Harbour Porpoise Abundance in Portugal over a 5-Year Period and Estimates of Potential Distribution

Abstract

The Iberian porpoise population is small and under potentially unsustainable removal by fisheries bycatch. Recently, a marine Site of Community Importance (SIC) was legally approved in Portugal, but no measures ensued to promote porpoise conservation. Information about porpoise abundance and distribution is fundamental to guide any future conservation measures. Annual aerial surveys conducted between 2011 and 2015 show a low overall porpoise abundance and density (2254 individuals; 0.090 ind/km2, CV = 21.99%) in the Portuguese coast. The highest annual porpoise estimates were registered in 2013 (3207 individuals, 0.128 ind/km2), followed by a sharp decrease in 2014 (1653 individuals, 0.066 ind/km2). The porpoise density and abundance estimated in 2015 remained lower than the 2013 estimates. A potential distribution analysis of the Iberian porpoise population was performed using ensembles of small models (ESMs) with MaxEnt and showed that the overall habitat suitability is particularly high in the Portuguese northern area. The analysis also suggested a different pattern in porpoise potential distribution across the study period. These results emphasize the importance of further porpoise population assessments to fully understand the spatial and temporal porpoise habitat use in the Iberian Peninsula as well as the urgent need for on-site threat mitigation measures.

Harbor porpoise losing its edge: Genetic time series suggests a rapid population decline in Iberian waters over the last 30 years

“…Annual aerial surveys in Portuguese continental waters between 2011 and 2015 estimated an overall average abundance of 2254 porpoises and a corresponding density of 0.09 ind/km 2 (CV = 21.99%) with large interannual fluctuations. The highest annual abundance estimated for that period was 3207 individuals (CV = 38.14%) in 2013, followed by a sharp decrease in 2014, when only 1653 individuals (CV = 43.27%) were estimated (Torres‐Pereira et al., 2022 ). López et al.…”

“…Furthermore, Iberian harbor porpoises are regularly found stranded, often with signs of bycatch mortality, potentially indicating unsustainable mortality rates (IMR‐NAMMCO, 2019; López et al., 2002; Pierce et al., 2020; Read et al., 2020; Torres‐Pereira et al., 2022; Torres‐Pereira, Araújo, et al., 2023; Vingada & Eira, 2018). In fact, based on Portuguese stranding data, an average of 9.19% (CI = 5.25%–16.10%) of the porpoise population was estimated to be removed annually by fisheries between 2011 and 2015, corresponding to an average of 207 porpoises removed per year, which largely surpasses the Potential Biological Removal rate conservatively estimated for the same period (22 porpoises, CI: 12–43) (Torres‐Pereira, Araújo, et al., 2023). An additional cause for concern is related to the potential overexploitation of the feeding resources (Méndez‐Fernandez et al., 2013; Santos & Pierce, 2003).…”

Assessment of Harbour Porpoise Bycatch along the Portuguese and Galician Coast: Insights from Strandings over Two Decades

“…A deeper analysis was dedicated to porpoise strandings data in Portugal, particularly because porpoise annual population abundances were available from 2011 to 2015 [ 43 ] in Portugal but not in Galicia. The proportion of stranded bycaught animals (N bycatch ) was estimated [ 23 ] and is most likely underestimated, since only fresh to moderately decomposed carcasses can be evaluated to determine the cause of death.…”

“…Then, the carcass detection rate (CDR), the number of animals dying per year (estimated annual mortality (EAM)) and the percentage of animals removed from the population due to bycatch based on strandings data (annual population removal based on strandings data (APR strandings )) were also estimated based on the number of stranded porpoises evaluated for bycatch evidence (strandings), as described in Table S1 . To obtain EAM, porpoise annual and overall abundance data (with respective confidence intervals) from 2011 to 2015 [ 43 ] and a specific mortality rate (Mr = 0.18) for the study area [ 33 ] were used. The estimated annual mortality due to bycatch (EAM bycatch ), carcass detection rate (CDR, relevant for estimating EAM bycatch ) and APR strandings were also recalculated considering the total number of stranded porpoises instead of the number of stranded porpoises evaluated for bycatch evidence.…”

“…It was also possible to estimate the potential biological removal (PBR) in Portuguese waters because population abundance estimates ( N ) and the respective coefficient of variation (CV N ) were obtained from aerial surveys performed in Portuguese waters during the period 2011–2015 [ 43 ]. To estimate PBR (i.e., the number of animals that “may” be removed from a cetacean population, not including natural mortalities, without compromising the population at the biological level [ 44 ]), the following formula was used: PBR = N min 1/2 R max f , where N min is the minimum population abundance estimate (lower 20th percentile of a log-normal distribution) estimated by: N min = N /exp [0.842 √ln(1 + CV N 2 )], …”