Spatio-temporal variation in lesser sandeel growth and demography, causes and consequences

  • Agnes Olin

Student thesis: Doctoral Thesis


In large parts of the north-east Atlantic, the lesser sandeel (Ammodytes marinus) is an important prey for seabirds, marine mammals and fish. A. marinus shows strong spatio-temporal variation in abundance and size, including a sustained decline in size in several locations in the North Sea. The variation in size has been hypothesised to be mainly driven by variability in food conditions, but exploring this hypothesis on a larger spatial scale has so far been hampered by the lack of zooplankton data of sufficient spatio-temporal and taxonomic resolution.;Further, the extent to which the clear spatial structure in the sandeel population is reflected in the populations of their seabird predators is not clear. This thesis aims to address these gaps and contribute to the mechanistic understanding of bottom-up effects in the zooplankton-sandeel-seabird food chain. As declines in the abundance and size of sandeels as a result of changes in their zooplankton prey are thought to have played a large role in driving the dramatic declines of seabirds in large parts of north-east Atlantic, understanding drivers and bottom-up processes in this food chain is of large importance.;The thesis first addresses the lack of the kind of high-resolution zooplankton data required for exploring the role of food conditions in sandeel dynamics. For this purpose, an approach for the generation of prey fields from spatially aggregated Continuous Plankton Recorder data is developed. The generated prey fields are then used to examine spatio-temporal patterns in sandeel food conditions, focusing mainly on the North Sea and covering the time period 1975 to 2016.;In the western North Sea, there have been clear declines in both the total amount of energy available to sandeels and the abundance of small copepods, which make up a large proportion of the sandeel diet. In terms of Calanus spp., which are also an important part of the sandeel diet, there was no clear change in abundances of Calanus finmarchicus in the examined locations, while abundances of Calanus helgolandicus showed a clear increase in most of the study area around 2000. The average prey size generally increased over time in the western North Sea, whereas it instead declined in the north-east.;Further, due to the differences in the timing of the feeding seasons, it is clear that 0 group and 1+ group sandeels experienced different prey fields, with, for example, a larger abundance of smaller copepods during the 0 group feeding season.;To explore to what extent this variation in food conditions can explain spatiotemporal variation in sandeel size, a dynamic energy budget growth model is then developed. This model estimates size daily throughout the first sandeel growth season as a function of food conditions, temperature, light conditions as well as size at and timing of metamorphosis. The model is run in six locations: southern Iceland, the Faroes, Shetland and three locations further south in the North Sea, including Dogger Bank, the Firth of Forth and the East Central Grounds.;In the more southerly locations considered, model predictions agreed well with observations in terms of long-term mean lengths and spatial differences in length and the model also reproduced a previously observed decline in length in the north-western North Sea. Agreement with observations in the Faroes and Iceland was poorer. Food conditions played the main role in driving predicted variation in size, with Calanus spp. being particularly important. Timing of metamorphosis also had a substantial impact on predicted sandeel size. In contrast, the direct effect of temperature was negligible.;Finally, the thesis explores the extent to which spatial patterns in the sandeel population along the coast of the UK propagate up to their seabird predators. To do this, geographical patterns in the synchrony of breeding success in black-legged kittiwake (Rissa tridactyla) colonies are examined in areas where sandeels are an important part of the diet. The distance between colonies was a strong determinant of between colonysynchrony, with the scale of synchrony in kittiwake and sandeel populations being similar. Further, the colonies also formed clusters with synchronous breeding success with a clear spatial pattern, which generally aligned with the spatial structure of the kittiwake,' sandeel prey.;The results of the thesis thus suggest that food conditions play an important role in driving observed variation in sandeel size. This implies that past and ongoing climate change-driven changes in the sandeel prey field are likely to have a large impact on sandeel growth rates, with potential knock-on effects on demographic rates. Further ,the results suggest that processes occurring at the level of the sandeel are reflected in the structure of the local kittiwake population, indicating that the sandeels are able to mediate changes occurring at lower trophic levels up to the level of their seabird predators.
Date of Award7 Sept 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorNeil Banas (Supervisor) & Mike Heath (Supervisor)

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