Stomach content composition and prey‐specific consumption rates of juvenile and adult harbor porpoises (Phocoena phocoena) were estimated from a data set including 339 stomachs collected over a 32 yr period (1980–2011) in the western Baltic Sea. The stomach contents were mainly hard parts of fish prey and in particular otoliths. The bias originating from differential residence time of otoliths in the stomachs was addressed by use of a recently developed approach. Atlantic cod and herring were the main prey of adults, constituting on average 70% of the diet mass. Juvenile porpoises also frequently consumed gobies. Here, the mass contribution by gobies was on average 25%, which was as much as cod. Other species such as whiting, sprat, eelpout, and sandeels were of minor importance for both juveniles and adults. The diet composition differed between years, quarters, and porpoise acquisition method. Yearly consumption rates for porpoises in the western Baltic Sea were obtained in three scenarios on the daily energy requirements of a porpoise in combination with an estimate including the 95% CLs of the porpoise population size. Cod of age groups 1 and 2 and intermediate‐sized herring suffered the highest predation from porpoises.

The stock of North Sea autumn spawning herring Clupea harengus (L.) has shown an unprecedented 10 yr sequence of sharply reduced recruitment, in spite of a high spawning biomass. Recent work has identified this below-expected recruitment as being determined during the larval phase: however, the underlying mechanism remains elusive. In this study we analysed archived larval samples captured before and after the onset of the reduced survival to test the hypothesis of a concurrent change in the larval growth rate. Individual larval growth rates, averaged over the 21 d prior to capture, were estimated for ~200 larvae from 4 different years using a model-based analysis of otolith ring widths. Hydrographic backtracking models complemented the otolith analysis by reconstructing the environmental history and spawning origin of each larva. A significant reduction in net larval growth rate of 8%, concurrent with the reduced larval survival and recruitment, was identified: after correcting for the effect of other explanatory variables (e.g. temperature changes), the gross reduction was found to be 12%. This reduction is most probably due to changes in either the amount or quality of available food. The study demonstrates the potential of coupling 2 different techniques for affording new insights into fish early life history: otolith microstructure analysis and hydrographic modelling. Finally, the study provides a novel indication of the association between reduced growth and larvae survival, thereby narrowing the range of potential mechanisms underlying the observed reduction in the recruitment of North Sea autumn spawning herring. KEY WORDS: North Sea herring · Otolith microstructure · Hydrographic backtracking · Larval growth · Larval mortality · RecruitmentResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 489: 197-211, 2013 2009) (Fig. 1). Such a reduction has occurred previously, albeit over a much shorter period e.g. the 1988 to 1990 year classes (Nash & Dickey-Collas 2005). The increased larval mortality has also been traced further back into the early life-history through analysis of field-based observations, and may occur closer to spawning than to metamorphosis (Fässler et al. 2011).However, in spite of the insight into the dynamics of this stock and its long history of scientific investigations (Cushing & Bridger 1966, Sinclair 2009, Dickey-Collas et al. 2010) a mechanistic understanding of the recruitment process remains elusive. Many hypotheses have been proposed; however, there is no clear consensus. Temperature increases in the North Sea have been widely reported and shown to be associated with increased larval mortality (Payne et al. 2009, Fässler et al. 2011). Bioenergetics models have suggested that the time around yolk-sack absorption and first feeding may represent a critical juncture for survival, again possibly related to temperature (Hufnagl & Peck 2011). Changes in the zooplankton community in the North Sea are also known to have occurred (E...

The diet of whiting Merlangius merlangus in the western Baltic Sea was investigated and compared to the diet in the southern North Sea. Clupeids were important prey in both areas, but especially in the western Baltic Sea where they constituted up to 90% of the diet of larger individuals. Gobies, brown shrimps and polychaetes were the main prey of juveniles in the western Baltic Sea, while a wider range of species were consumed in the North Sea. The shift to piscivory occurred at smaller sizes in the western Baltic Sea and the fish prey consumed was proportionately larger than in the southern North Sea. Estimates of prey abundance and food intake of M. merlangus are required to evaluate its predatory significance in the western Baltic Sea, but its diet suggests that it could be just as significant a fish predator here as in the southern North Sea.

Haemoglobin polymorphism in cod (Gadus morhua L) has been investigated throughout the last 50 years. Field studies have shed light on the geographic distribution of the two common alleles (HbI 1 and HbI 2 ), and laboratory studies have shown effects of genotype on physiological traits such as growth, reproduction and hypoxia tolerance. The geographic distribution of alleles shows a correlation with temperature, with increasing frequency of HbI 1 in warmer areas. This is likely due to temperature related differences in oxygen affinity of the three genotypes. We provide a general ecological introduction to cod haemoglobin polymorphism and a detailed discussion of physiological studies, particularly laboratory growth studies. Although differences in oxygen uptake are almost certainly a contributory mechanism to observed differences in traits such as growth rate, many other environmental, behavioural and social factors may also contribute, making it difficult to quantify the effect of HbI either experimentally or in the field.

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