bly related to themal requirements for matura- Life history observations
tion and early development, which generally
need temperatures below 20° C (e.g., Artyukhin We make seven general observations about acipen-
1988, Dettlaff et al. 1993). seriform life history and spawning biology:
- With the exception of Acipenser ruthenus, which 1. Acipenseriformes spawn repeatedly, but most
lives in both Europe and Asia, no species within females do not spawn annually. This pattern re-
Acipenseriformes is known to spawn in rivers on sembles that for anadromous fishes such as shad
two continents and few species spawn in more (Alosa; e.g., Leggett 1976) but is different from
than two of the biogeographic provinces that we that typical for Pacific salmonids (Orcorhyn-
define below. This situation is unlike that, for ex- chus; see Groot & Margolis 1991).
ample, for salimonids of the North Pacific Ocean, 2. All Acipenseriforms spawn in freshwaters of low
several of which spawn in both North American salt content (0–0.1‰) even though adults of
and Asian rivers (e.g., chum salmon.Oncorhyn- some species may migrate to feed in estuarine or
chus ketaspawns in rivers along the east and brackish waters (approximately 14 to 27‰,
north coast of Asia as well as the west coast of Pearse & Gunter 1957) or seawater (35‰).
America, Salo 1991). Our interpretation con- 3. The timing of spawning for Acipenseriformes is
cerning sturgeons and continents is subject to highly variable, equaling or exceeding the varia-
falsification, but all detailed work to date sug- bility round in any other group of fresh water or
gests that this pattern will hold true. For instance, diadromous fishes. They spawn in all seasons
Acipenser medirostris (west coast of North and in highly variable conditions of water flow
America) andA. mikarioi(Sea of Okhotsk and and temperature.
Sea of Japan) were at various times considered 4. Characteristics of spawning migrations vary
to be conspecific, but recent genetic and mole- greatly among Acipenseriformes in total dis-
cular data confirm that they are distinct species tance migrated, the distance upstream from salt
(Birstein 1993b, Birstein et al. 1997 this volume). water, etc. Several evolutionary scenarios and
- Much of the historic work concerning the distri- sets of terminology have been proposed to de-
bution of different species of sturgeons (e.g., scribe these variations in spawning migration
Berg 1948a, 1948b, 1959) predatescontemporary pattern (reviewed below).
concepts of continental drift. More recent ac- 5. The few studies done to date indicate that the
counts (e.g.,Berra1981,Hocutt& Wiley 1986, availability of suitable spawning habitat is crit-
Banarescu 1990, 1992, 1995) predate contempo- ical to reproductive success. Spawning sites are
rary phylogenetic interpretations of acipenseri- characterized byareas with hard substrate of
forms. If one restricts analysis to †Chondrostei- gravel to boulder size rocks containing many
dae, †Peipiaosteidae, Polyodontidae and the crevices. The water velocity near the bottom is
tribe Scaphirhynchini, all of which are small typically moderate (Kynard 1997 this volume).
groups with intriguing but fairly simple biogeo- 6. Annual spawning success and recruitment is
graphic distributions(Grande& Bemis 1991, Jin highly unpredictable, and may be zero if river
1995, Bemis et al. 1997b), then historical bioge- flows are too high during the brief reproductive
ography is easy to contemplate. The widely rang- window of females. High flows, whether caused
ing genusAcipenser, however, imposes many by natural phenomena or controlled releases by
difficult biogeographic questions, which is why it dams, can create high bottom velocities that pre-
is a focus in our present analysis. clude or greatly reduce spawning success (Ky-
nard 1997 this volume).
- Aparticular spawning site is usually used from
year to year. Such site fidelity might derive either
from the particular characteristics of the site or
from homing. Sturgeons are believed to have
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