Historically, shortnose sturgeon inhabited most major rivers on the Atlantic coast of North America south of the Saint John River, Canada. Today, only 16 populations may remain. Major anthropogenic impacts on shortnose sturgeon are blockage of spawning runs by dams, harvest of adults (bycatch and poaching), dredging of fresh/saltwater riverine reaches, regulation of river flows, and pollution. The pattern of anadromy (adult use of salt water) varies with latitude. The pattern may reflect bioenergetic adaptations to latitudinal differences between fresh and salt water habitats for thermal and foraging suitability. The greater adult abundance in northern and north-central populations likely reflects a historical difference with southern populations that is currently accentuated by increased anthropogenic impacts on southern populations. Adult abundance is less than the minimum estimated viable population abundance of 1000 adults for 5 of 11 surveyed populations. and all natural southern populations. Across the latitudinal range, spawning adults typically travel to about river km 200 or farther upstream. Dams built downstream of spawning reaches block spawning runs, and can divide amphidromous populations into upand downstream segments. Conservation efforts should correct environmental and harvest impacts. not stock cultured fish into wild populations.

Sex determination in an atherinid fish, the Atlantic silverside (Menidia menidia), is under the control of both genotype and temperature during a specific period of larval development. The sex ratios of the progeny of different females are variable and differ in their responsiveness to temperature. This demonstrates that sex ratio in fishes that normally have separate sexes can be influenced by the environment.

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Many of the most important commercial and recreational species of the megadiverse Brazilian freshwater fishes migrate in rivers among essential habitats during all life stages. These movements, however, have been severely blocked by hundreds of hydroelectric dams and reservoirs and they will be even more obstructed due to hundreds of new developments. Fishways have been used in many countries to allow fish to pass around dams. Fishway construction is booming in Brazil, but poor understanding of migrations by Brazilian fishes has led legislators, scientists, and the public to several misconceptions about the rules of fishways in fisheries conservation. First, is a belief that fishways are only needed to facilitate upstream spawning migrations. Also, it has been suggested that upstream passage for Neotropical migrant fishes is not useful if there is no large free-flowing stretch upstream of a dam that contains spawning habitat and has a large natural floodplain (nursery habitat). In this paper, we discuss that, in addition to providing passage for pre-spawning migrants, upstream fishways also provide passage for other fish migrations (e.g. foraging), and that all up-and downstream migrations during life history need to be addressed at dams to conserve fish resources. We also argue that an upstream fishway is important even if the upstream reach does not have spawning or nursery habitats. In addition, we discuss the need for protection of downstream migrant fish, and the importance of fish behaviourists and engineers working together on fishway design and operation to solve fish passage issues.

Summary Understanding the drift dynamics of pallid sturgeon (Scaphirhynchus albus) early life intervals is critical to evaluating damming effects on sturgeons. However, studying dispersal behavior is difficult in rivers. In stream tanks, we studied the effect of velocity on dispersal and holding ability, estimated swimming height, and used the data to estimate drift distance of pallid sturgeon. Dispersal was by days 0–10 embryos until fish developed into larvae on day 11 after 200 CTU (daily cumulative temperature units). Embryos in tanks with a mean channel velocity of 30.1 cm s−1 and a side eddy could not hold position in the eddy, so current controlled dispersal. Late embryos (days 6–10 fish) dispersed more passes per hour than early embryos (days 0–5 fish) and held position in side eddies when channel velocities were 17.3 cm s−1 or 21.1 cm s−1. Day and night swim‐up and drift by embryos is an effective adaptation to disperse fish in channel flow and return fish from side eddies to the channel. Early embryos swam <0.50 cm above the bottom and late embryos swam higher (mean, 90 cm). A passive drift model using a near bottom velocity of 32 cm s−1 predicted that embryos dispersing for 11 days in channel flow would travel 304 km. Embryos spawned at Fort Peck Dam, Missouri River, must stop dispersal in <330 km or enter Lake Sakakawea, where survival is likely poor. The model suggests there may be a mismatch between embryo dispersal distance and location of suitable rearing habitat. This situation may be common for pallid sturgeon in dammed rivers.

Abstract.-It is difficult for agencies to evaluate the impacts of the many planned dams on São Francisco River, Brazil, migratory fishes because fish migrations are poorly known. We conducted a study on zulega Prochilodus argenteus, an important commercial and recreational fish in the São Francisco River, to identify migrations and spawning areas and to determine linear home range. During two spawning seasons (2001)(2002)(2003), we radio-tagged fish in three main-stem reaches downstream of Três Marias Dam (TMD), located at river kilometer (rkm) 2,109. We tagged 10 fish at Três Marias (TM), which is 5 km downstream of TMD; 12 fish at Pontal, which is 28 km downstream of TMD and which includes the mouth of the Abaeté River; and 10 fish at Cilga, which is 45 km downstream of TMD. Late-stage (ripe) adults tagged in each area during the spawning season remained at or near the tagging site, except for four Cilga fish that went to Pontal and probably spawned. The Pontal area at the Abaeté River mouth was the most important spawning site we found. Prespawning fish moved back and forth between main-stem staging areas upstream of the Abaeté River mouth and Pontal for short visits. These multiple visits were probably needed as ripe fish waited for spawning cues from a flooding Abaeté River. Some fish homed to prespawning staging areas, spawning areas, and nonspawning areas. The migratory style of zulega was dualistic, with resident and migratory fish. Total linear home range was also dualistic, with small (,26-km) and large (53-127-km) ranges. The locations of spawning areas and home ranges suggest that the Pontal group (which includes Cilga fish) is one population that occupies about 110 km. The Pontal population overlaps a short distance with a population located downstream of Cilga. Movements of late-stage TM adults suggest that the TM group is a separate population, possibly with connections to populations upstream of TMD.

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