Article Published on Negative Trends in the Vulnerable Harbour Porpoise Population Showing Urgent Need for Action

Bycatch in fisheries is one of the largest threats to marine biodiversity globally. The management and conservation of threatened species relies on information such as abundance and mortality. A new paper, A negative trend in abundance and an exceeded mortality limit call for conservation action for the Vulnerable Belt Sea harbour porpoise population, has been published in Frontiers of Marine Science (open access). The study found that over the past 18 years, there has been a strong negative trend in abundance of the vulnerable harbour porpoise (Phocoena phocoena) population in the Belt Sea due to bycatch. Examining abundance data from 2005-2022, the authors concluded that the mortality limit is currently exceeded by far due to bycatch in fisheries, necessitating informed decisions on management actions to be taken without delay. ASCOBANS makes an important contribution to the conservation of the harbour porpoise through the Conservation Plan for the Harbour Porpoise Population in the Western Baltic, the Belt Sea and the Kattegat.

Citation: Owen K, Gilles A, Authier M, Carlström J, Genu M, Kyhn LA, Nachtsheim DA, Ramírez-Martínez NC, Siebert U, Sköld M, Teilmann J, Unger B, Sveegaard S (2024) A negative trend in abundance and an exceeded mortality limit call for conservation action for the Vulnerable Belt Sea harbour porpoise population. Front. Mar. Sci. 11. doi: 10.3389/fmars.2024.1289808.

Abstract:

The management and conservation of biodiversity relies on information on both the abundance of species and the potential impact of threats. Globally, one of the largest threats towards marine biodiversity is bycatch in fisheries. Under the Marine Strategy Framework Directive (MSFD), EU Member States are required to assess the status of species, such as the harbour porpoise (Phocoena phocoena), in relation to their abundance and mortality due to bycatch every six years. The Vulnerable (HELCOM) Belt Sea population of harbour porpoise has been surveyed to determine its abundance six times using dedicated aerial or ship-based line-transect distance sampling surveys. Here, we estimated the first trend in population abundance over an 18 year period (2005-2022). Using the most recent abundance estimate, we computed a mortality limit applying the modified Potential Biological Removal (mPBR) method based on the regionally agreed conservation objective to restore or maintain 80% of carrying capacity over 100 years with an 80% probability. Over the past 18 years there has been a strong negative trend (-2.7% p.a.; 95% CI: -4.1%; + 1.3%) in abundance, with a 90.5% probability. The mortality limit was estimated to be 24 animals, which the current bycatch estimates (~900 porpoises/year from the commercial Danish and Swedish set net fishery fleets, with no data from Germany and other fishery types) exceed by far. The frequency and quality of data available on abundance for this population are higher than those available for the majority of marine species. Given the observed population decline and likely unsustainable levels of bycatch, the results presented here provide a strong basis to make informed, evidence-based management decisions for action for this population. Such action is needed urgently, before the dire situation of other porpoise species and populations around the globe is repeated.

Last updated on 18 December 2024

Type: 
News item
Region: 
Baltic Sea
Species: 
Phocoena phocoena