Lake sturgeon (Acipenser fulvescens) COSEWIC assessment and status report: chapter 8
Biology
Several good descriptions of the morphology of lake sturgeon are found in Harkness and Dymond (1961), Houston (1987) and Scott and Crossman (1998). The original populations of lake sturgeon had some very large individuals, and specimens weighing in excess of 100 kg were not uncommon in the early 20th century. Female lake sturgeon are usually larger than males (Harkness and Dymond 1961; Mosindy and Rusak 1991), but if females are heavily exploited, males may become larger (Bruch, pers. comm.). A downward trend in size and age of both sexes is evident wherever lake sturgeon are harvested (e.g. Dumont et al. 1987; Patalas 1988; Sopuck 1987; Wallace 1991).
Since eggs take several years to develop in lake sturgeon, there is a difference between the age or size at maturity (with eggs under development) and age at first spawning (actual depositon of eggs – sexual maturity). Age at (sexual) maturity of lake sturgeon varies. Sexual maturity in females occurs between 14 to 33 years of age, but more often at 21 to 26 years of age, while in males, it occurs at age 8 to 12 years, but may take up to 22 years (U.S Fish and Wildlife Service (2006). Harkness and Dymond (1961) reported age at sexual maturity for males ranged from 14 to 22 years and females from 14 to 33 years. Studies using a gonad index at Cumberland House, Saskatchewan, found that females begin to spawn somewhere between 26 to 30 years of age (Dick, unpubl. data). Wallace (1991) noted similar results in the Saskatchewan River, with females maturing around 25 years of age at weights of 13.6 kg and lengths of 1.3 m, and males maturing earlier at smaller sizes. In lakes Saint-Pierre and Saint-Louis, the female median size and age at maturity are 1.3 m and 27 years, respectively (Goyette et al. 1987; Guénette et al. 1992). Similar ages of maturity were noted for females in the Timiskaming area (D. Nadeau, Regional Biologist, Quebec Wildlife and Parks, Rouyn-Noranda, QC; personal communication). Harkness (1923) tabulated weight/length at age data for fish from Lake Nipigon where individuals at the age of maturity were about 0.8-1 m in length and weighed around 4.5 to 5.5 kg. The mean weight of 1 m sturgeon in Rainy Lake was recently estimated to be 5.97 kg (Adams et al. 2006), comparable to 6.1 kg for Lake of the Woods/Rainy River (Fortin et al. 1996).
Houston (1987: Table 1) provided approximate age and size of sexual maturation for six populations in Quebec, Ontario, Manitoba and Wisconsin. Mosindy and Rusak (1991) used a maturity index based on mean weighted age at onset of sexual maturity for male and female spawners and found that mature males (average age 16.8 years) spawn every 2.2 years and mature females (average age 25.8 years) spawn every 3.75 years in Lake of the Woods. Wallace (1991) noted that females spawn every 4 to 8 years and males every 2 years in the Saskatchewan River. In the St. Lawrence River, spawning periodicity is estimated at 1 to 3 years for males and is likely higher than 4 years for females (Fortin et al. 2002).
Lake sturgeon congregate on, or close to, the spawning site at temperatures of 7-10°C, and spawn during May and early June at 9-18°C (Scott and Crossman 1998; Fortin et al. 2002; Erickson, pers. comm.). Males arrive first, and individual spawning females have been observed surrounded by 2-8 males in fast waters near shore (Dick, unpubl. data; Bruch and Binkowski 2002). During the spawning act, the males respond to cues from the female to participate in 2-4 spawning bouts during which a relatively small number of eggs (947-1444) are released into a cloud of sperm (Bruch and Binkowski 2002). The total number of eggs varies from ~50 000 in a 5.2 kg female to over 1 000 000 in very large fish (Scott and Crossman 1998). Harkness and Dymond (1961) reported that two 50 kg females averaged ~650 000 eggs each, roughly 9 000 to 15 000 eggs per kg of body weight (U.S. Fish and Wildlife Service 2006). Fecundity has been measured for 18 females caught in Quebec from the 1940s to the 1970s. Fecundity varied between 48 420 to 670 450 eggs for females between 0.9 and 2.0 m. It is related to length (Log10 F=3.70214 Log10 TL (in cm) – 2.62905; R2 =0,90) and weight (F=11921.2 + 13079.6 W (in kg); R2=0.94) (Cuerrier 1966, Fortin et al. 1992, 2002). According to these relations an average size mature female of 1.3 m on the Des Prairies River spawning ground produces 170 000 eggs. Preliminary observation in the Des Prairie River indicated that, to maximize egg to larvae survival, the average female requires between 13 to 48 m2 of spawning area (Fortin et al. 2002).
