Patterns and causes of spatial and temporal variation in growth rates of early juvenile European plaice Pleuronectes platessa

Juvenile growth rate can be an important determinant of fish population processes, but in many species this life stage occupies inshore areas where growing conditions are extremely heterogeneous. In this study, patterns and causes of growth variation in young-of-the-year European plaice Pleuronectes platessa were examined at nursery beaches along the west coast of Scotland and in the Irish Sea, at several spatial and temporal scales. To characterize growth patterns at these scales, an RNA-based index was developed and validated in laboratory and field trials (Chapter 2). The index estimated instantaneous growth rate of individual plaice from white muscle RNA and DNA concentrations, body mass and water temperature, and was sensitive to short-term variations in food conditions. A field application of the index at small scales (Chapter 3) detected significant alongshore (500 m scale), daily and depth-related growth variation, indicating fine structure in functional attributes of sandy beaches as fish nurseries. An investigation of temporal growth dynamics at seven beaches in two years (Chapter 4) found a general, linear decline in growth rate during late summer. The slope and intercept of this decline varied among beaches and years, but growth ceased in late August in most cases. These growth dynamics were not related to water temperature, total length or intra- or inter-specific (brown shrimp Crangon crangon) competitor densities. In a broad spatial context (Chapter 5) growth rate in mid-August varied considerably among 22 beaches (25 km scale) and three years, but not at coarser spatial scales (50 - 100 km scale). Growth limitation was widespread in mid-August and was related to intra-specific competitor density and physical characteristics of beaches (tidal range and wave fetch), but not inter-specific competitor density or environmental productivity. In conclusion (Chapter 6), a review of this dissertation and research published since 1960 indicated that growth of early juvenile plaice is variable at several scales, but underlying causes still require clarification. Through the development and application of an RNA-based index, this dissertation identified dominant spatial and temporal growth patterns, rigorously tested possible underlying causes and therefore provided a framework for future efforts to understand growth variation in juvenile plaice.