Peter HendersonPisces Conservation Ltd &University of Oxford

Fish and Crustacean Dynamics in the Severn

Estuary & Bristol Channel

35 years monthly sampling at Hinkley now completed..

Sampling started in October 1980 and is still ongoing. Data are available for about 80 species of fish, 17 macro-crustaceans and about 50 species of plankton including 9 mysid species.

The results show a system with many stable features, but with individual species showing a rich and variable range of dynamics

Species richness and diversity is notably stable

Change in the Hinkley fish community through time: overall community view. (a) species richness per year of time series (Hill’s N0 index) ; (b) exponential form Shannon index (Hill’s N1 index; (c) reciprocal of Simpson diversity index; (d) Berger-Parker index (Hill’s N infinity index);(e) mean rank shift index, sequential pairs of years; (f) Bray-Cutis dissimilarity values between sequential pairs of years. From: Magurran, A. E., & Henderson, P. A. (2010). Temporal turnover and the maintenance of diversity in ecological assemblages. Philosophical Transactions of the Royal Society B: Biological Sciences, 365(1558), 3611-3620.

Blue are older than 1 year. Red are in first year of life

All species are seasonal – e.g. sprat, Sprattus sprattus

The fish fall into 4 groups each abundant in a different season

The dendrogram shows the 4 seasonal groupings identified by cluster analysis. Box plots show ln scaled relative abundance for each seasonal cluster .

From:Shimadzu, H., Dornelas, M., Henderson, P. A., & Magurran, A. E. (2013). Diversity is maintained by seasonal variation in species abundance. BMC biology, 11(1), 98.

Abundance through time. The individual seasonal components are shown in the lower plot

While the overall system attributes can show notable temporal stability There are marked differences in the stability of individual species. Abundant core species show density-dependent regulation and high stability. Lower abundance species are more variable.

However species vary greatly in temporal stability

Relationship between mean (±sd) coefficient of variation (COV) and mean (±sd) biomass. Core species showing density-dependence shown in blue. Core species with no evidence of density-dependence in red. Transient species in grey.

From: Henderson, P. A., & Magurran, A. E. (2014). Direct evidence that density-dependent regulation underpins the temporal stability of abundant species in a diverse animal community. Proceedings of the Royal Society of London B: Biological Sciences, 281, 20141336.

These blue dots are high stability core species

These grey dots are highly variable transient species

Trends in 15 most abundant species

From : Henderson, P. A., & Bird, D. J. (2010). Fish and macro-crustacean communities and their dynamics in the Severn Estuary. Marine Pollution Bulletin, 61(1), 100-114.

Trends in next 15 most abundant species

In part these changes can be related to physical change in temperature, salinity and NAO

1980s rapid warming period 1990-2006

stability period

2007 start recent cooling period

The fish and crustaceans are responded to recent cooling.

Each time series can be decomposed and break points and trends identified – for example the Atlantic prawn P. serratus

Warmer waters allowed a change in seasonality to occur around 1997 and this produced a rapid increase in abundance During the 1998-2007 period of high abundance prawns were caught in high numbers in January and February.

Cooling post 2007 has resulted in switch to

1980s/90s seasonality and a decline in

abundance