Effects of Artificial Propagation and the Weather on Recruitment in the Lake Ontario Whitefish Fishery

Abstract

No significant relationship could be found between variations in the level of fry planting and ensuing variations in the level of catch in the long-term statistics of the Lake Ontario whitefish gill-net fishery. An alternate-year planting experiment in which fry were planted in the even-numbered years 1944 through 1954, similarly failed to show a detectable level of contribution of the hatchery fish to the fishery. The supported year-classes averaged larger than those not given hatchery support but this was judged coincidental because of a phasing with an alternate-year periodicity which characterized the catch statistics over the whole series examined, and was present in the six years prior to the start of the alternate-year plantings. Estimates of spawning stock were calculated and it was not found that the progeny-per-parent ratios were significantly affected by the fry plantings.A possible explanation for the periodicity was suggested by a significant correlation between air temperatures at the times of spawning and hatching, and the strengths of the produced year-classes. Cold Novembers followed by warm Aprils appeared to provide conditions associated with the production of larger year-classes, with the opposite combinations relating to the weaker broods.The disappearance of lake trout and ciscoes from the commercial catch resulted in increased fishing pressure directed towards the capture of the whitefish. Increases in the efficiency and intensity of the fishing were observed during the period of the study and these were likely responsible for the reduction of the average age of the fish in the catch by almost one year, and the resultant restriction of the annual catch to one year-class in recent years. Probably because most of the fish are currently caught before first spawning, a decline in spawning stock was almost continuous during the study period. The year-to-year fluctuations in the level of the catch increased both because of the reduced average age, and because of a greater variation in year-class strength. The year-classes produced in favourable years tended to increase, down to quite low levels of stock. This gave a configuration similar to Ricker's Type C reproduction curve, and which differed chiefly in that the limiting diagonal representing minimum reproduction fell below the replacement diagonal. It was suggested that the whitefish may require more than one spawning, to achieve stock replacement. The present instability of the catch, and the potentially serious effect of any sustained break in the rhythm of the climatic conditions suggest a condition of over-exploitation in this case.