Avlijaš, S., A. Ricciardi, and N. E. Mandrak. 2018. Eurasian tench (Tinca tinca): the next Great Lakes invader. Canadian Journal of Fisheries and Aquatic Sciences, 2018, 75(2): 169-179, https://doi.org/10.1139/cjfas-2017-0171
The Great Lakes are a hot bed for invasions, and aquatic invasive species (AIS) from the world-over have ‘hitchhiked’ on shipping vessels, or have accidentally been released into the lakes over many years. AIS can severely affect the water quality, food-webs, nutrient cycling, and fish productivity of invaded waters. Notable examples in the Great Lakes basin include zebra mussels (Dreissena polymorpha) and Asian carp (which actually consists of four species from the family Cyprinidae). In fact, up to now the Great Lakes have been invaded by at least 188 AIS, out of which 28 are fishes. In this paper Avlijaš and colleagues (2018) identify and review threats posed by the Eurasian tench (Tinca tinca), as they appear to be the most likely invader to expand into the basin next.
The tench is another species of cyprinid fishes originating from parts of Europe and Asia and has been used in aquaculture and sport fishing since the Middle Ages. As a result of human-mediated expansion of its range the tench is often absent from its native regions but successfully outcompetes other non-natives in other countries like Norway. The North American tench population was established through several introductions, starting in the 1980’s when they were part of watershed stocking programs in the United States. Since then tench have been introduced to Quebec through the illegal activities of a farmer and were detected in the St. Lawrence River in 2006 and have recently been found in Ontario. Likelihood of tench expanding to nearby lake watersheds remain high because illegally rearing tench in ponds appears to be quite widespread.
Similar to the other successful AIS, tench have a wide range of environmental tolerances. Tench have the ability to survive in low oxygenated waters, helping them outcompete native species that are more sensitive to oxygen depletion and are unable to survive in highly productive waters. Part of their survival strategy involves tench burying themselves in the mud to outlive anoxic conditions, partial drying and/or freezing in shallow waters. Tench also have a long life span and can survive up to 20 years! They are prolific reproducers and can spawn multiple times during a season, potentially releasing hundreds of thousands of eggs per fish. Experimental evidence suggests that warming water as a result of climate change may favor a longer reproductive season for tench to increase their reproductive fitness. Additionally, tench are non-specialist generalist predators, exploiting a wide range of prey items including zooplankton (microscopic animals), aquatic insects, crayfishes, snails, and mussels.
Although the ecological impacts of tench in North America are largely unknown, we can extrapolate from the impacts observed in other regions. For one thing, tench can establish in such high numbers that they’re considered detrimental to native sport fish species. In addition to tench outcompeting native fishes they can also consume eggs of native fishes. All these characteristics make tentch ideal candidates for AIS and give Avlijaš and colleagues good reason to fear for yet another species spreading through the Great Lakes.
Several risk assessments have established the habitat suitability of the Great Lakes basin for tench and at least one assessment found that the risk of tench invading the Great Lakes was high. Using computers to simulate their potential distribution in the basin and researchers found that shallow areas are particularly at risk of tench establishment. This is attributed to the illegal activities of individuals, such as rearing tench in ponds, which are potential pathways for tench introduction in to the basin. Once a viable population is established in the St. Lawrence River they will likely spread westward. With this spread of tench comes decreasing water clarity, transmitting diseases and parasites, and outcompeting of native fishes.
Early detection and education may be the key to preventing establishment of viable tench populations in the Great Lakes basin. Since fish farming ponds are known entry points, strategically sampling these sites may serve as an early warning system. However, since tench are introduced through human actions, educating and successfully communicating the risks of a tench invasion and their potential ecological impacts may be the most instrumental part of the fight to keep these invaders out of the Great Lakes.