MacDonnell (1998) and Barth and MacDonnell (1999) observed spawning in the Weir River in 1997 and 1998. In 1997 spawning occurred from 14 - 24 June at water temperatures of 11 – 17°C. Eggs were collected downstream of the rapids where the sturgeon were believed to have spawned until 25 June and larvae were first noted on 24 June (MacDonnell 1998). In 1998 spawning occurred between 8 May and 12 June at water temperatures of 16.5 – 17°C. A decrease in water temperature between 28 May and 5 June delayed spawning (Barth and MacDonnell 1999). Eggs were captured below the rapids where the sturgeon were believed to have spawned from
9 – 16 June and larvae were captured from 14 – 26 June.
In the Des Prairies River, between 1982 and 1999, spawning activity occurred during the second to the fourth week of May at temperatures varying between 12 and 17° C and lasted from 8 to 19 days (Fortin et al. 2002). Generally two peaks of spawning activity (as measured by catch per unit of effort of spawners and egg deposition) were observed. This bimodality is not related to water temperature or to river discharge and recapture data from multiple years suggest some fidelity of lake sturgeon to the first or second spawning period. Spawners of the first peak are generally larger than those of the second peak (Fortin et al. 2002).
In the St. Lawrence River mainstem spawning is delayed by about 2 weeks, as the water warms more slowly than in its tributaries (LaHaye et al. 2004).
Egg incubation requires 7-10 days in water temperatures of 13-15°C; after hatch, the negatively buoyant larvae move relatively little in the water column (U.S. fish and Wildlife Service 2006). Upon absorption of the yolk sac, loss of the intestinal plug and elongation of the snout, the larvae begin to feed (Dick 1995). They remain negatively buoyant until the swim bladder starts to form about 60 days post-hatch (Dick 1995).
The sex ratio is more or less 1:1 at birth but, following maturation, begins to widen in favour of females (Mosindy and Rusak 1991; Fortin et al. 1993). This may be explained by the greater longevity of females (Probst and Cooper 1954; Dumont et al. 1987). Fortinet al. (1993) found that by the age of maturation (20-29 years) in southern Quebec, the female:male ratio was in the order of 2:1 and by age 40, it was 6:1. Mosindy and Rusak (1991) found a similar increase in Lake of the Woods.
The young-of-the-year (YOY) grow rapidly reaching 15-20 cm by the end of the first summer. Juvenile lake sturgeon grow more rapidly in length than in weight during the first five years of life (Scott and Crossman 1998). It has been observed that the body shape of wild and cultured lake sturgeon becomes deeper with faster weight gains relative to increases in length (Dick 2004). After about 5 years, the rate of weight gain usually increases and the rate of gain in length decreases: the two rates become more uniform after the onset of sexual maturity when weight gain becomes even more rapid relative to increase in length (Scott and Crossman 1998). Royer et al. (1968) found that the growth of sturgeon in the Saskatchewan River generally was faster than that reported for Quebec and Ontario, but slower than in Wisconsin.
Power and McKinley (1997) demonstrated that there is a latitudinal gradient in growth rates with declines in weight and length at age at higher latitudes, but the difference is inverse when adjusted for thermal opportunity (total degree-days > 5°C, i.e., number of days in the year where the temperature exceeds 5°C). This counter-gradient partially compensates for reduced growth opportunities in more northern populations and may be genetic, selecting to reduce size-selective, over-wintering mortality (Power and McKinley 1997).
Fortin et al. (1996) studied the determinants of variations of growth in length and body condition of lake sturgeon on 32 lakes and river systems covering most of the distribution of the species. Growth decreased with mean annual air temperature and latitude but was generally faster in the western part of the distribution area. Condition decreased with latitude in the east but remained relatively stable in the west.
