Factors affecting the degree of vertical stratification of fatty acids in grey seal blubber

Abstract

The biochemistry of marine mammal blubber differs vertically from skin to muscle, which forms a challenge for using fatty acids (FAs) from differently sampled blubber as a proxy for dietary studies required for ecosystem-based management of coastal resources. In the blubber of some phocid seal individuals, the vertical stratification of several FAs is pronounced whereas in others the FAs distribute almost evenly through the blubber column. Using gas chromatography, we analysed the blubber vertical FA profiles of 30 adult male grey seals from the Baltic Sea, and examined which factors induced the largest vertical change of FA composition detected at the depth of 15-18 mm (outer and middle blubber boundaries). It was revealed that the degree of this compositional shift did not depend on the blubber thickness. Seal age only affected the vertical distribution of the FAs 16:0 and 16:1n-7. However, the outer blubber ratio of ∆9-desaturated monounsaturated FAs (MUFAs) to their saturated FA (SFA) precursors was not increased by grey seal age, contrasting earlier findings for ringed seals. A major determinant of the degree of FA stratification between the outer and middle blubber was the mismatch between the individually varying FA composition of the innermost blubber, regarded to reflect the dietary FA supply the most, and the uniform FA composition of endogenously regulated MUFA-rich outer blubber. Thus, discarding a fixed-depth layer of the grey seal outermost blubber, which we here show to span 0-18 mm from skin and which to a lesser extent reflects the diet of the individual, may in the case of small pinnipeds improve the sensitivity of the FA analysis in assessing spatial, temporal and individual dietary differences. When studying the outer blubber samples using only the diet-derived PUFA variables (SFAs and MUFAs omitted), the sensitivity of the analysis was better than when using this sample type with all main FA variables included.

Fatty Acid Composition in Blubber, Liver, and Muscle of Marine Mammals in the Southern Baltic Sea

“…In contrast, in the outer layer, the MUFAs are more enriched, which do not seem to be readily mobilized by the seals and therefore accumulated in the blubber tissue [26]. These higher MUFA concentrations suggest a more functional role of thermoregulation of the outer blubber layer of seals [27,48]. In the current study investigating grey seals and harbour porpoises of the southern Baltic Sea, no investigations were performed on the different fatty acid composition in the inner and outer blubber layer.…”

“…To date, only limited results on fatty composition in tissues of the top predators in the Baltic Sea are available, with the exception of blubber in grey seals [27,40]. As we present here a preliminary study on the fatty acid composition in different tissues of grey seals and harbour porpoises in the southern Baltic Sea, the study has a descriptive character based on the limited number of animals in both species.…”

“…In general, blubber tissue in marine mammals has two important biological functions: It firstly acts as an energy storage and secondly as a thermal regulator [47]. By the use of stratification analysis, it could be revealed that the fatty acid composition in the blubber of seals (grey seals, crabeater seals) shows large differences in biochemistry between the outer and inner blubber layer [26,27]. The inner blubber layer, containing high lipid proportions of longer chain SFAs and PUFAs, seems to reflect the fatty acid composition of the diet of the seals more than the outer blubber layer.…”

“…In contrast, the levels of SFA and MUFA increase in the skeletal muscle of adult animals as a possible protection against reactive oxygen species damage due to high rates of sustained lipid metabolism associated with the end of dives [51,52]. To our knowledge, only a limited number of studies on fatty acid de novo lipogeneses and lipid metabolism in different tissues of marine mammals have been conducted to date [27]. Marine mammals typically consume a high-fat low-carbohydrate diet and seem to show lower rates of fatty acid de novo synthesis compared to terrestrial mammals [53].…”

“…Marine mammal blubber is not homogenous and is functionally a complex tissue: The inner and outer blubber layers are significantly different, suggesting a stratification of the fatty acid composition of the layers [25]. The pattern of stratification in the marine mammals is prevalent, with increasing proportions of monounsaturated fatty acids (MUFAs) to the outer layer and increasing proportions of saturated and polyunsaturated fatty acids (SFAs, PUFAs) to the inner layer [26,27]. This variation in the fatty acid composition between the inner and outer blubber layer seems to be an indicator of different metabolic pathways, as recently shown in grey seals [28].…”