Relationships between fish stock changes in the Baltic Sea and the M74 syndrome, a reproductive disorder of Atlantic salmon (Salmo salar)

Abstract

Mikkonen, J., Keinänen, M., Casini, M., Pönni, J., and Vuorinen, P. J. 2011. Relationships between fish stock changes in the Baltic Sea and the M74 syndrome, a reproductive disorder of Atlantic salmon (Salmo salar). – ICES Journal of Marine Science, 68: 2134–2144. The M74 syndrome of Baltic salmon (Salmo salar), which appears as increased yolk-sac fry mortality (YSFM), impairs the reproduction of salmon stocks. Changes in the prey stocks of Baltic salmon in its two feeding areas, the southern Baltic Proper (BPr), where sprat (Sprattus sprattus) was the main prey species during the high incidence of M74, and the Bothnian Sea, where herring (Clupea harengus) is the dominant species, were analysed in relation to salmon growth and size and in relation to the incidence of M74. The high condition factor (CF > 1.05) of prespawning salmon predicted high YSFM. From the various stock factors of sprat and herring in the southern BPr, the biomass of sprat had the strongest positive relationships with the CF of prespawning salmon, and the total prey biomass with YSFM. It is concluded that the ample but unbalanced food resources for salmon in the BPr, primarily sprat, induce M74. By reducing the fishing pressure on cod (Gadus morhua) and by more effectively managing the sprat fishery in years when the cod stock is weak, the incidence of the M74 syndrome could be reduced and even prevented.

Fatty acid composition of sprat (Sprattus sprattus) and herring (Clupea harengus) in the Baltic Sea as potential prey for salmon (Salmo salar)

“…Moreover, Pickova et al [15] found an imbalance between the oxidative potential and antioxidant capacity in the eggs of salmon whose offspring suffered from M74. The M74 syndrome has been linked to a large food biomass and an abundance of sprat (Sprattus sprattus) in the Baltic Proper (BPr) [16,17]. According to Keinänen et al [18], a high lipid content in the prey fish biomass of salmon and an abundant supply of lipid-rich food explains the incidence of thiamine deficiency.…”

“…There, the proportions of sprat and herring have mainly varied as a consequence of large changes in the size of the sprat stock. Sprat has been the most prevalent species in the BPr since the end of the 1980s, i.e., after the collapse of the stock of cod (Gadus morhua), which is its main predator [17,24].…”

“…However, some of them halt their feeding migration in the southern part of the Gulf of Bothnia, the BS [23,25], where herring comprise almost the exclusive prey fish of salmon [17,19,20]. Sprat do not reproduce in the BS due to the low salinity [23].…”

“…Sprat do not reproduce in the BS due to the low salinity [23]. Following the explosive growth of the Baltic sprat stock in the early 1990s, some sprat have annually dispersed to the BS from more southern parts of the Baltic Sea [19,20], but the proportion of sprat in the salmon prey biomass in the BS has remained <5−10% [17]. In the GoF, sprat might have been more abundant than herring from the years of strong sprat recruitment onwards, although herring was previously the dominant prey species of salmon in the area [26,27].…”

“…Even salmon with a total length of >60 cm prefer prey fish with a total length of 4-15 cm, and only seldom consume fish with a total length of more than 19 cm [19]. Thus, all age groups of sprat are appropriate as salmon prey, but in general only young age groups of herring [17,26]. Previous studies on the FAs of Baltic herring, including that by Røjbek et al [35], have not considered whether the analysed fish were of a suitable size to be preyed on by salmon.…”