Fishing for food : feeding ecology of harbour porpoises Phocoena phocoena and white-beaked dolphins Lagenorhynchus albirostris in Dutch waters

Abstract

Harbour porpoises and white-beaked dolphins are the most common small cetaceans in the North Sea and Dutch coastal waters. The distribution and relative abundance of harbour porpoises and white-beaked dolphins from the Dutch coastal waters has changed significantly over the past decades. This thesis describes the past and present feeding ecology of these two species in Dutch coastal waters and investigates whether changes in abundance and relative distribution of porpoises reflect changes in their foodbase. For porpoises, three techniques for dietary analyses were combined, including stomach contents-, stable isotope- and fatty acid analysis, providing the most detailed description of their diet in time and space, elucidating differences between their short- and longer term diet. Stable isotope analysis (δ13C and δ15N) were carried out in bone and muscle samples collected from porpoises stranded along the Dutch coast. Muscle δ15N values revealed that neonatal enrichment occurred and that larger porpoises, in particular males, seem to feed on lower trophic level species, compared to smaller individuals. Also bone δ15N values show that larger animals had fed on lower trophic levels in distant times. Seasonal variation in bone δ15N and δ13C values revealed two distinct groups of porpoises along the Dutch coast, a winter group (mainly males) that migrated from neighbouring regions and a Dutch subpopulation in summer (Chapter 2). To assess the contribution of prey species to the porpoises’ diet, stable isotope analysis in both porpoise muscle and prey were carried out. With the use of a mixing model (Stable Isotope Analysis in R, SIAR), we revealed that 70-83% of the diet of porpoises consisted mainly of poor cod, mackerel, greater sandeel lesser sandeel, sprat and gobies. This highlights a higher importance of pelagic, schooling species in the porpoises’ diet compared to stomach contents, where 90.5% of the diet consisted of gobies, whiting, lesser sandeel, herring, cod and sprat. Porpoises thus also feed offshore on pelagic, schooling species, while they feed closer to shore on more benthic and demersal species shortly before they strand. This could be due to the distribution of prey species as well as differences in behaviour of porpoises and their prey between the coastal zone and offshore waters (Chapter 3). The use of Quantitative Fatty Acid Analysis (QFASA) showed that the diet of porpoises consisted mainly of gobies, mackerel, smelt, herring and dragonet, pointing towards profound differences between the diet as estimated by QFASA and as deduced from stomach contents. This study revealed that the longer term diet of porpoises in Dutch coastal waters consists both of coastal species (e.g. gobies, smelt and dragonet) and also pelagic, schooling species (e.g. mackerel and herring). The results also brought to dawn possible methodological problems in using QFASA for porpoise diet estimation, emphasizing the importance of applying different dietary analysis techniques when studying marine mammal diets and the need for controlled feeding experiments in order to improve the interpretation of dietary analysis results (Chapter 5). Besides new insights in the feeding ecology of porpoises, stable isotope analysis also elicited a non-food related conservation ecology issue. Distinct δ13C values in muscle of porpoises stranded in the Eastern Scheldt revealed that these porpoises foraged there for a longer period. This distinct δ13C signature of animals from the Eastern Scheldt was not observed in bone tissue, which suggests a relatively recent shift in habitat use rather than life-long residency of porpoises within the Eastern Scheldt. The high number of strandings within the Eastern Scheldt revealed a higher mortality rate compared to the Dutch coastal zone, indicating that along with other changes in the physical environment, the building of the storm surge barrier may play an important role in determining the residency of porpoises in the Eastern Scheldt, and that the area might act as an ecological trap for porpoises entering it. This is an example of the impact on marine species due to protection structures that emerge and respectively increase worldwide in response to the effects of global warming and climate change. It highlights that even semi-open structures, which are meant to ameliorate habitat loss, degradation and fragmentation, may still affect the abundance and distribution of migratory marine mammal species (Chapter 4). The analysis of stomach contents of white-beaked dolphins showed that their diet was dominated by Gadidae. All other prey species combined contributed little to the diet by weight. The two most important prey species based in weight were whiting and cod. In numbers, gobies were most common, but these contributed little to the diet by weight. The overall diet showed a lasting predominance of whiting and cod, without clear changes over time (35 years) or differences between sexes or size-classes of dolphins and revealed that white-beaked dolphins in the south-eastern North Sea are specialist feeders, with a strong preference for whiting and cod (Chapter 6). Stomach contents of juvenile white-beaked dolphins in our study revealed that at the age of about 1.5 years old, they had started feeding on solid food by taking a variety of small fish and invertebrate prey, mostly shrimp and squid. Immatures in our study, estimated to be 2-4 years old, still take small prey, including small gadoids, but also take larger gadoids. Calves apparently gradually learn to eat big fish by taking prey that is much smaller than those normally taken by their mothers. This study illustrates novel techniques for diet estimation to reconstruct sizes of shrimp and whiting from tail flaps and eye lenses, respectively (Chapter 7). Most dietary studies on porpoises and white-beaked dolphins are deduced from stomach contents. This thesis has demonstrated that using indirect methods for studying the feeding ecology of marine mammals is a valuable addition to the more direct approach using stomach contents. It supports the need for multi-method approaches because by using only one technique, key prey species in the predator-prey relation may be missed or underestimated. Future ecological and fishery impact assessment studies and management decisions for the conservation of porpoises and white-beaked dolphins should acknowledge a difference between their long- and short-term diet. Large improvement in the interpretation of the results from diet analyses can be established either by controlled feeding experiments with animals in captivity or by studies that help to understand the common principals in dietary analyses (e.g. digestion rates, turnover rates of tissues, tissue-dependent isotopic fractionation between predator and prey and lipid metabolism within the animal) and variation of these between species.</p