Annls Limnol. 28 (2) 1992 : 135-146

Long-term and short-term fluctuations in the numbers and catches of Arctic charr, Salvelinus a/pin us (L.), in Windermere (northwest England)

J . M . Elliotti E . Ba roudy 1

K e y w o r d s : A r c t i c c h a r r , W i n d e r m e r e L a k e , a n g l e r s ' c a t c h e s , g i l l - n e t c a t c h e s , e c h o s o u n d i n g .

A l l E n g l i s h p o p u l a t i o n s o f A r c t i c c h a r r a r e f o u n d i n t h e L a k e D i s t r i c t ( n o r t h w e s t E n g l a n d ) . T h e r e a r e a t l e a s t f o u r

r a c e s o f c h a r r i n W i n d e r m e r e , t h e l a r g e s t n a t u r a l l a k e i n E n g l a n d , w i t h t h e n o r t h a n d s o u t h b a s i n s o f t h e l a k e e a c h

c o n t a i n i n g t w o d i s t i n c t r a c e s t h a t s p a w n i n a u t u m n a n d s p r i n g r e s p e c t i v e l y . T h r e e m e t h o d s h a v e b e e n u s e d t o e s t i m a t e

l e v e l s o f t h e c h a r r s t o c k s ; n a m e l y g i l l - n e t t i n g e a c h N o v e m b e r ( 1 9 3 9 - 1 9 9 1 ) f o r a u t u m n s p a w n e r s i n t h e n o r t h b a s i n , a n g l e r s '

c a t c h e s f r o m b o t h b a s i n s ( 1 9 6 6 - 1 9 9 1 ) , e c h o - s u r v e y d a t a f r o m b o t h b a s i n s ( J u l y 1 9 8 9 - D e c e m b e r 1 9 9 1 ) . A n i n c r e a s e i n

g i l l - n e t c a t c h e s f r o m 1 9 4 5 t o 1 9 6 5 w a s a s s o c i a t e d w i t h a c u l l o f l a r g e r p i k e , Esox lucius L . ; s i n c e 1 9 6 5 , c a t c h e s h a v e

f l u c t u a t e d a r o u n d a h i g h m e a n l e v e l . T h e r e w a s g o o d , i f n o t e x c e l l e n t , a g r e e m e n t b e t w e e n t h e a n g l e r s ' c a t c h e s a n d t h e

g i l l - n e t c a t c h e s . T h e r a t i o o f a n g l e r s ' c a t c h e s i n t h e n o r t h a n d s o u t h b a s i n s s h o w e d t h a t c a t c h e s h a v e b e e n r e l a t i v e l y

l o w i n t h e s o u t h b a s i n s i n c e 1 9 8 4 . A l t h o u g h t h e r e h a s a l s o b e e n a n i n c r e a s e i n b r o w n t r o u t , Salmo trutta L . , t a k e n b y

t h e c h a r r a n g l e r s s i n c e 1 9 8 4 i n t h e s o u t h b a s i n , t h i s i n c r e a s e w a s n o t r e s p o n s i b l e f o r t h e l o w e r c h a r r c a t c h e s . W h e n e s t i ­

m a t e s o f f i s h d e n s i t i e s w i t h a n e c h o s o u n d e r w e r e r e s t r i c t e d t o f i s h > 2 0 c m a n d d e p t h s d o w n t o 2 0 m , t h i s b e i n g r o u g h l y

t h e s t o c k a v a i l a b l e t o c h a r r a n g l e r s , t h e r e w a s g o o d a g r e e m e n t b e t w e e n m o n t h l y e s t i m a t e s a n d a n g l e r s ' c a t c h e s , e s p e ­

c i a l l y i n t h e n o r t h b a s i n . T h e e c h o - s o u n d e r d a t a c o n f i r m e d t h e h i g h e r n u m b e r s i n t h e n o r t h b a s i n , a n d l o w e r n u m b e r s

i n w i n t e r i n b o t h b a s i n s . T h e t h r e e d i f f e r e n t m e t h o d s t h e r e f o r e p r o v i d e d c o m p a r a b l e d a t a o n t h e f l u c t u a t i o n s i n l e v e l s

o f c h a r r s t o c k s i n W i n d e r m e r e . P o s s i b l e r e a s o n s f o r t h e s e f l u c t u a t i o n s a r e d i s c u s s e d b r i e f l y .

F l u c t u a t i o n s h i s t o r i q u e e t r é c e n t e d e s n o m b r e s e t p r i s e s d ' o m b l e c h e v a l i e r , Salvelinus alpiaus ( L . ) , d a n s le l a c W i n d e r ­

m e r e ( n o r d - o u e s t d e l ' A n g l e t e r r e )

M o t s c l é s : O m b l e c h e v a l i e r . L a c W i n d e r m e r e , p r i s e p a r p ê c h e u r , p r i s e a u filet, é c h o - s o n d a g e .

T o u t e s l e s p o p u l a t i o n s a n g l a i s e s d ' o m b l e c h e v a l i e r s o n t s i t u é e s d a n s l a r é g i o n d e s l a c s , a u n o r d - o u e s t d e l ' A n g l e t e r r e .

