Early detection of reproduction deficits and the compensatory potential of enhancement stocking for vendace, Coregonus albula, fisheries in German lakes

Abstract

Enhancement stocking is a widespread but rarely evaluated practice in German vendace, Coregonus albula L. fisheries, which is precautionarily carried out to buffer yield fluctuations. However, it is assumed that stocking of larval vendace can elevate year‐class strength only in the presence of substantial reproductive deficits, which so far cannot be detected prior to the spring time period when stocking is conducted by fisheries managers. Simple measures for the early quantification of natural reproduction were tested, and the efficiency of larval enhancement stocking in a lake with strong reproduction deficits was analysed. Analysis in four lakes over 3 years revealed a significant positive correlation between standardised catches of vendace larvae caught with pump‐driven illuminated traps in spring and the abundance of young‐of‐year (YoY) vendace in the following autumn, whereas standardised catches of spawners were insufficient to forecast YoY recruitment. In a lake with low natural vendace reproduction, stocked vendace contributed 73–100% of year‐class strength and fisheries yield. Accordingly, enhancement stocking can efficiently compensate for reproduction deficits, and the simple larvae sampling method may facilitate early detection of these deficits, which would allow fisheries managers to respond accordingly through larvae stocking.

A matter of time—Efficacy of whitefish stocking in a large pre‐alpine lake

“…A relatively low proportion of stocked whitefish of one cohort, as we found for Lake Constance with 17.4% for the 2016 cohort and 24.5% for the 2018 cohort, has also been reported for other lakes in the alpine region (Meng et al, 1986;Müller, 1990) and elsewhere (Salojärvi, 1992;Wanke et al, 2016). Most of the time, natural recruitment occurred in those lakes and several authors argue that stocking is superfluous under these conditions (Gerdeaux, 2004) or that stocked fish only survive in high numbers if natural recruitment is deficient (Wanke et al, 2016;Wedekind et al, 2022). Based on these results and the fact that natural born offspring seem to dominate the whitefish cohorts in Lake Constance, higher proportions of stocked fish in the future are unexpected.…”

“…Meanwhile, other studies have shown that additive effects of stocking with larval fish are generally unlikely, because early life stages are vulnerable to density‐dependent mortality bottlenecks (Beddington et al, 2005; Lorenzen, 2000; Lorenzen & Camp, 2019). In general, additive effects of stocking are only achievable where natural reproduction is severely limited (Beddington et al, 2005; Hühn et al, 2014; Wanke et al, 2016). In ULC, natural reproduction is most likely several times higher than stocking (Baer, Gugele, et al, 2021).…”

“…A similar approach is most likely feasible in other lakes where large, cost‐intensive mark‐recapture studies could not be realised. Furthermore, it is highly beneficial if the bottleneck for low stocking success is known (Wanke et al, 2016) to adapt the stocking strategy (Hunt & Jones, 2018). If for example the stickleback predation on whitefish eggs (Baer, Gugele, et al, 2021) and larvae (Roch et al, 2018; Ros et al, 2019) is confirmed to be a bottleneck for whitefish cohort size in Lake Constance (Rösch et al, 2018), increasing the size at which whitefish are stocked (>40 mm, see Roch et al, 2018) might reduce predation losses and thereby increase the efficiency of stocking.…”

“…Most of these stocking programs are intended to support local whitefish populations to compensate for a shortfall in natural recruitment or to stabilize fluctuating fishery yield by reducing annual variation in cohort size (Eckmann et al, 1988). Many previous evaluations of effects of larval stocking on yield or year‐class strength in fisheries around the world have found variable effects, with some showing little or no effect (Salojärvi, 1992; Wanke et al, 2017), others showing negative effects (Anneville et al, 2015), and others showing increased harvest (Anneville et al, 2009; Wanke et al, 2016; Wedekind et al, 2022). Furthermore, the impact of stocking programs can vary over time, as in Lake Geneva, where whitefish abundance and fishery yield both increased over the first 10 years of stocking, but subsequent increased fishery yield was mostly driven by increased recruitment resulting from improved limnological conditions (Anneville et al, 2009; Gerdeaux, 2004).…”

How Many Ciscoes Are Needed for Stocking in the Laurentian Great Lakes?

“…Northern Europeans have a long history of rearing coregonines, such as Vendace C. albula ("European cisco") and European Whitefish C. lavaretus, to advanced life stages on a large scale (e.g., Salonen et al 1996;Wanke et al 2016), methods of which are being implemented in the Great Lakes. Bloaters from Lake Michigan and Ciscoes from remnant Lake Ontario populations have been reared and stocked in Lake Ontario since 2012 (OMNRF 2016;Bronte et al 2017), although the Cisco stocking program was recently discontinued.…”