Edinburgh Napier University

  The nursery function of the Forth estuary and Firth of Forth for juvenile plaice was investigated using several methods. Growth rates of juvenile plaice in the estuary were compared with growth rates of juvenile plaice in the Firth of Forth. Growth was estimated for comparisons between sites because growth is expected to indicate the quality of a particular habitat as a nursery. Growth was estimated at four sites in 2005 using two methods and in 2006 using a single method: 1) Tracking changes in length - frequency distributions of newly - settled plaice over time was used in both years and 2) Plaice were aged to the day using otolith microstructure in 2005 only, and the ages were regressed against length to estimate growth rates as a 'ground - truthing' of method 1). The growth rates of plaice were found to be similar between estuarine and outer firth sites, but lower at all four sites in 2006 than 2005. A temperature - dependent model was used to compare maximum growth rates with estimated growth rates to determine if plaice were growing optimally, and growth was less than optimal at all four sites in both years, with some evidence of density - dependence. A push net was used to sample plaice for length measurements at each site on each sampling date, and efficiencies of the net were estimated to determine actual densities of plaice and whether the net was length selective. No length selectivities were found, however, the efficiency of the net was less than 100 % and lower on muddy sites than on sandy sites. The effect of salinity on growth of juvenile plaice was assessed in a laboratory experiment. Plaice were found to grow faster at salinity 25, next fastest at salinity 30 and lowest at salinity.
Assessment of consumption rates showed that the differences were likely to be caused by the effects of salinity on physiological performance of fish. A new method of inferring contribution of juvenile plaice from each habitat to adult recruitment, using stable isotopic composition of plaice otoliths was assessed.
Plaice from the estuary could not be distinguished from outer firth plaice on the basis of otolith isotopic composition on the small scale used here, however, the work provides a baseline for this area in larger scale studies. Sediment isotopic composition showed differences between the estuary and outer firth, with differences between sediment types and a detectable influence of marine photosynthesis.