I l e x i s t e a u m o i n s q u a t r e r a c e s d ' o m b l e c h e v a l i e r à W i n d e r m e r e , l e p l u s g r a n d l a c n a t u r e l d ' A n g l e t e r r e . L e s b a s s i n s n o r d

e t s u d d u l a c c o n t i e n n e n t d e u x r a c e s d i s t i n c t e s : T u n e f r a y e e n a u t o m n e , l ' a u t r e a u p r i n t e m p s . T r o i s m é t h o d e s o n t é t é

u t i l i s é e s p o u r é v a l u e r l e s s t o c k s d ' o m b l e c h e v a l i e r , p r i n c i p a l e m e n t p a r c a p t u r e a u filet c h a q u e m o i s d e n o v e m b r e ( 1 9 3 9 - 1 9 9 1 )

p o u r l e s g é n i t e u r s d ' a u t o m n e d a n s l e b a s s i n n o r d , p a r p r i s e s d e s p ê c h e u r s d e s b a s s i n s n o r d e t s u d ( 1 9 6 6 - 1 9 9 1 ) e t p a r

é c h o - s o n d a g e d e s b a s s i n s n o r d e t s u d ( j u i l l e t 1 9 8 9 - d é c e m b r e 1 9 9 1 ) . U n e a u g m e n t a t i o n d a n s l e s p r i s e s a u f i l e t d e 1 9 4 5

à 1 9 6 5 e s t l i é e à l ' é l i m i n a t i o n d e s g r a n d s b r o c h e t s , Esox lucius L . ; d e p u i s 1 9 6 5 , l e s p r i s e s o n t f l u c t u é a u t o u r d ' u n n i v e a u

m o y e n é l e v é . U n t r è s b o n r a p p o r t e s t t r o u v é e n t r e l e s p r i s e s d e s p ê c h e u r s e t l e s c a p t u r e s a u f i l e t . L e s r a p p o r t s d e s p r i s e s

d e s p ê c h e u r s d a n s l e s d e u x b a s s i n s m o n t r e n t l a d i m i n u t i o n d e s p r i s e s d a n s l e b a s s i n s u d , d e p u i s 1 9 8 4 . B i e n q u ' i l y a i t

e u u n e a u g m e n t a t i o n d e t r u i t e s , Salmo trutta L . , d a n s l e s p r i s e s d e s p ê c h e u r s d e p u i s 1 9 8 4 d a n s l e b a s s i n s u d , c e t t e a u g ­

m e n t a t i o n n ' é t a i t p a s r e s p o n s a b l e d e l a b a i s s e d e s c a p t u r e s d ' o m b l e . L o r s q u e l ' é v a l u a t i o n d e s d e n s i t é s d e p o i s s o n p a r

é c h o - s o n d a g e s ' e s t l i m i t é e a u x p o i s s o n s d ' u n e t a i l l e s u p é r i e u r e à 2 0 c m e t s e t r o u v a n t e n t r e 1 e t 2 0 m è t r e s de p r o f o n d e u r ,

c e q u i r e p r é s e n t e l e s t o c k d i s p o n i b l e p o u r l e s p ê c h e u r s d ' o m b l e , i l e x i s t e u n e b o n n e r e l a t i o n e n t r e l e s é v a l u a t i o n s m e n ­

s u e l l e s e t l e s p r i s e s d e s p ê c h e u r s , s u r t o u t d a n s l e b a s s i n n o r d . L e s d o n n é e s d e l ' é c h o - s o n d a g e c o n f i r m e n t l e s c h i f f r e s

é l e v é s d a n s l e b a s s i n n o r d e t l e s c h i f f r e s p l u s f a i b l e s e n h i v e r d a n s l e s d e u x b a s s i n s . A i n s i , l e s t r o i s m é t h o d e s u t i l i s é e s

f o u r n i s s e n t d e s d o n n é e s c o m p a r a b l e s p o u r l e s f l u c t u a t i o n s d e s n i v e a u x d e s s t o c k s d ' o m b l e c h e v a l i e r à W i n d e r m e r e . L e s

r a i s o n s d e c e s f l u c t u a t i o n s s o n t b r i è v e m e n t d i s c u t é e s .

1 . N E R C I n s t i t u t e o f F r e s h w a t e r E c o l o g y , T h e W i n d e r m e r e L a b o r a t o r y , A m b l e s i d e , C u m b r i a L A 2 2 O L P , U . K .

Article available at http://www.limnology-journal.org or http://dx.doi.org/10.1051/limn/1992012

136 J . M . E L L I O T T , E . B A R O U D Y (2)

1. Introduction

The Arctic char r , Salvelinus alpinus (L.) , is a holarct ic species tha t is frequently anad romous in nor the rn latitudes higher than 65°N. At lower lati­tudes , this species forms numerous land-locked popu la t ions , usually found at high alti tudes. In the British Isles, there are several populations in Ireland a n d Scot land, only five recorded in Wales, and all t he English popula t ions are found in eight lakes in nor thwest England (Mait land & Lyle 1991). There a re at least four races of charr in Windermere , the largest natural lake in England, with the nor th and south basins of the lake each containing two races tha t spawn in spring and au tumn respectively (Frost 1965, Child 1984, Par t ington & Mills 1988). Most char r in the lake are au tumn spawners with spring spawners representing 4-6 °/o of the adult popula­t ion (Mills 1989, Mills & Hurley 1990). Rough esti­ma tes of the adul t charr in the lake are 132,000 a u t u m n spawners and 8,000 spring spawners with a total biomass of 30 - 35 Mg (tonnes) (Mills & Hur­ley 1990).

Windermere is divided by shallows and islands (Fig. 1) into a north basin (area 8.1 km 2 , mean depth 25.1 m, max imum depth 64 m) and a south basin (area 6.7 km 2 , mean depth 16.8 m, maximum depth 42 m) . A t an al t i tude of only 39.3 m , Windermere is one of the few lowland lakes with a substantial popula t ion of charr . The lake is situated in a Natio­nal Pa rk and is therefore a focus for tourism and recreat ion. Since 1945, regular measurements from b o t h basins have shown that the lake has become nutr ient enriched (Lund 1972, Sutcliffe et al. 1982, Heaney et al. 1988, Tailing & Heaney 1988). The increasing concentrat ion of NO3-N may be due to a large increase in the use of ni t rogenous fertilizers with subsequent winter fluctuations conditioned by climatic factors, including tempera ture . Large increases in winter concentrat ions of PO4-P, espe­cially in the south basin, are probably due to increa­ses in sewage-borne phosphorus discharged to the lake (see also Mills et al. 1990). Associated with this increasing enr ichment , there has been an increase in hypolimnetic deoxygenat ion during summer and a u t u m n in the sou th basin, first recorded in 1981. Complete anoxia has not occurred in the north basin because it is less product ive , deeper, and its ratio of epilimnetic to hypolimnetic volume is lower than tha t of the sou th basin (Heaney 1987).

