Abstract
Sprat, Sprattus sprattus, is the dominant pelagic species in British inshore and
estuarine waters. Within the Bristol Channel the population is almost totally
composed of fish <3 years old with the adults overwintering in Bridgwater Bay.
Sprat follow regular seasonal migrations and occasionally form huge aggregations
which together generate considerable between sample variability. Using a 36-year
monthly time series collected in the Bristol Channel since 1980, together with two
periods of intensive daily and weekly sampling, sprat growth is shown to have
declined almost linearly over the last 36 years coincident with increasing late
summer-autumn seawater temperatures. Longevity has also declined, with age 3+
sprat >140 mm standard length lost to the population by 1999. Further, adult
condition, measured as the average weight of a 103 mm standard length adult,
declined rapidly from 13.7 g in 2007 to 9 g in 2011. Despite these changes, which
would have reduced age-specific fecundity, a sign-rank test showed abundance of
adult sprat has shown no long-term trend and Bulmer's test indicates
density-dependent regulation is operating. While sprat recruitment is shown to be
responding to the sunspot cycle, the North Atlantic Oscillation and sea water
temperature, the impact of these variables on adult population density is damped
because of density-dependent regulation. The result is that sprat respond to
environmental change with large changes in their growth and condition, but the
adult abundance is constrained and shows no long-term trend. Recruitment was
modelled by combining a Ricker curve with terms for the response of sprat to
solar activity, the North Atlantic Oscillation and spring temperature. It is shown
that the stock-recruitment relationship does not form a simple curve, but is
bounded within a region in which the upper and lower constraints are defined by
environmental conditions. Within this bounded region the population trajectory
under differing environmental regimes can be predicted.
Keywords:
Sprat growth
Population dynamics
Long-term sampling
NAOI
Power station monitoring
Salinity
Seasonal variation
Water temperature
Density-dependence
http://dx.doi.org/10.1016/j.seares.2016.11.003
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