Stomach analyses of Baltic salmon from 1959–1962 and 1994–1997: possible relations between diet and yolk‐sac‐fry mortality (M74)

Abstract

In recent years, Baltic Sea salmon Salmo salar have suffered high larval mortality (M74) which can be cured by thiamine treatment. Analyses of long term mortality records from two salmon hatcheries suggest that before the 1970s M74 did not occur, or was less frequent. This indicates that varying M74 did not cause the long-term fluctuations of Baltic salmon catches in this period. The frequency of M74 has been correlated positively to the abundance of the salmon's primary prey, sprat Sprattus sprattus. Sprat, herring Clupea harengus and three-spined stickleback Gasterosteus aculeatus dominated the diet in both the periods of study, 1959-1962 and 1994-1997. The mean size of consumed sprat was significantly smaller in 1994-1997 compared with 1959-1962. Herring and, to a lesser extent, three-spined stickleback increased in the diet of salmon, while sprat appeared to constitute a smaller part of the diet in 1994-1997. The cause of M74 and the thiamine deficiency involved remains unknown, but is thought to be related to changes in thiamine or thiaminase content in forage fish, winter-feeding of salmon or general changes in the pelagic food web, caused by overfishing or eutrophication. 2001 The Fisheries Society of the British Isles

Fractionation of δ15N and δ13C for Atlantic salmon and its intestinal cestode Eubothrium crassum

“…Atlantic salmon from the Baltic Proper feed on a diet consisting mainly of sprat, herring and three‐spined stickleback Gasterosteus aculeatus L. Sprat may comprise between 75 and 95% of the total diet depending on size of Atlantic salmon and time of the year (Hansson et al , 2001). After analysing a total of 6000 stomachs, Hansson et al (2001) found that the proportion of sprat in the Atlantic salmon diet decreased with Atlantic salmon size, while herring Clupea harengus L. and three‐spined stickleback proportion increased. Once Atlantic salmon had reached a size of 60 cm, however, the diet composition of the fish became relatively stable (average L F in the present study was 79 cm), with sprat as the dominating food source ( c. 70%), whereas herring and three‐spined stickleback made up c. 25 and 5%, respectively.…”

“…Atlantic salmon from the Baltic Sea feed primarily on sprat Sprattus sprattus sprattus (L.) (Hansson et al , 2001), and as E. crassum ingest nutrients from Atlantic salmon intestines, they have access to the same nutrients as their host. Traditionally, gut content analyses are used to identify ingested prey, but cestodes possess no gut (Pappas, 1983), which makes this type of analysis impossible for studies of nutrient uptake by the parasite.…”

Specific gravity and vertical distribution of sprat eggs in the Baltic Sea

“…Stock development of cod Gadus morhua L., the main piscivore in the Baltic Sea, and sprat are closely related (Sparholt, 1996) as cod feed to a large extent on sprat and sprat prey on cod eggs and early larvae (Sparholt, 1994; Köster & Möllmann, 2000). Depending on recruitment success of sprat and cod, respectively, as well as fishing mortality, the Baltic Sea can be shifted towards either a cod or a clupeid dominated system with implications on the entire ecosystem (Rudstam et al ., 1994; Köster & Möllmann, 2000; Hansson et al ., 2001; Österblom et al ., 2001).…”

Influence of the marine feeding area on the muscle and egg fatty‐acid composition of Atlantic salmonSalmo salarspawners estimated from the scale stable isotopes

“…Oleic acid is reported to occur in higher proportions in S. sprattus than in C. h. membras in the Baltic Sea proper and hence is a characteristic FA for S. sprattus while ARA and n ‐6 PUFA are typical for C. h. membras (Røjbek et al ., ; Keinänen et al ., in press). This was not evident in this study, however, even though S. sprattus is the main prey fish in the Baltic Sea proper whereas in the Bothnian Sea C. h. membras is almost the sole prey species (Hansson et al ., ; Salminen et al ., ). A possible reason for nearly similar proportion between these areas may be that those FA are not good indicators for differentiating among fish species (Arts & Kohler, ) since they are the most common FA components amongst flora and fauna (Arts et al ., ).…”

“…Although it is difficult to attribute DHA in S. salar to particular diet sources, DHA in every analysed tissue was highest in those fish that most probably derived from feeding on S. sprattus , which is the main prey fish in the Baltic Sea proper (Hansson et al ., ; Mikkonen et al ., ; Vuorinen et al ., ). A large amount of DHA in S. salar muscle reflects a high‐fat marine fish‐based diet (Alvarez et al ., ; Hemre & Sandnes, ).…”