Sveegaard, S. 2011: Spatial and temporal distribution of harbour porpoises in relation to their prey. PhD thesis. Dep. of Arctic Environment, NERI. National Environmental Research Institute, Aarhus University, Denmark. 128 pp.
The population status of harbour porpoises has been of concern for several years due to anthropogenic influences, especially incidental bycatch in gillnet fisheries. Proper management of a wide-ranging species such as the harbour porpoise requires reliable information on distribution, migrations, status of biological populations, and habitat preferences. This PhD thesis discusses these issues.
Paper I provides an overview of the present knowledge on harbour porpoise distribution, the methods of examining this distribution, and the underlying ecology of prey and marine environment potentially affecting harbour porpoise distribution. Furthermore, the use of this information in the protection and management of harbour porpoises in Denmark is discussed, as well as future perspectives for this research to aid the protection of harbour porpoises.
The remaining parts of the PhD thesis are divided into two: one examining the harbour porpoise distribution and the usage of, and agreement between, methods for studying this distribution. The second part explores the underlying ecological causes governing harbour porpoise movements.
In part one, Paper II examines the movements and area preferences of 64 satellite tagged harbour porpoises, in order to determine the distribution and identify high density areas in the eastern North Sea, the western Baltic, and the waters in between. Results show an uneven harbour porpoise distribution, with concentrated occurrences in nine high density areas within the study area. Several of these areas are subject to significant seasonal variation. Paper III applies acoustic vessel surveys as an independent method to test the temporal and spatial stability of the distribution found by satellite telemetry in Paper II. The comparison of the two methods reveals a strong spatial agreement between them, which confirm the presence and stability of areas of high porpoise density, and, furthermore, validates the applicability of the two methods as tools for studying the distribution of harbour porpoises. In Paper IV, data from satellite tracked harbour porpoises are used to define the population boundaries of the genetically distinct population inhabiting Kattegat, Belt Sea, the Sound and the western Baltic Sea. This population is of particular importance in conservation efforts since it represents the only possible new gene flow into the endangered population in the Baltic Sea. Using the new identified boundaries, abundance estimates for the population were calculated based on two large-scale visual surveys in 1994 and 2005, to be 27,767 (CV=0.45) in 1994 and 10,865 (CV=0.32) in 2005. Although not statistically significantly diffe-rent in a statistical sense, the declining trend gives reason for concern.
The main drivers governing harbour porpoise movements are hypothesised to be prey-related. In the second part of the PhD thesis, this hypothesis is tested. Paper V reviews all available studies on harbour porpoise prey preferences based on analysis of stomach content for the genetically distinct harbour porpoise population discussed in Paper IV. Furthermore, the seasonal prey preferences are recalculated using accessible data from two of the reviewed studies. Cod, herring, gobies, and whiting are identified to be the primary prey species, although the relative importance of each species varies across seasons. These results are subsequently compared to available knowledge on the distribution of these fish species and the spatial and temporal correlation between porpoise and their prey is assessed. However, a serious lack of information on prey species distribution prevents any detailed analysis of temporal or spatial correlations between predator and prey. Paper VI compares the distribution of satellite tracked harbour porpoises with distribution of a main prey species, herring, obtained through annual acoustic surveys. Depth and density of a non-prey species, mackerel, are also included in the analysis to examine if the interactions between density of herring, mackerel (which preys on herring) and depth affect the distribution of harbour porpoises. It is found that densities of porpoises and mackerel are positively correlated with herring densities, which in turn is correlated with depth. Paper VII builds onto this analysis, and examines why the seasonal variation in harbour porpoise occurrence in a Danish strait does not correspond with the seasonal distribution of fish abundance (especially herring). By examining the harbour porpoise stomach content, it is found that in the high porpoise density season (April-October), mean prey weight per stomach is larger and the frequency of occurrence as well as the diversity of prey species is higher than in the low density season (November-March). Furthermore, cod is found to be the main prey species, in terms of weight in the high season, and herring in the low season. The development of frontal zones in the spring in the northern part of the Sound is suggested to aid the porpoises in locating their prey, and unavailability of the overwintering herring due to heavy boat traffic is suggested to be the cause of the low winter abundance.
In conclusion, this PhD thesis introduces several new applications for satellite telemetry data that - in combination with acoustic surveys - has significantly contributed to the current knowledge of harbour porpoise distribution. Furthermore, the thesis provides evidence of a porpoise-prey relationship which is important information in the conservation of the species, due to its influence on harbour porpoise distribution.
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