Just as the effectiveness of stocking varies from one species to another, huge differences can also be seen when the same species is stocked in different lakes. A study investigating the effectiveness of whitefish stocking – commissioned by the Hunting and Fisheries Section of Canton Aargau – was recently carried out in Lake Hallwil. Here, oxygen depletion in the depths of the lake had led to the extinction of the original whitefish population. For over 30 years, efforts have been made to improve oxygen availability by aeration of the lake. Whitefish populations have been supported by stocking measures. In recent years, water quality has continuously improved and whitefish yields have risen again. Does this mean that natural reproduction has recovered and stocking measures can be stopped? To answer this question, all the hatchery fish reared in one year were otolith-marked, so that the proportion of stocked fish in the lake could subsequently be determined. The results clearly show that natural reproduction of whitefish in Lake Hallwil remains severely restricted. More than 90 per cent of the juveniles were found to be hatchery-reared. However, whitefish capable of reproducing naturally in Lake Hallwil do exist, and the proportion of wild fish is expected to increase further in the future as oxygen conditions in the lake continue to improve.
In 2003, a similar project was carried out by the University of Konstanz and the St Gallen Fisheries Authority on behalf of the International Conference of Deputies for Fishery in Lake Constance (IBKF). The relative proportions of (otolith-marked) stocked and wild fish were determined six months after stocking. In the “0+” age group, over 60 per cent of the whitefish caught were hatchery-reared. How many of the stocked fish would subsequently be caught by professional fishermen or anglers? When spawners were captured in Lake Constance two and three years later, the proportion of marked whitefish was again assessed. The results were similar to those obtained for the “0+” fish. The stocked whitefish thus appeared to fare very well over a period of several years in Lake Constance.
The same question was explored in the study of Arctic char in Lake Geneva. Here, the findings were rather different: while stocked fish accounted for over 80 per cent of the “2+” age group, the proportion fell to less than 50 per cent two years later (“4+” age group, see Fig. 4). Over the years, Arctic char derived from natural production thus fare better than their hatchery-reared counterparts.
Assessments of the performance of a fish species in different lakes indicate that the effectiveness of stocking largely depends on the conditions prevailing in a given lake. In Lake Geneva, where natural reproduction is highly successful, the proportion of stocked whitefish is negligible. In Lake Hallwil, by contrast, stocking measures remain indispensable to the survival of whitefish populations for the time being.
Moving in the right direction
But why does natural reproduction not succeed in some lakes even though phosphorus levels have returned to the original range? If spawning sites are not otherwise degraded (e.g. due to a lack of gravel), the answer probably lies primarily in the sediments where egg development should take place. In lakes which were formerly heavily polluted, sediments remain virtually anoxic and continue to cause oxygen depletion. In cases where pollution levels were extremely high in the past – e.g. Lakes Hallwil, Baldegg or Sempach – this condition is likely to persist. However, the fact that whitefish can now reproduce successfully in Lake Geneva, where pollution levels used to be fairly high, gives grounds for hope that natural reproduction could be equally or even more successful in many other less polluted lakes. Future assessments of stocking outcomes planned for Lakes Constance and Hallwil, and hopefully also elsewhere, should indicate whether particular species can reproduce naturally in the various lakes or whether supportive measures are still required.
This article is based on a number of studies and reports, which the authors will be happy to make available on request.