Although nutrient enrichment is not directly harmful to charr, the associated anoxia will restrict the water space available to the fish. This space will be reduced further in hot summers when surface temperatures may be higher than the preferred values for this species. It is therefore important that levels of charr stocks in the lake are monitored so that suitable measures may be taken to ensure the survival of this species in Windermere. Evidence from other European lakes indicates that increasing enrichment can lead to a reduction and eventual extinction of charr populations (Mills et a l . 1990).

In marked contrast to stream salmonids that can be sampled by electrofishing, it is not possible to obtain absolute estimates of charr numbers in lakes. Three methods have been used in Windermere to obtain relative estimates of the levels of charr stocks. The chief objective of this investigation is to com­pare da ta obtained by these different methods . If results are similar, then conclusions on the short-term and long-term fluctuations in stock levels can be drawn with greater confidence.

2 . Methods

Since 1939, gill nets with a 32 mm (bar) mesh have been used each November to catch autumn spaw­ning charr in the north basin of Windermere. The results are expressed as catch-per-unit-effort (CPUE) which is the mean catch per gill-net day. The origi­nal monitoring site was just south of Low Wray Bay (1939-1973) but was moved south to North Thomp­son Holme (1975-1991) because of interference with the gill nets (see sites in Fig. 1).

Charr were last netted commercially in Winder­mere in 1921, but they are still caught by fishermen using plumb lines (for detailed account of methods, see Kipling 1984). Fish are caught down to a depth of 15-20 m depending upon the rowing speed of the angler 's boat and the minimum size limit for charr is 203 mm (equivalent to 8 inches). Anglers catch-per-unit-effort (CPUE) data are expressed as mean catch per boat per hour . Data are now available from 1966-1991 for angler A and from 1975-1991 for angler B.

The third and final method was echo sounding, using a Simrad EY-M echo sounder. This equipment has been used extensively to estimate the density of pelagic Fish communities dominated by coregonids,

F i g . 1. M a p o f t h e n o r t h a n d s o u t h b a s i n s o f W i n d e r m e r e s h o w i n g e c h o - s u r v e y t r a n s e c t s ( b r o k e n l i ne ) a n d s i t e s f o r a n n u a l g i l l - n e t

c a t c h e s o f s p a w n i n g c h a r r in N o v e m b e r ( n o r t h b a s i n o n l y ) ; a s i t e s o u t h o f L o w W r a y B a y w a s u s e d f r o m 1 9 3 9 - 1 9 7 3 a n d a s i t e

n o r t h o f N o r t h T h o m p s o n H o l m e w a s u s e d f r o m 1 9 7 5 t o t h e p r e s e n t .

F i g . 1. C a r t e s d e s b a s s i n s n o r d e t s u d d e W i n d e r m e r e i n d i q u a n t l e s t r a n s e c t s p a r c o u r u s p o u r l ' é c h o - s o n d a g e ( e n p o i n t i l l é s ) e t l e s s i t e s

d e s p r i s e s a n n u e l l e s a u filet d e s g é n i t e u r s d ' o m b l e c h e v a l i e r e n n o v e m b r e ( b a s s i n n o r d s e u l e m e n t ) ; l ' o m b l i è r e s u d d e L o w W r a y

B a y a é t é s u i v i e d e 1 9 3 9 à 1 9 7 3 et l ' o m b l i è r e d e N o r t h T h o m p s o n H o l m e d e 1975 j u s q u ' à a u j o u r d ' h u i .

138 J . M . E L L I O T T , E . B A R O U D Y (4)

osmer ids and cyprinids in Scandinavia (e.g. Lindem 1982, Lindem & Sandlund 1984, Brabrand 1986, Jurvel ius et al. 1987, Bjerkend et al. 1991). Few echo sounding studies have been performed on charr (Brabrand 1991). The Windermere surveys were m a d e a long Fixed transects (broken lines in Fig. 1). Surveys of both basins in daylight were performed once every two weeks throughout most of the year a n d every week dur ing the spawning season. Data f rom July 1989 to December 1991 are used in the present investigation. Night surveys have also been m a d e each month from October 1990. A survey was pos tponed on some occasions because of dangerous weather or the absence of the boat during its annua l ma in t enance (the echo-sounder is mounted on the hull o f the b o a t ) . A H y d r o - A c o u s t i c - D a t a -

Acquisition-System (HADAS) provided quantitative information about the fish in the water column by t ransforming and processing the received echo. Results are expressed as mean density of fish per hectare.

3. Results

3.1 . Gill netting and anglers' catches

Gill-net catches of autumn spawning charr in the north basin showed a general upward trend from 1945 to about 1965 (Fig. 2a). This increase was asso­ciated with a cull of larger pike, Esox lucius L. , and the reduction of the biomass of this major preda­tor by about 65 % could be responsible for the increase in charr numbers (Kipling 1984). Since

Y e a r

F i g . 2 ( a ) G i l l - n e t c a t c h - p e r - u n i t - e f f o n ( C P U E ) f o r c h a r r a t W i n d e r m e r e n o r t h b a s i n s p a w n i n g s i t e s f r o m 1 9 3 9 - 1 9 9 1 . C P U E is t h e m e a n

c a t c h p e r g i l l - n e t d a y i n N o v e m b e r n e a r L o w W r a y B a y ( 1 9 3 9 - 1 9 7 3 ) a n d N o r t h T h o m p s o n H o l m e ( 1 9 7 5 - 1 9 9 1 ) s p a w n i n g s i t e s , ( b )

G i l l - n e t C P U E ( s o l i d l i n e ) a n d a n g l e r C P U E ( b r o k e n l i n e ) f rom 1 9 6 6 t o 1 9 9 1 . A n g l e r C P U E is t h e m e a n c a t c h p e r b o a t p e r h o u r .