Little is known regarding the natural mortality of juveniles or larger lake sturgeon. Wallace (1991) indicated that the total annual mortality rate in the lower Saskatchewan River was in the order of 4.8% around 1963 and increased to 18.9% by 1980, while annual recruitment was in the order of 3.5%. Adams et al. (2006) calculated the total annual mortality for lake sturgeon in Rainy Lake from 1965 to 1984 to be 4.7%, for ages 18 – 39. Throughout this period a commercial harvest on the U.S. side of the lake was in operaton and harvest levels declined from 1007 kg in 1964 to zero from 1974 to 1978, but then averaged 345 kg annually from 1979 to 1990. Most researchers agree that the maximum mortality rate should not exceed 5%, since recruitment in the few self-sustaining populations is in the order of 4.7 to 5.4% (Sunde 1961; Priegel 1973; Priegel and Wirth 1975; Baker 1982). MacDonnell (1998) indicated that tagging studies in the Nelson River in 1997 suggest a mortality rate of 3.5% and expressed a concern regarding the sustainability of the harvest in the domestic fishery.
Historically, the typical lifespan of lake sturgeon was believed to be in the neighbourhood of 55 years for males and 80 to 150 years for females (U.S. Fish and Wildlife Service 2006). However, in exploited populations, most sexually mature sturgeon seen today are less than 2 m in length and 36 kg in weight, although a few larger, and fewer still, very large sturgeon, are occasionally reported (Dumont et al. 1987; Scott and Crossman 1998;). Studies on natural populations of lake sturgeon in western Canada (Choudhury and Dick 1993) and elsewhere (Magnin 1966) found that females live considerably longer than males, average lifespans being in the order of 55 years for males and 80 years for females, although older fish have been collected. In the past 25 years, maximum age and weight observed in the St. Lawrence River commercial harvest were 96 years and 90 kg.
There is no evidence for reproductive senescence; for example, the largest and perhaps the oldest fish ever caught in Manitoba was estimated to be 150 years old, measured 4.6 m, weighed 184.6 kg, and was “full of caviar” (Stewart and Watkinson 2004: 46; Figure 14).
Figure 14: A 184.6 kg Female Lake Sturgeon Measuring 4.6 m, Caught in the Roseau River, MB in 1903
The age of the fish was estimated to be 150 years, and it was reported to be full of roe. Original photograph by George Barraclough, now in the collection of the Franklin Museum, Dominion City.
Given the mean ages at which males and females reach the onset of sexual maturity as stated above (16.8 years of age for males and 25.8 years for females), and an average life span of 55 to 80 years, the natural average generation time (average age of parents of a cohort) would be in the order of 35 to 54 years. The average age of fish in unexploited populations may have been even greater since there are records of fish of up to 150 years of age (Scott and Crossman 1998; Stewart and Watkinson 2004). The average today would appear to be more in the range of 25 to 50 years giving an average generation time in the range of 26 to 30 years (see Fortin et al. 1996; Scott and Crossman 1998). The average age of fish from the study of Adams et al. (2006) in Rainy Lake would be in the order of 19 to 24 years and the oldest fish captured was 59 years old. Generation times at, or near, the mean age of the onset of sexual maturity are indicative of stress.
Plohman et al. (2001a,b) compared plasma samples from wild caught lake sturgeon to plasma and other tissues from lake sturgeon reared under controlled conditions and fed diets of different nutrient quality. These studies found that nutritionally poor diets decreased the levels of thyroid hormone and reduced growth. This suggests that environmental modifications through changes in habitat could alter the type and quantity of food available and, consequently, have a detrimental effect on sturgeon survival.
Lake sturgeon are adapted to water temperatures ranging from near 0 to 24ºC. Laboratory studies indicate that cultured juvenile and young adults continue to grow at temperatures of 4-6°C, and that growth continues even when temperatures change by as much as 8°C (in a range of 12 - 20°C), over periods as short as 24 hrs (Dick et al. 2002). Cultured fry and larval lake sturgeon grow rapidly at temperatures up to 25°C, but juvenile lake sturgeon grow most rapidly at 22°C. However, there is a tendency for buoyancy disorders (i.e. floating and rolling onto their dorsal surface) at higher temperatures (≥ 22°C). Lake sturgeon reared at 15 - 19°C did not exhibit this disorder. Optimal survival was observed between 14-17ºC and incipient mortality at 20ºC (Wang et al. 1985).
Traditional knowledge of Elders and observations of lake sturgeon feeding in shallow areas during the summer months indicate that they can utilize habitats with suboptimal levels of oxygen, at least for feeding (Dick, unpubl. data). Lake sturgeon have been known to survive several hours out of water in the bottom of a boat (Mackay, pers. comm.).
