Quantification des rejets d'une ferme piscicole de loup (Dicentrarchus labrax l. ) bilans de masse : n, p et c

INTRODUCTION: Norway Lobster fisheries in the Celtic Sea are mostly located in the Vllg and Vllh subdivisions. The communal regulation applying to Norway Lobster mesh size has been in effect since 1978. The use of derogative mesh size to catch Norway Lobsters is only allowed if the percentage of species protected by marketable size caught at the same time as Norway Lobsters is inferior to a fixed threshold in relation to the mesh size in effect. This regulation has been applied according to a specific schedule. With a percentage of protected species higher than 60 %, catching is considered as geared towards fish and mesh size is what the global regulation requires, i.e. 80 mm. As a matter of fact, as it has been mentioned many times, under the pressure of this constraining regulation, fleets and fishing techniques have quickly evolved resulting in those vessels' considerable increase in catching ability for Norway Lobsters as well as fish. In most cases, flexibility is ruling and 80 mm mesh tends to become common practise. Those vessels always temporarily go towards the most profitable type of fishing; Norway Lobsters remain the targeted specie, even if not always substantially as a main catch. One of the greatest blames brought against Norway Lobster vessels in the Celtic Sea was the potential destruction of immature fish. Those rejects were evaluated in 1980, thus with mesh sizes ranging between 55 and 60 mm. For most species, rejects are anecdotal apparently due to different spatial distribution of immatures and adults. For Whiting, immatures concentrate in the coastal zones during the winter and are only caught if Norway Lobster vessels have to leave the central shoal zone because of bad weather. Only Hakes and Sardines account for a very high reject percentage since they dwell in the same silty bottoms as Norway Lobsters and their trophical migration, if any, takes place in those sedimental pile areas at the foot of shallow waters. In those first evaluations, the volume of Hakes rejects detected was, to say the least, alarming and led to believe that either enormous quantities of immatures were present on the sea bottom at the time of catching or that trawl selectivity was lessened because of Norway Lobsters being present. This second point has been since verified and it seems that Hakes selectivity with or without Norway Lobster trawling was exactly the same. Norway Lobster rejects were not counted since values would have been unuseable on account of the difference between communal marketable size, i.e. 25 mm for cephalothoracic length and 34.5 mm for French size. They each would be 75 % for the first case against 55 % in the second case. Because of great modifications brought in fishery development in this area, it was crucial to redo a substantial evaluation of rejects: To obtain size composition of species not suitably tested through standard age analyses. To calculate manual sorting curves for species which rejects are hard to evaluate routinely, but only as long as rejects coincide with the commercial fraction of the catch.

Si la production piscicole du loup {Dicentrarchus labrax L.) connaît actuellement une croissance exponentielle en Europe du Sud, il est indispensable, pour mieux réfléchir les structures d'élevage, d'en maîtriser les rejets. Pour cela, il est primordial de quantifier les rejets de cette espèce de manière à proposer des ratios d'excrétion simples, facilement exploitables par l'éleveur. C'est à partir des bilans de masse de l'azote, du phosphore et du carbone établis sur une pisciculture travaillant de façon intensive et 4 lots (24,5 à 362,5 g de poids moyen) appartenant à son cheptel, que nous avons construit ces ratios. Chaque lot a été suivi pendant 1 mois et la pisciculture dans sa globalité 2 mois. Les bilans de masse établis pour l'azote et le phosphore sont équilibrés (intrants = extrants) mais jamais avec précision contrairement à ce que Ton peut lire dans la bibliographie. Les principaux rejets déterminés ont été liés aux biomasses en présence ainsi qu'à la quantité d'aliment distribué. L'excrétion carbonée du loup n'a pas pu être clairement définie. D'après ces suivis, le phosphore serait l'élément le mieux utilisé par le loup au vu des apports alimentaires. Les quantités d'azote, phosphore et carbone rejetées par l'élevage sont considérables puisque pour l'azote et le carbone 90 % de l'ingéré qui seraient rejetés dans le milieu ambiant.