Changes in main biological indices and nutrition of european perch (Perca fluviatilis linnaeus, 1758) in operating special table fish farm

Abstract

The perch (Perca fluviatilis Linnaeus, 1758) is a small predator that is widespread in small reservoirs of Ukraine. Unfortunately, available information does not give a clear proved notion of its impact on the smelt of traditional polyculture of bigheaded and common carps in the small reservoirs, which are actively used in commercial activities, such as special table fish farm (hereinafter referred to as STRG). This is a special procedure for fisheries on water bodies that has an intermediate position between fisheries and aquaculture: the STRG user is harvesting by commercial fishing fish species, which he himself stocked in the water body. Furthermore, Ukraine is not unique in that kind of activities. It is close to the globally recognized concept of "Culture based fisheries" or "fisheries based on aquaculture". However, it should be noted that the reservoirs themselves have their native fish fauna, which will undoubtedly affect commercial activities. This is especially true of the water body has some predatory fish species in the fish fauna. This raises the question of age group selection and the appropriate average weight of stocking material. The issue is very simple: stocking material with less weight is cheaper and significantly increases profits. On the other hand, small abundant predators, especially European perch (P. fluviatilis), quickly consume it and can reduce the stocking process. In this occasion, the cost of error or failure to comply with a proposed set of measures for fish planting material can have significant negative consequences, which in some cases can significantly reduce the efficiency of stocking. The main objective was to investigate changes not only in basic biological indices, but also in fish diversity of the perch diet. In addition, authors also wanted to test and the effectiveness of regulation measures for P. fluviatilis abundance in actively functioning STRG. Similar series of studies with a 10-year interval have not been conducted before.

Materials and methods of research. The primary materials were collected during 2019 during commercial fishing, reclamation fishing and angling. The sampling of ichthyologic samples were carried out from the catch of a seine (50/40/35 mm, h = 1.5, l = 150 m) and a juvenile seine (10/10/5 mm, h = 1,0, l = 50 m).

The STRG user followed the proposed reclamation activities: specialized fishing in juvenile seine in summer and specialized ice-angling in winter for 10 years. As a result, the perch stock has decreased by almost half since 2010 [7], amounting to 1.45 t. in 2019. This data gave a proof that the activities undertaken in 2010-2019 were effective and contributed to a significant downsizing of the of the P. fluviatilis population. Despite the sufficient feeding conditions, progress and upward trend in quantity were not detected. This was a positive effect, because European perch is dangerous for the cyprinid juveniles that were stocked in the STRG.

The research gave a This allows us to state that, despite the considerable potential to increase in quantities, the species composition of fish in perch feeding varied significantly between pre-STHG and STRG mode. In 2010, there were some unintentional stockings in the reservoir, but there was no formed stock of traditional polyculture fish species, thus the basis of the structure of the food was roach (Rutilus rutilus). It ranged from 40 to 55% in terms of occurrence and 30-50% in content. In general, it can be stated that the perch feeding of the reservoir near Dedivtsi was mainly due to the native fish fauna. These data showed that that during the decade of intensive fish stocking with a traditional polyculture of bigheaded and common carps, the ratio of fish species in the perch diet had changed significantly and began to be based largely on the STRG cultivation objects that definitely had a potential negative economic effect. Individual improvements in size and weight of the perch were also noted against the background of the decrease in the number and elimination of elder age groups from the reservoir.

Massive planned stocking of the reservoir with traditional polyculture juveniles during ten years of stable STRG operation allowed to form commercial stocks of bigheaded and common carps that could be effectively harvested by commercial fishing. From the other hand, the high concentration of these juveniles made them more accessible to European perch; consequently, they took more significant role in the perch's diet. Thus, according to 2019, representatives of traditional polyculture were 60-75% in terms of attendance and 63-74% in content. It should be noted that in 2010 these figures were respectively 8-18% and 15-37% respectively. This fact indicates that the perch feeding of this reservoir as of 2019 is predominantly based on the youth of the AWGS cultivation facilities. This negative impact is greatly reduced by land reclamation measures that keep the perch population low. Individual improvements in size and weight of the perch were also noted against the background of the decrease in the number and elimination of elder age groups from the reservoir. As plans for future, it was foreseen to monitor the size and biomass of the studied population to ensure the balance of economic feasibility and biological diversity in the future. In addition, gained positive experience of population abundance control could be implemented in numerous reservoirs-analogues.