Does migration promote or inhibit diversification? A case study involving the dominant radiation of temperate Southern Hemisphere freshwater fishes

Although theory predicts that dispersal has a pivotal influence on speciation and extinction rates, it can have contradictory effects on each, such that empirical quantification of its role is required. In many studies, dispersal reduction appears to promote diversification, although some comparisons of migratory and nonmigratory species suggest otherwise. We tested for a relationship between migratory status and diversification rate within the dominant radiation of temperate Southern Hemisphere freshwater fishes, the Galaxiidae. We reconstructed a molecular phylogeny comprising >95% of extant taxa, and applied State-dependent Speciation Extinction models to estimate speciation, extinction, and diversification rates. In contrast to some previous studies, we revealed higher diversification rates in nonmigratory lineages. The reduced gene flow experienced by nonmigratory galaxiids appears to have increased diversification under conditions of allopatry or local adaptation. Migratory galaxiid lineages, by contrast, are genetically homogeneous within landmasses, but may also be rarely able to diversify by colonizing other landmasses in the temperate Southern Hemisphere. Apparent contradictions among studies of dispersal-diversification relationships may be explained by the spatial context of study systems relative to species dispersal abilities, by means of the “intermediate dispersal” model; the accurate quantification of dispersal abilities will aid in the understanding of these proposed interactions.     

Growth hormone and cortisol treatment stimulate seawater tolerance in both anadromous and landlocked Arctic charr

In a comparative experiment the effect of cortisol and growth hormone (GH) on the hypo-osmoregulatory ability of a landlocked and an anadromous strain of Arctic charr (Salvelinus alpinus) was investigated. Cortisol and GH were implanted either alone or in combination, and the fish were exposed to a 24 h seawater challenge test (SWT) on days 14 and 28 after implantation. Hypo-osmoregulatory ability, measured as plasma osmolality and chloride concentration after the SWTs, was better in the anadromous than in the landlocked strain, irrespective of treatment. However, cortisol provided a strong stimulation of hypo-osmoregualtory ability in both strains, and this stimulation seemed to be potentiated by GH in an additive manner. Improved hypo-osmoregulatory ability in GH + cortisol treated anadromous Arctic charr was accompanied by increased gill Na⁺, K⁺-ATPase activity and Na⁺–K⁺–2Cl⁻ cotransporter protein abundance, but no changes in gill Na⁺,K⁺-ATPase α1a and α1b mRNA levels. For landlocked charr the improved hypo-osmoregulatory ability in GH +cortisol treated fish was accompanied only with an increase in gill Na⁺–K⁺–2Cl⁻ cotransporter protein abundance. Hormone treatment caused an improvement of hypo-osmoregulatory ability that was of approximately the same magnitude in the landlocked as in the anadromous Arctic charr. This suggests that the lack of spontaneous development of hypo-osmoregulatory ability often seen in landlocked populations of Arctic charr may depend, at least partly, on a lack of the hormonal activation seen in anadromous populations.     

Otolith Sr : Ca ratios of the freshwater and anadromous cyprinid genus Tribolodon

The ratio of strontium to calcium concentration in the otolith of four species of Tribolodon (Cyprinidae) was examined to reveal their migratory histories. The ratio was relatively stable from the core to the edge of the otolith in fluvial Tribolodon ezoe and T. hakonensis, whereas it changed from a low level to a higher level in anadromous T. brandtii and T. hakonensis. Four specimens of T. nakamurai from the Mogami River, Yamagata Prefecture, northern Japan, showed relatively stable ratios, indicating a fluvial life mode. On the other hand, the ratio increased once in only one specimen of T. nakamurai from the Koyoshi River of Akita Prefecture, suggesting the possibility that it was born in the Mogami River, the nearest major habitat of this species, and accidentally migrated to the Koyoshi River through the sea.     

