Feeding strategies and body condition of juvenile European flounder Platichthys flesus in a nursery habitat

Estuarine habitats are major nurseries for the European flounder Platichthys flesus, with different year classes sharing food and space resources. Hence, an understanding of feeding strategies that optimize resource use and maintain carrying capacity is fundamental for sustainable and successful ecosystem management. The main feeding areas of juvenile European flounder (including 0-group and 1-group age classes) in the Lima estuary (northern Portugal) nursery ground were investigated by integrating stomach content analyses with stable isotopic values (δ13C and δ15N) and fish condition indices (Fulton K and RNA:DNA ratio). The 0-group flounder that were associated with the upstream section of the estuary presented the lowest δ13C value (−25.58 ± 1.86‰), while 1-group flounder exhibited a higher δ13C value (−22.59 ± 2.51‰), indicating use of the more saline areas of the estuary (lower and middle sections). The two age groups did not differ in terms of δ15N (0-group: 13.93 ± 0.29‰; 1-group: 13.50 ± 0.96‰), indicating similar trophic levels. The low salinity upper estuary was the main feeding area of 0-group flounder (74%), while 1-group flounder fed along the estuary both upstream (52%) and downstream (48%). Juvenile flounder showed high individual condition based on the Fulton K index (0-group: 1.05 ± 0.08; 1-group: 1.07 ± 0.05) and RNA:DNA (0-group: 1.70 ± 0.70; 1-group: 1.41 ± 0.47). These indices deal with fish health, and hence indicate nursery habitat quality. It is concluded that in this temperate nursery habitat, different feeding strategies sustained the condition of the European flounder juveniles, compared with other flounder populations.

Effect of starvation on condition and growth of juvenile plaice Pleuronectes platessa: nursery habitat quality assessment during the settlement period

In order to assess the effect of feeding deprivation on growth and condition of field-caught newly settled juvenile plaice Pleuronectes platessa, a 3 week starvation experiment was conducted and changes in RNA/DNA ratio, Fulton's K condition index and recent otolith growth were measured. Fed ad libitum and starved fish were analysed after 0, 1, 2, 3, 4, 5, 7, 9, 14, 16 and 22 days of starvation. In parallel, field newly settled juvenile plaice were collected weekly over the settlement period (between April and May 2005) in the adjacent intertidal zone of the Canche estuary (eastern English Channel, France) and fish performances were estimated. After starvation day 4, Fulton's K condition index and RNA/DNA ratio of juvenile plaice significantly differed between treatments while recent otolith growth differed between treatments after day 9 of the experiment procedure. Results indicate that the three biological parameters are sensitive indices and can be used to accurately assess nutritional condition of wild juvenile fish. Values of 0.32 for RNA/DNA, 0.83 mg. mm−3 for Fulton's K index, and 3.99 µm for recent otolith daily growth were defined as critical threshold values below which juvenile plaice can be classified as ‘starving'. When comparing these experimental values with those from field-caught newly settled juvenile plaice, less than 1% of wild individuals can be classified as ‘starving' whatever the index. Our results suggest that feeding conditions throughout the settlement period on the Canche intertidal nursery ground are favourable to juvenile plaice development and survival, and hence to the recruitment success.