Harbor seal predation and fishery effects on Kattegat-Skagerrak cod abundance : in the aftermath of Phocine distemper virus

• Main Author: Hökby, Lovisa
• Format: Bachelor Thesis
• Language: English
• Published: Uppsala universitet, Institutionen för biologisk grundutbildning 2021
• Subjects: Harbor seals; Phoca vitulina; PDV; phocine distemper virus; Kattegat; Skagerrak; cod; Gadus morhua; human-wildlife conflicts; Natural Sciences; Naturvetenskap; Biological Sciences; Biologiska vetenskaper; Ecology; Ekologi; Zoology; Zoologi; harbor seal
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-444104

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.