Physical and Biological Constraints
The Baltic herring is adapted to reproduce in low salinity, which makes it capable for spawning in all parts of the Baltic Sea, including such areas where nearly freshwater conditions prevail. Experimental work has shown that eggs of the Baltic herring are fertilized in a wide range of salinity, but the best result is obtained in salinity of ca. 8 PSU, which probably is the salinity optimum.
The reproductive success is modified by many different factors starting from the fertilization of eggs during the spawning act. The development of eggs from fertilization to hatching takes several days and even weeks, depending on the water temperature. During this period eggs are exposed to the surrounding environment rather unprotected. All eggs deposited on the spawning substrate don’t survive but are eliminated by different factors, including fish predation. For instance in our study area, perch (Perca fluviatilis) and whitefish (Coregonus lavaretus) feed on herring eggs during the spawning time.
Even the species composition of the plant community on the spawning beds may affect the reproductive success of the herring. As shown by our studies (see publications list below) and those made in Sweden (G. Aneer, 1987), mortality of herring eggs varies depending on the plant species that forms the egg substrate. The obvious cause is allelopathy; i.e. toxic substances that are excreted by the plants either actively or through decomposing. Filamentous brown algae (Pilayella, Ectocarpus) and red algae (Furcellaria) are an example of this; experimental studies have shown that mortality of eggs increases when eggs are attached to these algae.
Determinants of Egg Survival
The reproductive success is not dependent on the external factors only, but also different parental properties can modulate the result of reproduction. In the experiments, we found differences among females both in fertilization rate and in embryonic mortality, and there were also differences among males in fertilization success. In our experiments, the overall reproductive result (‘hatching success’; the proportion of hatched eggs of all spawned eggs) of the “poorest” individuals was only 10% of the result of the best females, which implies that the parental differences may be high enough to contribute to the large variation observed in the samples collected from the spawning beds. Based on the analyses, the fertilization success of Baltic herring eggs is a joint effect of both female and male properties, whereas the embryonic mortality seems to be a maternal trait.
Also the composition of the egg seems to affect embryonic survival. A high fat content of eggs, measured as the total amount of lipids per an unfertilized egg, was connected with lower egg mortality during later embryonic stages (after eye pigmentation), as was also a high amount of polar lipids, suggesting that energy demands are high and that intense growth of the embryo may deplete egg yolk reserves if the lipid content is too low. However, a high fat content may also be a disadvantage in the early stages. In Baltic herring, high triacylglycerol (TAG) levels in the unfertilized eggs of the mother fish were associated with increased egg mortality during early embryonic development (before eye pigmentation). Although TAGs are important as storage lipids, and are used as an energy source during embryonic development, it is possible that their high amount reflects deficiencies in other yolk components necessary for the successful completion of embryogenesis.
Although the fertilization rate of herring eggs in the experiments was largely variable among parents, we were not able to point out parental properties connected with high success. However, female size, condition factor and fat content were linked with embryonic development. Eggs from larger fish had a better survival during the early embryonic stages. Females with a higher condition factor or muscle fat content produced eggs showing lower early embryonic mortality and also a better total survival and hatching success, although there was no direct correlation between these female properties and the fertilization percentage or the late stage mortality. Partial correlation coefficients further suggest that high female condition is related to good reproductive success irrespective of fish size (see table below). Obviously, females in good condition can provide their eggs with more of the constituents that are important for successful development especially during the sensitive early embryonic stages.
Table: parameters of reproductive success in relation fo female properties: Pearson’s partial correlation coefficients marked with * are significant at level p=0.003
|n=55||Fertilization rate (n=70)||Early Mortality||Late Mortality||Survival||Hatching success|
|Muscle fat DWt %||0.039||-0.503*||-0.167||0.406*||0.398*|
- Rajasilta, M., Mäkinen, K., Ruuskanen, S. Hänninen, J., Laine, P. (2021) Long-term data reveal the associations of the egg quality with abiotic factors and female traits in the Baltic herring under variable environmental conditions. Frontiers in Marine Science; Marine Fisheries, Aquaculture and Living Resources. Doi: https://doi.org/10.3389/fmars.2021.698480
- Rajasilta, M., Laine, P. & Eklund, J. 2006. Mortality of herring eggs on different algal substrates (Furcellaria spp. and Cladophora spp.) – an experimental study. Hydrobiologia 554: 127-130.
- Laine, P. 2003. Regulation of reproduction in Baltic herring – individual effort meets the environment. Annales Universitatis Turkuensis Ser AII TOM 166. Turun yliopisto, Turku (academic dissertation).
- Laine, P. & Rajasilta, M. 1999. The hatching success of Baltic herring eggs and its relation to female condition. J. Exp. Mar. Biol. Ecol. 237: 61-73.
- Laine, P. 1999. Paternal effects on the hatching success of Baltic herring eggs. ICES J. Mar. Sci. 56 (Suppl.): 142-146.
- Laine, P. & Rajasilta, M. 1998. Changes in the reproductive properties of Baltic herring females during the spawning season. – Fisheries Research: 36: 67-73.
- Rajasilta, M., Paranko, J. & Laine, P. 1997. Reproductive characteristics of the male herring in the northern Baltic Sea. – J. Fish. Biol. 51: 978-988.
- Rajasilta, M., Eklund, J., Kääriä, J. & Ranta-aho, K. 1989. The deposition and mortality of the eggs of the Baltic herring, Clupea harengus membras L., on different substrates in the south-west archipelago of Finland. – J. Fish. Biol. 34: 417-427.