lost to all obligatory saltwater fish. Baby salmon can gorge themselves on insects at the right time of year, and the right time of year is the time when glaciers are melting, right up near the headwaters. Just think of the number of insects in a Siberian summer for example.
OK, so my logic has got me to the point where the ideal arrangement from an evolutionary point of view is to breed in river headwaters at the time of year when the glaciers are just starting to melt, and to exit those rivers for the open sea as you get larger. Ideal, except for the return journey, which is frought with danger. The sooner an adult salmon can get up to the river headwaters, the more chance the baby salmon have of getting fed by the burgeoning insect population there.
From a circle of life point of view, the drying adult salmon provide good food for the insects that their young will feed from.
Other fish have other survival strategies, but the salmon one is a good one.
> 4. Many other species breed in a different location to where they spend most of their lives, but are there any others that return to a specific location, and only breed once?
The European eel might, or the American eel, who do the salmon journey in reverse. I think the jury is still out on that one.
I haven’t yet seen any evidence to contradict the hypothesis that European and/or American eels only breed once. They certainly return to a specific location to breed, well distant from where they spend most of their lives.
As for the hazardous nature of the breeding journey, the hazards to European eels swimming to their breeding sites in the Sargasso Sea are easily as large as the hazards to breeding salmon swimming upriver. A large fraction of the eels fall prey to sharks.
