The evaluation of the stocking of pike fingerlings

Summary In order to evaluate the stocking of artificially propagated northern pike (Esox lucius L.) fingerlings (4–6 cm), of which 1–1,5 million/year are produced by the Organization for the Improvement of Inland Fisheries (O.V.B.), the composition and abundance of northern pike populations in four shallow waters were monitored during a 5–8 year period. The specimens stocked were marked by amputation of a ventral fin. At the end of their first growing season the numbers of these and of those that were naturally reproduced (O⁺ pike) as well as the abundance of larger specimens were estimated using mark-recapture methods. Sampling was executed by electrofishing, seining and (incidentally) trawling. The O⁺ pike was caught quantitively by electrofishing.Based on the distribution pattern observed during the sampling period, four age/length classes were distinguished. (1) O⁺ pike, caught almost without exeption within emerged or submerged vegetation; (2) O⁺<pike<41 cm (forklength) found within submerged, floating and ingrowing vegetation; (3) 41 cm<pike<54 cm caught within or in the vicinity of vegetated areas; (4) pike>54 cm, the majority of which — in two experimental waters — were found outside the vegetation. Within the length range of 0–54 cm, the biomass of O⁺ pike and of 0–41 cm pike appeared to be negatively correlated with those of larger pike. The standing stock<54 cm pike was found to be determined by the amount of aquatic vegetation. This phenomenon was still more pronounced when the different habitat preferences of the various length classes were taken into account (GRIMM, 1981).Based on the negative correlations it is concluded that the biomass of 0–41 cm pike/ha preferred habitat is determined by the biomass of larger northern pike and that the stocking of fingerlings does not influence this relationship.In one of the experimental waters a high mortality occurred due to the severe winter of 1978/–1979, causing a decline of at least 50% of the abundance of legal-sized (48 cm) northern pike. In the following year (1979) the numbers and biomass of the O⁺ class amounted to 10 and 5 times the previously recorded highest values, respectively. In 1980 the O⁺ class was virtually absent. As a result of these extreme values the negative correlation was found to be exponential, indicating a density-dependent relationship between smaller (0–41 cm) and larger (41–54 cm) pike. It was shown that, in the 3 years fingerlings were stocked, the abundance of 0–41 cm pike (kg/ha preferred habitat) relative to the abundance of larger pike, did not differ from that in the 4 years in which fingerlings were not introduced.In the two years that pike-fingerlings were introduced in the beginning of May in this water, the frequency of occurrence of O⁺ pike, originally stocked as fingerlings, amounted to ca. 80%. In the year stocking took place at the end of May, introduced pike constituted only ca. 6% of the O⁺ population. These high and low frequencies are explained by the fact that the first and last introduced specimens are either ahead of or behind the ecological time table: they are on the average larger or smaller then the naturally reproduced specimens.Intraspecific predation within the O-class can be a factor of importance (GRIMM, 1981). Therefore, it is supposed that the frequency of occurrence reflects the number of naturally reproduced pike that were replaced by the stocked ones.