Sandeels comprise a quarter of North Sea fish biomass and are vital prey for a number of marine mammals and birds. However, in recent decades there have been significant declines in sandeel abundance and energy value. These declines have been linked to climate change, however, it is unclear what the relative influence of changes in food and temperature is. Here we examine the role of these factors on different aspects of the sandeel lifecycle and determine how robust sandeels are to expected climate warming. First, we review the evidence for climate impacts on sandeels and discuss the implications for higher trophic levels with particular reference to seabirds. Evidence summarised demonstrates that sandeels are a critical food source for many seabirds, and that declines in sandeel populations have negatively impacted seabird breeding populations. Lack of existing quantitative understanding of the influence of food and temperature on sandeels demonstrates the need for a new mathematical model to predict the outcome of climate warming on sandeel stocks. Second, we model changes in spawning and hatch dates off the Scottish east coast, which have been proposed as a contributor to the long-term decline in sandeel energy content. Results indicate that spawning and hatch dates do not explain this decline. Instead, the timing of both lifecycle events is relatively fixed and is governed by predictable environmental cues. Moreover, given the weak temperature effect on spawning and hatching, future temperature rises appear unlikely to significantly affect hatch date. The central part of this thesis is the development and use of a new dynamic energy budget model to unravel the influence of food and temperature on sandeel abundance and energy content. An important application of the model was the examination of mortality rates between 2001 and 2007, a period of pronounced stock decline off the Scottish east coast. The model was driven by food and temperature. Support was found for the hypothesis that overwinter starvation mortality contributed towards a recent decline in sandeels in northern UK waters. Highest over-winter mortality rates were recorded for juveniles and not individuals aged 1 or over due to the effect of weight-specific metabolism. However, a sensitivity analysis of the model suggests that mortality rates are more sensitive to changes in copepod abundance in the build up to overwintering rather than temperature during overwintering. We suggest that food-driven size-selective starvation mortality may have contributed to the stock decline off the Scottish east coast. We therefore conclude that indirect food web effects of climate change are likely to be greater than direct physiological effects on sandeels.
|Date of Award||5 Oct 2017|
- University Of Strathclyde
|Sponsors||University of Strathclyde & Fisheries Research Service (FRS)|
|Supervisor||Mike Heath (Supervisor) & Douglas Speirs (Supervisor)|