Fishing for Food

Abstract

Shelf waters are rich marine ecosystems. World-wide, major changes in number and distribution of species have occurred and raised concern about the state and stability of these ecosystems (Worm et al. 2006). Questions are arising as to what factors are causing these changes. Understanding these factors and under- lying mechanisms and the adaptability of species can enable us to improve our conservation measures as the environments change. As a contribution to such understanding I studied the feeding ecology of porpoises, a sentinel species in many coastal marine systems. The diet of ma- rine mammals is fundamental to understand their ecology and their role and function in the marine ecosystem. It serves to investigate scientific questions on feeding strategy, predator-prey relationships, and responses of marine mam- mals to changes in food web dynamics, thereby delivering insight in functional relationships to community structure and ultimately state and stability of the ecosystem they inhabit. In that way marine mammals can be regarded as sen- tinel species for the state of the ecosystem they live in and can contribute to the assessment of e.g. the responses of the system to a topical issue such as climate change or fishery interactions (Moore 2008; Bossart 2011). Apart from the direct competition for resources, marine mammals and fisheries can have an impact on each other, due to by-catch of marine mammals during fishery ac- tivities or the damage of fishing gear. The effect of by-catch on marine mammal populations is relatively well documented (Northridge 1991; Matthiopoulos et al. 2008), while studying the competition for resources remains difficult due to problems in quantifying marine mammal diets that are often variable in space and time. In this thesis, the focus lies on the ecosystem of the north-eastern At- lantic, in particular of the North Sea. Large changes have been documented in the abundance and distribution of marine mammals in this area. Quantitative abundance estimates of marine mammals in this area have only recently been done (Hammond et al. 2002; Hammond 2006), therefore earlier trends in abun- dance are mainly based on archived and anecdotal documentation of strandings. Strandings of marine mammals are usually rather well documented in densely populated areas and can yield information on species diversity as well as local occurrence and densities of marine mammals in the area (Pyenson 2010; Peltier et al. 2012). Dedicated surveys can be used to obtain information on changes in abundance and distribution of marine mammals. Stranded animals provide ample opportunity to study a whole suite of parameters, including diet. In the North Sea, distinct changes in stranding frequencies of several ce- tacean species have occurred during the twentieth century. On the Dutch coast (Figure 1.1), numbers of stranded harbour porpoises Phocoena phocoena in- creased gradually again since the 1990s, and showed an abrupt increase in 2006 with numbers ranging between 336 and 860 animals per years between 2006 and 2011 (walvisstrandingen.nl 2012). Strandings of white-beaked dolphins Lagenorhynchus albirostris have become more abundant since the 1960s. Before this, bottlenose dolphins Tursiops truncatus used to strand more frequently than white-beaked dolphins, but they have disappeared from the Dutch coastal wa- ters in this period (Camphuysen et al. 2008). Since 2006, between one and four white-beaked dolphin strandings are recorded each year (walvisstrandingen.nl 2012).