The third spawning bed, which is dominated by F. lumbricalis, was visited twice within a period of three days at the end of April 2009. The sample taken on 21 April 2009 contained eggs AZD8055 mouse in the embryonic developmental stages (m–n), and 3 days later the eggs had embryos already in developmental stages (o–p). According to Rajasilta et al. (1989, 1993, 2006) red algae (including F. lumbricalis) have a negative effect on Baltic herring eggs, causing higher egg mortality. However, in this study, the embryos in the eggs collected from F. lumbricalis thalli developed normally to the very last developmental stages, resulting in successful mass hatching. One of the advantages of F. lumbricalis as a spawning

substrate could buy Epacadostat be the extensive 3D structure of the firm F. lumbricalis thalli. This can accommodate a larger number of eggs while ensuring their proper aeration compared with other spawning surfaces, on which eggs may be laid in multilayers. It is known that embryonic oxygen uptake increases in the later development stages ( Silva & Tytlerb 1973); in multilayer mats only the eggs in the upper layers develop successfully to the last stages, whereas the eggs in the deeper layers abort (the abortion stage is layer-dependent: the deeper the egg, the earlier the abortion stage) and/or show severe embryonic abnormalities ( Messieh & Rosenthal 1989), most likely due to the lack of

oxygen. This is less likely to happen when F. lumbricalis is used as a spawning substrate. The locations of Baltic herring spawning beds are

usually very specific (Geffen 2009), and there are reports that Baltic herrings return to the same spawning beds year after year (Oulasvirta & Lehtonen 1988, Bergstrm et al., 2007). This occurs even if there is a strong anthropogenic impact in the area (Rajasilta et al. 2006). During this study the hydrological conditions between two spawning seasons were very different: in 2009 there was strong upwelling resulting in colder water and higher salinity. In 2010 the upwelling was not significant and the water in the coastal area was mixed with hyper-eutrophic Curonian Lagoon waters, resulting in greater turbidity and lower salinity. Moreover, the winter in 2010 was much colder and longer compared with 2009, resulting in later spawning (Figure 4). Despite these differences, 4��8C the spatial pattern of the spawning persisted, indicating that there are more stable factors determining the distribution of the spawning beds than just the hydrological conditions. One such factor could be the bottom geomorphology, which was tested in this study in terms of the bottom slope. Table 3 shows the average 100 m profile slopes to the east and west of the sampling points, the average profile depth gradients as well as the average maximum westward and eastward slopes values for 10 m segments with corresponding standard deviations. Graphical representations of the bottom profiles are shown in Figure 5.