鹽度變化對大吻鰕虎魚脊索上屈後期仔魚成長、死亡與甲狀腺發育之影響

Effects of Salinity Changes on Growth, Mortality, and Thyroid Gland Development of Rhinogobius gigas Postflexion Larvae

大吻鰕虎魚（Rhinogobius gigas）爲台灣特有種生物，主要分布在台灣東部河川之中、下游水域，具有兩側洄游與高豐度特性。爲了解大吻鰕虎魚仔魚在其溯河洄游過程中，經歷海河鹽度變化的影響，本研究於花蓮溪河口採集大吻鰕虎魚脊索上屈後期仔魚，將之馴化在30 ppt之水槽中，進行4個不同鹽度變化處理組實驗：一、15 ppt 10天後轉爲2 ppt 15天；二、15 ppt 10天後回到30 ppt 15天；三、15 ppt 25天；以及四、30 ppt 25天。大吻鰕虎魚仔魚之日成長、日死亡及甲狀腺發育等均被記錄。研究結果顯示，接受處理組1（鹽度由15 ppt轉爲2 ppt）之仔魚比接受其他3個處理組者，有顯著較高的日平均成長率與數目較多但形狀較小的甲狀腺濾泡。日平均死亡率在此4個處理組間無顯著差異。這些結果顯示，在上溯洄游進入河川時，經由幾乎是海水環境轉變爲淡水環境之自然鹽度變化過程的大吻鰕虎仔魚，具有較快速的成長率與較多但較小的甲狀腺濾泡；滯留在較高鹽度之沿岸水域與河口域環境中的大吻鰕虎魚仔魚，會付出成長率延遲與甲狀腺濾泡變大等仔魚停留在高鹽度水域之代價。

Rhinogobius gigas is an endemic species of amphidromous goby commonly found in mid- and lower reaches of rivers in the eastern Taiwan. It is one of the dominant species in the fish larval assemblages of the rivers. To investigate the effects of salinity changes on the larvae during their upstream migration, its postflexion larvae were collected from the Hualien River estuary. They were acclimated at 30 ppt, and then, treated with 4 different regimes of salinity changes: 1) 15 ppt for 10 days and then 2 ppt for 15 days; 2) 15 ppt for 10 days and then back to 30 ppt for 15 days; 3) 15 ppt for 25 days; and 4) 30 ppt for 25 days. The daily growth, daily mortality and thyroid gland development of the larvae were measured. The larvae that received Treatment 1 (salinity changes to 15 ppt and 2 ppt) had significantly higher growth rate, and higher number but smaller size of the thyroid follicles than those received the other three treatments. There was no significant difference in the daily mortality rate among the 4 treatments. The results indicated that R. gigas larvae that encounter natural changes in salinity from nearly sea water to freshwater in their upstream migration in river have a faster growth rate with increase in number but decrease in size of thyroid follicles. For those remain in high salinity coastal and estuarine waters, the growth rate retards and the thyroid glands enlarge, the cost of the larvae to stay in the high salinity zones.