T h e s c a l e s f o r g i l l - n e t a n d a n g l e r C P U E h a v e b e e n a r r a n g e d s o t h a t t h e o v e r a l l m e a n s f o r e a c h c o i n c i d e .

F i g . 2 . ( a ) U n i t é s d ' e f f o r t d e p è c h e a u f i let s u r l e s o m b l i è r e s d u b a s s i n n o r d d e W i n d e r m e r e d e 1 9 3 9 à 1 9 9 1 . L ' u n i t é d ' e f f o r t d e p ê c h e

c o r r e s p o n d à la p r i s e m o y e n n e p a r j o u r d e p ê c h e a u f i let e n n o v e m b r e s u r l ' o m b l i è r e d e L o w W r a y B a y ( 1 9 3 9 - 1 9 7 3 ) et N o r t h T h o m p s o n

H o l m e ( 1 9 7 5 - 1 9 9 1 ) . ( b ) U n i t é s d ' e f f o r t d e p ê c h e p a r f i le t ( t ra i t c o n t i n u ) et p a r p ê c h e u r ( t i r e t s ) d e 1 9 6 6 à 1 9 9 1 . L ' u n i t é d ' e f f o r t

d e p ê c h e p a r p ê c h e u r c o r r e s p o n d à la p r i s e m o y e n n e p a r b a t e a u et p a r h e u r e . L e s é c h e l l e s d e l ' u n i t é d ' e f f o r t d e p ê c h e d e s f i l e t s

e t d e s p ê c h e u r s s o n t a j u s t é e s p o u r q u e l e s m o y e n n e s c o ï n c i d e n t .

( 5 ) F L U C T U A T I O N S O F A R C T I C C H A R R I N W I N D E R M E R E 139

1965, gill-net catches of charr have fluctuated con­siderably with no obvious discrepancy caused by a change of site from 1973 to 1975 (break in series in Fig. 2a) .

Gill-net and angler ' s catches (CPUE for whole lake) generally followed a similar pattern (Fig. 2b), but there was a marked discrepancy about 1980 when the relative catches crossed (note that both sets of catches were scaled so that their overall means coincided ; means indicated by horizontal line on Fig. 2b) . There was a positive relationship between the two sets of catches when the periods 1966-1980 and 1981-1991 were treated separately (angler A in Fig. 3). A similar relationship was not obtained for a second angler, probably because fewer data points were available (angler B in Fig. 3). This explanation was supported by the s t rong positive relationship between catches by the two anglers even though the C P U E for angler B was generally lower than that for angler A (Fig. 3). It was therefore concluded that there was good, if not excellent, agreement between the anglers ' catches and the gill-net catches.

As the anglers ' catches generally reflected chan­ges in charr abundance , they could be used to com­pare popula t ion changes in the two basins of the lake. A simple, but useful, index of these changes was provided by the ratio of mean catch in the south basin t o mean catch in the north basin. Values of one angler 's catches from 1966 to the present sho­wed tha t , prior to 1983, catches were similar in both basins or were higher in the south basin (angler A in Fig. 4). The marked decrease in the ratio for the next five years indicated the relative decline in south basin catches from 1984 to 1988. The ratio of cat­ches for a second angler over a shorter period (1975 to 1988) followed a similar pat tern (angler B in Fig. 4). There was a clear improvement in 1989 with simi­lar catches in both basins (ratio = 1) for both anglers, — but a decline in 1990 and 1991 to earlier values indicated that the 1989 improvement was tem­porary, not permanent .

Char r fishermen on Windermere have expressed the general, but subjective, view that more brown t rout , Salmo trutta L., have been taken on charr tackle in recent years . Records were obtained from the two anglers and these confirmed an increase in the percentage of t rout t aken in the catches, espe­cially in the south basin (Fig. 5). It is notable that the latter increase commenced in 1984, the same year

which marked the onset of changes in the rat io of catches in the north and south basins of Windermere (cf. Fig. 4). The change in the latter ratio could have been due to the increased propor t ion of t rou t in the catches, but a comparison of the ratio for total cat­ches (charr + trout) in both basins revealed little change in the temporal pat tern from that obta ined for charr alone. Further comparison of the two ratios revealed few differences but the presence of trout slightly increased the ratio in a few years (e.g. 1988, 1990, 1991 in Fig. 6). However, the general conclusion must be that the increases in the catches of t rout in the south basin were not responsible for the lower catches of charr relative to those of the north basin.

Monthly catches were compared for both anglers in the fishing season (March /Apr i l to September) in 1989, 1990, 1991. These years were chosen for more detailed examination because the anglers ' cat­ches could also be compared with fish density esti­mated by the echo sounder (Fig. 7). Apart from one large catch by angler B in April 1991, b rown t rout were rarely taken by both anglers in the north basin. They were, however, frequently taken in the catches in the south basin, especially in spring a n d early summer. These comparisons also show that in most monthSj catches were lower in the south basin than in the nor th basin. Comparisons with the results from the echo sounder are made in the next section.

3.2. Echo sounding

The echo sounder provides estimates of the total number of fish present in the open water of the lake but cannot identify the species. Shoaling fish reco­gnized with the echo sounder were undoubtedly perch, Perca fluviatilis L. , and these fish could be easily deleted from the records. Pike, Esox lucius, were probably rare in the open water of the lake. It was impossible to separate charr from pelagic brown trout and the echo sounder therefore provi­ded estimates of the total number of pelagic salmo-nids (charr + trout) in the north and south basins of Windermere. As mentioned earlier, the me thods used by anglers to catch charr t ake fish at water depths about 1 m to no more than 20 m, and charr taken from the lake have a fork length greater than about 20 cm. For these reasons, the results from the echo sounder were filtered to leave those from water depths down to 20 m and fish longer than 20 cm. The final estimates of fish density were therefore

140 J . M . E L L I O T T , E . B A R O U D Y (6)

Angler A

I I I I I I I I ; i L_

40 60 80 20 40 60

& 21 5 » ^ « «78

Angler

o 2

1 9 8 1 - 1 9 9 1 (.. 12)

