Prey patch and distribution (SP 8)

Prey patch and distribution (SP 8)

The aim of the project is to collect data on the distribution of harbour porpoise prey patches in order to improve the DEPONS modelling framework. More specifically we aim (1) to quantify the spatial and temporal dynamics of potential porpoise prey patches in relation to natural environmental variation and (2) to evaluate to what extent patches with different fish species compositions are selected by porpoises.

Prey dynamics are known to influence the porpoise population dynamics in our simulation framework, so availability of better data on the spatial and temporal distribution of prey will significantly improve the realism of the population estimates. In the current version of the model these are obtained using a landscape with randomly distributed food patches, which is unlikely to be realistic.

The main aim of this project is to determine how the size, dynamics and distribution of the fish patches that porpoises potentially prey upon is related to natural environmental variation in selected parts of the North Sea. A secondary aim of the project is to investigate to what extent the prey species composition affects whether the prey patches are selected by porpoises.

The study will be used for building a realistic map of porpoise prey patches, where the size and spatial distribution of the patches corresponds to that in the North Sea and where their suitability to porpoises is explicitly incorporated. This map will be used in subsequent simulations of porpoise population dynamics in the North Sea.

Background

Harbour porpoises are affected by a wide range of anthropogenic disturbances such as noise from wind farms and ships. These disturbances are likely to have a direct impact on the individual porpoises, but until recently their cumulative impact on the population level had never been investigated. An indepth understanding of the joint effect of different kinds of disturbances on the population’s dynamics is necessary for assessing whether its long-term survival is jeopardised.

Methods

The use of hydro acoustic equipment to estimate abundance of fish offers high resolution vertical and horizontal observations of fish in the water column that cannot be achieved using any other method (Simmonds & MacLennan, 2005). The general principle of acoustic surveys is to continuously integrate the backscatter from the echo-sounder along a predetermined track. The backscatter is filtered to remove unwanted echoes from e.g. the bottom, airbubbles and zooplankton after which it yields a proxy for fish abundance.

To convert the integrated backscatter to true fish abundance, the acoustic reflection (target strength) of the fish species needs to be considered. Depending of the fish species characteristics (e.g. size, shape, occurrence of swimbladder) they will reflect different amounts of sound. Trawl hauls are typically used to characterise the species and size composition in the survey area. These data are then used to determine the target strength distribution of the fish community, which allow for species and size specific estimates of fish density.

The spatial distribution and dynamics of porpoise prey patches will be mapped using the scientific echo-sounder, Simrad EK60, which during the last 15 years has been the de-facto standard for fisheries research. All procedures follow the recommendations in Simmonds & MacLennan (2005). The survey will be conducted in the summer of 2016 using the R/V Aurora.

Schedule and Deliverables

The project will be initiated in November 2015. The results of the project shall be delivered to the DEPONS steering group in the form of a submission ready manuscript for at peer-reviewed scientific journal by June 2017. After the submission the simulation framework will be updated to take advantage of the new input data; the model will be re-parameterized based on the new information, and it will be investigated how sensitive it is to variations in the size and spatial distribution of prey patches. The project will end by June 2018.