Ecology of North American Freshwater Fishes. Stephen T. Ross Ph. D.
The family includes both freshwater and anadromous species, has a Holarctic distribution, and includes some of the largest and longest-lived species of freshwater fishes (Berra 2001). In fact, the Beluga Sturgeon (Huso huso) of the Black and Caspian seas reaches 9 m and 1300 kg and may live for nearly 100 years (Berra 2001). North American sturgeons in the genus Acipenser also reach large sizes (> 2m TL) and may live 100 years or more (Sulak and Randall 2002). Anadromous North American forms (all in the genus Acipenser) are derived from freshwater forms and were one of the first groups of fishes to solve the physiological challenges of moving from fresh to salt water (Bemis and Kynard 1997; Sulak and Randall 2002).
CLUPEIDAE (10 SPECIES) Although this family is primarily marine, there are a number of freshwater species worldwide, all considered to be derived from marine forms. In North America, clupeids first appeared in the fossil record in the middle Paleocene (ca. 60 mya) in what is now Montana and apparently occupied western North America until the middle Eocene (Grande 1982). After the middle Eocene, freshwater clupeid fossils did not show up again in the fossil record for approximately 40 million years until the Pliocene/Pleistocene (Cavender 1986). The Plio/Pleistocene fossil was identifiable as a modern species—the Threadfin Shad (Dorosoma petenense) (Miller 1982).
ICTALURIDAE (46 SPECIES) Ictalurid catfishes date from the Paleogene and are restricted to North America. The earliest North American fossils referable to the Ictaluridae date from the late Paleocene (60 mya) (Cavender 1986; Lundberg 1992). The most complete specimens are of the extinct genus Astephus and are from Eocene lake deposits in the Green River Formation, a large system of lakes located in intermontane basins, in what is now Utah, Colorado, and Wyoming, that were formed by the uplift of the Rocky Mountains in the Tertiary (Grande 1984; Grande and Lundberg 1988). From the Paleocene through the late Eocene, the Green River system comprised one of the world’s largest and longest-lived Great Lakes systems (Grande 2001). Ictalurids likely originated in North America (Gilbert 1976; Burr and Mayden 1992; Lundberg 1992) and, although there were connections with Asia and Europe in the Cretaceous and Paleocene (Figs. 2.5C, D), this group has never been found outside of North America. Of the modern genera, Ictalurus, Ameiurus, and Trogloglanis occurred in the early Oligocene, and Pylodictis fossils are known from the middle Miocene (Lundberg 1992). The genus Noturus (madtom catfishes) is younger, with fossil material dating only from the early Pleistocene (1–2 mya) (Cavender 1986).
SALMONIDAE (38 SPECIES) The oldest fossil salmonids are of the extinct genus Eosalmo, found in Eocene deposits located in what is now British Columbia and northern Washington (Wilson and Williams 1992). Eosalmo apparently occupied Pacific drainages and, based on phylogenetic analysis, was basal to all other members of the Salmonidae (i.e., considered a stem group) (Wilson 1992). Although Gilbert (1976), among many others, considered the Salmonidae to likely have a marine origin, later work on salmonid phylogeny and ecology points to a freshwater origin for the group. All primitive salmonids are restricted to freshwater habitats, whereas derived groups all contain anadromous species (i.e., those spawning in fresh water and then moving to the sea to feed) (Stearley 1992). The fossil species Eosalmo provides additional support for the freshwater origin hypothesis because of the discovery of a large size series of specimens, ranging from young to adult fish, in the same lake deposit (Stearley 1992; Wilson and Williams 1992). The anadromous life cycle shown by some modern members of the family, such as Steelhead (Oncorhynchus mykiss), Pacific salmon species, and Atlantic Salmon (Salmo salar), is thus considered a secondary adaptation. Anadromy may have been triggered by increased seasonality caused by the cooling of the climate in the middle Cenozoic, such that the marine habitat offered greater productivity and more constant temperatures that would have favored increased growth in the marine compared to the freshwater habitats (Gross et al. 1988; Stearley 1992).
Based on the worldwide distribution of the family throughout northern Asia and also Europe (Berra 2001), and the presence of the stem-group fossils in western North America, the family likely evolved in the region of Laurasia that included western North America and perhaps northern Asia (Figure 2.5E). The modern species of Pacific trout (genus Oncorhynchus) likely originated in the Miocene and have had at least six million years of history (Stearley and Smith 1993). The Atlantic basin salmonids (genus Salmo) are primarily a European lineage, and the separation of the eastern Salmo and the western Oncorhynchus lineages likely occurred via a vicariant event across the northern coast of Asia in the Miocene (Stearley 1992).
