The population dynamics of Nassarius reticulatus (L.) (Gastropoda, Prosobranchia) was investigated in Gullmar Fjord on the Swedish west coast from 1973 to 1979. A fairly well-defined population on a shallow, sandy bottom in Kvarnbukten Bay (36 000 m*) was intensively studied. Settling of veliger larvae was very irregular and normally only took place where the organic content of the substrate was fairly high, i. e. in deeper water. The snails, especially small ones ( S 15 mm), were greatly attracted to detritus-rich substrates, whereas larger ones congregated on canion. The snails in Kvarnbukten Bay spent their first 3 years in the settling area living mainly on detritus. The change in diet may be a factor influencing the intraspecific competition between small and large snails. When nearly 4 years old (ca. 15 mm) they become sexually mature. From then on most of them participated in the yearly migrations: in April and May from winter quarters in deeper water (1-5 m depth) to the shallow part of the bay (0-1 m) where they spent the summer, and then back again in September to December. Growth rate was fast during the first 3 years with a length increment of about 5.5 mm year-', but then decreased considerably, also becoming individually very variable. Maximum age was about 15 years. Fecundity was high: ca. 6000 eggs female-' year-'; females spawned every year. Mortality rate during the planktonic phase was high but there were few predators on larger snails, parasitism in shallow water probably being the most important cause of losses at this age. Temperature was one of the main proximate factors affecting locomotory activity, migration, spawning and growth. Threshold temperatures for various activities were narrowly defined. Irregular settling of larvae and hence sporadic recruitment to the shallow parts of the bays seemed to be the main reason for population fluctuations in different bays. Advantages of inshore migration (temperature and availability of food) seemed to balance disadvantages (mainly parasitism) and this may explain the dimorphism in migratory habits. As a whole these snails show several special adaptations to the varying environment in this non-tidal, soft bottom littoral.

The seasonal variations in numbers and biomass of the mobile epifauna of a shallow, sandy bay in Gullmar Fjord (Sweden) was investigated during 1976–1977 using the drop‐net technique. Juvenile plaice, Pleuronectes platessa L., sand goby, Pomatoschistus minutus (Pallas), and brown shrimp Crangon crangon L. are the dominant epifaunal species. A net enclosing an area of 100 m2 was used, and the organisms captured were collected with a small trawl. Three stations with a water depth≤ 1 m were regularly sampled with a total number of 73 drops. Maximum abundance was for plaice in June (6 ind m‐2), for the shrimp in July‐August (61 ind m‐2), and for sand goby in September (2 ind m‐2). Standing stocks (dry wt) were 0.2, 1.5 and 0.2 g m‐2 respectively. Production estimates were based on seasonal changes in size frequency and information on specific growth rates. The production rate was for plaice (0‐group) 0.3 g m‐2 yr‐1, for the shrimp 1.5, and for sand goby 0.2 g m‐2 yr‐1. Reliable estimates of the absolute abundance can be derived from the mean catch per unit area only when the efficiency of the fishing gear is known. After 6 hauls with the trawl in the drop‐net enclosure more than 85% of the shrimps were caught and after the fifteenth haul less than 1% were still left. The fishing efficiency of a push‐net in relation to the drop‐net was for the shrimp 50%, for plaice 35% and for sand goby 17%.

No abstract

Atlantic salmon living in the brackish Baltic Sea have lower muscle pigmentation than populations elsewhere. The pigment in question is the antioxidant and vitamin A precursor astaxanthin, which is synthesized by crustaceans from algal carotenoids. Baltic salmon feed nearly exclusively on the clupeids sprat and herring. To evaluate astaxanthin availability to salmon we assessed astaxanthin levels and isomeric composition in their prey fish. We also analyzed astaxanthin dynamics in the dominant piscivorous fish in the Baltic Sea, the Atlantic cod.The geometrical E-(trans-) and Z-(cis-) isomers were distributed selectively in fish tissues, with highest E : Z ratios in salmon gonads (82 : 18) and lowest in herring gonads (24 : 76). Sprat and herring are not ideal prey with respect to their high whole-body concentrations of Z-isomers, which have low bioavailability for salmon and cod. These Z-isomers predominantly accumulate in the clupeid gonads. A crucial mechanism for the transport of astaxanthin from clupeids to piscivores is the direct transfer of crustacean astaxanthin (mainly all-E) from the clupeid stomachs. Low stomach astaxanthin content in clupeids decreases total astaxanthin transfer to higher trophic levels. In autumn, herring stomachs (including contents) had 12.5 times lower astaxanthin concentrations than sprat stomachs, and herring had 2.8 times less whole-body all-E-astaxanthin (by weight) than sprat. These results confirm recent reports of starvation in the Baltic herring, which may further decrease astaxanthin levels in the Baltic salmon. Cod did not have lower astaxanthin levels than their Atlantic counterpart, which may be attributed to their lower need for astaxanthin and higher food diversity.

Factors affecting the predator-related mortality of sand goby Pomatoschistus minutus (Pallas), common goby P. rnicrops (Kreyer) and brown shrimp Crangon crangon L. were investigated in Gullmar Fjord, Sweden. Substrate selection and locomotory activity was recorded in the 3 species on sand and on mud. Sand goby and brown shrimp preferred sand to mud, while common goby preferred mud to sand. Activity was clearly correlated to substrate type. In all 3 species, swimming activity was increased by a factor 4 when kept on the less preferred substrate. Predation experiments were performed with sand or mud as bottom substrate and with cod as predator. On sand, common gobies were more rapidly eliminated by the predator than were sand gobies, and sand gobies more rapidly than brown shrimps. On mud, sand gobies were more rapidly taken than were common gobies and brown shrimps. Staying on the 'wrong' substrate will result in increased locomotory activity which will in turn increase the risk of predation. We suggest that the microgeographical distribution of members of the epibenthic guild is primarily a consequence of various behaviour patterns. Predators act to restrict the habitat use of these fish and shrimp species. Differences in behaviour between the species when encountering a predator reduce their vulnerability to predators on their respective preferred substrate. This may in part explain the spatial segregation and will contribute to the maintainance of community structure.