The recovering harbor seal population along the Swedish west coast has coincided with declining cod stocks. Although the cod stocks were primarily depleted by intense fisheries, some stakeholders argue that reduced seal populations could have direct, positive effects on the cod stocks. The outbreaks of Phocine distemper virus (PDV) in 1988 and 2002 when up to 66% of the harbor seals along the Swedish west coast died, could be viewed as natural experiments allowing for assessment of any positive impacts of reduced mortality from seal predation on cod abundance. I quantified removal from seal predation and fishery landings and analyzed correlations to cod abundance of different length classes. Analyses were conducted for a longer time- series, 1979-2019, reaching back to when removal from fisheries was substantially larger than that from seal consumption. I also analyzed the data with a more recent perspective, 2003-2019, starting after the later PDV outbreak when seal predation relative landing sizes has increased. During this period, fishery discard data is available and included in the models. Methods include graphical presentation, multiple linear regression analyses, and to expand the existing data on seal diet, I collected diet samples of harbor seal feces from two haul-outs in Skagerrak. No statistically significant, negative correlations between seal predation and cod abundance were found in any data set, hence, I cannot support the idea of harbor seals having an impact on cod abundance. Also, the significant correlations between cod abundance and landings were all positive. I argue that culling of seals cannot be motivated as a strategy to help cod recover, as this would jeopardize the persistence of the seal population rather than assuring cod stability.

It has been assumed that the considerable dispersal ability of many marine species would prevent genetic divergence in the absence of geographic isolation. However, recent work has shown that many marine species often develop differentiation among areas within their known dispersal range. This 'paradox' of marine divergence is particularly important among marine mammal species where behavior can restrict gene flow. To investigate genetic substructure within such a species, I used tissue samples collected from 63 stranded Pacific harbor seals (Phoca vitulina richardsi) in Washington and Oregon between 2006 and 2010 for population stock analysis. DNA was extracted from frozen tissues, and a 551 bp fragment of mtDNA control region sequence and eight microsatellite loci were amplified to investigate localized genetic structure. Minimum spanning network and haplotype frequency analyses of mtDNA sequences indicated that while haplotype lineages are not isolated within a sampling region, there is some evidence of regional differentiation. On the other hand, microsatellite data suggest a lack of substructure among the animals sampled, with only a weak signal of limited gene flow between Puget Sound and coastal areas. Biased dispersal among age classes was also suggested, with juveniles showing less differentiation among areas than adults. Regardless of the historical scenario which has led to this complex pattern of genetic structure in Pacific harbor seals across the Pacific Northwest, my results suggest higher levels of exchange among areas than previously suspected, and will have important consequences for future management considerations for these stocks.

Muscle samples from 105 marine mammals stranded along the Oregon-Washington coasts (2002-2009) were tested for levels of total mercury by Cold Vapor Atomic Fluorescence Spectrometry. The total mercury present is predominantly in the form of highly toxic methylmercury. After normalizing muscle tissue to 75% water weight, due to variance in water content, Steller sea lions (Eumetopias jubatus) and northern elephant seals (Mirounga angustirostris) exhibited the highest mean concentrations of total mercury followed by harbor seals (Phoca vitulina), harbor porpoises (Phocoena phocoena), and California sea lions (Zalophus californianus); 0.34 ± 0.278, 0.34 ± 0.485, 0.21 ± 0.216, 0.17 ± 0.169 and 0.15 ± 0.126 mg/kg normalized weight, respectively. Mean normalized values demonstrated limited muscle methylmercury accumulation in these species in the Pacific Northwest. Normalizing muscle mercury concentrations eliminated variability from desiccation, and allowed for a clearer indication of the amount of mercury the animal accumulated before stranding. However, actual wet weight concentrations in some of the stranded carcasses were high enough to pose a risk to scavengers.