In laboratory studies, lake sturgeon juveniles grew normally at 12 ppt salt water for 3 months and 18 ppt for 2 weeks (Dick, unpubl. data). Lake sturgeon have been reported from the estuaries of the Nelson, Gods and Hayes rivers (D. Macdonald, Manitoba Conservation, Thompson, MB; personal communication) and occasionally enter the brackish waters of Hudson Bay and Gulf of St. Lawrence (Page and Burr 1991; LeBreton and Beamish 1998).
Seasonal movements are not well known, but lake sturgeon probably move from shallower to deeper waters in warmer temperatures, returning to the shallows when temperatures decline in the fall. Other than that, movement appears to be limited, with the exception of spawning migrations (Fortin et al. 1993). Spawning migrations of over 100 km have been recorded (Scott and Crossman 1998). However, there is thought to be strong site fidelity with spawning fish returning to the same sites year after year, although the occasional fish may wander from lake to lake to spawn (Swanson et al. 1991; Rusak and Mosindy 1997). In the Moose River, juveniles and adults appear to occupy the same areas and there is no evidence of juvenile dispersion (Seyler 1997a). Radio telemetry and tag-recapture date from the Weir River indicate that lake sturgeon congregate near spawning sites in late spring in relation to water temperature and flow regimes and disperse throughout the Nelson River system, moving as far upstream as dams and power generating facilities will allow (MacDonnell 1998). Lake sturgeon from the Hayes River may also utilize the Weir River for spawning (Barth and MacDonnell 1999).
Barth and Murray (2005), based on studies in the Nelson River from 2001 – 2003, suggested that in spring, lake sturgeon (both spawners and non-spawners) are attracted to areas of higher water velocity and spawning fish move upstream to potential spawning areas. Some non-spawners may accompany spawners, but usually remain down river. The lake sturgeon move downstream to deeper, lower water velocity habitats for the summer. Relocation data (Barth and Murray 2005) suggest that lake sturgeon make frequent localized movements (1 – 20 km) during the summer in association with feeding, and in the fall, move to areas of deep, medium-velocity habitats to overwinter. Results of the four-year study also indicated that most lake sturgeon exhibit site fidelity; although they frequently make localized movements, they demonstrated a preference for certain areas (Barth and Ambrose 2006). Borkholder et al. (2002) and Knights et al. (2002) have documented similar findings.
In the Lake Winnebago system larval lake sturgeon drift downstream 9 to 30 days post-hatch (Kempinger 1988) and lake sturgeon fry have been located 40 km downstream of hatch sites (Seyler 1997a). In the Des Prairie River, larval drift generally occurs at night and lasts 14 to 30 days, between the third week of May and the third week of June (La Haye et al. 1992, 2004; D’Amours et al. 2001; Fortin et al. 2002; Garceau and Bilodeau 2004).
Radio and sonar tagging studies indicate that lake sturgeon of earlier life stages do not move as far as that reported for some larger, older individuals (Mosindy and Rusak 1991; Swanson et al. 1991; Smith and King 2005; Benson et al. 2005). Seyler (1997a) found that young lake sturgeon move little over a year. This also was evident from studies in Round Lake, a small lake on the Pigeon River, Manitoba, where tagged sturgeon did not leave the lake, and near Numao Lake on the Winnipeg River (Dick 2004). Populations upstream and downstream from Sipiwesk Lake on the Nelson River did not seem to mix as tagged fish showed little movement between the regions (Macdonald, pers. comm.). Rusak and Mosindy (1997) observed segregation of lake and river populations based on the commencement of spawning migrations and winter habitat preferences in the absence of physical barriers in the Lake of the Woods.
In the Quebec part of the St. Lawrence River, mark-recapture experiments indicate that, with the exception of spawning migrations which are extensive, movements are restricted (Magnin and Beaulieu 1960; Dumont et al. 1987; Fortin et al. 1993). In this system, lake sturgeon occur in large numbers in small localized sites, and this condition increases their vulnerability to fishing and to any degradation of these local habitats (filling, dredging, toxic outflows, etc.). Some lake sturgeon seem to form stable groups; for example, on at least three occasions, pairs of fish tagged simultaneously were recaptured together (Dumont et al. 1987).
