Wild Trout in an English Chalk stream: Modelling Habitat Juxtaposition as an Aid to Watershed Rehabilitation A.Burrows, S.Kett and M.A.House Flood Hazard Research Centre, Middlesex University, Queensway, Enfield London, UK,

The River Piddle: A Classic English Chalkstream Stable flow regime gravel substrate high primary and secondary production complex structure of riparian zone providing great “edge” habitat cover

RIVER PIDDLE Location of study sites

Problems of bankside grazing Poached unfenced banks Bank collapse Absence of littoral margin Wider & shallower channel

Research Aims Analyse response of a wild brown trout population to temporal and spatial habitat variation in an English chalk-stream using Physical Habitat Simulation Model Examine habitat factors limiting carrying capacity for adult trout Assess importance of juxtaposition of spawning and nursery habits for 0+ recruitment Quantify importance of reach scale meso-habitats for different life stages and seasons. Improve understanding of natural mechanisms regulating brown trout populations to improve effectiveness of habitat rehabilitations

Stages in Methodology Quantitative electrofishing

Transect Based Meso-Habitat Survey Depth Column velocity Cover Substrate

Cells defined by surveying channel cross sections

Habitat Time Series Generation

POPULATION DYNAMICS

Length - frequency plot Upstream sector - 1999 35 30 age 0+ = 115cm age 1+ = 210 age 2+ = 265 age >2+ = 325 25 20 15 10 5 50 100 150 200 250 300 350 400 450 500

Population Structures

Length – frequency histograms Comparison of length-frequencies between river sectors

Von Bertalanffy Growth Curve from mark-recapture data showing length gain for tagged trout in the Cobbs Wood reaches 1993 - 1999 Result - Brody-Bertalanffy Results k (growth rate) = 0.329 L infinity = 420.3 To (Time when length is 0) = 0.866 95% Confidence for K +/- 0.2114 95% Confidence for L infinity +/- 65.59

RELATIONSHIPS BETWEEN HABITAT AND POPULATION STRUCTURE ANALYSIS II RELATIONSHIPS BETWEEN HABITAT AND POPULATION STRUCTURE

Adult Habitat Overhanging cover Pool Deep undercut bank Half-log cover board

Adult Winter Habitat Durations 500 1000 1500 2000 2500 93 94 95 96 97 98 99 Cover Near shore Fig.1. Adult winter habitat durations for marginal and total reach habitats (WUA) in the Upper River

Adult Marginal Habitat-Upper River 200 400 600 800 1000 1200 1400 93 94 95 96 97 98 99 2000 Summer Winter Seasonal Time Series comparison for Adult Marginal Habitat

Winter view

Adult density v marginal habitat Relationships between adult density and mean marginal habitat in winter

Fry Habitat Medium gravel - good spawning habitat

Spawning Quality by Habitat Type

Riffles - spawning R2 = 0.65; p=0.05 Fry 56.00 57.00 58.00 59.00 Mean Riffle Habitat during hatching period -1.00 0.00 1.00 2.00 Fry ^ R2 = 0.65; p=0.05

Young of Year Growing Season Relationship between 0+ density and minimum summer habitat R2= 0.49; p=0.01 1000.00 1500.00 2000.00 2500.00 3000.00 Growing Season Habitat Minima -1.00 0.00 1.00 2.00 3.00 Fry ^ fryd = -0.40 + 0.00 * ys.min R-Square = 0.49

Meso-habitat Time Series (GS)

Spawning and Nursery Habitat Related to Fry Density in Lower River

SUMMARY Adult carrying capacity at reach scale related to winter marginal habitat Refugia from winter peak flows more important than from summer low flows Riffles are critical meso-habitats for early life stages (especially during hatching period) Glides are critical meso-habitats during first growing season at low flows Glides are more important for juveniles age1+ Sequence of meso-habitat juxtaposition important for early life stages Lower sector reaching biological carrying capacity ?

CODA: Wild Trout 8 context Similar anthropogenic problems as in US BUT: relatively few, small catchments with wild trout populations populations are isolated and fragmented (‘contracted’) limited refugia from which potential re-colonisation can occur In tiny catchments meso-habitat juxtaposition is more important to maximise recruitment and offers an effective management technique with potential catchment wide benefits No second chance - chalk catchments are a small and unique ecotype

FISHERIES SCIENTIST FOR SALE 27 years fly fishing experience Salmonid habitat assessment and mapping Channel survey and habitat enhancement Electofishing Habitat modelling Population dynamics analyses

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