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Understanding migratory behavior, mortality, and physiology is essential for conservation of many species, particularly anadromous fish. In this study, freshwater and marine migrations of the endangered salmonid North Sea houting (Coregonus oxyrinchus (Linnaeus, 1758)) were investigated using telemetry. Furthermore, physiological samples were collected from North Sea houting and from resident and anadromous populations of the closely related European whitefish (Coregonus lavaretus (Linnaeus, 1758)) to compare hypo-osmotic tolerances. On average, North Sea houting spent 193 days at sea where the mortality was 36%. Most fish returned from sea in the autumn, and river entry correlated inversely with river temperature and positively with discharge. Fish spent an average of 49 days in the estuarine area. Artificial lakes negatively affected migration speeds. Migration speeds did not differ consistently between individuals (i.e., not a repeatable trait) but correlated positively with water temperature. Fish arrived at spawning areas in November. In the post-spawning state, Na+/K+-ATPase activities were elevated in North Sea houting and anadromous whitefish compared with resident whitefish, while osmolality was elevated only in North Sea houting. Our study provides important information for conservation planning related to the Habitat Directive of the European Union that lists the North Sea houting as critically endangered. [ABSTRACT FROM AUTHOR]

We report the incidence of male intersex in adult Pacific lamprey ( Entosphenus tridentatus (Richardson, 1836)) during their pre-spawning migration in fresh water. Although 'hermaphrodites' have been suggested in other adult lampreys, this is the first detailed description and discussion of this phenomenon. A total of 0.5% of our adult Pacific lamprey from Willamette Falls (2 out of 427 adults) were intersex, with oocytes in the testes. This phenomenon was identifiable only by histological examination. The testes of the intersex males were immature, in the beginning stages of meiosis. One intersex male possessed primary growth or perinucleolar stage oocytes loosely interspersed throughout the testes, and the other possessed at least 6 mid-vitellogenic oocytes (0.6 mm, mean long diameter) separate from the testes. Because premetamorphic lamprey can possess both female and male gonial cells, we hypothesize that intersex is a remnant larval trait and that these fish failed to fully develop into males during metamorphosis. [ABSTRACT FROM AUTHOR]

Anadromous fishes migrate to sea, apparently to take advantage of growing conditions, and return to fresh water to spawn. Migration is an energetically demanding part of the life cycle, and animals tend to evolve physiological and behavioral traits that suit the environmental conditions they experience during migration. We hypothesized that this premature migration is a consequence of the fish having to avoid stressful summer temperatures in order to access certain suitable areas for spawning in the fall. We tested this idea in sockeye salmon, Oncorhynchus nerka, by compiling data on the timing of migration and spawning and the freshwater temperature regime along the migration route in populations throughout the species North American range. The timing of migration varied among populations and was primarily related to temperature regime during migration and the timing of spawning. When temperatures were moderate, sockeye salmon tended to migrate to the vicinity of the spawning grounds about 1 month prior to spawning, regardless of the length of the freshwater migration. A long delay between migration and spawning seems to be an adaptation to avoid migration when temperatures are high. The results supported the hypotheses that the mean and variance of delay among populations from warm areas would be greater than those for populations from cold areas. At the southern end of the North American range of sockeye salmon, populations in Washington State and southern British Columbia experience higher water temperatures than other populations, and may also have evolved greater tolerance of warm water than northern populations. In summary, patterns of migratory timing among populations reflect both passage conditions and spawning time for adults.