Evidence of downstream migration of Sakhalin taimen, <Emphasis Type="Italic">Hucho perryi</Emphasis>, as revealed by Sr : Ca ratios of otolith

The migratory history of Sakhalin taimen, Hucho perryi, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith by using wavelength dispersive X-ray spectrometry on an electron microprobe. Otolioth Sr:Ca ratios of freshwater-reared samples remained consistently at low levels throughout the otolith. The Sr:Ca ratios of samples from Lake Aynskoye of Sakhalin Island showed a low value from the core up to a point of 700–2140µm. Thereafter, the ratios increased sharply and remained at higher levels up to the outermost regions. The difference in Sr:Ca ratio might be the result of the presence of individuals that underwent seawater and freshwater life history phases, probably reflecting the ambient salinity or the seawater–freshwater gradient in Sr concentration. Otolith Sr:Ca ratio analysis revealed downstream migration history in H. perryi.     

Diverse migratory histories in a brackish water type of the ninespine stickleback, <Emphasis Type="Italic">Pungitius pungitius</Emphasis>

The life history in a brackish water type of the ninespine stickleback, Pungitius pungitius, was studied by examining the strontium (Sr) and calcium (Ca) concentrations in the otoliths. The fluctuating patterns of Sr/Ca ratios along the life history transect in the otoliths varied widely among fish in spite of their identification as brackish water type as estimated by morphological characteristics. More than 70% fish showed the intermediate otolith Sr/Ca ratio throughout, averaging 5.23–7.71 × 10⁻³. Besides this brackish water resident life history type of P. pungitius, other sticklebacks had anadromous (25%) and freshwater amphidromous (2.5%) life history types. These findings clearly indicate that the migration of the ninespine stickleback between fresh and sea waters is obligatory but facultative having an ability to utilize the full range of salinity in its life history.     

Variation in migratory history of Dolly Varden in a stream with an artificial dam in the Shiretoko Peninsula,Hokkaido, Japan

The variation of migration patterns in Dolly Varden Salvelinus malma in a stream with an artificial dam (erosion-control-dam) in the Shiretoko Peninsula, Hokkaido Island, Japan, at the southernmost part of its distribution, was examined by otolith Sr:Ca ratio analysis. All specimens from the above-dam area showed consistently low Sr:Ca ratios (<5.0 × 10⁻³) throughout the otolith. In contrast, many specimens from the below-dam area had higher ratios (>5.0 × 10⁻³) throughout the otolith or only at the outer part than do those from the above-dam area. This higher ratio is probably due to salinity effects. Specimen from the below-dam area could be divided into three types: (1) Consistently low Sr:Ca ratios throughout the otolith, being freshwater residents. (2) Low Sr:Ca ratios around the inner part of the otolith, and thereafter higher ratios toward the outer part, being typically anadromous. (3) Relatively higher Sr:Ca ratios throughout many or in some parts of the otolith, indicating that these have migrated from the freshwater to brackish water or seawater from the early life stage. These findings suggest that Dolly Varden has a high degree of plasticity in its migratory behavior.     

Identifying sea-run brown trout, Salmo trutta, using Sr : Ca ratios of otolith

 The migratory history of the brown trout, Salmo trutta, collected from Japanese rivers, was examined in terms of strontium (Sr) and calcium (Ca) uptake in the otolith, by means of wavelength dispersive X-ray spectrometry on an electron microprobe. Sea-run (anadromous) and freshwater-resident (nonanadromous) types of S. trutta were found to occur sympatrically. Otolith Sr concentration or Sr : Ca ratios of anadromous S. trutta fluctuated strongly along the life history transect in accordance with the migration (habitat) pattern from sea to freshwater. In contrast, the Sr concentration or the Sr : Ca ratios of nonanadromous fish remained at consistently low levels throughout the otolith. The higher ratios in anadromous S. trutta, in the otolith region from the core to 1500 μm, corresponded to the initial seagoing period, probably reflecting the ambient salinity or the seawater–freshwater gradient in Sr concentration. The findings clearly indicated that otolith Sr : Ca ratios reflected individual life histories, enabling the sea-run S. trutta to be distinguished from the freshwater-resident brown trout. Received: March 18, 2002 / Revised: May 15, 2002 / Accepted: June 5, 2002 Acknowledgments The authors are grateful to Messrs. T. Ikeda, S. Kudo, Y. Miyakoshi, M. Nagata, K. Shimoda, T. Takami, K. Takeuchi, and M. Ueda for their assistance in collecting specimens. This work was supported in part by Grant-in-Aid No. 13760138 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Correspondence to:Takaomi Arai     