CATOSTOMIDAE (71 SPECIES) Suckers represent the median in terms of age in North America (Figure 2.3). The oldest North American fossils, of the extinct genus Amyzon, date from the middle Eocene (ca. 49 mya). Amyzon is represented in various western fossil deposits, including the Green River Formation of Wyoming (Grande et al. 1982; Grande 1984; Cavender 1991). The habitat was likely swamp-like and included crocodiles and alligators (Grande et al. 1982). Like the modern subfamily Ictiobinae with which it is closely related, Amyzon was a fairly large-bodied fish. More derived species of suckers have tended toward smaller body sizes (G. R. Smith 1992). Catostomids occupied much of western North America and eastern Asia by the late Eocene, a time when the Asian and North America landmasses were connected via Beringia (Figure 2.5E) (Cavender 1986, 1991). Of the modern genera of suckers, Ictiobus occurred by the middle Miocene and Chasmistes by the late Miocene (Cavender 1986). Suckers are thought to have originated in Eurasia and then reached North America via the Pacific connection of Beringia (Figs. 2.4 and 2.5E) (Gilbert 1976; Briggs 1986; Burr and Mayden 1992; Berra 2001).
CENTRARCHIDAE (31 SPECIES) The centrarchids are endemic to North America and likely evolved there (Gilbert 1976; Burr and Mayden 1992). The earliest fossils of this primarily eastern North American family date from the Eocene epoch of northwestern Montana (ca. 45 mya) in drainages that flowed eastward from the continental divide (Cavender 1986). During this time, North America had separated from Europe but was still connected to Asia via Beringia (Figure 2.5E). Based on a fossilcalibrated molecular phylogeny, Near et al. (2005) estimated the age of the most recent common ancestor to the Centrarchidae to be 33.6 million years, providing another line of evidence supporting the Eocene age estimate for the group. Because the earliest fossils have not been linked to species, they could not be used in calibrating the molecular phylogeny. By the Miocene, modern genera including Lepomis, Micropterus, and Pomoxis were well represented and, especially by the early Pleistocene, centrarchids had become a dominant element in the North American freshwater fish fauna (G. R. Smith 1981). Ages of modern species, based on molecular phylogenies, are 8–11 million years for Micropterus, at least 11 million years for Pomoxis, and 14 million years for Lepomis (Near et al. 2003, 2005). Centrarchids also were widespread by the middle Miocene, based on fossils found west of the continental divide and including fossils of the extant western genus Archoplites (Cavender 1986).
CYPRINIDAE (297 SPECIES) The largest family of North American fishes has a Eurasian origin and likely reached North America via Beringia. The earliest fossil evidence in North America is from several Oligocene deposits in the northwestern United States in a region that in the middle Tertiary would have been near the western continental margin (Cavender 1986, 1991). By the late Miocene and Pliocene, cyprinids had taken their place as a major component of the North American fish fauna (Cavender 1986, 1991). In the New World, cyprinids are restricted to North America with no records, past or present, from South America. They are well represented both in lineage and species diversity throughout Europe, Asia, and sub-Saharan Africa (Howes 1991; Berra 2001). Although there are a variety of hypotheses of relationships within the Cyprinidae, there appear to be two main lineages (treated as subfamilies), the Leuciscinae and the Cyprininae (Cavender and Coburn 1992). North American minnows are all within the subfamily Leuciscinae, which is also well represented in Eurasia. Two phyletic groups are recognized within the subfamily Leuciscinae, the Phoxinini and the Leuciscini. The majority of North American minnows are phoxinins, with only the monotypic genus Notemigonus placed in the Leuciscini (Cavender 1991).
Cyprinids likely reached North America via Beringia during periods of lowered sea level that occurred coincident with a period of climatic cooling during the late Eocene to early Oligocene—a cooling event perhaps caused by changes in ocean currents related to the separation of Australia from Antarctica and the opening of a seaway between Greenland and Norway, allowing an exchange between North Atlantic and Arctic waters (Figs. 2.5D, E) (Cavender 1991). By the time that cyprinids reached North America, the Atlantic Ocean had filled the gap between the North American and European plates, precluding