The present research adopts with the application of the sequential mesh regulations on bottom trawl to study how to protect economic immature fishes. The experimental design includes first the determination of the relationships of fish size and the price per weight (PPW) of the economic demersal fishes sold as the one-hundred Dollars-a-plate manner in Tunkang and Zihguan fish markets in the southwestern coast of Taiwan. The obtained results were used to yield the catch of smaller size fishes and the lower PPW for regulating the economic immature fishes. The three kinds of regulated fish size were defined as the smallest surveyed size, smallest economic size, and smallest matured size. These regulated fish size were used as the body lengths of 50% selection (L50) to calculate the regulating mesh size. This study presents that four steps are needed for the sequential mesh regulations of bottom trawl in the southwestern Taiwan. The first step is to regulate the mesh size between 35 and 45 mm for banning bottom trawler to catch fishes smaller than the smallest surveyed fish size. The objective of first step is to reduce the bycatch without affecting the landings of economic immature fishes sold in the fish market. The second step is to regulate the mesh size between 50 and 60 mm for banning bottom trawler catch fishes smaller than the smallest economic fish size, and to reduce the catch of smaller size fishes and the lower PPW of economic immature fishes. The third step is to regulate the mesh size between 65 and 70 mm for banning bottom trawler to catch fishes smaller than the smallest matured fish size. This is also to reduce the catch of economic immature fishes, and to increase the recruitment. The fourth step is to regulate the mesh size between 70 and 82 mm for banning bottom trawler catching the lower PPW of economic fishes that is not large enough to reach their optimal PPW. The present research provides important data results for regulating the mesh size of bottom trawler operation for the achievement of sustainable coastal fishery in Taiwan.

The Wing Trawling System (WTS) was tested as an alternative to traditional shrimp capture methods in the Gulf. Compared to an otter trawl, this trawl was conceived to reduce bycatch, retain shrimp catch, and minimize seafloor disturbance. Through seventy-one paired tows, the WTS was assessed against a standard otter trawl. The WTS was found to reduce bycatch by 63-65% and reduce shrimp catch by 30-35%. Additionally, I measured the depth of the scars produced by both trawls and quantified the turbidity of the plumes behind them. The scars left by the WTS and the otter trawl were between 9.9 cm-13.6 cm. The turbidity behind the WTS was 18.6 NTU, while the turbidity behind the otter trawl was 206.8 NTU. In conclusion, the WTS offers an alternative to an otter trawl that reduces bycatch and the impact trawling has on the seafloor but results in a significant amount of shrimp loss.

L'industrie de la pêche est confrontée à plusieurs contraintes comme l'augmentation du prix du carburant et la diminution des stocks de poisson. Pour améliorer la rentabilité, nous avons fait l'optimisation des chaluts de pêche (pélagique et de fond) afin d'augmenter leurs efficacités énergétiques. Deux facteurs principaux ont été utilisés : la traînée du chalut et sa surface balayée. Notre travail porte sur l'optimisation du plan du chalut par modification du nombre de mailles des pièces du filet et par changement de la longueur des câbles. Nous avons développé une méthode de recherche successive appelée SOT et nous l'avons comparée à une méthode de recherche aléatoire dans le cas d'un chalut pélagique avec un gain net de 54%. La SOT a été utilisée pour optimiser le plan des pièces du filet d'un chalut de fond. Une amélioration du gain énergétique de 38% a été obtenue, ainsi qu'une réduction de 45 jours de pêche. Par souci de risque de baisse de capturabilité, nous avons développé une fonction objectif qui dépend de la distribution de poisson. Trois fonctions de même densité surfacique ont été testées. L'amélioration de l'efficacité énergétique calculée par la méthode SOT sur un chalut de fond est respectivement de 52%, 16% et 32% selon les trois distributions. Avec la méthode SRT (méthode de raffinement séquentiel), nous obtenons des réductions de 56%, 30% et 39% pour les trois distributions. Finalement nous avons adapté la méthode SOT pour optimiser les longueurs des câbles du chalut. Nous obtenons dans un cas une amélioration de l'efficacité énergétique de 49%. L'utilité de ce travail devrait être étendue hors de la pêche comme l'écologie en général. / Nowadays, fishing industry is facing several constraints such as increased fuel prices and declining fish stocks. In this work, we perform optimization of fishing trawls (pelagic and bottom) in order to improve their energy efficiencies. Two main factors were used: the trawl drag and its mouth swept area. Our work focuses on optimizing trawl design by altering mesh numbers and changing cable length. We have developed a method called SOT and compared it to a random search method in the pelagic trawl case with a net energy gain of 54%. The SOT was used in design optimization of bottom trawl panels. Improved energy gain of 38% was obtained and a reduction of 45 total number of fishing days. In order to bypass catchability reduction, we developed an objective function which depends on fish distribution. Three functions with same bottom surface density were tested. Improving energy efficiency calculated by the SOT method on a bottom trawl is respectively 52%, 16% and 32% for the three distributions. With the SRT method (sequential refinement), we obtain reductions of 56%, 30% and 39% for the three aforementioned distributions. Finally we adapted the SOT method to optimize trawl cable lengths. In one case, we obtain energy efficiency improved by 49%. The usefulness of this work should be extended beyond fishery to ecology in general.