40 60 80 20

Gill Net CPUE

40 60

F i g . 3 . R e l a t i o n s h i p b e t w e e n a n g l e r c a t c h - p e r - u n i t - e f f o r t ( C P U E ) a n d g i l l -ne t C P U E f o r t h e p e r i o d s 1 9 6 6 - 1 9 8 0 a n d 1 9 8 1 - 1 9 9 1 f o r a n g l e r

A , a n d f o r t h e p e r i o d s 1 9 7 5 - 1 9 7 9 a n d 1 9 8 1 - 1 9 9 1 ( e x c l u d i n g 1982) for a n g l e r B , t o g e t h e r w i t h t h e r e l a t i o n s h i p b e t w e e n C P U E f o r

a n g l e r A a n d B ( a n g l e r C P U E is t h e m e a n c a t c h p e r b o a t p e r h o u r , g i l l - n e t C P U E is t h e m e a n c a t c h p e r g i l l - n e t d a y ) . R e g r e s s i o n

l i n e s f o r a n g l e r A ( 1 9 6 6 - 1 9 8 0 ) : A n g l e r C P U E = 0 . 7 9 1 + 0 . 0 2 7 ( g i l l - n e t C P U E ) , r = 0 . 6 6 ; f o r a n g l e r A ( 1 9 8 1 - 1 9 9 1 ) : A n g l e r

C P U E = 1 . 6 6 6 + 0 . 0 3 2 ( g i l l - n e l C P U E ) , r = 0 . 5 0 ; f o r ang le r B ( 1 9 8 1 - 1 9 9 1 ) : A n g l e r C P U E = 1.607 + 0 . 0 1 1 (g i l l -ne t C P U E ) ,

r = 0 . 2 0 ; f o r b o t h a n g l e r s : C P U E a n g l e r B = 0 . 2 6 8 + 0 . 5 5 9 ( C P U E a n g l e r A ) , r = 0 . 7 0 .

F i g . 3 . R e l a t i o n e n t r e l ' u n i t é d ' e f f o r t d e p ê c h e p a r filets m a i l l a n t s e t p a r p ê c h e u r s p o u r les p é r i o d e s 1 9 6 6 - 1 9 8 0 e t 1 9 8 1 - 1 9 9 1 p o u r

le p ê c h e u r A , et p o u r l e s p é r i o d e s 1 9 7 5 - 1 9 7 9 et 1 9 8 1 - 1 9 9 1 (1982 e x c l u ) p o u r le p ê c h e u r B ; r e l a t i o n e n t r e u n i t é d ' e f f o r t d e p è c h e

p o u r l e s p ê c h e u r s A e t B ( l ' u n i t é d ' e f f o r t d e p ê c h e p o u r les p ê c h e u r s e s t la p r i s e m o y e n n e / b a t e a u / h e u r e et p o u r les f i l e t s , l ' u n i t é

d ' e f f o r t d e p ê c h e e s t l a p r i s e m o y e n n e p a r j o u r d e p ê c h e ) . D r o i t e s d e r é g r e s s i o n p o u r p ê c h e u r A ( 1 9 6 6 - 1 9 8 0 ) : u n i t é d ' e f f o r t d e

p ê c h e d u p ê c h e u r =- 0 . 7 9 1 + 0 . 0 2 7 ( u n i t é d ' e f f o r t d e p ê c h e d e s f i l e t s ) , r = 0 . 6 6 ; p o u r p ê c h e u r A ( 1 9 8 1 - 1 9 9 1 ) : u n i t é d ' e f f o r t

d e p ê c h e d u p ê c h e u r = 1 .666 + 0 . 0 3 2 ( u n i t é d ' e f f o r t d e pêche d e s f i l e t s ) , r = 0 . 5 0 ; p o u r p ê c h e u r B ( 1 9 8 1 - 1 9 9 1 ) : u n i t é d ' e f f o r t

d e p ê c h e d u p ê c h e u r = 1.607 + 0 . 0 1 1 ( u n i t é d ' e f f o r t d e p ê c h e d e s filets), r = 0 . 2 0 ; p o u r p ê c h e u r s A e t B , u n i t é d ' e f f o r t d e p ê c h e

d u p ê c h e u r B = 0 . 2 6 8 + 0 . 5 5 9 ( u n i t é d ' e f f o r t d e p ê c h e d u p ê c h e u r A ) , r = 0 . 7 0 .

F i g , 4 . R a t i o o f a n g l e r C P U E f o r s o u t h b a s i n t o n o r t h b a s i n f o r a n g l e r A ( s o l i d l i n e ) f r o m 1 9 6 6 t o 1991 a n d a n g î e r B ( b r o k e n l i n e )

f r o m 1975 t o 1 9 9 1 . ( N o t e : t h e h i g h e n c i r c l e d v a l u e f o r a n g l e r B i n 1986 w a s d u e t o t h e i n c l u s i o n o f o n e e x c e p t i o n a l l y h i g h c a t c h

i n t h e s o u t h b a s i n ; w h e n t h i s w a s e x c l u d e d f r o m t h e a n a l y s i s t h e e s t i m a t e d r a t i o w a s v e r y s i m i l a r t o a d j a c e n t v a l u e s ) .

F i g . 4 . R a p p o r t d e l ' u n i t é d ' e f f o r t d e p ê c h e d u b a s s i n s u d a u b a s s i n n o r d p o u r le p ê c h e u r A ( t r a i t c o n t i n u ) d e 1966 à 1 9 9 1 e t p o u r

le p é c h e u r B ( t i r e t s ) d e 1 9 7 5 à 1 9 9 1 . ( N o t e : l a v a l e u r e n c e r c l é e p o u r le p ê c h e u r B e n 1986 e s t d u e à l ' i n c l u s i o n d ' u n e p r i s e e x c e p t i o n ­

n e l l e d a n s l e b a s s i n s u d ; q u a n d c e t t e v a l e u r é t a i t e x c l u e d e l ' a n a l y s e , le r a p p o r t e s t i m é é t a i t s e m b l a b l e a u x v a l e u r s a d j a c e n t e s ) .

