Don\u2019t forget the porpoise: acoustic monitoring reveals fine scale temporal variation between bottlenose dolphin and harbour porpoise in Cardigan Bay SAC

Hanna K Nuuttila,Peter G H Evans,Sarah Baulch,Winnie Courtene-Jones,Malene Simon

doi:10.1007/s00227-017-3081-5

Abstract

Populations of bottlenose dolphin and harbour porpoise inhabit Cardigan Bay, which was designated a Special Area of Conservation (SAC), with bottlenose dolphin listed as a primary feature for its conservation status. Understanding the abundance, distribution and habitat use of species is fundamental for conservation and the implementation of management. Bottlenose dolphin and harbour porpoise usage of feeding sites within Cardigan Bay SAC was examined using passive acoustic monitoring. Acoustic detections recorded with calibrated T-PODs (acoustic data loggers) indicated harbour porpoise to be present year round and in greater relative abundance than bottlenose dolphin. Fine-scale temporal partitioning between the species occurred at three levels: (1) seasonal differences, consistent between years, with porpoise detections peaking in winter months and dolphin detections in summer months; (2) diel variation, consistent across sites, seasons and years, with porpoise detections highest at night and dolphin detections highest shortly after sunrise; and (3) tidal variation was observed with peak dolphin detections occurring during ebb at the middle of the tidal cycle and before low tide, whereas harbour porpoise detections were highest at slack water, during and after high water with a secondary peak recorded during and after low water. General Additive Models (GAMs) were applied to better understand the effects of each covariate. The reported abundance and distribution of the two species, along with the temporal variation observed, have implications for the design and management of protected areas. Currently, in the UK, no SACs have been formally designated for harbour porpoise while three exist for bottlenose dolphins. Here, we demonstrate a need for increased protection and species-specific mitigation measures for harbour porpoise.

Highlights

In Cardigan Bay, West Wales, the two most common cetacean species, bottlenose dolphin (Tursiops truncatus) and harbour porpoise (Phocoena phocoena) are observed near shore throughout the year (Simon et al 2010; Baines and Evans 2012)
This study examined the spatio-temporal fluctuations in the distribution and occurrence of bottlenose dolphins and harbour porpoises within Cardigan Bay Special Area of Conservation (SAC) with implications for future management
During January to March 2009, T-PODs were deployed at all sites surveyed the previous year apart from New Quay fish factory (Table 1) with only single T-POD logging in Mwnt inshore between April and September 2009 and in Ynys Lochtyn between October and end of December 2009

Summary

Introduction

In Cardigan Bay, West Wales, the two most common cetacean species, bottlenose dolphin (Tursiops truncatus) and harbour porpoise (Phocoena phocoena) are observed near shore throughout the year (Simon et al 2010; Baines and Evans 2012). In Wales ( Cardigan Bay), porpoise deaths caused by bottlenose dolphin attacks increased notably between 1991 and 2004, when peak mortality was recorded (Deaville and Jepson 2006; Penrose 2006, 2014) Within their population range, bottlenose dolphins and harbour porpoises show high variability in their distribution, with seasonal and diel fluctuations which typically correspond with the seasonal occurrence of prey species (Evans 1980; Northridge et al 1995; Goodwin 2008; Sveegaard et al 2012; Pirotta et al 2014; Norrman et al 2015). Visual surveys cannot inform us of the nocturnal activity or diel changes in habitat use of the two species and it is only from acoustic studies that such data can be achieved (Bräger 1993; Philpott et al 2007; Todd et al 2009)

Methods

Results

Conclusion