Potential effects of hydroelectric dam development in the Mekong River basin on the migration of Siamese mud carp (Henicorhynchus siamensis and H. lobatus) elucidated by otolith microchemistry.

The migration of Siamese mud carp (Henicorhynchus siamensis and H. lobatus), two of the most economically important fish species in the Mekong River, was studied using an otolith microchemistry technique. Fish and river water samples were collected in seven regions throughout the whole basin in Thailand, Laos and Cambodia over a 4 year study period. There was coherence between the elements in the ambient water and on the surface of the otoliths, with strontium (Sr) and barium (Ba) showing the strongest correlation. The partition coefficients were 0.409-0.496 for Sr and 0.055 for Ba. Otolith Sr-Ba profiles indicated extensive synchronized migrations with similar natal origins among individuals within the same region. H. siamensis movement has been severely suppressed in a tributary system where a series of irrigation dams has blocked their migration. H. lobatus collected both below and above the Khone Falls in the mainstream Mekong exhibited statistically different otolith surface elemental signatures but similar core elemental signatures. This result suggests a population originating from a single natal origin but bypassing the waterfalls through a passable side channel where a major hydroelectric dam is planned. The potential effects of damming in the Mekong River are discussed

Age and growth estimates from three hard parts of the spotted catfish, Arius maculatus (Actinopterygii: Siluriformes: Ariidae), in Songkhla Lake, Thailand’s largest natural lake

The spotted catfish, Arius maculatus (Thunberg, 1792), is a euryhaline fish that is economically important in the Indo-West Pacific. Population dynamics studies and stock assessments of this species have focused on marine stocks, but not those from fresh water. In this study, the age and growth of A. maculatus were, therefore, investigated for the inland stock in Songkhla Lake, Thailand. A total of 213 individuals ranging between 35 and 238 mm TL were used. The length–weight relation indicated positive allometry of this population. Three hard parts (otolith, dorsal- and pectoral-fin spines) were used for aging. The marginal increment ratio confirmed that an annulus was deposited once a year in all three hard parts. All of the samples were aged between 0+ and 6+ years. Verification of age estimates from three readers showed that the otolith was the most suitable part for age estimation. Three growth models (von Bertalanffy, Gompertz, and logistic) were applied in the study. The von Bertalanffy model best described the growth of this fish in Songkhla Lake. The obtained asymptotic length was 290.87 mm TL and the relative growth rate parameter was 0.166 year–1. Our results will be applied as inputs for fish stock assessment models. The obtained growth parameters also can serve as a reference for A. maculatus stocks elsewhere

Age and growth estimates from three hard parts of the spotted catfish, Arius maculatus (Actinopterygii: Siluriformes: Ariidae), in Songkhla Lake, Thailand’s largest natural lake

The spotted catfish, Arius maculatus (Thunberg, 1792), is a euryhaline fish that is economically important in the Indo-West Pacific. Population dynamics studies and stock assessments of this species have focused on marine stocks, but not those from fresh water. In this study, the age and growth of A. maculatus were, therefore, investigated for the inland stock in Songkhla Lake, Thailand. A total of 213 individuals ranging between 35 and 238 mm TL were used. The length–weight relation indicated positive allometry of this population. Three hard parts (otolith, dorsal- and pectoral-fin spines) were used for aging. The marginal increment ratio confirmed that an annulus was deposited once a year in all three hard parts. All of the samples were aged between 0+ and 6+ years. Verification of age estimates from three readers showed that the otolith was the most suitable part for age estimation. Three growth models (von Bertalanffy, Gompertz, and logistic) were applied in the study. The von Bertalanffy model best described the growth of this fish in Songkhla Lake. The obtained asymptotic length was 290.87 mm TL and the relative growth rate parameter was 0.166 year–1. Our results will be applied as inputs for fish stock assessment models. The obtained growth parameters also can serve as a reference for A. maculatus stocks elsewhere

Sr-Ba elemental signatures of river water samples collected from six regions during 2007-2010.

<p>Sr-Ba elemental signatures of river water samples collected from six regions during 2007-2010.</p

Fish sampling sites in the Mekong River basin.

<p>Black square: <i>Henicorhynchus siamensis</i>. Star: <i>H. lobatus</i>. A total of 29 sites are serially numbered toward upstream within each of seven regions: N =  Thai North, S =  Songkhram, G =  Gam, U =  Mun, X =  Xekong, T =  Tonle Sap and M =  Mekong mainstream (MM). Location of the Khone Falls is indicated.</p

Otolith surface elemental signatures plotted against river water signatures.

<p>Open circle: <i>H. siamensis</i>. Closed circle: <i>H. lobatus</i>. Linear regression lines based on combined data from both species are superimposed, for which statistical significance and the coefficient of determination are shown above each panel.</p

Pairwise differences (mmol/mol) between regions in <i>H. siamensis</i> otolith core elemental signatures for Sr/Ca (above diagonal) and Ba/Ca (below diagonal).

<p>Significant differences are in bold. Values are average elemental molar ratios for regions in the row minus those in the column headings.</p

Sr-Ba elemental profiles along <i>H. siamensis</i> otolith transect.

<p>Triangle: otolith core. Circle: otolith surface. From top to bottom, otolith samples from Thai North (2), Songkhram (3), Gam (2), Mun (2), Tonle Sap (3), and Mekong mainstream (2) were analyzed with the number of sites in a parenthesis. Within a given site, different colors denote different individuals.</p

Pairwise differences (mmol/mol) between regions in <i>H. siamensis</i> otolith surface elemental signatures for Sr/Ca (above diagonal) and Ba/Ca (below diagonal).

<p>Significant differences are in bold. Values are average elemental molar ratios for regions in the row minus those in the column headings.</p

Water sampling sites in six regions.

<p>S =  Songkhram, G =  Gam, U =  Mun, X =  Xekong, T =  Tonle Sap and M =  Mekong mainstream.</p