F i g . 5 . P e r c e n t a g e o f b r o w n t r o u t t a k e n in t h e n o r t h a n d s o u t h b a s i n s i n a n n u a l c a t c h e s b y a n g l e r s A a n d B .

F i g . 5 . P o u r c e n t a g e s d e t r u i t e s d e s p r i s e s a n n u e l l e s d e s p ê c h e u r s p o u r l e s b a s s i n s n o r d et s u d .

142 J . M . E L L I O T T , E . B A R O U D Y (8)

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F i g . 6 . C o m p a r i s o n b e t w e e n r a t i o s ( s o u t h b a s i n : n o r t h b a s i n = S B / N B ) o f C P U E f o r a n g l e r s A a n d B u s i n g t o t a l c a t c h ( c h a r r +

t r o u t ) a n d c h a r r o n l y .

F i g . 6 . C o m p a r a i s o n e n t r e l e s r a p p o r t s ( b a s s i n s s u d : n o r d = S B / N B ) d e l ' u n i t é d ' e f f o r t d e p ê c h e p o u r l e s p ê c h e u r s A et B , à p a r t i r

d e l a p r i s e t o t a l e ( o m b l e + t r u i t e ) et d e l a p r i s e s e u l e d ' o m b l e .

for those fish that should, in theory, be available t o the angler (Fig. 7).

T h e results conf i rmed the earlier conclusion from the anglers ' catches that the density of fish was con­sistently higher in the north basin. In the north bas in , there was also a similar pat tern in the fluc­tua t ions in fish densi ty and anglers ' catches, e.g. a late summer peak (August), especially in 1989, 1990. Similarities were less evident for the lower numbers in the south basin, but more fish were usually avai­lable in mon ths in which anglers fished. Est imates of fish density at night were usually higher than the cor responding est imates during the day but there were three exceptions in which the reverse was t rue (October 1990, September, October 1991).

These compar i sons are generally encouraging because they show that the results obtained with the echo sounder a re generally comparab le t o angler 's ca tches . T h e echo sounder d a t a not only confirmed

the higher numbers in the north basin but also demonstrated lower numbers in winter in both basins (Fig. 7). It is therefore concluded that the three different methods provided comparable data on the fluctuations in levels of charr stocks in Windermere .

4. Discussion

Nutrient enrichment is not directly harmful to charr and has indeed been used to increase produc­tivity for charr in some extremely oligotrophic lakes in Scandinavia (Milbrink & Holmgren 1984). Howe­ver, in many lakes, any short-term benefits to charr from increasing enrichment are soon outweighed by lower concentrations of oxygen, siltation of spaw­ning grounds and increased competit ion from other fish, especially cyprinids. Examples include Lake Neuchatel where charr became extinct in 1970

(9) F L U C T U A T I O N S O F A R C T I C C H A R R I N W I N D E R M E R E 143

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F Î £ . 7 . C o m p a r i s o n s o f m o n t h l y C P U E f o r c h a r r ( o p e n c o l u m n s ) a n d t r o u t ( b l a c k p o r t i o n o f c o l u m n s ) t a k e n i n n o r t h a n d s o u t h b a s i n s

b y a n g l e r s A a n d B ; e s t i m a t e s o f f i sh d e n s i t y f r o m e c h o - s o u n d i n g a r e a l s o p r o v i d e d .

F i g . 7 . C o m p a r a i s o n d e l ' u n i t é d ' e f f o r t d e p ê c h e m e n s u e l l e e n t r e l e s o m b l e s ( c o l o n n e s b l a n c h e s ) et l e s t r u i t e s ( p a r t i e no i r e d e s c o l o n n e s )

p r i s e s d a n s les b a s s i n s n o r d et s u d p a r les p ê c h e u r s A e t B ; l ' e s t i m a t i o n d e l a d e n s i t é d e p o i s s o n p a r é c h o - s o n d a g e es t a u s s i p r é s e n t é e .

144 J . M . E L L I O T T , E . B A R O U D Y (10)

(Pedro l i 1983), Lake Constance where the charr popu l a t i on is main ta ined only by cont inual restoc­king ( H a r t m a n n 1984), and Lake Zug where catches of charr have fallen by 98 % (Ruhlé 1984). Other , more local factors may be responsible for the extinc­t ion of a charr popula t ion . For example, charr were once present in Ullswater in the English Lake Dis­trict, but have been absent for at least 50 years. The mos t p robab le reason for their d isappearance was pol lu t ion of their spawning grounds by suspended solids a n d lead f rom mine workings.

Al though there is no evidence to suggest that Win­de rmere charr a re endangered, the loss of o ther charr popula t ions th roughout Europe provides an obvious reason for moni tor ing the levels of charr s tocks in b o t h basins of the lake. The three diffe­rent me thods of gill-netting, angling and echo soun­ding have been s h o w n in this investigation to pro­vide comparab l e d a t a on the f luctuations in levels of charr stocks. This similarity increases confidence in the conclusions d rawn from one or more of the sampl ing me thods .

T h e long-term records for gill-net catches are essential for the assessment of the relevance of short-t e rm change . For example , low gill-net catches were obta ined in the nor th basin in 1987, 1988, and 1989, bu t the higher catch in 1990 demonstrates that there has been n o pe rmanen t decline in catches. The 1990 value is similar to values obtained in the early 1980's (Fig. 2 ) .

T h e ra t io of anglers ' catches in the nor th and south basins of the lake clearly identifies the persis­tently lower catches in the south basin from 1984 t o 1988, a n d demons t r a t e s that the improvement in 1989 was t e m p o r a r y , not permanent (Fig. 4). Such compar i sons highlight the need for long- term records . I t is r emarkab le that the start of lower cat­ches of charr in the sou th basin coincided with an increase in t rout catches taken on charr tackle (Fig. 5) . Replacement of charr by t rout would have been an obvious reason for the lower charr catches but fur ther analyses showed that this was not the case (Fig. 6 ) .

