台灣西南海域刺公鯷稚魚之日成長模式

Abstract

刺公鯷(Encrasicholina punctifer)為台灣沿岸水域常見的小型魚類，為魩仔漁業中重要的捕獲物種之一。 刺公鯷的外型和異葉公鯷(E. heteroloba)相似，兩物種經常於同地先後或同時出現。 先前對於刺公鯷年齡及成長的研究多限於30日齡內的階段(仔魚時期)，對於30日齡以上的階段(稚魚時期)，由於不是漁業之主要利用對象及漁具選擇性的關係，非魚網所能網羅，因此其在沿岸水域之成長狀態並不十分明瞭。 現今台灣的魩仔漁業資源處於過漁狀態，瞭解刺公鯷及相關魚種進入稚魚時期後的命運為一亟待探討的問題。 本研究為進一步探究刺公鯷稚魚階段的成長模式，於2002到2006年間，在屏東枋寮漁獲物中採集刺公鯷稚魚(耳石日齡判讀>30天)，共獲得258尾標本。 標本經測量其標準長、體重之後觀察其卵巢成熟度，並取其耳石。 耳石經處理後，取其影像，判讀各日週輪，計算耳石之總輪數，並取得各日週輪直角切Sulcus軸(S-axis, SU)之相對位置座標。 由各點之座標經計算取得各日週輪間之輪寬，依時間序列逆算其孵化日期，並將逆算體長套適成長模式。 由結果顯示本採樣之範圍為31.1 – 78.8 mm 標準體長，孵化後生存日齡天數範圍為34 – 81天。 由孵化日之頻度分布顯示，刺公鯷之稚魚依其孵化時間可區分為春季群及秋季群。 由卵巢成熟度鑑定，兩亞成魚標本具有發育中的卵巢，推估刺公鯷至少需要十週(70 輪)以上的時間才能夠發育為具生殖力的個體。 以耳石輪寬時序列，逐一分析個體的成長事件，可以鑑識出尾脊索上屈期(flexion)及仔魚變態期(metamorphosis)。 由輪數–體長關係、SU–體長關係及逆推的體長–時間關係之變異數分析顯示，2003年與其他年間差異最顯著，其次為季節群之區分。 由此可分為三群：2003年年級群、春季群(2002及2004)及秋季群(2005及2006)。 由SU –體長關係估計三群的成長曲線發現，2003年仔魚期的生長速率為0.385 mm d-1，進入稚魚期後成長率會加快為1.675 mm d-1。 春季群在仔魚期的生長速率為0.469 mm d-1進入稚魚期後成長率會加快，為1.187 mm d-1。 秋季群由仔魚期進入稚魚期之成長速率則沒有太大的變化，為0.714 mm d-1。 總結台灣西南海域的刺公鯷其五十天內的成長率平均為0.758 mm d-1。 群間成長率差異推測可能是因為基因上、溫度及食物供應水平的影響，且由枋寮港附近林邊溪的歷年流量推知營養可能佔較大的因素。 刺公鯷的成長率、成熟度與其他熱帶的鯡亞目相近，具演化之保守性(coservatism)，由這些相近物種的成長曲線可推測其壽命大約在五個月左右。

Arrow anchovy (Encrasicholina punctifer) is one of the major species caught by coastal fisheries in Taiwan. The morphology of arrow anchovy is similar to its congener E. heteroloba, which is also caught jointly most of time. Previous fisheries studies focused on the larval stages that were less than 30 days and thus hardly data are available for the growth pattern of anchovy after this stage. However, for understanding the resource, investigate what the ecological fate of the survival population is after local exploitation becomes a common need for conservation management purpose. In order to explore the growth pattern after larval stage, 258 specimens of juvenile arrow anchovy (>30 days) from the waters off southwestern Taiwan (Fang-liao) during 2002 and 2006 were collected. For each specimen, standard length (SL) and body weight (BM) were taken, maturity stage was extracted, and sagitta was picked for microstructure analysis. The length composition of specimens ranged 31.1 - 78.8 mm SL, with an age ranged from 34 to 81 days. Based on frequency of back-estimated hatching date, two cohorts of spring and autumn were discerned. Shown by two specimens having developing ovaries, and thus postulated that more than 10 weeks were about to reach adult stage. Four typical growth patterns described by having checks of notochord flexion and metamorphosis or not, were adopted to quantify group differences. Based on the analysis of variances applied to, 1) the relationships of increment counts versus standard length (SL), 2) the size of sagitta versus SL, and 3) array of backcalculated size-at-age, that 2003 annual group had the primary difference from other year's samples, while the spring cohort and autumn cohort had secondary seasonal variations. The result indicated that the growth rate of 2003 group was 0.385 mm d-1 for larvae and 1.675 mm d-1 for juvenile, respectively. For spring cohorts of 2002 and 2004, at larval stages the growth rate was 0.469 mm d-1 and juvenile period 1.187 mm d-1. Since a continuous growth from larval to juvenile was found in the autumn cohorts of 2005 and 2006, the overall growth rate was estimated to 0.714 m mm d-1 as whole. In summary, the average growth rate of juvenile anchovy was 0.758 mm d-1 in the waters off the southwestern Taiwan. The differences among groups may ascribe to the genetic variation, and ambient environment factors, such as temperature and ration levels. Local river runoff was used to mimic an average ration level to access the cause of growth difference. In case of clupeoid's evolutionary conservation in demographic traits, E. punctifer is considered to live up to about 5 months.