The eastern Baltic (EB) cod (Gadus morhua) stock was depleted and overexploited for decades until the mid-2000s, when fishing mortality rapidly declined and biomass started to increase, as shown by stock assessments. These positive developments were partly assigned to effective management measures, and the EB cod was considered one of the most successful stock recoveries in recent times. In contrast to this optimistic view, the analytical stock assessment failed in 2014, leaving the present stock status unclear. Deteriorated quality of some basic input data for stock assessment in combination with changes in environmental and ecological conditions has led to an unusual situation for cod in the Baltic Sea, which poses new challenges for stock assessment and management advice. A number of adverse developments such as low nutritional condition and disappearance of larger individuals indicate that the stock is in distress. In this study, we (i) summarize the knowledge of recent changes in cod biology and ecosystem conditions, (ii) describe the subsequent challenges for stock assessment, and (iii) highlight the key questions where answers are urgently needed to understand the present stock status and provide scientifically solid support for cod management in the Baltic Sea.

The bird-lemming hypothesis postulates that breeding success of tundra-nesting geese and waders in Siberia follows the cyclic pattern of lemming populations, as a result of predators switching from lemmings to birds when the lemming population crashes. We present 50 years of data on constant-effort catches of red knot Calidris canutus and curlew sandpiper C. ferruginea at an autumn migratory stopover site (Ottenby) at the Baltic Sea, supplemented with literature data on winter censuses of dark-bellied brent goose Branta b. bernicla and white-fronted goose Anser albifrons in northwestern Europe, and waders in Germany and Southern Africa. Number and proportion of juveniles in these bird populations (both our own and literature data) were compared with an index of predation pressure (calculated from the abundance of lemmings on the Taimyr peninsula), and climate indices for the North Eurasia and the North Atlantic regions. The index of predation pressure correlated significantly with the number of juveniles of red knot and curlew sandpiper, but not with number of adults. Also, this index correlated with the reproductive performance of geese and waders reported in the literature. Fourier analysis revealed a significant deviation from random noise with the maximum spectral density at the period length of 3 years for number of juvenile red knots and curlew sandpipers captured at Ottenby, abundance of lemmings, reproduction in arctic fox Alopex lagopus, and reproductive performance in geese on the Siberian tundra. Also, the date of passage at Ottenby for adult red knot and curlew sandpiper showed a spectral density peak at a period length of 3 years, the latter species also showing a peak at a period length of 5-6 years. Passage dates for adult red knot and curlew sandpiper were earlier in years of high predation pressure compared with years of low predation pressure. The fluctuations in reproductive success of the studied Siberian goose and wader species appear to be primarily influenced by biotic factors in the breeding area, rather than by abiotic factors, such as climate oscillations. Annual variations in migratory arctic bird populations may have far reaching effects in habitats along their migration routes and in their wintering areas. We suggest a link between lemming cyclicity in the Northern Hemisphere and predation pressure on Southern Hemisphere benthos, in which the signal is carried between continents by long distance migrating waders.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. The spatial distribution of foraging predators differing in foraging traits was investigated theoretically. Three types of individual differences were investigated: dominance that determines the outcomes in fights for food, searching efficiency and prey handling efficiency. The basic assumptions were: (i) predators search for prey; (ii) encountered prey takes time to handle; (iii) the handling process may be interrupted by a searching competitor initiating a conflict about the prey; (iv) conflicts take an expected time to settle; and (v) the winner resumes handling and the loser resumes searching. 2. Stable equilibrium distributions were found in all cases, in the sense that no competitor could achieve a higher intake rate from switching patch. When it was assumed that each of the predators was different from the others, the ideal free distributions of the dominance model and the handling efficiency model resembled one another. A boundary phenotype existed with more dominant/efficient competitors only occurring in the high prey density patch and less dominant/efficient ones occurring mixed across patches. This was labelled semitruncated distribution.In the search efficiency model, the distribution was truncated, i.e. competitors less efficient than the boundary phenotype only occurred in the high prey density patch whereas the more efficient competitors occurred in the low prey density patch. 4. The models were also analysed with the predators classified into two types. As the density of competitors increased, the solution approached a set of neutral equilibria where a few predators of the more efficient type could be exchanged for a larger number of the less efficient type to reach a new equilibrium. 5. The density of predators may be negatively correlated with patch productivity and prey density when predators differ in handling efficiency. Prey-searching efficiency of predators was negatively correlated with predator density, patch productivity and prey density. Its consequences for the conservation of species are discussed.

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