Poor water quality constrains the distribution and movements of twaite shad <Emphasis Type="Italic">Alosa fallax</Emphasis><Emphasis Type="Italic">fallax</Emphasis> (Lacépède, 1803) in the watershed of river Scheldt

Worldwide, river fragmentation is primarily responsible for the decline of populations of migrating fish. In particular, anadromous fish species, which necessarily migrate to fresh water to reproduce, are endangered since many are no longer able to reach their natural spawning sites. In addition, pollution of rivers effectively prevents upstream or downstream movements and blocks access to spawning grounds. This article investigates how poor water quality interferes with the life history cycle of twaite shad Alosa fallax fallax (Lacépède, 1803), an anadromous clupeid fish, in the watershed of River Scheldt, a heavily impacted environment in West Europe. We used two models based on known ecological and environmental information to explain past and present twaite shad distribution within the watershed and to make inferences about a future population recovery and juvenile habitat value. We demonstrated that historical spawning areas satisfy water quality conditions necessary to support spawning and successful development of early life history stages of the twaite shad. However, poor water quality conditions just upstream the freshwater–saltwater boundary still act as an effective migration barrier for upstream movement. As a consequence, spawning grounds are inaccessible and the population is dominated by seasonal adults occurring in the lower estuarine part of the watershed. This article provides testable and diagnostic information to the watershed management in that it identifies habitat and water quality requirements needed to support the expected recovery of an endangered anadromous fish population. Guest editors: S. Dufour, E. Prévost, E. Rochard & P. Williot Fish and diadromy in Europe (ecology, management, conservation)     

Distinguishing between juvenile anadromous and resident brook trout (Salvelinus fontinalis) using morphology

Phenotypic variation linked to habitat use has been observed in fish, both between and within species. In many river systems, migratory and resident forms of salmonids coexist, including anadromous (migrant) and resident brook trout, Salvelinus fontinalis. In such populations, juvenile anadromous (migrant) brook trout, prior to migration, inhabit regions of higher current velocity than residents. Because it is more costly to occupy fast currents than slow currents, differences in morphology minimizing the effects of drag were expected between the two forms. As predicted, migrant brook trout were found to be more streamlined (narrower and shallower bodies) than resident brook trout, and these differences persisted into the marine life of the fish. Migrants also exhibited shorter pectoral fins, which facilitate pelagic swimming, indicating that migrants, prior to their migration to the sea, possess the appropriate morphology for swimming in open water habitats. The reported differences between migrants and residents were powerful enough to derive discriminant functions, using only five of the seven measured traits, allowing for accurate classification of brook trout as either migrants or residents with an overall correct classification rate of 87%. Importantly, this study contributes to the notion that a link exists between morphology, habitat use, metabolic costs and life-history strategies. Contribution to the program of CIRSA (Centre Interuniversitaire de Recherche sur le Saumon Atlantique).     

EVIDENCE FOR SYMPATRIC GENETIC DIVERGENCE OF ANADROMOUS AND NONANADROMOUS MORPHS OF SOCKEYE SALMON (ONCORHYNCHUS NERKA)

Anadromous and nonanadromous morphs of the Pacific salmon Oncorhynchus nerka spawn in close physical proximity in tributaries to Takla Lake, British Columbia, yet differ in morphology, gill raker number, allozyme allele frequencies, and reproductive traits. Both morphs are semelparous typically maturing at age four, the anadromous morph (sockeye) at fork lengths of 38–65 cm and the nonanadromous morph (kokanee) at 17–22 cm. When reared together, pure and hybrid morphs also exhibited different growth rates and maturity schedules. Collectively, these large differences between the morphs confirm that sockeye and kokanee exist as reproductively isolated populations. Average gene flow (m) was estimated to be 0.1–0.8% between morphs, 1.7–3.7% among tributaries for kokanee, and 0.3–5.6% among tributaries for sockeye. We conclude that divergence has occurred in sympatry and examine potential isolating mechanisms.