The Population Biology and Reproductive Success of Pike in Gravel Pit Lakes.
The population biology of pike was studied in two gravel pit lakes of contrasting habitat type. The Main Lake of 17 ha has very little aquatic vegetation and is vey exposed; St peters Lake covers 2 ha and is a sheltered complex of interconnected lagoons with dense stands of aquatic vegetation. A comparison was made between the growth, mortality, population structure, numbers and biomass of pike in the two lakes and factors affecting recruitment are discussed.
Power law relationships were estimated between fecundity and fish length, weight and age, for pike from the two lakes. Relative fecundity did not differ significantly between the lakes and was estimated at 17.6 eggs g-1 for Main Lake pike and 19.4 eggs g-1 for St Peters Lake pike. Experimental work using samples of eggs stripped from 18 female pike and artificially fertilized showed a significant relationship between egg diameter and female length. Samples of eggs were taken from a pike spawning area in the Main Lake at two day intervals between 6 April – 5 May. These showed that egg distribution was clumped with average egg densities ranging from 14 – 671 m-2 on sand/silt substrates. The stage of development of these eggs was monitored over the sampling period. Egg losses were estimated at 9 - 10% per day. Survival from Stage 1 to hatched fry was estimated at 3 – 6%. Survival of eggs on aquatic plants, settled silt and disturbed silt were determined experimentally and the effect of silt deposition on egg hatching is discussed.
Survival of 0 group pike was studied in aquaria, experimental ponds and lakes to determine the main phases of mortality and consequent effects upon pike recruitment and subsequent year class strengths.
Yolk sac fry kept at densities of 15 – 200 fry per 0.054 m3 aquarium showed significant density-dependent mortality due to cannibalism. The role of cannibalism in the population regulation of pike is discussed. Yolk sac fry were stocked in early May in experimental ponds at densities of 0.74 – 81.4 m-2. The highest mean daily mortality rates occurred between May – July. Survival in December ranged from 0.059 – 11.25% of the initial stocking density.
Survival of 0 group in the St Peters lake was higher than that in the Main Lake in both 1986 and 1987. Recruitment in the Main Lake is variable. This is probably due to the lack of aquatic vegetation both as a spawning substrate and as cover from predators for 0 group pike.
The implications for management of pike fisheries are discussed and recommendations for future work presented.