T h e increase in b r o w n t rout in the anglers ' cat­ches f rom 1984 t o t he present has been most m a r ­ked in the south bas in (Fig. 5). A n explanat ion for this increase has yet t o be found, but there could have been a general increase in t rout density in the sou th basin a n d / o r a shift in the habitat frequented

by the t rout . Trout have been thought traditionally to inhabit the shallow littoral regions of the lake but if the shore areas become unsuitable for some unk­nown reason, then the fish would move t o the open water to jo in the pelagic charr . Fishermen on Win­dermere have expressed the subjective view that brown t rout are now sparse in the littoral region of the lake. There are also energetics reasons why brown trout should move into cooler water so that their conversion of food into growth is more effi­cient (Elliott 1979, 1981). Such speculations indicate a need for further research to determine why brown trout appear to have become more numerous in the open water of the south basin, but not the nor th basin, in recent years.

The echo-sounding technique provides, for the first t ime, estimates of fish density in the open water of the lake. Previous methods all used nets or t raps, and usually provided an index of abundance , most frequently of spawning adults. Such methods can­not provide estimates of absolute numbers . Echo-sounding overcomes this problem and provides fre­quent estimates of fish density at different depths as well as the size frequency distribution of the fish in the water column. The chief disadvantage of this method is that it cannot separate charr from pela­gic brown trout in Windermere . A similar problem exists in other studies of pelagic fish stocks in lakes (see references in Brabrand 1991, Brandt et al. 1991).

In spite of this problem, the results from the echo rounder a re encouraging. Compar isons with the anglers ' catches produced no major problems in interpretation (Fig. 7). The echo-sounding data sho­wed that fish densities in the open water of the north basin were usually markedly higher than those in the south basin, that few fish were present outside the fishing season at depths usually fished by the anglers, and that fish densities at night were usually higher than those recorded in the day.

It can be inferred from the angler 's catches that most of these pelagic fish are charr, especially in the north basin (Fig. 7). The seasonal, and even daily, fluctuations in the fish densities recorded by the echo sounder indicate that the behavioural movements of the charr must be extensive. The causes of such movements are as yet unknown but could be rela­ted to fluctuations in the food supply or environ­mental factors such as temperature and oxygen. These two latter factors would be limiting in the

(11) F L U C T U A T I O N S O F A R C T I C C H A R R I N W I N D E R M E R E 145

south basin of Windermere in years in which deoxy-genat ion occurs at water depths below 20 m and water temperatures near the surface exceed 16°C (Mills et al. 1990). It is clearly impor tant to esta­blish the tolerance limits for different life-stages of charr to both temperature and oxygen. Experiments have almost been completed to provide such values.

This investigation has shown that by combining results from three different sampling methods , it is possible to describe quantitatively the long-term and shor t - term f luctuat ions in the density of charr in Windermere . Such a quantitative description is only a first step in investigating the ecology of charr in Windermere. Several important aspects must be exa­mined as soon as possible, and these include infor­mat ion on the t empera ture and oxygen tolerance limits of the char r , an explanat ion for the increa­sing numbers of pelagic brown trout in the south basin of the lake, and a detailed analysis of the diel and seasonal movements of different size groups of charr th roughout the lake. W o r k is proceeding to answer all these p rob lems .

A c k n o w l e d g e m e n t s

T h i s w o r k is p a r t o f a l a r g e r i n v e s t i g a t i o n w h i c h is f i n a n c e d j o i n t l y b y t h e N a t u r a l E n v i r o n m e n t R e s e a r c h C o u n c i l a n d N o r t h W e s t W a t e r L i m i t e d . W e a r e e s p e c i a l l y g r a t e f u l t o t h e l a t t e r f o r t h e i r s u p p o r t . W e a r e a l s o g r a t e f u l t o M r J o h n C o o p e r a n d M r G o r d o n K y d d f o r a l l o w i n g u s t o u s e t h e i r r e c o r d s o f c h a r r c a t ­c h e s , a n d t o t h e f o l l o w i n g c o l l e a g u e s for t h e i r a s s i s t a n c e w i t h t h i s p r o j e c t : P . V . A l l e n , P . R . C u b b y , J . M . F l e t c h e r , P . A . T u l l e t t . T h i s w o r k w a s c o m p l e t e d w h i l s t E l l y s a r B a r o u d y w a s in t e n u r e o f a N E R C R e s e a r c h S t u d e n t s h i p .

R e f e r e n c e s

B j e r k e n g B . , B o r g s t r o m R . , B r a b r a n d A . & F a a f e n g B . 1 9 9 1 . — F i s h s i z e d i s t r i b u t i o n a n d t o t a l fish b i o m a s s e s t i m a t e d b y h y d r o a c o u s t i c a l m e t h o d s : a s t a t i s t i c a l a p p r o a c h . Fish. Res., 11 : 4 1 - 7 3 .

B r a b r a n d A . 1 9 8 6 . — E s t i m a t i o n o f f i s h d e n s i t y i n t h e L a k e s V a n e r n a n d H j a f m a r e n , u s i n g e c h o s o u n d i n g e q u i p m e n t . Inf. Inst. Freshwat. Res., Drottninghoim, 7 , 2 6 p ( I n S w e d i s h w i t h E n g l i s h s u m m a r y ) .

B r a b r a n d A . 1 9 9 1 . — T h e e s t i m a t i o n o f p e l a g i c f i sh d e n s i t y , s i n g l e f i s h s i z e a n d fish b i o m a s s o f A r c t i c c h a r r (Salvelinus alpinus ( L . ) ) b y e c h o s o u n d i n g . Nord. J. freshwat. Res., 6 6 : 4 4 - 4 9 .

