For many studies, it is important to measure the total lipid content of biological samples accurately. The Bligh and Dyer method of extraction was developed as a rapid but effective method for determining total lipid content in fish muscle. However, it is also widely used in studies measuring total lipid content of whole fish and other tissues. Although some investigators may have used modified Bligh and Dyer procedures, rarely have modifications been specified nor has their effectiveness been quantitatively evaluated. Thus, we compared this method with that of the classic Folch extraction in determining total lipid content of fish samples ranging from 0.5 to 26.6% lipid. We performed both methods as originally specified, i.e., using the chloroform/methanol/water ratios of 1:2:0.8 and 2:2:1.8 (before and after dilution, respectively) for Bligh and Dyer and of 8:4:3 for Folch, and with the initial solvent/sample ratios of (3+1):1 (Bligh and Dyer) and 20:1 (Folch). We also compared these with several other solvent/sample ratios. In samples containing <2% lipid, the results of the two methods did not differ. However, for samples containing >2% lipid, the Bligh and Dyer method produced significantly lower estimates of lipid content, and this underestimation increased significantly with increasing lipid content of the sample. In the highest lipid samples, lipid content was underestimated by up to 50% using the Bligh and Dyer method. However, we found a highly significant linear relationship between the two methods, which will permit the correction of reported lipid levels in samples previously analyzed using an unmodified Bligh and Dyer extraction. In the future, modifications to procedures and solvent/sample ratios should be described.

We determined the fat content and fatty acid composition of 26 species of fish and invertebrates (n = 1153) that are primary forage species of piscivorous seabirds and marine mammals in Prince William Sound (PWS), Alaska. Flatfish, shrimps and octopus had the lowest average fat contents (~1.0%), although some cods, as well as juvenile walleye pollock Theragra chalcogramma, Pacific herring Clupea harengus pallasi and pink salmon Oncorhynchus gorbuscha also ranged as low as 0.5 to 0.7% fat. The highest fat contents were found in eulachon Thaleichthys pacificus (25%), adult herring (21%) and the squid Berrytheuthis magister (5 to 13%). Within species, fat content varied mostly with season, but also with size. Fatty acid signatures generally distinguished forage species, with up to 95% of individuals correctly classified using either discriminant or classification and regression tree (CART) analyses. Discriminant plots provided insight into the relationships between fatty acid signatures of different species. Species with similar life histories and diets clustered closer together, while those with the greatest differences in ecology differed most in their fatty acid patterns. Within some species, changes in fatty acid signatures were apparent with increasing size and were consistent with known dietary shifts reported from stomach contents analyses. Furthermore, fatty acid signatures of Age 0 (yr) pollock and herring in PWS were consistent with previous stomach contents analysis that indicated annual differences in the timing of dietary changes from eating zooplankton to piscivory. Overall, when size/age classes were taken into account, species classification using fatty acid signatures was improved. Our findings have important implications for evaluating diets and food web interactions of fish stocks, as well as at higher trophic levels. Despite individual variation within species, our results indicate that fatty acid signatures accurately characterize forage species in this ecosystem, and consequently can be used to study and perhaps estimate the species composition of diets of their predators. KEY WORDS: Fatty acids · Forage fish · Food webs · Trophic interactionsResale or republication not permitted without written consent of the publisher

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Although it is well known that responses to ethologically-relevant odors are influenced by endocrine factors, it has not been clear whether these hormonal effects might be mediated at the level of the peripheral sensory neurons. During an investigation of hormonal pheromones in South-East Asian Cyprinids, we observed that in adult male Puntius schwanenfeldi, an androgendependent sex character was correlated with electro-olfactogram response to a putative sex pheromone (15-keto-prostaglandin-F2a). As secondary sex characteristics are androgen-dependent in male teleosts, this observation suggested a functional relationship between androgen and peripheral olfactory receptor response. We therefore investigated this possibility using androgen implants.In laboratory-raised juveniles, androgen treatment increased the magnitude and sensitivity of electro-olfactogram response to prostaglandin without affecting responses to other odors. Furthermore, androgen-treated juveniles performed pheromone-dependent sex behavior in the presence of a prostaglandin-injected stimulus fish. For the first time in vertebrates, the present data demonstrate hormone-induced plasticity of primary chemosensory neuronal responsiveness to an ethologically relevant compound.

The collapse of various stock complexes of cod (Gadus morhua) in the northwest Atlantic has prompted a clarification of relationships among stock components. Here we examine the genetic composition of >2300 cod collected during 1994–1997 in the Gulf of St. Lawrence and its approaches to determine whether: (1) stock components can be genetically identified; (2) population structure is temporally stable; (3) components are always separated and, if not, where and when are they mixed; and (4) component contributions to mixtures can be estimated. We use polymorphism at six microsatellite DNA loci from cod collected on or near their spring and summer spawning grounds to examine structure and then employ maximum likelihood analyses to estimate contributions of each component to mixtures overwintering near the entrance to the Gulf. Estimates of genetic structure (FST and RST) reveal significant differences among cod populations during stock‐separated periods, and the structure appears to be temporally stable. Multidimensional scaling analysis of estimates of genetic distance (DA) suggest that the structure results from differences among cod collected within the Gulf of St. Lawrence and those collected near the entrance to the Gulf on either side of the Laurentian Channel in the Cabot Strait, as well as among cod collected south of Newfoundland along the north side of the Channel. Weak genetic heterogeneity among seven regional mixed‐stock collections during the overwintering period suggests that cod aggregations characteristically found in the overwintering region represent population mixtures that differ in the proportion of cod contributed to them by the various stock components. Maximum likelihood estimates indicate no significant temporal changes in component contributions to the mixed‐stock samples between 1996 and 1997 when all of the winter mixed‐stock samples were pooled. The combined contribution of cod from the southern and northern Gulf of St. Lawrence to the mixed‐stock samples ranged between 46% and 71% (expected 64%). More precise estimates of contributions from these two regions are precluded by the weak genetic differentiation detected in our samples. The contribution by cod from the Cape Breton Island region was small and estimated at 3%. Contributions by cod from the eastern Scotian Shelf, southwest Newfoundland and south‐central Newfoundland were in the range of 13–14%, 4%, and 8%, respectively. Contributions by inshore cod from Placentia and Fortune Bays in south Newfoundland were small to negligible (∼3% each). The results indicate that future management could be designed around the spatial and temporal scale of the stock structure identified during the stock‐separated period and around the spatially varying contributions to the overwintering mixed‐stock fishery.