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Recruitment
Bass post-larvae resulting from offshore spawning recruit from June onwards into coastal and
estuarine nursery habitats (Jennings and Pawson, 1992; Reynolds, Lancaster and Pawson,
2003). The survival of sea bass through the first winter is reduced at temperatures below 5–
6°C (Kelley, 2002; Lancaster, 1991), which may well explain the attraction of warm water for 0group
(first year) sea bass, especially in cold winters in the North Sea, where they may
otherwise not survive(Kelley, 1986; Pawson and Eaton, 1999). Incidental catches of juvenile
bass taken during demersal fish surveys (DFS) suggest that the estuaries in the south of the
Netherlands (Easter Scheldt and Wester Scheldt) have become increasingly important as
nursery areas for sea bass (ICES, 2008).
Juvenile bass emigrate from these nursery areas at around 36 cm, aged 3-5 years depending
on growth, which is strongly related to summer temperatures (Reynolds et al., 2003). In
contrast to the regular migrations shown by adult bass, a substantial proportion of pre-adult sea
bass emigrate from their respective ―stock‖ areas and disperse widely, and do not necessarily
recruit to their specific parent spawning stock (Pawson et al., 1987; Pickett, Kelley and
Pawson, 2004). It appears that there is considerable mixing of sea bass at this stage
throughout large parts of the populations' range, and pre-adult fish have been observed to
emigrate from the Thames Estuary (southern North Sea) along the English Channel coast and
even into the eastern Irish Sea. After 4-7 years, or at approximately 35 cm for males and 42 cm
for females, sea bass attain maturity (Pawson and Pickett, 1996).
Feeding
Bass are opportunistic predators throughout life, feeding on the species of crustaceans and
fish which are the most readily available in any particular environment (Pickett and Pawson,
1994). Bass larvae feed on small zooplankton whilst 0-group bass take larger, epibenthic
organisms that are abundant in the near-shore environment, particularly small crustaceans
(Roblin and Brusle, 1984).Whilst small fish such as gobies and juvenile herring and sprat are
found more regularly in the diet as the bass grow, crabs are the most common food of juvenile
bass in UK waters, followed by brown shrimp Crangon crangon. Pickett and Pawson (1994)
found shore crab (Carcinus meanas) and brown shrimp occurring at a similar frequency
(around 40% of stomachs) in the southern North Sea, with relatively few fish, though they did
note a predominance of fish (particularly mackerel Scomber scombrus) in the diet of adult bass
taken 10-30 miles offshore in the western English Channel. Low temperatures inhibit feeding in
juvenile bass (Lancaster, 1991), whilst adults continue to feed throughout the year, except
possibly when spawning (Pawson and Pickett, 1996).
Climate effects on sea bass production in the North Sea
Studies on the migration of sea bass in the late 1970s and early 1980s around the coasts of
England and Wales, and on the species‘ reproductive physiology (Devauchelle, 1984) and the
distribution of bass eggs in relation to water temperature (Thompson and Harrop, 1987), were
used by Pawson et al. (1987) to suggest that the southerly/westerly migrations between
summer feeding areas (in the North Sea and the offshore spawning areas in the English
Channel were related to the necessity for adult female bass to seek out water above
approximately 9 C, both for final gonad maturation and spawning. Pawson, Pickett and
Witthames (2000) showed that ambient water temperatures in winter strongly influence the
onset of maturity in female bass, so that females that stayed in the North Sea are unlikely to
mature until they either adopt the autumn migrations to the south west, or if warmer winters
occur.
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