B r a n d t S . B . , M a d s o n D . M . , P a t r i c k E . V . , A r g y l e R . L . , W e l l s P . A . , U n g e r P . A . & S t e w a r t D . J . . 1 9 9 1 . — A c o u s t i c m e a s u ­r e s o f a b u n d a n c e a n d s i z e o f p e l a g i c p l a n k t i v o r e s in L a k e M i c h i g a n . Can. J. Fish, aquat. Sci.. 4 8 (5 ) : 8 9 4 - 9 0 8 .

C h i l d A . R . . 1 9 8 4 . — B i o c h e m i c a l p o l y m o r p h i s m in c h a r r (Sal­velinus alpinus L . ) f r o m t h r e e C u m b r i a n l a k e s . Heredity, 5 3 ( 2 ) : 2 4 9 - 2 5 7 .

E l l i o t t J . M . 1 9 7 9 . — E n e r g e t i c s o f f r e s h w a t e r t e l e o s t s . Symp.

zool. Soc., Lond., 4 4 : 2 9 - 6 1 .

E l l i o t t J . M . 1 9 8 1 . — S o m e a s p e c t s o f t h e r m a l s t r ess o n f r e s h w a ­t e r t e l e o s t s . In Stress and Fish ( P i c k e r i n g A . D . e d . ) , 2 0 9 - 2 4 5 . L o n d o n : A c a d e m i c P r e s s .

F r o s t W . E . . 1 9 6 5 . — B r e e d i n g h a b i t s o f W i n d e r m e r e c h a r r , Sal­velinus willughbii ( G i i n t h e r ) , a n d t h e i r b e a r i n g o n t h e s p e -c i a t i o n o f t h e s e fish. Proc. R. Soc., B 163 : 2 3 2 - 2 8 4 .

H a r t m a n n J . . 1 9 8 4 . — T h e c h a r r s (Salvelinusalpinus) o f L a k e C o n s t a n c e , a l a k e u n d e r g o i n g c u l t u r a l e u t r o p h i c a t i o n . I n Bio­logy of the Arctic Charr ( J o h n s o n L . & B u r n s B . L . e d s ) , 4 7 1 - 4 8 6 . W i n n i p e g ; U n i v e r s i t y o f M a n i t o b a P r e s s .

H e a n e y S . I . 1 9 8 7 . — T h e i n f l u e n c e o f l a k e m o r p h o l o g y a n d a l g a l c o m p o s i t i o n o n n i t r o g e n - c y c l i n g a n d h y p o l i m n e t i c d e c o m p o ­s i t i o n . Schweiz Z. Hydro!., 4 9 : 3 8 4 - 3 8 5 .

H e a n e y S . I . , L u n d J . W . G . , C a n t e r H . M . & G r a y K . 1 9 8 8 . — P o p u l a t i o n d y n a m i c s o f Ceratium s p p . in t h r ee E n g l i s h l a k e s , 1 9 4 5 - 1 9 8 5 . Hydrobiologia, 161 : 1 3 3 - 1 4 8 .

J u r v e l i u s J . , H e i k k i n e n T . , V a l k a e j a r v i P . & L i n d e m T . 1 9 8 7 . — F i s h d e n s i t y a n d f i s h s p e c i e s c o m p o s i t i o n i n r e l a t i o n t o t h e p h o s p h o r u s c o n c e n t r a t i o n i n s o m e p e l a g i c a r e a s o f l a k e P a i j a n n e . Biol. Res. Rep. Univ. Jyvaskylii, 10 : 1 8 9 - 1 9 9 .

K i p l i n g C . 1 9 8 4 . — S o m e o b s e r v a t i o n s o n a u t u m n - s p a w n i n g c h a r r , Salvelinus alpinus L . , i n W i n d e r m e r e , 1 9 3 9 - 1 9 8 2 . J. Fish Biol., 24 : 2 2 9 - 2 3 4 .

L i n d e m T . 1 9 8 2 . — S u c c e s s e s w i t h c o n v e n t i o n a l in s i t u d e t e r m i ­n a t i o n s o f f i sh t a r g e t s t r e n g t h . I n t e r n a t i o n a l C o u n c i l f o r t h e e x p l o r a t i o n o f t h e s e a ( I C E S ) : N ° 5 3 S y m p o s i u m o n F i s h e ­r i e s A c o u s t i c s , B e r g e n , N o r w a y , 2 1 - 2 4 J u n e 1 9 8 2 .

L i n d e m T . & S a n d l u n d O . T . 1 9 8 4 . — E c h o s o u n d i n g o f p e l a g i c f i sh p o p u l a t i o n s i n l a k e s . Fauna, 37 : 1 0 5 - 1 1 1 . ( I n N o r w e ­g i a n w i t h E n g l i s h s u m m a r y ) .

L u n d J . W . G . 1 9 7 2 . — E u t r o p h i c a t i o n . Proc. R. Soc., B 1 8 0 : 3 7 1 - 3 8 2 .

M a i U a n d P . S . & L y l e A . A . 1 9 9 1 . — C o n s e r v a t i o n o f f r e s h w a t e r f i s h in t h e B r i t i s h I s l e s : t h e c u r r e n t s t a t u s a n d b i o l o g y o f t h r e a t e n e d s p e c i e s . Aquat. Conserv., 1 : 2 5 - 5 4 .

M i l b r i n k G . & H o l m g r e n S . 1 9 8 4 . — R e s t o r a t i o n o f c h a r r p o p u ­l a t i o n s i n i m p o u n d e d l a k e s in S c a n d i n a v i a b y l o c a l l y a p p l i e d f e r t i l i z a t i o n . In Biology of the Arctic Charr, J o h n s o n L . & B u r n s B . L . e d s , 4 9 3 - 5 0 8 . W i n n i p e g : U n i v e r s i t y o f M a n i t o b a P r e s s .

M i l l s C . A . 1 9 8 9 . — T h e W i n d e r m e r e p o p u l a t i o n s o f A r c t i c c h a r r ,

Salvelinus alpinus. Physiol. Ecoi, J a p a n , S p e c . V o l . 1 :

3 7 1 - 3 8 2 .

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