Fish Migration - Classification

Classification

Classifications can be either fundamental (like biological classification that rests on a phylogenetic basis), or are merely heuristic typologies (as here, about migrations) to assist communication about complex issues. Classifications are judged according to their fundamental accuracy, whether they are convenient or not. Typologies in contrast are essentially arbitrary and their effectiveness is to be judged solely by the problems they solve or create. Secor and Kerr (2009) for example show several typologies that encapsulate various aspects of fish life history.

Migration is a word used in multiple senses. It is important to distinguish "true" migration, i.e. a life-history-structured or at least patterned activity such as seen in anadromous species like salmons, from mere movement or wandering as may happen, say, with euryhaline species that easily move between fresh and salt water but not necessarily or with regularity. "Vertical migration", for example, the phenomenon of plankton and fishes regularly changing their depth throughout the 24h day, is a special usage unlike migrations of (e.g. salmons) that range over distances in migrations that may cover river, lake, and sea, or the great migrations of game through Africa's Serengeti.

Anadromous and catadromous are words that have been commonly used for centuries. They are slightly more narrowly used in the following classification of migrating fish by Myers 1949:

"Diadromous. Truly migratory fishes which migrate between the sea and fresh water. There has been no English term by which one can refer collectively and briefly to anadromous, catadromous and other fishes which truly migrate between fresh and salt water, and this new term is now proposed ... . Like the two well known ones, this adjective is formed from classical Greek (, through; and, running). ...

Anadromous. Diadromous fishes which spend most of their lives in the sea and migrate to fresh water to breed (From, up ...). ...

Catadromous. Diadromous fishes which spend most of their lives in fresh water and migrate to the sea to breed. (From, down ...). ...

Amphidromous. Diadromous fishes whose migration from fresh water to the seas, or vice versa, is not for the purpose of breeding, but occurs regularly at some other stage of the life cycle. (From, around, on both sides ...) ... ]

Potamodromous. Truly migratory fishes whose migrations occur wholly within freshwater. (From, river ... ) ... This term and oceanodromous received the fishes excluded by the narrowing of anadromous and catadromous]

Oceanodromous. Truly migratory fishes which live and migrate wholly in the sea. (From, the ocean ... ) ... This term and potamodromous received the fishes excluded by the narrowing of anadromous and catadromous] ”

The "-ous" endings are for the adjectival form of the terms; nouns are obtained by replacing that ending with "-y", e.g. anadromy.

The terms anadromous and catadromous were of long standing (and similar but not identical usage); the other terms were coined by Myers. Myers was hesitant about introducing new terms however, saying:

"The writer's strong aversion to the infliction of new scientific terms upon the scientific public has, however, caused him to proceed with great circumspection. ... Finally, the writer has ventured to propose new terms only because he feels that certain new terms will be of distinct advantage in some specialized types of ichthyological work, and that the general biologist and fishery worker will seldom or never have to bother his head about them. Catadromous and anadromous will almost certainly always remain the most important and probably the only widely used terms of their class."(emphasis added)

Myers' term diadromous has proved useful as an inclusive term. But his prediction was otherwise accurate: anadromous and catadromous, are indeed the main widely understood terms. Secor and Kerr (2009) found only one paper that used oceanodromous, 18 using potamodromous, and 122 using amphidromous compared to 985 times for anadromous, 143 times for diadromous, 71 times for catadromous. Obviously the amount of work done on a group is one source of this variation, but another source is the perceived utility and standing of the term itself.

Amphidromy is rarely understood by the wider fish biology audience, or alternatively "There seems to be some reluctance to use the term amphidromy". Myers' definition's ambiguity ("or vice versa") and teleology ("for the purpose of") is one part of that problem. Its meaning/usage has been adjusted (but two such adjustments were found zero times by Secor and Kerr (2009)), and recently it has been stated as differing from anadromy simply because return to fresh water occurs at a juvenile or immature stage; other differences have been claimed (such as where most growth occurs) but they are necessary consequences of the stage at return and therefore are not independent. Some authors have therefore eschewed amphidromy in favour of the more widely understood terms: either anadromy, with or without a remark that return to rivers occurs at an earlier stage (e.g. "juvenile-return anadromy"), or diadromy which discards information that would be conveyed by anadromy with or without remark.

It has to be borne in mind that any typological system has heuristic value as a convenience for description, and need not reflect phylogeny: each category (term) may include representatives of many distantly related taxa, each of which may well have close relatives that are in another category (term) or that are not migratory and thus fall completely outside the typology. These terms are therefore of limited safety when used alone to screen data to be considered for an analysis, or to decide which species should be read about to explore a phenomenon or possible comparison.

The limitations of typologies were clearly stated by Myers himself. Each may be useful in one context, for one purpose, but not another. I.e. they may categorise along different axes (see Secor and Kerr (2009)). For example, Myers in the same year devised another interesting set of terms, also directed at fishes, but on the basis of their salt-tolerance.

As a footnote it is interesting to note that George S. Myers had previously been one of the degree supervisors for Porfirio Manacop, whose ground-breaking Master's work (in 1941, later published 1953) on a Sicyopterus species overturned the prevailing notion that it was catadromous. It is likely that Myers was impressed by this, and used Sicydium as the "type" genus for amphidromous.

And, as a contrary footnote showing how science fails: regrettably, Manacop's work seems to have been as locally unpopular as it was innovative, because over 20 years later the group he worked on was still being referred to by a colleague in the Philippines who would certainly have known of his work, incorrectly and without evidence, as catadromous.

And although these systems were originated for fishes, they are in principle applicable to any organism.