Size and age distribution of juveniles in the experimental samples (mostly age 2 to 8) and of subadults in the commercial harvest samples suggests that, in the Quebec part of the St. Lawrence River system, lake sturgeon are mainly produced in the upper part of the system. Larvae drift downstream of the main spawning area of the Des Priaires River towards the lower part and gradually colonize the river along a downstream-upstream gradient. Most of the juvenile concentrations are found in the lower part, between the fresh water of the Lac Saint-Pierre archipelago and the estuarine brackish waters near Iles d’Orléans (Dumont et al. 2000a). In the upper section, in Lac Saint-Louis, males and females are longer, heavier and older and almost half of the females (45%) are maturing. In the most downstream commercial fishing sector, lake sturgeon are smaller, lighter and younger and only 2.4% of the females are maturing. Intermediate values are observed in the two median fishing sectors (Dumont et al. 1997; Dumont et al. 2006). These observations suggest that most individuals in a population may be more localized than previously thought, and that there is some natural separation of populations in the absence of physical barriers. They also suggest use of different parts of the river by different life stages.
During the major growth period, adult and juvenile lake sturgeon generally are thought to frequent shallow areas of lakes and rivers 4.6 - 9.2 m in depth (Scott and Crossman 1998). However, Choudhury et al. (1995) suggested that juveniles may stay in the river for several years, especially in systems where adults migrate substantial distances to spawn. In contrast, there was no evidence of migration in Round Lake (on the Pigeon River in Manitoba) and, while both adults and juveniles frequented the same areas of the lake, the larger fish utilized more of the lake (Dick 2004).
Larval lake sturgeon feed on invertebrates about 400 to 500 μm in size, moving to larger invertebrates as they grow (Dick et al. 2002). Thibodeau (1997) examined the gut contents of 797 larvae (<20 mm) caught in drift nets in Des Prairies River. Only 18 had begun exogenous feeding, but prey could be observed in only six of them, consisting of small chironomid larvae and amphipods. Very little is known about the food, growth and habitat of YOY during summer in the St. Lawrence system. The upper estuary is the only site where summer and fall YOY lake sturgeon (120-180 mm TL from July to September) have been captured in significant numbers in the St. Lawrence River. Nilo et al (2006) and Guilbard (2002) observed that they feed mostly on amphipods and chironomid larvae.
In the St. Lawrence River, diet of juvenile lake sturgeon is highly diversified and composed of at least 75 taxa, of which more than 50 were found to comprise more than 5% of the diet in individual fish. This probably reflects both the high diversity and density of the benthic fauna of this system [~2400 g/m2 compared for example to < 100 g/m2 in Northern Ontario watersheds (Beamish et al. 1998; Nilo et al. 2006)]. The most exploited prey are amphipods, aquatic insect larvae, molluscs and oligochaetes. Fish and microcrustaceans are also eaten, but in much smaller proportions (Mongeau et al. 1982; Nilo et al. 2006). Prey composition varied according to sampling site and period, and to fish size. Diet composition is only partly determined by benthos (molluscs, oligochaetes, burrowing insects larvae) availability, suggesting that there is a positive selection for drifting prey (amphipods and ephemeropteran nymphs). The tendancy of juvenile lake sturgeon to aggregate locally (Dumont et al. 1987; Nilo 1996) cannot be fully explained by their food habits, which include prey species that are widely distributed throughout the St. Lawrence River. Other factors certainly affect the juvenile lake sturgeon distribution in this river. In the upper estuary, Atlantic sturgeon and lake sturgeon use the same major prey in different proportions (Guilbard. 2002). A comparative morphometric study of the digestive tract suggests that the thicker gizzard wall of the lake sturgeon facilitates access to hard preys, while for Atlantic sturgeon a longer intestine and a higher development of the spiral valve favour chemical digestion (Guilbard 2002). Zebra mussels (Dreissena polymorpha) have been found in the digestive tract of lake sturgeon but is not a preferred prey item (Guilbard 2002; Nilo et al. 2006).
