Investigating the influence of the tidal regime on harbour porpoise Phocoena phocoena distribution in Mount’s Bay, Cornwall

Abstract

Investigating the influence of the tidal regime on harbour porpoise Phocoena phocoena distribution in Mount’s Bay, Cornwall

Abstract

Unintentional by-catch in fishing gear is a significant cause of mortality of harbour porpoises in UK waters. Understanding the spatial distribution of harbour porpoises at fine scales and how this changes over time is essential when trying to understand where these lethal interactions might occur.

Mount’s Bay in Cornwall, UK is an important area for harbour porpoises. However knowledge of how they use the Bay is limited. Effort and sightings data were collected over eight years between 2011 and 2018 from a wildlife watching vessel. The data shows a high sightings rate for the Bay and confirms the relative high density of porpoises when compared to areas of the UK traditionally considered as strongholds.

Harbour porpoises are present in high numbers year round. However, a seasonal pattern in their occurrence was identified, with numbers peaking in September. Areas where porpoises consistently occur are considered to be important foraging habitat due to their high foraging nature.
To better understand the distribution of porpoises in the bay and how it changes with the tide cycle, Maximum Entropy Modelling (MaxEnt) with files correcting for survey biases was used. By analysing various environmental variables, the models identified the most important factors that influence the likelihood of porpoises being present. These include the longitudinal position within the bay, the depth of the water, the type of substrate on the seafloor, the distance from the nearest significant seafloor slope, and the distance from the nearest tide front.
In addition, a General Additive Model for Location Scale and Shape (GAMLSS) was also used to model the likelihood of porpoise occurrence in relation to thermal front features remotely sensed by satellites.

Identifying Important Areas for Harbour Porpoises

The variables longitude, depth, distance from the nearest significant seabed slope, and distance from the nearest tide front can be combined to describe the location of turbulent oceanographic processes. The models identify specific spatially focused areas that are important for porpoises and demonstrate that the position of these areas changes throughout the tide cycle. The results indicate that these changes are associated with tidal processes. Tidal processes have been found to concentrate plankton and nekton, which would enhance the foraging success of harbour porpoises and likely explain this relationship.

Key Words: Geography, Geographical Information Systems, GIS, Habitat Modelling, Harbour Porpoise, MaxEnt, Tidal Processes, Conservation

Advisor: Valentijn Venus
Master degree project 30 credits in Geographical Information Sciences, 2023
Department of Physical Geography and Ecosystem Science, Lund University
Thesis nr 168

Popular Abstract

Investigating the influence of the tidal regime on harbour porpoise Phocoena phocoena distribution in Mount’s Bay, Cornwall

Harbour porpoises, small cetaceans characterised by a high surface area to body mass ratio, face challenges in colder waters where they risk energy loss. Maintaining adequate energy levels becomes crucial for these mammals, necessitating constant foraging to regulate body temperature, ensure vital nutrient availability, and sustain essential life-cycle processes, such as reproduction. This perpetual foraging also preserves their blubber, serving as insulation against energy dissipation. Mount's Bay in Cornwall boasts a notable porpoise population comparable to West Scotland, acknowledged globally for its significant porpoise presence.
However, one of the primary human-induced threats to porpoises lies in entanglement within fishing gear, notably gill nets—anchored fixed nets equipped with floats along their headline, trapping fish upon entry. Unfortunately, harbour porpoises also fall victim to entanglement in these nets, often resulting in fatal drownings. To mitigate this issue, noise deterrents known as pingers have been used as a solution in some fisheries. The theory posits that these devices startle porpoises away from the nets, proven effective in reducing by-catch in various global locations. Notably, porpoise by-catch remains a recognised concern in Mount's Bay, Cornwall.
While employing pingers to prevent by-catch appears reasonable, a potential issue arises due to the porpoises' high energy demands, leading them to need to target the most productive feeding areas. It is theorised that tidal dynamics interacting with the seafloor shape create conditions that aggregate zooplankton, attracting fish—a crucial resource for successful foraging. During the day, zooplankton tends to hide near the seabed, migrating vertically, while tidal currents force it towards the surface, luring fish upward.
If fishing nets overlap with these critical foraging areas, the use of pingers might deter porpoises precisely when they require nourishment. In the absence of alternative foraging zones without pingered nets within reach, porpoises lose crucial feeding opportunities. Each instance of pingered nets obstructing porpoises from foraging not only deprives them of vital feeding time but also prompts energy expenditure in seeking new foraging grounds. Depletion of energy reserves leads to a downward spiral, reducing foraging efficiency, rapidly deteriorating a porpoise's body condition, ultimately resulting in starvation.
While the use of pingers may decrease by-catch, it could inadvertently escalate porpoise starvation occurrences. Proponents argue that porpoises return to an area once the pingered net is removed, but this argument holds no weight if the net interferes during the area's productive feeding phase.
This study aims to pinpoint where harbour porpoises forage within Mount's Bay with fine-scale precision and how this behaviour fluctuates across the tide cycle. Data collection for this study relied on a platform of opportunity, a vessel primarily engaged in activities other than data collection. While the collected data are valuable, they may exhibit biases, often due to uneven survey area coverage. Consequently, the study employs the Minimum Cross Entropy modelling technique to predict porpoise distribution. This method uses observed porpoise locations and environmental variables, calibrated against the surveyed area distribution. The model outputs a relative probability distribution of porpoise occurrence, shedding light on influential variables dictating their distribution and their prey species. The data and variables were analysed hourly across the tide cycle, with model accuracy assessed through 100 runs, withholding 25% of the data each time for validation against observed locations.
Advisor: Valentijn Venus
Master degree project 30 credits in Geographical Information Sciences, 2023
Department of Physical Geography and Ecosystem Science, Lund University
Thesis nr 168

• author: Jones, Duncan
• supervisor: Valentijn Venus
• organization: Dept of Physical Geography and Ecosystem Science
• course: GISM01 20232
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Earth and Environmental Sciences
• keywords: Geography, Geographical Information Systems, GIS, Habitat Modelling, Harbour Porpoise, MaxEnt, Tidal Processes, Conservation
• publication/series: GIS Thesis
• report number: 168
• language: English
• id: 9141722
• date added to LUP: 2023-11-28 13:25:10
• date last changed: 2023-11-28 13:25:10