戟捕蝦漁法之研究－I.電戟捕蝦漁法對資源影響之探討

Abstract

Electric shrimp fishing in Taiwan has a short history of about 10 years, but the rate of expansion has been very rapid, especially since 1969. The number of vessels equipped for electric shrimping has increased from less than a dozen in 1962 to over 2,000 in 1973; horsepower of vessels has increased from 4 hp to 120 hp, and the power of the alternating current generator used from 2.8 kw to 40 kw. This rapid development and a possible further expansion of this new type of shrimping prompted the initiation of the present studies to find out the possible effects of electric shrimping on the shrimp resources. Experiments on the effects of the various intensities of DC, AC, OC and PC (pulsed direct current) on the life and habits of the various stages of growth of shrimps were conducted and the results obtained are summarized as follows: 1. Gravid females (Penaeus japonicus) a. Subjected to: Type of current-DC and OC Intensity of current-3A Duration-1-3 seconds b. Results: No harmful effects observed; spawning normal; 5 out of 6 treated specimens spawned, number of eggs spawned and hatching rate all normal; survival rates of 1st stage nauplii reared to 3rd mysis stage same as those of normal hatchery specimens. 2. Eggs (P.japonicus, P.monodon, P.penicillatus) and nauplii (P.japonicus, P.monodon): No harmful effects observed on specimens subjected to 3A electric intensity and within a duration of 4 seconds. 3. Zoea, mysis, post-larva, juvenile (P.japonicus): No harmful effects observed on specimens subjected to DC, AC, OC, PC, within an intensity of 3A and a duration of 5 seconds. 4. Adult shrimp (Metapenaeus monoceros), body length 7.6-9.5 cm: Able to recover from shock and paralysis when subjected to electric stimulation of (1) not more than 4A intensity and 5-second duration, or (2) intensity increased to as high as 15A but duration reduced to 1 second. 5. Substance produced around electrodes of DC: When DC is used, substances such as C12, CuC1, CuCl, and Cu(OH)2 are found around the electrodes. These products, especially CuCl, are harmful to eggs and larvae. Fortunately, because of economic reasons the use of DC has been discontinued. 6. It was found that to maintain similar electric intensity, the AC and DC type of shrimping gear required the largest amount of electricity, PC type next and OC type the least, and is therefore recommended for future use. 7. The amount of electric stimulation received by the shrimps (and fish also) is proportional to its size; small shrimps receive smaller amount and thus less affected than the larger ones, while eggs and larvae because of their small size are the least affected. About 4-5 v of electricity is enough to force shrimps to jump out of the sand bed, but has extremely little effect on eggs and larvae. Fishing with properly controlled voltage would be possible to catch mostly large specimens and leave small ones behind to grow and replenish the stock. 8. Basing on the results obtained, it seems unlikely that electric shrimping would produce harmful effect on the shrimp resource, provided that the electronic oscillatory type of gear is used within the safety limit of 3A intensity and duration of 4 seconds. However, this type of fishing has very high efficiency, therefore the number of fishing vessels should be limited to avoid over-fishing and depletion of resource.

本篇係從事＂電戟捕蝦漁法＂之一系列研究中，探討對蝦類資源之影響一題。以直流電、交流電及振動電流或脈動電流等四種電流，並以不同之電戟時間，分別電戟抱卵母蝦、卵、各期幼蟲，稚蝦及成蝦，獲得如下之結果： 1.抱卵母蝦經電流強度 3A，電戟時間 1～3 sec 之直流電或交流電分別電戟結果仍能正常產卵，並且6尾中竟有5尾產卵。卵之孵化率也達 68.1～90.0％，又其第一期無節幼蟲培育至第三期糠蝦期幼蟲之存活率達 29.5～71.2％，此存活率不下於一般大量繁殖蝦苗者，因此，電戟對抱卵母蝦似無不良影響。 2.電流強度 3A，電戟時間在 4 sec 之範圍內之電戟，似可視為無害於蝦卵及無節幼蟲。 3.無論是直流電、交流電、振動電流或脈動電流，在電流強度 3A，電戟時間在 5 sec 之內之電戟對眼幼蟲、糠蝦期幼蟲、後期幼蟲及稚蝦，似無不良之影響。 4.成蝦（體長 7.6～9.5 cm 之砂蝦為例）在電流強度不超過 4A，電戟時間不超過 5 sec，或電流強度雖增加至 15A，但電戟時間縮短於 1 sec 之內為安全範圍，在此範圍內受到電戟麻痺後仍能回復。 5.使用直流電電戟時，會有氯氣、CuCl、CuCl2 及 Cu(OH)2 等生成物產生，此等生物對蝦卵及各期幼蟲具有不可忽視之危害作用，期中以 CuCl 之毒害為甚，但，直流電式捕蝦器，因其耗電量大又其電極易於腐蝕，故迄今已完全被淘汰，因此，此等生成物之毒害因素，可不必再考慮。 6.依據電戟頻度，在同樣之電流強度下，直流電式及交流電式捕蝦器之耗電量較大，振動子式捕蝦器次之，已電子振盪式捕蝦器耗電量為最小，又，電子振盪式捕蝦器具有將於下文所述之諸多優點，所以今後之電戟捕蝦器改以此式為宜。 7.電戟時魚、蝦在水中惑受之電壓和體長之立方成正比，因此，小蝦較大蝦所受之電戟影響為小，卵及各期幼蟲所受之影響更比小蝦者為小。一般迫使蝦類由砂中躍啟之電場強度約為4～5V，在此電場強度中，各期幼蟲所受之影響實為微乎其微。因此，電戟捕蝦漁法如能妥善運用，對資源保護反有益處。 8.綜合此次之試驗結果，今後以電子振盪式捕蝦器，而在電流強度 3A，電戟時間 4 sec 之安全範圍內作業，則電戟捕蝦漁法對蝦類資源似可視為無不良之影響，但此法效率非常高，故應限制單位作業範圍內之蝦曳網船數目，以免導致嚴重影響資源之＂過漁＂現象產生。