Adult lake sturgeon consume a wide range of benthic organisms (Harkness and Dymond 1961). The type of food consumed depends on seasonal and spatial availability as well as the nature of the benthos over which sturgeon feed (Harkness and Dymond 1961; Mosindy and Rusak 1991). Choudhury and Dick (1993) reported the percent of food items in the total diet of individual lake sturgeon from several commercial fisheries: ephemeropteran naiads (23-67.4% of ingested prey), chironomid larvae (7.7-45.4%), trichopteran larvae (1.9-9.1%), gammarids (0-18.9%), Orconectes spp. (7.7-26.4%), bivalves (7.7-13.6%), hirudineans (0-11.3%) and fishes (0-3.8%). In Lake of the Woods, mayfly larvae and crayfishes represented nearly 70% of the food items found in commercially harvested sturgeon; chironomids, fingernail clams and small minnows also were present (Mosindy and Rusak 1991). Sculpins, sticklebacks and other small benthic fish species have been reported in the diet, and fish eggs may be consumed infrequently (Harkness and Dymond 1961; Mosindy and Rusak 1991). Choudhury and Dick (1993) reported two lake sturgeon with fully distended stomachs containing numerous yellow perch (Perca flavescens) egg masses.
There is no evidence that adult lake sturgeon have natural predators, but historically they may have been vulnerable to black bear (Ursus americanus) while spawning in shallow waters in high concentrations. Commercial fishermen believed that parasitic lampreys [sea lamprey (Petromyzon marinus) and silver lamprey (Ichthyomyzon unicuspis)] were responsible for the poor condition of some fish as lamprey scars were found on most adult fish in the Great Lakes (Harkness and Dymond 1961) prior to sea lamprey control in the 1950s. However, more recently, the incidence of lamprey scarring is rare and in one study not a single scar was found among over 3000 individuals examined (L. Mohr, Ontario Ministry of Natural Resources, Owen Sound, Ontario, personal communication). Similar observations were also made in 2004 in the St. Lawrence River commercial catch. Vladykov (1985) reported an incidence of 61 silver lampreys attached to a single lake sturgeon caught in 1961 in the St. Lawrence River. The total weight of these lampreys (120 g) was only a small fraction of the 16 kg host sturgeon.
YOY are the most likely stage to be preyed upon by other fish species. Walleye (Sander vitreus) were reported to have lake sturgeon in their stomachs in the Abitibi River (Seyler 1997c), and the predation of 2.5 - 5 cm larval sturgeon by 9 - 12 cm yellow perch and 15 cm northern pike (Esox lucius) has been documented in laboratory studies (Dick, unpubl. data).
A number of parasitic trematodes, acanthocephalans, nematodes and cestodes have been reported from lake sturgeon (Harkness and Dymond 1961). Choudhury and Dick (1993) reported 19 parasite species from lake sturgeon and Choudhury et al. (1995) reported 10 parasite species in lake sturgeon from near the southern limits of its current range (Lake Winnebago, Wisconsin). Some of these parasites are common in other fishes with similar feeding patterns and habitat preferences, while other parasites are associated only with the sturgeons, indicating a strong phylogenetic relationship and specificity at the host family level. The coelenterate parasite, Polypodium hydriforme, infects the mature ova of lake sturgeon destroying the egg (Dick et al. 1991). This parasite has been reported from the United States, but has been reported in Canada only from the Rainy, Winnipeg, Nelson and Saskatchewan rivers; however, it probably occurs in other parts of Canada as well (Dick et al. 1991).
It is clear that lake sturgeon do not adapt readily to change, whether from fishing pressure or habitat alterations. Constructed barriers on streams restrict the movements of large, mature lake sturgeon; however, the importance of these barriers, if local spawning habitat remains intact, is unknown. Studies on tagged juveniles and adults indicated that most individuals do not move great distances and that the integrity of local habitat appears to be essential for survival (Dick 2004). Laboratory studies on the movements of wild and cultured lake sturgeon in the same location indicate that over a 10-day period, wild and cultured lake sturgeon moved over the same substrates and fed on the same natural food items (Dick 2004). Lake sturgeon have been reared successfully from hatch and have been returned to the wild as yearlings or older fish. This approach has been successful in the Assiniboine River near Brandon, but has proven difficult to monitor on the Winnipeg and Nelson rivers (Dick, unpubl. data). A major landslide occurred along the Ouareau River in March 1990 in the single lake sturgeon spawning ground of this river. Despite the fact that at least two thirds of the spawning substrate has been buried and that the flow regime has been deeply modified, the spawning site was still used and none of the alternative sites contained eggs or larvae (La Haye et al. 1990).
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