Dispersion of vendace eggs and larvae around potential nursery areas reveals their reproductive strategy

Depending on their reproductive strategy, different fish species aim to aggregate or disperse eggs and larvae in their reproductive habitat. Many pelagic species disperse their eggs widely around the potential nursery areas. Larval dispersion or aggregation affects population sub-structuring, which has important implications in fisheries management and conservation of the natural spatial diversity in populations. The dispersion of larval vendace (Coregonus albula) was quantified in two oligotrophic Finnish lakes, and effects of density and environmental variables on the inter-annual variation in the larval distribution were examined by analysing spatial abundance data from the lakes from 1999 to 2017. A 3-D hydrodynamic egg distribution model was used to simulate the larval transport after hatching. Vendace larvae dispersed lake-wide to both littoral and pelagic zones but, in some littoral hot spots, more larvae aggregated year after year. However, in years of high larval number, the densities increased not only in the hot spots, but generally at all sampling plots. An overall increase in abundance was observed at all sampling sites. The simulations of the egg distribution model supported the hypothesis that the dispersion of the eggs occurs by spawners, i.e. by spawning at several different spawning sites, which are located all around the lake. The dispersion of vendace eggs and larvae can be seen as a bet-hedging strategy in space and time since in boreal oligotrophic large lakes with fragmented morphology, weather and other environmental factors in spring during hatching varies from year to year spatially in unpredictable manner. Lake-wide larval dispersion suggests that the subpopulations of adjacent lake deeps may swap considerable amounts of individuals during early life and may not be closed units. Conservation of particular habitats seems unnecessary for Finnish vendace populations where large potential spawning areas in lakes are available.