MEASURES BENEFIT BOTH FISH AND ANGLERS
With a surface area of 594 km2 and an average depth of approximately 3.3 m, Lake Balaton, located in Hungary, is the largest shallow lake in Central Europe. The fish stocks of the lake are no longer self-sustaining, mainly due to significant pressure from angling.
Natural recruitment cannot replace the fish biomass removed from Lake Balaton by anglers, since earlier water regulation measures (the construction of the Sió-floodgate in 1863-64, and the draining of the surrounding wetlands) eliminated a considerable part of the natural spawning grounds. The spawning of native fish species subjected to heavy angling pressure can still take place under suboptimal conditions—for example, on the sheltered rip-rap (stony material placed along shorelines to prevent erosion) habitats of harbours. Altogether, 97 different harbours were built in Lake Balaton. These harbours could therefore be an important habitat type for the fish fauna.
Fish stocking is crucial to maintaining the fish stock structure expected by anglers. To improve sustainability and reduce the costs associated with this fishery management practice, recently EU-funded projects have established artificial reefs and spawning nests on Lake Balaton.
Fishery management of Lake Balaton and the importance of pike-perch
Lake Balaton has a long history as a fishing ground. The first decades of the 20th century can be considered its “golden era”. During this period, pike-perch (Sander lucioperca) from the lake were exported to France, Germany and Austria, serving as a famous ingredient in fine dining. Later, along with the eutrophication of the lake and intentional introduction of non-indigenous fish species (e.g. European eel, Asian carps), the target species of fishermen changed. The activity of commercial fisheries was completely stopped in 2013. From that point onwards, the main purpose of fishery management of the lake has been to develop and maintain the fish resources according to the necessities of anglers, and to develop angling tourism while considering ecological constraints.
Pike-perch is one of the most important apex predators of the lake’s fish community as well as one of the most important target species for anglers. Consequently, the pike-perch population has a key role in the ecosystem of the lake but is subject to high and constantly increasing pressure. To prevent the further overexploitation of pike-perch population, two projects were initiated between 2016 and 2020.
Artificial reefs: the old-new solutions
Water level regulations were initiated in Lake Balaton in 1963. Around the same time, wetlands along the southern shoreline were drained and canalised. Commercial fishermen were the first to realise the negative effects of these processes. The fishermen knew that fish, especially pike-perch, use reefs and rip-rap habitats as places to rest and spawn. As a result, the fishery management company established numerous artificial reefs of red sandstone throughout the lakebed. These structures were periodically renewed until the 1970s.
While no scientific survey of these structures was ever carried out, fishermen had continuously offered positive assessments regarding the fish population structure and biomass in surrounding areas. Based on this, recent fishery management decided to establish nine new artificial reefs on sandy or muddy bottoms of Lake Balaton in 2020. The new habitats were rapidly colonized by aquatic organisms; however, the fauna composition was strongly dominated by non-indigenous species.
Role of non-indigenous species in the life of artificial reefs
Biological invasions are often facilitated by disturbances. The construction of an artificial reef is a significant disturbance to the original bottom-dwelling community. The colonisation pattern is presumably similar to the well-studied nearshore structure of harbour rip-raps. The first inhabitants are macroscopic invertebrates of Ponto-Caspian origin, such as killer shrimp (Dikerogammarus villosus) and river nerite (Theodoxus fluviatilis). Initial fish assemblage of these habitats is also dominated by invasives. Black bullhead (Ameiurus melas), monkey goby (Neogobius fluviatilis) and pumpkinseed (Lepomis gibbosus) are the most abundant species.
Monitoring results suggests that the abundance of invasive species may decrease in time, however, they usually cannot be completely eradicated. These species may have a negative effect on the spawning success of native species such as pike-perch, including egg and fry predation. In short, while artificial reefs are indisputably attractive for pike-perch and other native species, they strongly enhance non-indigenous biodiversity.
Spawning nests to enhance the pike-perch population
Spawning nests have long been used to support pike-perch stocks. However, previous applications used organic materials, such as spruce or juniper bundles, which were suitable for only the given spawning season. In the case of Lake Balaton, a more sustainable option was necessary. Altogether, ten spawning nest prototypes were tested, considering different bottom conditions. A round nest covered in artificial grass, measuring 80cm in diameter, proved to be the most effective, with a mean number of 380 000 eggs produced per nest. Over 2,200 nests were produced based on these results. These nests have become an important element of pike-perch stock maintenance in Lake Balaton, without any known side effects.
Conclusions: the necessity of detailed research
While artificial reefs and nests are both useful tools in fishery management practice, there is a gap in the knowledge regarding their effects. Obligatory monitoring activities and tracking of project outcomes often cannot provide spatiotemporally relevant information on the colonisation processes and annual habitat usage of commercially important fish species. Additionally, very little information on the potential effects of architectural complexity (e.g. material, size distribution, emergence etc.) of artificial reefs is available.
If these questions could be addressed, artificial structures could potentially contribute to local biodiversity maintenance, without promoting biological invasions.
Árpád Ferincz1, Gábor Nagy2, Zsolt Szári2, Béla Urbányi1
1: Institute of Aquaculture and Environmental Safety, Hungarian University of Agriculture and Life Sciences, Gödöllő
2: Balaton Fishery Management Nonprofit Ltd., Siófok