Variation of blubber thickness for three marine mammal species in the southern Baltic Sea

Evaluating populational trends of health condition has become an important topic for marine mammal populations under the Marine Strategy Framework Directive (MSFD). In the Baltic Sea, under the recommendation of Helsinki Commission (HELCOM), efforts have been undertaken to use blubber thickness as an indicator of energy reserves in marine mammals. Current values lack geographical representation from the entire Baltic Sea area and a large dataset is only available for grey seals (Halichoerus grypus) from Sweden and Finland. Knowledge on variation of blubber thickness related to geography throughout the Baltic Sea is important for its usage as an indicator. Such evaluation can provide important information about the energy reserves, and hence, food availability. It is expected that methodological standardization under HELCOM should include relevant datasets with good geographical coverage that can also account for natural variability in the resident marine mammal populations. In this study, seasonal and temporal trends of blubber thickness were evaluated for three marine mammal species—harbor seal (Phoca vitulina), grey seal (Halichoerus grypus) and harbor porpoise (Phocoena phocoena)—resident in the southern Baltic Sea collected and investigated under stranding networks. Additionally, the effects of age, season and sex were analyzed. Seasonal variation of blubber thickness was evident for all species, with harbor seals presenting more pronounced effects in adults and grey seals and harbor porpoises presenting more pronounced effects in juveniles. For harbor seals and porpoises, fluctuations were present over the years included in the analysis. In the seal species, blubber thickness values were generally higher in males. In harbor seals and porpoises, blubber thickness values differed between the age classes: while adult harbor seals displayed thicker blubber layers than juveniles, the opposite was observed for harbor porpoises. Furthermore, while an important initial screening tool, blubber thickness assessment cannot be considered a valid methodology for overall health assessment in marine mammals and should be complemented with data on specific health parameters developed for each speciesinfo:eu-repo/semantics/publishedVersio

Correction: Coming of age: - Do female harbour porpoises (Phocoena phocoena) from the North Sea and Baltic Sea have sufficient time to reproduce in a human influenced environment?

[This corrects the article DOI: 10.1371/journal.pone.0186951.]

Coming of age: - Do female harbour porpoises (<i>Phocoena phocoena</i>) from the North Sea and Baltic Sea have sufficient time to reproduce in a human influenced environment? - Fig 1

<p>Mid-longitudinal section of a tooth from a neonate (A) and an adult (B) harbour porpoise (<i>Phocoena phocoena</i>).</p

Overview of analysed specimens.

<p>Overview of analysed specimens.</p

Binomial regression model showing the functional relationship between the presence of corpora (purple polygon) in female harbour porpoises from the German North Sea and Baltic Sea in relation to their age as determined by GLG.

<p>The filled area indicates the 95% confidence interval of predicted values. The red line within that area indicates the predicted average probability of finding corpora within specimens at the given age; the blue dashed line marks a probability of 50%, with the red solid line indicating the corresponding age based on the intersection of the blue dashed line with the x-axis; samples are indicated as dark grey dots; a purple circle around data points indicates animals below the age threshold showing at least one corpus, an orange circle indicates animals above the threshold showing no corpora; background histogram (orange) shows number of animals (secondary y-axis) across tooth age classes (at 1-year intervals).</p

Model diagnostics for the binomial regression model of the probability of detecting corpora (models c1 and c2).

<p>Model diagnostics for the binomial regression model of the probability of detecting corpora (models c1 and c2).</p

Percentile of all animals found between 1990 and 2017 that were older than the estimated threshold age of reaching sexual maturity of the population.

<p>Percentile of all animals found between 1990 and 2017 that were older than the estimated threshold age of reaching sexual maturity of the population.</p

Population structure of female harbour porpoises of the North Sea (green bars) and Baltic Sea (red bars).

<p>Proportion of animals indicates the proportion of animals relative to the total number of animals within a given age bracket. Round brackets indicate exclusive values, square brackets indicate inclusive values.</p

Quartile age distribution of harbour porpoises from the German North Sea and Baltic Sea between 1996 and 2017.

<p>Quartile age distribution of harbour porpoises from the German North Sea and Baltic Sea between 1996 and 2017.</p

Number of Primordial Follicles in Juvenile Ringed Seals (<i>Pusa hispida</i>) from the Gulf of Bothnia and West Greenland

Primordial follicles are important for the reproduction cycle and, therefore, also for the survival of the whole population of a species. Mammals have a large pool of primordial follicles, and it is thought that this pool represents the total number of oocytes. The aim of the present study was to determine the total primordial follicle number of juvenile ringed seals (Pusa hispida) from the Gulf of Bothnia and Greenland. Overall, 52 ovaries from two ringed seal populations (West Greenland (N = 6), Gulf of Bothnia, region in the Baltic Sea (N = 46)) were examined. All ovaries were cut into 2 mm thick slices and every slice was embedded in paraffin. Out of each tissue block, a 5 µm thick section was cut and stained with haematoxylin-eosin. The mean volume of the follicles and the total volume of primordial follicles per ovary were estimated by stereology and used to calculate the total estimated number of primordial follicles. The median of the total estimated number of primordial follicles seemed to be higher in Baltic individuals than in Greenland individuals (Gulf of Bothnia = 565,657; Greenland Sea = 122,475). This widens the total range of primordial follicles in ringed seals overall and might bear some potential for discussions regarding the influence of endocrine disruptors and environmental influences depending on different regions/populations and their exposure to various factors. Thus, this study aims to provide basic reference data of the number and mean volume of ringed seal primordial follicles