Variations In Otolith Morphology And Growth Characteristics Of Schizothorax Nukiangensis Along Riverine Longitudinal Gradients

Environmental factors,such as water temperature,altitude,dissolved oxygen,river gradient,flow velocity,and biogenic matter,differ greatly along the longitudinal gradients from headwater to mouth in river systems.The distributions of fish are generally constrained to the water system.Therefore,these environmental gradients likely result in adaptive alterations in fish fauna,community structure,and population biological characteristics.The phenotypic plasticity of organisms(e.g.,variations in morphological forms and life-history traits)has important ecological significance for biological adaptation to changing environments.Understanding how phenotypic plasticity variations in fish populations respond to environmental changes along the riverine longitudinal gradients,is not only an essential part of environmental biology,but also an important approach to study the adaptive evolution of fish.Schizothorax nukiangensis(Tsao,1964)is an endemic fish species inhabiting mainstream and tributaries of the Nu-Salween River.To date,it was found to be the Schizothoracinae fish with the widest distribution along the altitudinal gradient(from 700 m to 3,800 m).However,how it adapts to various riverine environmental gradients through phenotypic plasticity remains unclear.Hence,S.nukiangensis was selected to uncover the variation in its otolith(a hard-part in the inner ears of teleost fish and are characterized by balance and hearing function)morphology along the longitudinal gradient,and to reveal the relationships between variations in growth characteristics and environmental conditions via field sampling and experimental analysis.The main findings and conclusions are as follows:(1)The significant difference in otolith morphological characteristics(mostly at the rostrum,excisura and posterior rim)were found by reconstructed the mean otolith shape of S.nukiangensis based on wavelet transformation.The relationships between otolith morphology and habitat conditions were analyzed using multi-regression tree and multi-factor analysis.The results showed that the average river gradient was the main environmental factor affecting the otolith shape(especially the relative length of the otolith)of S.nukiangensis in the Nu-Salween River.Fish inhabiting in river sections with larger river gradient have relatively longer otoliths,which may be the result of adaptation to the fast-flow habitat.This is because fish with longer otoliths potentially have stronger abilities of balancing and swimming.Other environmental factors,such as altitude,latitude,and annual mean temperature,play a secondary role in the variation in otolith morphology.This study reveals the decisive role of environmental factors in otolith shape of riverine fish.Therefore,identification of species and populations using otolith morphology should be taken into account the variation in otolith morphology caused by environmental conditions(2)The annual age of S.nukiangensis among habitats was identified by otolith grinding method.Results showed that the age range of S.nukiangensis population was 1—22 years,and the otolith increment width of S.nukiangensis reflect this growth pattern by declining with age.The SLW(length-weight relationships)function equation of S.nukiangensis was W 0.0178SL2 98,n = 598,R2 = 0.99,P<0.01;Von Bertalanffy,growth equation Lt = 776.5154[1-e-0.0299(t + 0.5415)]was fitted to the growth pattern of the S.nukiangensis by compared the residual sum of squares(RSS)of three fish growth equations(Von Bertalanffy,Gompertz and Logistics function).Combining the parameter b value of the SWL equation with the growth performance index ￠.the results indicated that the growth rate of the fish population inhabiting in the middle stream(BZLZ)were faster than in other habitats(upstream and downstream sites).Therefore.high-altitude(lower water temperature)or low-altitude(higher water temperature)rivers is not conducive to the growth of fish which live in the same river due to the low-temperature restriction or high-temperature stress.Fish groups that were inhabiting in river sections at medium altitudes exhibit the fastest growth rate.which may owe to better temperature conditions and sufficient food resources.(3)Using the back-calculated length via otolith annuli of S.nukiangensis,it was found that the standard length in the first year(SLFY)of S.nukiangensis was positively correlated with the altitude and latitude of the habitat,which may be due to the results of size-selection of winter mortality.Larger individuals can survive after the first winter,while smaller individuals cannot do in river sections with a high altitude.Conversely,the increment of standard length in the second year(ISLSY)was negatively correlated with the altitude and latitude of the habitat,which may be interpreted as the lack of food in high-altitude river sections,leading to the decrease in growth rate.In other words,in the second year,the growth rate of groups in low-altitude river sections was faster than that of groups in high-altitude river sections owing to optimal temperature conditions and high food availabilities.In conclusion,this study investigated the phenotypic plasticity(i.e.,otolith morphology and growth characteristics)of S.nukiangensis along riverine longitudinal gradients.The results revealed the adaptation strategy of fish to longitudinal environmental gradients in rivers,and enriched the content of environmental biology research.The results can also provide a scientific basis for the conservation and management of S.nukiangensis and fishery resources in the Nu-Salween River.