Biomanipulation in Lake Årungen, Norway: A Tool for Biological Control

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This chapter deals with whole-lake experimentation for the restoration of the water quality and reduction of pollutants in fish from the eutrophic Lake Arungen, Norway, through biomanipulation of the top predator fish, the northern pike (Esox lucius). The manipulation was carried out during 2004–2006, with a main effort to remove large cannibal pike (>65 cm), and the cascading impacts on the major fish community were evaluated. The main fish species in Lake Arungen are roach (Rutilus rutilus), perch (Perca fluviatilis), pike, and rudd (Scardinius erythrophthalmus) with some additional fish species in low numbers. The main assumptions behind the removal of large pike were to obtain (i) increased number of small pike and large perch due to reduced cannibalism and predation pressure; (ii) reduced abundance of roach and small perch due to increased number of piscivores, i.e., small pike and large perch, and (iii) reduced levels of pollutants in the remaining fish community by increased growth rates and changes in diet. The recruitment success of pike after removal of large pike was evaluated by the Petersen mark–recapture method. The habitat utilization of the major fish species was studied based on gillnet catch data. The trophic position of different fish species was determined based on both gut content analyses and analyses of stable isotopes of nitrogen (15N/14N=δ15N) and carbon (13C/12C=δ13C). Important biological factors influencing the dynamics of mercury and organic pollutants through the fish community in the lake were identified as well. The estimated numbers of small pike (45–64.9 cm, mostly age class 3+) significantly increased in 2005–2006, after the experimental removal of the large pike in 2004 and 2005, and thus assumed to be a cannibal-driven response. The dominant prey fish of pike was roach, both in terms of numbers and biomass. The relative abundance of piscivores increased and as a consequence the relative abundance of small roach declined in 2005–2006. A marked shift in habitat utilization of roach and perch was recorded after the manipulation of the pike population. The gut content analysis was in agreement with the food web analysis based on the stable isotopes of nitrogen and carbon in 2005–2006. Pike (exclusively piscivores) and perch (feeding mainly on zooplankton, macroinvertebrates, and fish) occupied the top trophic positions in Lake Arungen, with pike at the highest position. Roach and rudd occupied lower trophic positions, with rudd at the lowest. Although there was segregation in diet between roach and perch, the increased importance of zooplankton in their diet during the summer 2006 may indicate an increased abundance of zooplankton. Mercury content in fish muscle increased with size and weight (bioaccumulation), as well as with an increased position in the food web (biomagnification), with pike having the highest concentrations. Growth rate of pike in age classes 1–3 increased in 2005, probably due to the lower predation risk, and thereby a more extensive use of the most favorable pike habitats. The faster growth rate might have played a role for a significant decrease in total mercury concentrations, probably as a result of growth biodilution (~50% reduction between same size groups of pike). None of the fish analyzed in 2005 had a mercury concentration which would have exceeded the FAO/WHO provisional tolerable weekly intake value (1.6 μg kg–1), whereas the mercury concentration in only four analyzed pike in 2003 would have exceeded the recommended limit. The metabolite pp’DDT formed the main part (88%) of the total concentrations of the DDTs (dichlorodiphenyltrichloroethane) in fish liver. Both ΣDDT and ΣPCB (polychlorinated biphenyls) were significantly higher in pike compared to perch and roach, whereas HCB (hexachlorobenzene) and HCE (heptachlor epoxide) were detected mainly in pike. In pike, males had significantly higher concentrations of all organochlorine compounds compared to females. Such a sex-dependent differentiation in the concentrations of organochlorine compounds may be attributed to physiological differences, for example, a higher elimination of the lipophilic pollutants in roe during spawning by female pike and higher growth rate of female pike. The lower levels of organochlorines in pike and perch in 2005 compared to analyses performed in 2004 might be an effect of the biomanipulation of Lake Arungen as well. Therefore, selective exploitation of a large top predator fish, e.g., large pike, may serve as a tool to reduce the concentrations of mercury and organochlorine compounds in the remaining fish community. To obtain a sustained positive effect both on the trophic status of the lake and on the pollutant levels in the fishes, as obtained by the manipulation of the large pike in 2004–2006, a high fishing pressure on the large pike has to be persistent and carried out annually.